Optical strength of semiconductor laser materials

Pergamon

0079-6727(95)oooo2-x

Pmg. Quanf. Elecfr. 1996, Vol. 20, No. I, pp. 1-82 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0079-6727196 $32.00

OPTICAL STRENGTH OF SEMICONDUCTOR LASER MATERIALS P. G. ELISEEV* Center

for High Technology Materials, University of New Mexico, Albuquerque, NM 87131, U.S.A.

Abstract-A review of phenomena of optical damage in semiconductor laser materials is presented with compilation of empirical data, discussion on theoretical explanation and modeling of related processes. A self-consistent model is considered for quantum-well lasers. The protection technique is also reviewed concerning the elimination of most important facet damaging which is a factor of power performance and reliability of semiconductor lasers.

CONTENTS 1. Introduction 1.1. Introductory comments to the problem I .2. Definitions and measurements of related parameters 1.2.1. External illumination 1.2.2. Self-damaging 1.2.3. Relationship of external and internal power 2. Optical damage and self-damage in semiconductors: Phenomenology 2.1. Comparable survey of LIDT in dielectrics and semiconductors 2.2. Catastophic optical self-damage in semiconductor lasers 2.3. Defects of catastrophic optical damage 3. Sudden failure due to optical damage 3.1. Phenomenology of sudden failures due to optical damage 3.1.1. Power-dependent operation lifetime 3.1.2. The dark region growth near the facet and the kinetics of the facet temperature 3.1.3. Bum-in effect in uncoated devices in inert atmosphere 3.2. Laser mirror facet heating 3.3. Surface degradation processes 4. Phvsical nrocesses of ontical damaee 4.1: Prehminary comments 4.2. Thermal mechanism, runaway and “microexplosion” 4.2.1. External illumination 4.2.2. Physical model for thermal runaway (microexplosion) for self-damage in laser diodes 4.3. Quantum-well lasers: features of increased optical strength of mirror facets 4.3.1. A self-consistent model for facet region 4.3.2. Main results and discussions 4.4. Surface recombination and model for surface degradation 4.4.1. Surface recombination velocity 4.4.2. Comments on the surface degradation model 4.5. Discussion on damaging mechanisms 5. Improvement in optical strength 5.1. Imurovement of COD intensitv 5.1:1. Material optimization 5.1.2. Mirror facet protection techniques 5.1.3. Interference coating 5.1.4. Passivation 5.1.5. Window-layer lasers 5.1.6. Window-type and NAM structures 5.1.7. Laser-irradiated NAM laser 5.1.8. NAM lasers of various types 5.1.9. Bent-waveguide NAM laser 5.1.10. Disordered NAM lasers 5.1.11. Non-injection region laser

*On leave from P. N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow, Russia.

2 2 5 5 6 7 8 8 13 18 23 23 23 25 2-l 28 32 36 36 40 40 43 55 56 58 61 61 62 63 66 67 67 68 69 69 70 70 71 71 72 72 72

2

P. G. Eliseev

5.2. Configuration designing 5.2.1. Thin-active-layer and tapered configurations 5.2.2. QW optimization 5.2.3. Non-waveguide and surface-emitting configurations 6. Conclusions Acknowledgements References

1.

13 73 73 74 75 76 76

INTRODUCTION

1.1. Introductory comments to the problem Optical strength (capability of resisting optical damage) is an important characteristic of semiconductors used in semiconductor lasers. Optical strength is a matter of emission power limitation and of device reliability, as discussed in a number of reviews and monographs(‘-9). The progressive improvement of the high-power performance of semiconductor lasers was accompanied by noticeable progress in optical strength of semiconductor materials during past years. From a more general point of view, this technical parameter of materials is a basic characteristic which has to be understood in terms of the physics and chemistry of substances, surfaces and structures. It will be seen below that the scientific picture of optical damage, and especially self-damage in semiconductors, is far from complete, in spite of the great achievements in high-power laser developments, testing and production. Semiconductors are widely used materials in active and passive optical devices including lasers, amplifiers, modulators, receivers, elements and circuits of integrated and nonlinear optics, optical lenses, mirrors, windows, etc. Some attractive parameters of semiconductor optical materials (large absorption/gain, large refractive index, high nonlinear and electrooptical refraction, photorefractivity, etc.) follow from their strong interaction with the optical field. This occurs especially in the range of intrinsic absorption, where “giant” coefficients can be achieved for optical gain or absorption (more than 104cm - ‘), for nonlinear refraction (n2 as large as lo-’ cm’/W), etc. Numerous applications could be classified by the relation of the operation wavelength to the fundamental edge. Photodiodes and most other types of photoreceivers operate within the range of intrinsic absorption. Lasers, amplifiers and most LEDs operate at the intrinsic absorption edge, as do electroabsorption and excitonic-band modulators. In the intrinsic absorption band the radiation does not penetrate deeply into matter and most of the absorbed energy converts into heat near the surface. Such interaction is used in laser annealing and laser ablation technologies@‘5), which are the subject of developing models and theories. In the case of transparent materials, the radiation can pass through the sample and linear absorption can be sufficiently low (10 -*-lo - 3 cm- ‘) to allow applications of semiconductors in windows, lenses, waveguides, etc. Most nonlinear optical devices and electro-optical modulators also operate in the transparency region where photon energy is less than the bandgap. On the other hand, the strong interaction with the radiation means that most semiconductors have relatively low optical strengths compared to transparent dielectrics. This results in output power restriction and operation lifetime limitations due to sudden failure in semiconductor lasers. Laser-induced damage is also a factor limiting the performance in a number of semiconductor devices and optical elements operating under laser beams”~30’. Besides above-mentioned laser processing technologies, laser-induced damage in semiconductors is always an undesirable phenomenon. Attempts to use the optical damage to fabricate device structures (for example, in the preparation of periodic structures for distributed feedback lasers) were ineffective for practical purposes. One measure of the optical strength is obtained from the critical power density of the optical flux (radiation intensity, measured in W/cm*) corresponding to optical damage. This is a physical characteristic of materials which is important in practice, as the damage leads

Optical

strength

of semiconductor

laser materials

3

to the device failure. Optical damage phenomena for a number of optical and photoelectrical devices are itemized in Table 1. The optical strength at external illumination by laser beam is characterized by laser-induced damage threshold (LIDT) which may be expressed in either intensity or energy density (J/cm2) units. For emitting devices, the characteristic is obtained from self-damage observation and is commonly recognized as catastrophic optical damage (COD) intensity (or energy density). Table 1 shows that typical values of the radiation intensity at the damage threshold are in the range l&100 MW/cm’ being several orders of magnitude lower than the LIDT in high-quality dielectric materials. Most characteristic COD events in semiconductor lasers occur at mirror facet (sometimes called “catastrophic optical mirror damage” (COMD) to distinguish it from “catastrophic optical bulk damage” (COBD)). The critical power for such damage appears to be sensitive to the state of the surface. The practical need to elevate the power performance characteristics of lasers has led to numerous design and technological improvements for facet protection. COBD phenomena appear to be initiated at imperfect sites in the crystal. They have become more important because of the great progress in the elevation of the optical strength at facets. However, there have been only a few observations clearly related to bulk self-damage in laser materials, most of them being made using optical excitation. Comparing COD characteristics of different laser materials, it is apparent that the majority of measurements concern the GaAs-based materials including heterojunction systems of GaAlAs/GaAs, InGaAs/GaAs, InGaAsP/InGaP/GaAs, InGaP/InGaAlP/GaAs (all grown on GaAs substrates). In other materials, COD characteristics have not been studied systematically, for different reasons. One is their high COD level, which is typical for the practically important group of InGaAsP/InP and InGaAs/InP materials. The output power in these laser diodes is often limited, either by thermal saturation due to the total heating of the active region and the strong temperature-dependence of laser emission characteristics (threshold current and slope efficiency), or by catastrophic degradation under current overload of a non-optical nature. Consequently, there are no empirical data on COD power in laser diodes operating at 1.5 pm and at longer wavelengths. Another reason for the lack of data is the low power level of laser emission, which automatically excludes COD occurrence in a number of mid-IR laser diodes (mainly on lead-salt-based crystals) used in high-resolution spectroscopy. Typically in these diodes, the thermal saturation is the limiting factor of emission power. Finally, laser materials like II-VI and other compound materials are at an early stage of technological development. Therefore, most attention to date has been given to the development of devices with practically acceptable performance characteristics. Data for some of these materials are obtained with electron-beam or optical pumping. We have no experimental data on COD in electrical-breakdown-pumped semiconductor laser materials, as rapid degradation occurs in these devices due to mechanisms other than optical damage. Studies on COD phenomena in the GaAs family of materials were performed from the mid 1960~‘~‘-~~)beginning with those concerning GaAs homojunction laser diodes. After the development of heterostructure lasers and the achievement of continuous-wave operation at room temperature (early 197Os), the problem of rapid degradation of these lasers became apparent. Since that time, the COD power of practical laser devices has increased greatly due to (i) the introduction of new materials such as InGaAsP/InP which has a stable surface and does not suffer from COD, (ii) the use of ultra-thin active layer laser structure of the quantum-well type, and (iii) the application of special techniques of facet protection for window-like types. In spite of this progress the COD remains an issue for both output laser characteristics: maximal instantaneous power and maximal operation lifetime at high-power regimes. In this paper we shall present the up to date knowledge and understanding of optical damage phenomena in semiconductor materials with an emphasis on laser materials. A survey of the corresponding data on LID and COD measurements will be given, as well as a

GaAs and InCaAs laser diode, optical amplifiers

InGaAa and Si photodiodes

Absorption

Intrinsic absorption range

Laser-induced thermal damage

LIDT: < 20-30 in 100 ns pulses

Examples

Mechanism of interaction

Spectral range

Most characteristic damage

Typical values for the damage threshold, MW/cm*

COD: IO at CW mode, up to 80 at 100 ns pulses

Self-damage (micro-melting) at mirror facets

Intrinsic absorption edge

Stimulated emission

Laser, amplifiers

LIDT: 3&50 at pulses in range of 15~500 ns

Laser-induced breakdown damage

Transparency range

Nonlinear interaction

Phase conjugation mirror, E/O modulator

Nonlinear and E/O devices

Passive elements

LIDT: 5-20 at CW mode,%&100 at short pulses

Macro damage, Laser-induced breakdown, etc.

Transparency range

Transmission, minor interaction

waveguides

GaAs and ZnSe windows, lenses,

Table I. Optical damage phenomena in optical and photoelectric devices: attempt of rough classification

Photoreceivers

Type of device

F w <

‘FI 0

P

Optical

strength

of semiconductor

laser materials

5

description of typical defects generated by laser radiation (Section 2) consideration of the involvement of COD in long-term operation limitations (Section 3), consideration of theoretical issues of the problem (Section 4) and a survey of advanced laser structures with an enhanced optical strength and reliability (Section 5), followed by concluding remarks. An extended bibliography is given on related subjects which may be useful itself for more detailed study of the problem. 1.2. Dejinitions and measurements of related parameters 1.2.1. External illumination. The phenomenon of laser-induced damage in solids has attracted attention for many years, and there is a wide range of literature on this subject, mainly concerning damage under external illumination”‘, “, ‘s30.40-49). We begin our review with this type of material characterization, and a survey of definitions of the main related parameters is given below. In LIDT measurements, the laser beam parameters are easily controllable and well known. The routine experimental conditions suggest the measurements of energetic beam parameters at the threshold of damage in the material undergoing the laser illumination. Ordinarily, the LIDT is defined at the critical values of the following parameters: peak-on-axis energy fluence F (J/cm2), irradiance or intensity @ (W/cm’), rms electric field E (V/cm), which result in irreversible change in a specimen. These changes may be identified as morphological ones seen in microscopic studies, as an increase in the optical scattering or transmittance of the specimen, or as changes in the device parameters (such as the electrical parameters of photodetectors and other optoelectronic devices). Accompanying effects may also be observed, such as a visible flash in the case of breakdown at surfaces, or a particle emission from the surface detectable by mass-spectrometry or ion current measurements. There are several specific issues concerning accurate determination of comparable damage characteristics: l l l l

l

the criteria of damage, the problem of damage under repetitive action of laser illumination, identification of damage site in the specimen, the reproducible preparation of specimen, especially its illuminated surfaces (front and rear), taking into account realistic illuminated spot shape and pulse shape of laser emission.

As mentioned above, empirical signs are used to identify the damage occurrence, giving the damage criteria. These signs considered simultaneously can give the distinguishing LIDT parameters, the lowest possible value of LIDT being the one used. In order to make comparisons easier, it is useful to mention whether the data relates to ordinary optical LIDT, or to specific morphological or electricaf LIDT. For example, in testing Si avalanche photodiodes for LIDT, different values for morphological and electrical LIDT were measured under Q-switching 10 ns pulses from Nd:YAG laser (49).Typically, the former was 0.2 J/cm’ (onset), 0.5 J/cm* (50% damage), and 1.5 J/cm2 (surface ripple pattern), whereas degradation for the electrical parameter was measured as 5.5 (onset), 9.5 (50% damage) and 13 (severe degradation). The reason for the difference is that the electrically active region of the device is distant from the illuminated surface, and this is a particular property of tested diodes and of the geometry of illumination in tests. Another interesting subject is the determination of LIDT under repetitive action of laser pulses. In the same example of Si APD’49’it was found that the onset of electrically detected LIDT under a single pulse was 5.5 J/cm2, as above, but after 30 pulses it decreases to 2.8 J/cm2 and after 3000 pulses to 0.4 J/cm2. This is an illustration of the “subthreshold” damage accumulation. It is seen that the damage criterion appears to be influenced by test condition and single-pulse measurements gives generally much higher LIDT that measurements at repetitive pulses. The repetition-rate effect will be discussed later. Optical damage can be

6

P. G. Eliseev

initiated at surface scratches, inclusions and internal structure defects, which are all weak sites in the material. Therefore it is not easy to determine an intrinsic LIDT in the bulk if the material is not of excellent quality. In order to measure LIDT, one has to choose the most perfect samples and to make measurements avoiding an involvement of surface damage (by accurate focusing of the beam into the bulk of the specimen, etc.). The majority of known experimental data on LIDT relates to extrinsic LIDT rather than to intrinsic. Examples will be given later. Sample preparation is of critical importance especially if surface damaging processes are under study. The surface mechanism can include the evaporation of adsorbed species (water molecules, for example, under COt laser illumination), followed by their ionization and heating by radiation. The heat can dissipate then from the plasma into the semiconductor producing a thermal damage of the latter even if it is highly transparent to the radiation. The LIDT measured in such a case will not relate much to the optical properties of the material, but rather to the surface state. The role of surface protection and passivation in particular cases will be discussed later. Aspects of LIDT determination are discussed in the literature (see, for example, Ref.@‘)). The measured energetic quantities are optical energy in a single pulse, averaged power in repetitive pulses and in CW laser beam. Measured spatial quantities are illuminated spot size (FWHM, l/e2-diameter for Gaussian beam) and normalized spot shape. Measured temporal quantities are pulsewidth (FWHM) and normalized pulse shape (including data on illumination background, tails, etc.). The physical critical parameters of LIDT F, @, E (see above) should be calculated from measured quantities, usually with some idealization of spatial and temporal shape characteristics. This can give a scattering of experimental data from different laboratories using laser sources of different quality. There also exists a size effect of illuminated spots on LIDT so sometimes an indication of this size is necessary in addition to the calculated data on LIDT. 1.2.2. Self-damaging. The internal processes of laser self-damage differs from the damage under external illumination in several aspects. l

l

l

l

The photon energy is not freely chosen but is determined by material and is situated at the edge of fundamental absorption. The photons pass the pumped region with an amplification of the flux, but will be strongly absorbed in unpumped or defect regions and also in region converted to absorbing state under the indirect influence of operation conditions like local overheating. The optical energy, accumulated in the laser cavity, is normally too small to cause the actual damage. Therefore the energy for optical damage has to be supplied during the damage process, with the laser continuing to operate under enhanced absorption in the damaging site. If total absorption occurs, the radiation intensity inside the cavity drops down within a few picoseconds. Thus the real process proceeds with a relatively low level of additional losses in the cavity (another version is a process initiated optically, but supplied with energy from another source like a pumping current). Power measurements concerning the COD can be made by registration of outside emission, whereas the acting quantity is the internal power. The relationship between external and internal powers is dependent on the cavity configuration and reflectivities, so the published data relate to externally measured powers, which coincide with internal ones in the particular case of well-made anti-reflection coated edge-emitting configurations. In lasers with high reflectivity, like in VCSELs, the internal power is significantly larger than that measured outside. As optical power in lasers can be restricted by causes other than COD, it sometimes happens that COD power cannot be measured at all, in contrast to LIDT measurements where sufficient power may be supplied by an independent source. As a result, COD

Optical strength of semiconductor laser materials

l

7

parameters are known for a few laser materials whereas several of them (mostly mid-IR materials) are not inspected for COD. Two factors prevent the measurement of the COD level in arbitrary samples: a sample having very strongly resistant facets against COD to this type of damage, or a sample being so bad that power does not reach the COD level. A possible means to enter the COD condition is to perform measurements at lower temperatures. In this case the output power performance may be significantly improved. In COD measurements we have no possibility of varying the spot size and shape, which must be determined from other measurements and calculations. A change in the spot shape can influence the measured COD parameters. For example, the measurements in broad-area laser (BAL) diodes give scattered averaged COD powers, correlated to . This is because the spotty near field is somewhat uniformity of the near-field pattern (50.5’) like a finger-print of a BAL diode, and local radiation intensity in individual spots may be much higher than averaged intensity. The actual factor of damage is a local intensity, but not an averaged one.

In edge-emitting laser diodes the shape of the self-illuminated spot is determined by optical confinement in the cavity, and transverse sizes are quite different in vertical (along a normal to junction plane) and in horizontal (in the plane) directions. In vertical directions, the laser beam is strongly confined by the waveguiding structure of the active region, and typical vertical size (spot heighr) is of the order of the value of the emission wavelength in the medium, for instance, in the range 200 nm to 1 pm in GaAs laser diodes (although not equal to, and sometimes hundreds of times larger than the active layer thickness d). This gives a large diffraction divergence of the beam in the far field. In the horizontal direction, the lateral confinement can be varied within large limits. In BAL diodes the spot width may be as large as several hundreds of micrometers, if multimode emission is acceptable, but in numerous types of stripe-geometry laser diodes the width is reduced to a few micrometers to provide a stable single-transverse-mode operation. There is a systematic problem in attempts to compare results in different papers where COD power but not intensity is represented. When no data about the spot height h, is given, we prefer to use a power density p per units of spot width (in W/cm or mW/pm), and if there is no indication about actual spot width u’, we shall use a nominal stripe width, w, (if indicated) (which is not generally coincident with a spot width) to calculate the nominal p. Quantities w and w, are close each to other in structures which are well confined laterally, such as buried heterostructures. However, in many contact-stripe and ridge-waveguide diodes the difference may be as large as factor 1.5-2. So nominal power density p in these cases may be somewhat overestimated. We assume that after measurements of COD power P cODone can calculate actual p (PcOD/ w) or nominal p (PcOo / w,) COD power density. Then, if h is known, one may calculate the COD intensity @ = p/h. In these calculations the filling of the spot is assumed to be uniform. Laser self-damage is identified firstly by sudden and irreversible deterioration of the light-current curve, and post-mortem by observation of damages at device facets. An example of the L-I curve with a COD event identified is given in Fig. 1. The output power drops during a gradual increase in current in the experiment and maximal measured power is assumed to characterize the COD level (power density is 375 W/cm). After the drop, the slope efficiency decreased irreversibly to a low value indicating that laser emission of a poor condition is generated over the active region and has a limited possibility of being extracted from the cavity (both are due to damaged mirror facets). 1.2.3. Relationship of external and internal power. As mentioned above, the power P measured outside the diode is the same as P, inside near the output mirror only if the latter is highly transparent. This is the case in travelling-wave amplifiers, but usually not the case in laser oscillators, where some level of reflectance is necessary to supply sufficient feedback. Very often in edge-emitting diodes the front facet reflectivity R is optimized to a small value

P. G. Eliseev GaAIAs/GaAs DH,

.-5 % ‘5

8 6

5 a 5

4

O

2 0

i

After COD 5

10 15 Pulse current, A

20

: 5

Fig. 1. The light-current characteristics of GaAlAs/GaAs DH laser diode in pulse operation mode before and after COD with an irreversible sudden decrease of power at 375 W/cm”).

( I lo%), so the difference of P and P,, may be neglected in comparison to other measurement inaccuracies. In other cases, the internal power exceeds the external power according to the expression’ 4.5’), P,” = P( 1 + R”2)2/(1 - R) )

(1)

and the same relationship is valid for intensities if spot size parameters are taken adequately. One consequence of this is that a simple AR coating can give a pronounced increase in measured COD power (maximum by n times, where n is the refractive index of laser medium) as it does not modify the internal power corresponding to COD, but increases the ratio of the external power to the internal one. In principle, the surface coating gives a positive effect of facet protection against the COD, so the coating neutral to the reflectance modification (1/Zthick layer) of proper material (like aluminum oxide) also gives an elevation of the COD leve1(54’. Therefore protection coating can give cumulative effect if reflectance is modified. The correlation of COD power to the mirror transmittance (1 - R) was demonstrated in Ref.‘S3’. If R is very high (99% or more in some VCSEL diodes), the formula (1.2.1) leads to internal power being larger than external by several hundred times. This has to be taken into account in comparative studies. Output facets in VCSELs are well protected against COD events by their window-type construction and the high reflectance Bragg-mirror is buried some distance from the surface. As a result there has been no observation of COD in these types of laser diodes up to now. 2. OPTICAL

DAMAGE

AND SELF-DAMAGE PHENOMENOLOGY

IN SEMICONDUCTORS:

2.1. Comparable survey of LIDT in dielectrics and semiconductors A number of optical materials are involved in semiconductor laser design: active semiconductor media themselves, waveguide, cladding and window semiconductors and some dielectric materials used as anti-reflection (AR), high-reflection (HR) and/or protection layer materials. However, the optical damage limitation comes exclusively from the restricted optical strength of active media where the damage process is easily assisted by pumping mechanisms. Nevertheless, we shall give here a short survey of laser induced damage in some dielectrics and bulk semiconductors related to laser materials. Transparent solid dielectrics are enable to transmit very high power laser beams. Some of these materials like Si02, A1203, etc. are quite stable and can serve as protection coatings in

9

Optical strength of semiconductor laser materials Table 2. Peak power threshold of the laser-induced damage in fused silica’*‘) Optical strength, GW/cm*

Minimal

Maximal

2 1 0.5 1.3 2

500 1

As measured in volume At defects in volume At inclusions Front surface Rear surface

1 24 >18

semiconductor lasers. It is interesting to compare their optical strength to that in semiconductors. Early experiments on LIDT in transparent materials were reviewed by Olness(“), and the subject was subsequently the focus of numerous publications(2’,23.24*28*M) and books”“,27). The laser-induced damage in a highly pure fused silica occurs quite sharply at intensities as high as 3.3 x lOI W/cm2 as reported in Ref. (56)where a LIDT under 40 ps pulses at 1.06 pm emission was determined. The laser beam was focused into a 5 pm diameter spot inside the specimen bulk. An energy fluence in this case could be as large as 132 J/cm*. The localization of the damage appears to be very important because of the influence of extrinsic objects like defects and inclusions. This influence gives a large scattering of empirical data not related to specific localization. The situation may be illustrated by data obtained from well-studied and widely used optical materials. Some representation is given in Table 2 on the optical strength of transparent material (also fused silica, as an example) under illumination by Q-switched Nd-laser operating in the nanosecond range with pulses at wavelength 1.06 pm (27).It is seen that the measured values cover three decades depending on localization of the damage. Intrinsic optical strength for these optical pulses is probably above 500 GW/cm* which is the largest measured value for volume damaging. Surfaces and imperfections in the bulk cause a strong decrease of the LIDT so the actual problem of optical strength in optical elements on the base of this typical material is presumably an extrinsic issue rather than intrinsic one. Large scattering of experimental values of LIDT for volume initiated damages may be explained by the presence of not easily identified defects influencing the LIDT. Another cause of the LIDT variation is nonlinear effects like self-focusing and

Table 3. Critical intensities for the modification of semiconductor mirror reflectivity under ruby laser illumination (pulsewidth was 30 ns, R was measured by probe Ar-laser beam at 514.5 nm)‘s8b Material

The threshold intensity for a reflection increase, MW/cm*

The threshold intensity for a reflection decrease, MW/cm2

InSb GaAs CdSe Ge Si B (~olv)

4.5 12.2 4.5 11.3 49.5 180

20.3 12 8.1 54 252 504

Table 4. Comparable laser-induced damage thresholds in some semiconductors under illumination by CW-operating CO* laser at 10.6 pm (exposition time 0.1 s)Q@ Material

Energy bandgap near 300 K, eV

Gt! Si GaAs GaAaP GaP

0.79 1.10 1.43 1.70 2.20

Electron concentration, Cm.’

4 X 10’4 1.3 x 10’3 10’6 2 x 10” 10’8

Electron mobility, at 300 K, cm/V.s 2300 1200 3500 2000 110

LIDT, kW/cm2 8.5 II 6 3.5 1.33

10

P. G. Eliseev

Table 5. Compiled data on the LIDT measurements in &As. Laser sources are: Ruby = Q-switched ruby laser, SRS = stimulated Raman scattering component of ruby laser emission, Nd = Q-switched neodymium-doped laser, Er = erbium doped lasers, CO1 = CO1 gas laser Temperature K 77 300 300

Pulsewidth ns 20 20 30 40 20

1.21 (SRS) 1.17 (Nd) 1.17

300 77 300 77 77 300

1.17 1.17

300 300

1.17 1.17

300 300

17.5 20 35 60 35 45

0.449 (Er) 0.421 (Er) 0.117 (COZ)

300

0.117

300

100

0.117 0.117

300 300

100 190-220

0.117

300

cw (0.1 s)

Photon energy, eV 1.79 (ruby) 1.79 1.79 1.79 1.49 (SRS)

30 20

90 10” 60

LIDT, MW/cm2 3 a+2 12.2 76 4.9 20 7 20 20 50 35 40 43 + 8 30 f 10 13 * 5 224 33 9-18 800 80 16500; 53600 1300-14500 10 + 5 30* 10 10-13 107 76 >63 0.006

Comments

increase of R decrease of R

References 10) ,611 08, (57, (40)

IU) (42, (65,

n-type single shot multiple shots

undoped, SI (high resistance) heavily doped p-type, n-type and undoped single shot, 100 pulses, 30000 pulses

(261 ,u, (66)

(67, (0, (42,

(26,

self-defocusing influencing the local light intensity inside the sample, which can deviate from the averaged measured value. The optical strength of crystalline A&O3 (sapphire) was measured to be as high as 20 GW/cm* for illumination by a Q-switched ruby laser at room temperature(23). By comparison, in similar tests the strength in GW/cm2 was measured to be 16 in ruby crystal, 41 in LiF pure crystal, 8 in NaCl, 13 in quartz, 5-17 in various types of glasses and 0.1 in Plexiglas. In semiconductor materials, LIDT is found to be a few orders lower (in MW/cm2) for peak power (see Tables 3-5). The first experimental observations, related to surface damage in semiconductors under external laser illumination, were probably made in connection to the application of the semiconductor mirror for Q-switching in solid-state lasers since 1964. It was noticed that the mirror changes the reflectivity R under intense illumination and the increase of R was interpreted as a result of melting at the surface, which give a metal mirror instead of a semiconductor one for a short time (“. 58). Laser-induced damage was also associated with an application of polished mirror of Ge or InSb in purpose of Q-switching of a ruby laser(59).It was observed that the mirror was actually working for the purpose, but degraded so rapidly that the practical application was not promising. The multiple-shot-illuminated surface lost a mirror quality and plural surface pits could be observed by optical microscopy. In experiments with different materials (see Table 3) two critical intensities were measured, one at the start of a noticeable increase of R and another at an irreversible decrease of R. Examples of the early determination of the optical strength of semiconductors are reported in Ref.@“).Laser-induced damaging had been studied in a number of semiconductors including Ge, GaAs, GaSb, etc. under illumination by ruby laser at free run operation (groups of

Optical strength of semiconductorlaser materials

I1

0.5 ms-long spikes at 694.3 nm wavelength). Several damage stages of mirror surfaces were identified by the visual inspection of samples: l l l

appearance of non-flat surface (energy density 5-10 J/cm’), formation of circular craters (10-20 J/cm’), formation of cracks propagating from craters (20-30 J/cm’).

These figures are useful to characterize a heavy macroscopic damage of semiconductor surface by light which is strongly absorbed by the material. More delicate but still significant surface transformations are known in the laser annealing processes where the energy density 0.1-I J/cm2 is just effective u4-‘5). These transformations (which often include a melting and recrystallization of some surface layer of material) are detectable by electric measurements but not always are visually seen. Laser annealing is usually used for the treatment of surface layers undergoing ion implantation and containing a high density of irradiation defects (the layer sometimes appearing amorphous). Experimental study of laser-induced damage was performed by Grasyuk and ZubareP) in GaAs, Si, CdSe under the action of intense pulses of radiation at different wavelengths (694 nm, 820 nm and 1060 nm) in connection with the development of optical pumping with one- and two-photon absorption. The LIDT was identified by the appearance of cracks on the illuminated surface. This damage was located at the surface but not in the bulk, even when the penetration depth was about 2 mm. Empirical data for GaAs are included in Table 5. It was established that LIDT increases with an increase of penetration depth of radiation into the semiconductor, i.e. the damage occurs, dependence on the optical energy density in the material. It was noticed also that LIDT is larger in materials with higher mechanical strength. Attempts to observe optical components which could be generated by stimulated Mandelshtam-Brillouin scattering (forward, backward and 90” components) were performed in this study. There were no signs of this type of stimulated scattering up to 30 MW/cm*. The damaging of GaAs and ZnTe under illumination by Q-switched Nd:glass laser was reported by Samc4’).It was noticed that damage occurrence was always accompanied by a visible plasma flash at the front surface. In addition, in thin GaAs sample (about 300 pm) the visible light could be observed from an exit surface which was identified as a second-harmonic emission at 530 nm. The exit surface was not damaged in GaAs up to about 50 MW/cm’, whereas in ZnTe the damage threshold at the exit surface appeared to be lower ( - 1 MWjcm’), than that at the front surface ( - 1.6 MW/cm2). In both materials, the front surface damage was represented by smooth craters, and exit face damage (of ZnTe) consisted of an aggregate of many small pits scattered over an area two or three time the size of the front face damage. Changes of material characteristics can happen at deep subthreshold power levels. Examples of gradual changes under laser emission can be found in the deterioration of edge luminescence emission in GaAs illuminated by laser light at hv > Ei60’ due to possible photochemical segregation of As at the surface and in photochemical improvement of the luminescence by so-called “photo-washing”@” applied also to the GaAs surface to improve its state. Another important observation of the effects accompanying the optical damage was the detection of ions and neutral particles from the illuminated surface’62-64). Emission of positively charged particles from GaAs was reported in Ref. c63) . In the study of ZnS, the emission of high-energy neutral particles (Zn, S, S2) was observed at intensities above 1040% of the critical value for morphological damage w . The flux of the particles increases sharply at the damage threshold, so the emission could be considered as a precursor to severe damage. In principle, the particle evaporation is also a type of optical damage, but it can lead to changes in the substrate which are not easily detectable by optical evaporation occurs uniformly. In this case the optical mirror

methods

especially

if the

is not distorted after many

12

P. G. Eliseev

pulses. The particle flux is a signature of the strong interaction between the optical beam and the solid matrix, which also takes place in dielectrics like Si02 (but not in Alt0#j3). Some comments are need in connection with the accumulation effects observable in LIDT measurements. First, the accumulation of action from previous illumination doses may be seen in the dependence of LIDT on a number of laser pulses (or on the integrated duration of illumination). A fatigue effect - gradual decrease of the optical strength in semiconductor lasers (due to surface degradation) leading to COD after long-term normal laser action will be discussed below. There is also a known pulse-repetition-frequency (PRF) effect’*‘) which is a dependence of the measured LIDT on the repetition rate during the measurement. It can be caused not only by the accumulation of subthreshold changes but also by some memory in the material of previous pulse action producing a decrease of damage threshold for the next pulse. The pulse repetition frequency effect is rather pronounced in semiconductors but not in dielectrics like NaCl. The drop of LIDT in GaAs was found to occur rapidly below 10 c/s and then more slowly to 25% of the single-pulse value as the repetition rate increases to 100 c/s (*‘).A fatigue effect is also a decrease of LIDT as a result of physical and chemical changes in the material produced by the previous action of laser beam. Numerous experiments have been performed on the damage in semiconductors under the action of high-power emission from CO, lasers at 10.6 pm (therefore in the transparency region for a number of laser semiconductor materials). Kovalev et 01.‘~~’reported threshold intensities for breakdown at surfaces in materials with a different bandgap (Ge, Te, InAs, InSb) under the illumination of pulse lasers at 10.6 pm (spot diameter 0.75 mm, pulse duration 150 ns for front spike and about 2 ms at the base). These thresholds appeared to be in a narrow range from 30 to 50 MW/cm* in spite of large differences in the optical properties of studied materials. It was noticed also that in these materials the free-carrier density generated by light and light-induced absorption at subthreshold intensities are very different. These factors are important in the consideration of the dielectric breakdown model, although in the study they appeared to be unimportant. The phenomenon of surface breakdown (visible by a light flash followed by surface damages) was suggested to be not attributed to nonlinear processes in semiconductors, but caused by a unique mechanism for numerous materials associated with an evaporation of species absorbed on the surface. Laser-induced gaseous plasma in the air volume adjacent to the surface give an increase in visible light emission and simultaneously produce surface damage or erosion by a plasma-etching process. The scale of damage is a result of accumulation at repetitive pulses. In experiments with InAs under 20 ms long pulses at 10.6 pm (spot diameter 5 mm), Kovalev@) found the damaging mechanism to be of a thermal nature, with a heating, melting and decomposition of a thin layer of material. The threshold intensities were measured for polished and etched surface to be 3.6 MW/cm* in the uncoated state and 2.7 MW/cm* in the AR-coated (by ZnS/BaF2 layers) state. The mechanism was stated to be common for a pulse duration of more than 1 ms. Simple calculations using sample parameters gave an estimation of critical energy fluence as large as 700 J/cm*(‘O”,which is significantly higher than the observed LIDT energy density of 72 J/cm*. Kovalev(46’postulated enhanced absorption at the surface layer due to mechanical treatment. Some results of LIDT measurements under illumination by CW laser beam at 10.6 ,urnu6’ are given in Table 4 with an indication of the electronic characteristics of the samples used. The low power (l-l 1 kW/cm*) of LIDT in this case corresponds to a rather large energy fluence, for example 600 J/cm* in the case of GaAs. Gallium arsenide was studied most comprehensively, and is represented in the literature on damage in semiconductor materials. A wide compilation of the measurements of LIDT in GaAs is given in Table 5. Selected data from this Table will be used later to compare theoretical estimations with experimental results (see Section 4).

13

Optical strength of semiconductor laser materials

2.2. Catastrophic optical self-damage in semiconductor lasers

A facet self-damage in GaAs laser diodes operating at low temperatures under pulse excitation was reported in 1966 by Bagaev et a1.(3’)and by Cooper et a1.(32’.A possible thermal origin of this type of degradation was suggested. Dobson and Keeble(33’had studied a surface self-damage in GaAs lasers and suggested that a negative temperature dependence of the bandgap in GaAs influenced the occurrence of the damaging. They also showed that the critical power of self-damage could be increased by rolling out the p-n junction near a surface facet so that the active region of the laser was separated from the illuminated spot on the facet (“trough” laser configuration). It was a first example of the window-type laser cavity being more resistant against surface damaging. Kressel and Mierop’34’and KresseF3” reported power density measurements for catastrophic optical damage (COD) in GaAs lasers (of homojunction type like in other early publications) and noticed a difference of COD level at room temperature and 77 K (experimental data are compiled in Table 6). They concluded that the SMBS may be responsible for the positive temperature dependence of the COD power density. It was also stated that the catastrophic self-damage is definitely caused by optical flux density and COD power is not a function of operation currenP4’. Defects produced by COD at the laser diode facets was studied in Ref.~‘~3’~35~‘7~39~5~55~68~78~. Shaw and Thornton’39’ reported signs of thermal decomposition of GaAs in a strongly-damaged region at the facet. It was established that a deviation of stoichiometry in the region in favor of Ga could be interpreted as a leakage of more volatile As at enhanced temperatures during the COD event. They also noticed an appearance of thin lines of damage extending from the facet mirror within the bulk of the active region. Ettenberg et al.“” observed and explained an influence of anti-reflecting (AR) coatings on the critical power of COD in GaAs lasers. It was taken into account that the power measured outside the active region is closer to the internal power (acting in the COD) in AR-coated lasers than in uncoated lasers, where internal power is higher than the external power. Therefore, the power extracted from an AR-coated unit with no COD appears to be larger than that from an uncoated unit. An improvement of externally-measured COD level by about 3 times was found in experiments by AR-coating. Borodulin et a1.‘73’reported observations of COD in

Table 6. Critical power densities and optical intensities for COD in laser diodes on the base of GaAs and GaAlAs active materials (with no “window’‘-like protection). The pulse duration is 100 ns if otherwise not indicated; d is active layer thickness Structure

Temperature K

Power density W/cm

Critical intensity, MW/cm’

230 800 400 100 600

1.8-2.5 5.8-8.0

Comments

References

GaAs

Homostructure SH DH DH DH DH DH

DH DH

77

300 300 300 77 300 300 300 300

-2 -5

-2 4.4 5-8 80-I 10 230-320 340-410

2.9-4.0 5.2-7.3 5.7-6.8

326-466 94-134

4-6.5 1.2-1.6 4.4

300 300

d = 0.5 pm -3mm At = 100 ns AI = 100 ns Ar = 200 ns d = 0.14 nm d = 0.12 pm d = 0.11 pm Al*03 -coated, d = 0.1 pm, Ar = 100 ns CW (E. = 870 nm) Ar = 100 ns, uncoated

1761

(94,

14

P. G. Eliseev COD power density, W/cm

1

10'

10

““’

GaAlAslGaAs DH

100

loo0

l(

00

Pulsewidth, ns Fig. 2. The dependence of COD power density on pulsewidth in GaAIAs/GaAs DH and SQW lasers: l-‘*), 2-““, 3 - P. G. Eliseev and G. T. Mikaelian (not published).

DH GaAIAs/GaAs laser diodes at room temperature in pulse mode of operation. It was demonstrated that the COD power per width units of diode cavity was dependent on the active layer thickness in proportion increasing from 100 to 600 W/cm when the thickness grows from 0.5 to 3 pm. It was noticed that an anticorrelation exists between slope efficiency and COD power in parallel samples from the same wafer: from this observation the estimation of maximal local COD intensity was made at about 10 MW/cm’. Another important observation was reported in Ref.(73)concerning the involvement of COD in the limitation of operation lifetime when the power of lifetime testing is established below the COD level as determined for an instantaneous degradation. The lifetime decreased strongly when power approached the COD level (observable from about 50% of the instantaneous COD power). Typical defects were found in units after long-time testing indicating the accumulation of self-damage defects during sub-critical testing. Eliseev”’ reported the decrease of COD intensity in reverse proportion to a square root of the pulse duration. This type of dependence was confirmed also in Ref.(4) for SH and SCH lasers in the pulsewidth range from 25 ns to 2 ps, whereas Imai et ~1.~~~~ had found a slower dependence to the fourth degree root of the pulse duration time. The latter measurements relates to a range from 100 ns to 10 ms where the slope is decreasing to approach asymptotically a value for CW mode of operation. Therefore the averaged slope may be less than in the range of shorter pulses (see Fig. 2). The COD level for CW DH lasers was reported at 0.2-0.4 MW/cm2 for uncoated devicesc4). HenshalP5’) has reported observations of COD in broad-area GaAlAs/GaAs separate-confinement DH lasers with sawn-side cavity and noticed that the catastrophic degradation at low power could be explained by the presence of internally circulating modes. When laser diodes were selected for uniform near-field pattern and assumed freedom from circulating modes, a good agreement was obtained between the COD power and effective optical width (mode spot size) perpendicular to the junction. A dependence of COD power density on active layer thickness is shown in Fig. 3 for a wide range 0.1-3 pm. An anticorrelation between burn-off power (in range 15-50 mW/pm) and vertical divergence angle (full beam width between half power points in range 16-48”) could be understood in terms of mode spot size (which determines the diffraction divergence of the beam) and of an invariant critical intensity 5.5 MW/cm’ for COD in the five-layer heterostructure under pulse excitation. A lower COD power in non-uniform-near-field samples was also explained by the invariant critical intensity which was achieved more easily in such samples. The development of dark (non-radiating) lines propagating inward at 200400 cm/s from

Optical

strength

of semiconductor

15

laser materials

0

m

Oo.5 0.1

1

5

Activelayer thickness, pm Fig. 3. The influence of the active layer thickness don COD power density in GnAL4s/GnAs DH lasers: open circles - W) at pulsewidth At = 100 ns, triangles -WI at pulsewidth At = 200 ns.

laser facet under damaging optical power was reported by Hakki and Nash(74).The COD intensity for GaAs active material at 100 ns long pulses of injection current was established to be about 2 MW/cm* in uncoated diodes at room temperature. The appearance of dark-line defects (DLD) in the bulk of the diode propagating from the surface in the < 110 > direction (along the active stripe) was also stated (74) . Another important observation was reported in’74’ that COD power might be enhanced by protection coating by A&O+ Yonezu et ~1.“~’reported in 1979 a significant increase of COD power in a window-stripe GaAlAs DH lasers, namely, from 510 mW/pm in ordinary units to 40 mW/pm (up to 10 MW/cm*) in window-stripe units at pulse duration 100 ns. A transient behavior of the optical power intensity under a one-shot-pulse excitation was investigated by Kamejima and Yonezu (‘I). It was observed that output power suddenly fell to about 40-50% of the initial signal when the applied current exceeded a value corresponding to the COD level. The time delay to the start of power fall was current-dependent and falling time was less than a few nanoseconds. Optical intensity at COD followed a reverse square-root dependence on the pulsewidth equal to 12-13 MW/cm2 at 15 ns and l-l.5 MW/cm2 at CW operation mode. It was concluded that the failure process is in agreement with the model of surface heating and material melting at facets producing mirror damage and dark-line defects. Wakao et 01.~‘~)studied COD levels in visible and IR lasers on the base of GaAlAs/GaAs DH materials. As measured in A120)J-coated diodes, the COD intensity at CW mode of operation was estimated to be equal to 1.2-1.6, 0.9-1.3 and 0.7-l. 1 MW/cm2 at wavelengths 870,790 and 760-780 nm, respectively. Pulse COD (100 ns) was found to have similar values: 6.5 MW/cm2 at 870 nm and 6 MW/cm2 at 750 nm. Thus the COD limitation of emission intensity in these “short-wave” laser diodes (with an appropriate dielectric protection) was established to be near 1 MW/cm2 in CW operation mode which corresponds to specific power limitation at 50 W/cm (5 mW/pm) if the vertical size (height) of the mode spot at the mirror is assumed to be typically equal to 0.5 ,um. CW-COD as large as 4 MW/cm2 for 830 nm lasers was stated by Shinozaki et ~1.‘~‘)for aluminum-oxide-protected facet and up to 4.7 MW/cm2 for the window GaAIAs lasers by Ueno(79).In Ref.@‘)the CW level of COD in quantum-well lasers with GaAIAs active layer was measured to be 8.3 MW/cm2 at room temperature which is probably the highest value JPQE 20/l-B

16

P. G. Eliseev

Table 7. Critical parameters of COD in single quantum-well lasers (compilation of measurements at 300 K at CW operation mode if otherwise commented, facet are coated if otherwise commented)

Materials GaAlAs/GaAs GaAlAs/GaAs GaAlAs/GaAs GaAIAs/GaAs GaAlAs/GaAs InGaAsP/InGaP InGaAlAs/GaAs InGaAs/GaAs InGaAs/GaAs InGaAs/hGa P InGaAs/GaAs InGaP/InGaAIP InGaP/InGaAIP

Wavelength, nm 830 860

Power density of COD, W/cm -500

Intensity at COD, MW/cmz

> 1370

Comments no COD

8 780

500-1000

800 810 980 960-980 980

1064 670 698

- 3000

813 8.6 >4 18-20 80

15 525 2500 250

2 7-10

uncoated

References 04, (95, (961 IP7, 181)

198, WI

Ar = 100 ns pulsed (pulsewidth is not indicated) cw Ar = 200 ms cw

At = 30-80

,100, ,101, ,102, ,103, ,104,

for this type of lasers. Some resuming data on COD measurements are given in Table 6. Further improvement in the optical strength at mirror facets of these lasers was possible by introducing special window configurations such as the “trough” laser’“), “crank” laser, “window_stl-ipe” lasers(‘5.79**-84),non-absorbing-mirror (NAM) lasers(8~92),etc. (see Section 5 for more detailed data). Temperature dependence of COD power was investigated in uncoated GaAlAs/GaAs DH lasers of stripe geometry with a 6 pm wide stripe and an 80 nm thick active layefi9)). The laser wavelength was 855 nm. Measurements were performed under 300 ns pulses in temperature range from 25°C to 183°C. It was found that COD power decreases in this range from 103 mW to 46 mW, whereas the current overdrive (difference between operation current at COD and threshold current) changes only a little within limits 230-257 mA. It was concluded that not only optical power but also injection current is responsible for the catastrophic degradation of laser diodes. Studies of quantum-well laser diodes revealed a pronounced improvement in COD characteristics as seen from the data compiled in Table 7. The COD intensity at room temperature is found to be in the range 2-20 MW/cm* for CW-operated diodes and up to 80 MW/cmZ for pulse-operated ones. This improvement may be considered as a continuation of such a tendency for thinner active layer DH lasers due to a smaller optical confinement parameter. The parameter characterizes a fraction of the power flux which propagates within the active layer. When the parameter decreases, the larger fraction of the emission power goes through a transparent waveguide and cladding materials. If the active material converts into an absorbing state, it can dissipate only a part of optical energy, but not the total flux. This may explain an advantage of quantum-well lasers. Another subject is better heat diffusion conditions for ultra-thin active layers if heat generation occurs within them. More detailed consideration of the physics of COD in QW devices will be given later. An example of high-power operation of GaAlAs/GaAs quantum-well lasers is given in Ref.(9’) where SQW-GRINSCH ridge-waveguide lasers were prepared and studied with frequency doubling to obtain high CW power operation in the blue light region. The single mode power was measured from a 3.1 pm wide stripe laser as large as 180 mW, and maximal power was 425 mW. Surface passivation and protection by dielectric films were applied to eliminate COD at mirror facets in these lasers. As a result, the power density reached more than 100 mW/pm with no COD, and CW emission up to 41 mW at 428 nm was measured

Optical strength of semiconductor laser materials

17

as the output of a second-harmonics generating device pumped by the laser diode. Active layer thickness in the laser was 7 nm. Summarising the experimental data on COD power in GaAlAs/GaAs and InGaAs/GaAs lasers dependent on the active layer thickness, one may conclude that the maximal power limit can be found in ultra-thin active layer quantum-well lasers (near l-3 kW/cm for the pulse regime) whereas with the increase in thickness the COD limit decreases and reaches a minimum below 100 W/cm at a thickness between 100 and 400 nm which corresponds with most closely confined optical distribution in the direction vertical to the active layer. As the thickness increases further, the COD power density increases to about 500 W/cm at the thickness of several micrometers. This tendency for DH lasers was shown in Fig. 3. Section 5 will discuss how laser COD power can be optimised by a proper choice of the ultrathin active layer structure. After introducing the quaternary materials like InGaAsP for laser diodes it was established that significant difference exist between “shorter wave” diodes with GaAs-based technology and “longer wave” diodes with InP-based technology. Laser material InGaAsP/InP has a wide application in fiber optical communications so its technology was developed rapidly in the 1970s and 1980s including a technology of low-power but long-operation-lifetime devices for 1.3 and 1.55 pm wavelength ranges. Some reliability advantages were found in these devices in contrast to shorter wave ones. It was important that COD limitations were not pronounced so the critical level for these devices was not well known, as an emission power was often limited by other causes than COD, particularly by thermal saturation and by electrical damage”05’. Only a few papers relate to COD observations in lasers on other than GaAs-based technology. From data on high-power operation of 1.3 and 1.45 pm quaternary InGaAsP/InP laser diodes of VIPS-type (V-grooved inner-stripe grown on p-InP substrate)(‘“’ it is seen that these diodes operate well at power levels as high as lo&150 mW/pm (at - 40°C). An upper limit mentioned was N 190 mW/pm as a result of thermal saturation. Rough estimation shows that lasers do not suffer from COD at intensities of 25-30 MW/cm* in CW operation mode. The diodes had an emitting area 2 pm wide and were coated at both cavity ends for reflectivity of 3-5% at the front facet and 90-95% at the rear facet. Nakano et u/.(‘~‘) reported CW COD in InGaAsP/InP lasers at more than 10 MW/cm* and under short (3.4 ns) pulses at 80 MW/cm*. A trend to use Al-free materials for the improvement of performance and reliability of lasers led to the development of a number of structures using lattice-matched or strained layers of quaternary alloys instead of GaAIAs. (‘08-“s’The advantage of materials containing In in the active region was revealed by a comparison of the ageing characteristics of structures with InGaAs (980 nm wavelength range)(‘“““4’ and InGaAIAs (810 nm) in the active layer”‘5’ to those with GaAlAs. The advantage is that there is no “dark-line desease” of slow (current-driven) degradation in In-containing materials in contrast to ordinary materials.“‘S~“6’.A possible explanation is the so-called dislocation pinning by In atoms in the crystalline lattice, as proposed by Kirkby. (“‘) . For shorter wavelength ranges, laser structures of InGaP/InGaAIP are suitable, and COD limitation in these structures appeared to exist”03,“8’.Later we shall give data from papers devoted to the possibility of improving the COD characteristics of lasers with a ternary alloy InGaP in the active region, using the effect of the change in the bandgap dependence on the ordering/disordering of the natural superlattice structure of the alloy. Selected data on COD power or intensity in QW lasers based on different materials are given in Table 7. Important contributions to the understanding of COD occurrence in semiconductor lasers were made by studies of damaging in laser materials by optical excitation (see(@),for example). These studies were performed with structure wafers of the same composition as laser diode wafers, but allowing scanning and selective pumping by high power short-wave emission. The

18

P. G. Eliseev

claddings of DH materials remain unpumped, as they are transparent for the pumping emission. In the studies, the signs of the damage were DLDs appearing immediately under a visual control, so to some extent the experimental observations concerned the in situ damage, which is not easy to perform in diode lasers. In lasers with electron-beam pumping the optical strength was found to be dependent on the density of the intrinsic (stoichiometric) defects and the density of dislocations.“‘9-‘22’In visible (i& = 530 nm) lasers of CdS the surface and bulk damage was observed at low radiation intensity, 0.2-3 MW/cm2(‘20),and surface degradation under subthreshold power was also noticed. When laser material CJS is grown under the sulfur or cadmium pressure providing desirable control of the crystal stoichiometry, the self-damage threshold could be improved to 1l-l 3 MW/cm2 at optimal pressure, whereas in ordinary materials it was 1S-2 times lower and only 5 MW/cm’ in crystals grown at excess cadmium vapor pressure.(‘20,‘22) It was found’also that doping by gallium leads to higher optical strength between 13 and 14.5 MW/cm2. Measurements were at a pulse width of 8 ns at room temperature and electron energy up to 200 keV. Samples were pumped in the longitudinal geometry and have rear facet reflectance 90% and front reflectance 30-50%. Cavity length was 0.20-0.22 pm and the electron-bombarded spot was 0.5 mm in diameter. In experiments with InGaAsP/InP electron-beam-pumped lasers, the COD level was found to be dependent on the type of starting material (thick films of the quaternary alloy epitaxially grown on InP substrate) and being in the range 5-6 MW/cm’ in the case of film thickness less than 10 pm and up to 8-9 MW/cm2 at a thickness of 20 pm (wavelength 1.0-l. 12 pm, temperature 298 K, pulse duration 100 ns). Bochkarev et ~1.~‘~~)reported increase in the optical strength of laser material based on heavily-doped GaSb, particularly in comparison to GaAs lasers. For comparison, in GaAs under electron-beam-pumping COD intensity was measured to be 5-l 5 MW/cm2 at 80 K and it decreased at 300 K to 2-6 MW/cm2 (100 ns pulses, repetition rate 50 c/s, electron energy 50 keV). It was noticed that electron pumping does not produce a damage up to a current density 40 A/cm’. The quality of used crystals and of surface preparation were identified to affect the COD level. In Ref.(“‘), for example, the COD variation in electron-beam-pumped GaAs lasers was reported to vary from 5-7 MW/cm’ in samples fabricated from noncrucible floating zone grown material to l&20 MW/cm2 in units fabricated from horizontal boat grown material. 2.3. Defects of catastrophic optical damage

Phenomena of COD are well-studied in GaAlAs/GaAs lasers. The damage responsible is known to occur at mirror facets in the form of dark lines which propagate inwards, normal to the facet mirror at velocities 200-400 cm/s. These COD-related dark line defects (DLDs) are thought to be due to localized melting initiated at regions of high nonradiative recombination such as cleaved surfaces or occasionally at material defects. As mentioned above, the optical self-damaging in semiconductor lasers results in the appearance of l

l

l

surface defects in the form of pits, arrays of pits or grooves and holes detectable on the mirror surface of facets;(3’-35) deposited products of destruction outside the pits, possibly extracted from a damaged spot, in some case droplets of melted and recrystallized material can be observed on the facet surface;(3s.39.124) dark line defects (DLDs) extending from the initiation point (presumably from a surface) backward into the active volume. The DLD are observed also after internal damage.(6*.72)

Surface studies”’ showed that damage is nucleated in the center of the active layer (as seen in Fig. 4,“‘) and light damage is localized in a submicrometer-wide region of the facet mirror. By other observations COD defects can extend to a size of a few micrometers (up to 10-20 pm,(39)). Fig. 5 gives an example of severe surface damage which was observed in

Optical strength of semiconductor

Fig. 4. Scanning electron micrograph

laser materials

of optical self-damage DH laser.

19

at the mirror facet of GaAlAs/GaAs

suddenly failed GaAMs/GaAs SQW laser diode, but the scale of the damage suggests not optical, but rather current-induced damage process. The amount of fused material on the facet is much larger than that which can be heated optically. Another typical example is given in Fig. 6 with a small droplet of the resolidified material near the damaged site at the facet. This is obviously a result of optically-induced damage tightly localized at the exit of the active

Fig. 5. Scanning electron micrograph of a severe damage at the mirror facet of not-optical nature (probably, due to current-induced mechanism) of InGaAs/GaAs SQW laser diode.

20

P. G. Eliseev

Fig. 6. Scanning electron micrograph of optical self-damage at the mirror facet of a ridge-waveguide InGaAs/GaAs SQW laser diode: above -’ scattered electron image, below secondary electron image. A resolidified droplet is supplied with higher contrast at lower image.

region (in a small area of mode spot). An example of damaged quantum-well laser facet is given in Fig. 7. It was noticed also that COD is accompanied by propagation of the damaged region inwards, the active region from a facet surface forming DLD on luminescence topograms of the active region. Hakki and Nash observed the DLD growth rate of 200400 cm/s along direction < 110 > of the active stripe under the action of high intensity radiation (damage threshold between 9 and 20 MW/cm ’ ). (74)In Ref.(“) some deficiency of As in the COD defect on the mirror of the GaAs laser diode was explained as a result of local heating during the damage of up to 1400°C. The deviation from a stoichiometry as detected by the x-ray microanalysis (XMA) was confirmed as evidence for thermal decomposition of the material in a number of studies.(3*.39,68) SEM and XMA are used in Ref.(39)to analyze heavy damage

Optical strength of semiconductor laser materials

21

Fig. 7. Scanning electron micrograph of optical self-damage at the mirror facet of GaAlAs/GaAs DH broad-area laser diode.

of GaAs laser facets and it was established that there are fractured regions, fissures across the junction, solidified droplets and continuous chains of solidified material beads. Henry et aZ.@*) carried out a detailed study of catastrophic degradation in double-heterostructure GaAlAs/GaAs laser material under optical excitation by a beam from an Ar laser (operating at 514.5 nm by 18 ns pulses) focused into a spot 74 x 24 pm large on the active layer of GaAIAs (x = 0.08) in the studied double heterostructure. It was established in the Ref.@*’that DLDs of COD are quite different from < 100 > and < 110 > dark lines found in noncatastrophic degradation at lower intensities of emission. The damage threshold by superradiant (SR) emission under the optical excitation was estimated at 9 MW/cm2 (with a factor 2 accuracy). A cleaved surface was damaged by letting the SR emission impinge on a cleaved facet and increasing a pumping power. When COD occurred, the reflected SR light decreased and bright spot of scattered light appeared at the cleave. The damage occurred abruptly with no visible signs of damage at lower power. This damage could be produced anywhere along the cleave at about the same power. The power increase from 5 to 10% was need to obtain the DLD propagation from the damaged facet to the center of the excitation spot. By translating the sample under a fixed excitation spot it was possible to cause the DLD to grow across entire sample (a few millimeters) in any crystallographic direction. Therefore the < 110 > DLDs are not defined by crystallographic direction but are the result of active stripe direction being perpendicular to the cleaved < 110 > facet. Four types of structural defects were identified using SEM and TEM observations:@) l

l

l

First type is composed of small dislocation loops (diameter less than 20 nm) and dipoles which are separated by regions containing no detectable defects. These defects are aligned along the dark line. Second type is composed of large dislocation loops (20&500 nm) elongated along the dark line. These loops lie on < 111 > planes with a Burgers vector a/2 < 110 > . Third type of damage is composed of narrow zones 150-250 nm wide and elongated along the propagation direction of the dark line. The damage is characterized by material contrast indicating composition shift. The latter was identified as an excess of AI in the center of circular defected spot surrounded by Ga-enriched material. This is explained as an indication for a local recrystallization.

22

P. G. Eliseev

l

Fourth type of defects in DLD was not detectable with usual TEM contrast, this was observed by the electron-beam-induced current and cathodoluminescence SEM techniques coupled to scanning TEM imaging. The dark regions in these images are found to contain dislocation loops or no detectable defects and to be 600-800 nm wide. Defects of this type are regions containing a high density of nonradiating centers (point defects or clusters of point defects which have a size smaller than 1.5 nm).

These observations related to GaAlAs/GaAs laser material are in agreement with thermal damaging mechanism assuming the local melting under intense illumination was followed by rapid recrystallization. Several kinds of structural defects was also described by Ueda et af.‘69’as observed using TEM technique in catastrophically degraded GaAlAs/GaAs DH lasers of diffusion-stripe geometry. They are arrays of dislocation tangles (about 2 pm in diameter), nearly perfect dislocation networks, pipe shaped defects with strong dark contrast, and simple dislocation dipoles, The first three kinds of dislocations are assumed to be caused by propagation of molten zone due to local heating at the mirror surface and the final defect may be caused by thermal stress. Glide motion is suggested to produce multiple dislocation dipoles in active region. Point defects and small dislocation loops are generated after the molten zone is cooled and recrystallized, and simple dipoles are assumed to be generated by the relaxation of stress due to lattice mismatching between layers. Therefore, the dislocation dipoles are considered to be generated from the facet by the thermal stress rather than by propagation of a molten zone. As to pipe shaped defects in heavily damaged sites of DLD, it was suggested that these defects may be inclusions in an amorphous state. A study in Ref.“‘) also presents a detailed defect analysis. Alternating dark sites along the line defects with a period that diminishes with an increasing distance towards the active region were noticed. The dark sites were identified as multiple dislocation loops containing a cluster “microdefect” at the center. There was a distance of 300-500 nm between the dark sites produced by the degradation under 100 ns pulses, and 3-5 pm under 1 ms pulses, i.e. in approximate proportion to the pulse duration. No line defects were observed from degradation at CW conditions in periodic structures. Applying a sequence of single pulses made it possible to identify individual dark sites as attributable to the action of a single pulse. These observations suggest the following picture of the line defect formation: A pulse of supercritical intensity heats the surface region and elevates nonradiative recombination causing absorption of laser radiation to the point where a small region near the surface melts. The heated region cools at the termination of the pulse and multiple dislocation loops were formed due to the slip that compensates the change in volume from recrystallization of the melted material. The multiple dislocation loops may serve as absorption regions that will be stronger than the surface region; consequently, the resulting dark site serves as a seed for the subsequent melting cycle. At a higher temperature (but below the melting point) absorption is so strong that the depth of penetration is less than 0.3 pm. The melting region is heated on one side and shifts towards the radiation. This produces the periodic self-induced defect structure under pulsed conditions. The penetration of the line defects into the material bulk is inhibited only when the intensity diminishes to the critical level due to higher cavity losses. Optically induced catastrophic degradation in InGaAsP/InP material was investigated by Temkin et al.“‘” An optical excitation was supplied by Q-switched Nd:YAG laser into spot 650 x 450 pm large. Pulse duration was N 100 ns at repetition rate 250 c/s. Peak power of pumping emission was up to 1.4 kW. Catastrophic degradation was observed in LPE-grown 1.3 pm emitting DH material at pumping 3 to 4 times above lasing threshold. DLDs are generated at the radiation flux in excess of 100 MW/cm*, significantly higher than in similar GaAlAs structures. The velocity of the DLD growth was of the order 200-400 pm/s. In contrast to GaAL4s these lines are shown to be due to localized melting only at material

Optical strength of semiconductor laser materials

23

defects and not at cleaved mirror facets. Due to this it cannot often be generated in large sections of the sample or at all in some of the studied samples. In view of the very high power threshold this type of catastrophic degradation should be of limited importance for the InGaAsP/InP lasers. It was noticed that in IRED devices (1.3 and 1.5 pm range) DLDs of gradual degradation were not generated even at much higher injection levels, by up to a factor of 8 compared to those sufficient for their creation in GaAlAs devices. The < 100 > DLD was observable only occasionally in InGaAsP lasers, so very short DLDs have been observed in 1.3 pm lasers under prolonged high injection and temperature stress at 250°C. Their propagation velocity of 0.3 pm/h is considerably smaller than that of similar GaAlAs/GaAs lasers. No < 100 > DLDs have been found in lasers emitting at 1.55 pm. It was concluded that defects are not seeded on faces, the threshold power for defect formation is much greater than in GaAlAs/GaAs lasers and the defect growth velocity was much less than in GaAIAs/GaAs lasers. Thus, in InGaAsP active material for long wavelength lasers the optical self-damage appears at very high power density to be not an actual degradation mechanism in laser devices. An absence of DLD nucleation at cleaved facets was interpreted as an indication that a surface recombination velocity in the InGaAsP is much lower than the estimated 4 x lo5 cm/s typical for GaAlAs material. As a result, the COD could be considered to be not a significant problem for these long-wavelength lasers until power density is below about 0.1 GW/cm’. As to the same quaternary, operating in shorter wavelength range ( N 800 nm) the COD level was measured in QW laser diodes to be 8.6 MW/cm2 in CW operation mode which was comparable to 8.3 MW/cm’ as measured in the same paper in GaAlAs QW material.@” COD occurrence was dependent on the cavity length and not observed in units with a long cavity.“‘) It was concluded also that the COD in InGaAsP/InGaP lasers could be explained by the conventional surface recombination model taking lower surface recombination velocity in such material as compared to GaAs and GaAIAs. 3. SUDDEN

FAILURE

DUE

TO OPTICAL

DAMAGE

3.1. Phenomenology of sudden failures due to optical damage 3.1.1. Power-dependent operation lifetime. The COD occurrence routinely results from output power increase during the laser testing. However it may play an important role in a long-term reliability limitation as a cause of sudden failure at constant power test because of certain process reducing the optical strength of the material at facets during the normal laser operation. This is the type of fatigue effect mentioned earlier in connection with a decrease of the optical strength characteristics under repetitive action of the laser emission. In some early studies of laser diode reliability it was noticed that the defect of COD can be found in failed units tested under constant current (decreasing power) or under constant power (increasing current) modes. This was stated particularly in Ref.“,“’ with respect to GaAs/GaAlAs DH lasers. A demonstration of accidental COD defects was also obtained in InGaAsP/InP DH lasers in Ref. (‘24) . The power-dependent operation lifetime was taken into account in high-power laser tests. Gradual and sudden modes of failure were found to be power- and current-dependent.‘73, ‘26) A systematic study of this behavior, along with the progress of high-power QW laser on the base of GaAs/GaAk and InGaAs/GaAlAs materials was performed by Moser et al.(‘27-129’, who reported that critical power of COD in QW lasers decreases gradually during long-term aging. The time dependence of COD was studied for the CW operation of GaAlAs single-QW lasers with air-cleaved mirrors. An empirical rule was proposed which yielded the time for the COD failure in the form of an Arrhenius-type expression containing a frequency factor (3.8 x lo6 s- ‘) and an activation energy (1.35 eV). In this way COD is expected as a JPQE 20, I --c

24

P. G. Eliseev

thermally activated process, depending on the mirror temperature. The mirror temperature rise, AT, is assumed to be proportional to the optical output power P, i.e. AT = cP, where c is a proportionality factor equal to 130 Kpm/mW (power was taken per width unit of diode in micrometers). According to Ref. (“‘), laser diodes on the base of InGaAs operating in the wavelength range 900-l 100 nm do not suffer sudden failure due to the growth of dark-line defects (DLD) along < 100 > direction (if tested at relatively low output power in respect to COD). Such DLDs were responsible for short-time sudden failures in laser diodes based on GaAs technology but not containing sufficient amount of In (less than 5%). In contrast to this, such DLDs are not found in diodes with larger In content in the active region. An extrapolated operation lifetime of the strained-layer InGaAs SQW laser diodes (x = 0.37, wavelength 1010 nm) was stated to be over 50 thousand hours at specific output power per width unit of the active region near 1.2 mW/pm.(“5) The rate of the operating current increase (to maintain 60 mW output power) is found near 1% per kh. The only requirement of the device structure was to be beyond the critical thickness (about 12 nm) of the active (strained) layer to avoid the strain relaxation through the misfit dislocation creation. The increase of the degradation rate when the active layer thickness approached the critical value was noticed in Ref.“-“‘. Some indication of DLD formation was reported, but at very low rate in InGaAs QW lasers.(13’) On the other hand, at higher output power per unit width (above 50% of the COD power) the dominating failure process is just optical damaging at the diode facets. In this case the laser degradation is driven by an optical and photochemical processes at and near the surface. The degradation may take place as an instantaneous failure due to the rapid COD occurrence or as a sudden failure after some operation time, as with the COD attributes (specific pattern of the damaged facet). The instantaneous COD pulse power was determined to be as high as 300 mW/pm in SQW broad-area diodes of 960-980 nm range.(‘“) The COD process was found to be responsible for sudden failures in samples of 980 nm range lasers of the ridge-waveguide configuration, operating at 50°C at power level of 50 and 75 mW/,nm.(‘32)The lifetime before failure was 30-50 h at 50 mW/pm and 18&300 h at 50 mW/pm, whereas at 30 mW/pm samples were able to operate over 5000 h with no degradation via the COD process. Thus, at high power level the dominating degradation goes into two steps: 1) slow pre-failure process, leading to the decrease of the COD power, 2) sudden power drop due to COD process. Experimental data for a first stage of the degradation at high output power (50 mW/pm) shows the relatively rapid increase of the operation current (at constant power regime) of about 1% with further slow growth according to a square root of time from 1 to about 5% increment during 150-250 h. In general, the lifetime z to the COD failure may be expressed in the following empirical relationship (for both InGaAs and GaAs/AlGaAs laser diodes, see Ref (127. 133) 1:

l/~ = v exp( - E/ka cP) ,

(2)

where v is a frequency factor, E is the activation energy, kB is the Boltzmann constant, c is the temperature proportionality term, and P is power per micrometer ridge width. The power dependence of the lifetime was found in Ref. (‘32,‘34) to be less steep than a simple exponential function of Eqn. (2). Similar behavior was related in Ref. (7)to the “stress-strength model”. Another identification is a “fatigue” effect as postulated in Ref. (I). The essence of the proposed models is a decrease of the optical strength of the laser material during a long-time operation. The microscopic modeling of the pre-failure process is not established ultimately. Some considerations on this subject are given in Ref. (‘32).The root-time dependence was identified as an indication of an involvement of the diffusion of some kind of defects. The defects are introduced from the facet surface or interface (if it is coated) and diffuse to the bulk via the recombination-enhanced defect motion. As a result, the fraction of the active region adjacent

Optical strength of semiconductor laser materials

25

to the facet appears to be deteriorated by the presence of non-radiative recombination centers reducing the local luminescence efficiency. This is experimentally identified as a growth of dark region near the facet. Okayasu and Fukuda(‘39) and Fukuda”) investigated the reliability of quantum well lasers operating at 980 nm wavelength with a strained layer. Ridge-waveguide diodes grown by MOVPE with facet protection by dielectric layers (antireflection aluminum-oxide coating at front facet and high-reflectivity aluminum-oxide/titanium-oxide multilayer coating at rear facet) were used throughout that study. Life-test was carried out in the automatic power control mode at constant front output power of 10,20,30 and 60 mW or in automatic current control mode at constant current of 150 mA. The ambient temperature was kept at 50°C. Analysis of constant power long-term aging test over 14 thousand hours shows that the degradation rate is proportional to the square root of the aging time irrespective of operating output power. The gradual degradation rate is represented in the form [Z(t) - lo] / lo = A t”exp( - Ea/kaT ) ,

(3)

where Z(t) is the operation current increasing along a time, lo is its starting value, A is a proportionality factor, t is time in hours, EA is the activation energy. Exponent n is found to be near 0.5, and the median degradation rate R = (I - ZO)/ZO t”* was around 10 - 3 h - ’ ’ at 50°C and at output power 30 mW (intensity about 2.5 MW/cm*). When the device lifetime is defined to be the time as operation current reaches 1.5 times the initial value it can be calculated to be 2.5 x 10’ h. Dependence of R on both power and current was found to be linear, but this does not necessarily indicate that gradual degradation is caused by both the optical flux and the injection carriers because, if only one of the two causes the degradation the same result would be conducted due to low threshold current, good linearity of light-current characteristics and weak temperature dependence of threshold. Thus the ultimate identification of the primarily factor of the gradual degradation does not follow from these measurements. In Ref.(‘32)the study of the degradation behavior of 980 nm lasers was continued at powers of more than 100 mW/facet (ridge width was 2 pm). It was stated that there was an absence of dark line defects like < 100 > DLD, but the dark region was observed to be generated around the facet. The defected region increase finally became the cause of the COD occurrence through the additional heat generation. From these experiments the limiting factor of the device life is confirmed to be the COD and it was demonstrated that the suppression of the defect diffusion from the facet is a key technology to lengthen the device life under a high output power operation of more than 100 mW. It was demonstrated that lasers aged under a constant output power of more than 50 mW/,um show stable aging characteristics in the initial stage, but all devices suddenly failed after the stable operation. The damaged part at the facet after COD was clearly observed in the light emitting region at the facet. The damage usually occurred at the facet with an antireflecting film, because optical power density at the facet with lower reflectivity is higher than that at the facet with higher reflectivity. 3.1.2. The dark region growth near the facet and the kinetics of the facet temperature. The appearance of dark region near mirrors is quite a routine degradation phenomenon typical for most kinds of laser diodes.(6,7.‘40) For example, the electroluminescent topography of cavity end in the InGaAs QW laser during the first stage of the degradation demonstrated in Ref.(13*’ indicates the development of eight 10 pm long dark regions extended from facet along the active stripe and formed in the high-power test during 100-200 h of CW operation. According to Ref.(13*),the nature of these defects has not been determined exactly but they may be introduced by an out-diffusion of host atoms of laser material into a coating films (as suggested in Ref.(‘34’)or by an inner-diffusion of some impurities, such as oxygen included either in ambient medium or in the coating film (as considered in Ref.(‘35)). The defect migration processes in the operating laser medium may be influenced by several

26

P. G. Eliseev

factors. These are: (i) elevated temperature, (ii) presence of high excess carrier density supplying a free energy for the recombination-enhanced diffusion, (iii) presence of the electric field supplying the electromigration, and (iv) high radiation energy density which can be converted into a heat at any absorbing point inside the medium and on the surface. One recent instance for the migration was given in Ref.(‘36’where the p-n junction displacement had been observed after long-term operation of GaAs/GaAIAs ridge-waveguide laser diodes with p-side doped with beryllium. The displacement over 0.2 pm into n-side just under a ridge was measured as a result of Be migration in devices operated 7000 h at constant ambient temperature 50°C and at constant optical power of 20 mW (the ridge width about 4 mm). The Be migration is estimated by the diffusion coefficient to be lo-l6 cm/s at an enhanced facet temperature of 175°C. Similarly a junction displacement was found under the electron-beam testing in the microscope during 36 h. In the latter case no thermal effect was expected to have played a role as the beam current was in the nanoampere range at an accelerating voltage of 29 kV. Therefore the Be migration cannot be attributed to the elevated temperature only, but the recombination-enhanced impurity diffusion was suggested. The p-n junction displacement was revealed by EBIC SEM technique. Probably Be atoms diffuse via the kick-out mechanism supported by Ga interstitial (see Ref.‘13”). The mirrors in the study of Ref.“36’ were prepared by the chemically-assisted ion-beam etching before a deposition of the aluminum oxide film. Proposed scenarios includes the enhanced concentration of these defects prior to laser operation, their appearance as a result of migration from the nonstoichiometric mirrors, or creation during laser operation, whereas the mirror may serve as a “heater” thus facilitating thermally activated defect formation. Thus, in the vicinity of the mirror facet during the laser operation a number of migration processes can take place, especially those accelerated by the local temperature rise in the facet region. Such processes include the recombination-enhanced diffusion of native crystalline defects and impurities. One of the most important impurities in the degradation occurrence is oxygen. The oxidation of the uncoated mirror facets leads to the power degradation of laser diodes”’ accompanied with the migration of oxygen atoms into inner region of the diode. The oxidation may be inhibited by the protective coating. However, processes including in-diffusion and impurity migration can occur at the interface between the semiconductor and the coating material. This is assisted by a large amount of the energy which is generated at the interface during a long-term high-power operation. This is a factor lowering the interface stability and enhancing the defect and impurity migration around the interface.“32’ In GaAs/GaAlAs devices the region at the interface often serves as an origin for generation of < 100 > DLD, manifesting the enhanced defect motion in the region. Another species acting as a surface recombination center is probably atomic As segregated at surface in result of chemical reactions. One possible consequence of the defect motion near the facet is the local release of the strairF3”, which reduces the bandgap estimated up to 40 meV and this is equivalent to additional local heating of 80°C. (13*) This band-shrinkage effect facilitates the local absorption of the laser emission near the facet as well as the local increase of the injection current density due to lowering of the diffusional potential of the p--n junction. It comes to an addition with the shrinkage originated by the local overheating of the facet region. The kinetics of summarized effect plays an important role in the degradation process prior to the COD failure. The direct determination of the facet temperature in laser diodes confirms the existence of the overheated region near the facet at sufficiently high output power. Examples of the local temperature measurements are given in Ref.(‘4’-‘44’,In Ref. (‘44’the result is presented on the evolution of the facet temperature during the high-power test at 46 mW in GaAs/GaAlAs SQW ridge-waveguide laser diodes (specific power is 9.2 mW/pm). The temperature

Optical strength of semiconductor laser materials

27

measurements are performed using the Raman microprobe in a backscattering geometry. The illuminated spot size was about 1.5 pm. The comparison had been made of the facet temperature behavior of uncoated units operating in an ordinary air ambient and in some inert atmospheres (He or dry N2). In the air ambient the COD failure occurred in about 20 min of operation, whereas in the inert atmosphere the failure was not observed during about 2 h. According to these measurements, the facet temperature rises AT in air-tested diode and increases before the failure from an initial value of about 70 to 140 degrees just at the start of the rapid thermal runaway at the COD process. The temperature rose to more than 300 degrees over the ambient (room) temperature. Thus there is additional evidence of the role of agents in the ambient air (oxygen and humidity) accelerating the surface degradation, as was established earlier in the case of GaAs/GaAlAs DH laser diodes.“45-‘47) This observation confirms the evolution of the temperature at the facet to somewhere about AT = 140 degrees at the COD threshold as measured in the 1.5 pm wide spot. Because the size of the active layer is much less than this spot diameter, the local temperature in the active medium at facet may be estimated to be higher. On the other hand the steady-state overheating can smooth the temperature differences in such a small spatial scale, so a comparable overheating also occurs in spacer, waveguide and adjacent part of cladding layers. The temperature evolution gives evidence that the thermal sources grow in the facet region during the device operation, and this is a direct result of the deterioration of the power conversion balance in favor of nonradiating processes, made possible due to an increase in the non-radiative recombination center concentration. As mentioned above, these centers. migrate to the inner part of the active region from the surface opened to ambient air. Microdefect analysis of degraded regions near the mirror facet was reported by Hwang et a1.“48’.Strained QW-type laser diodes were studied on the base of InGaAs active material grown at GaAs substrate. The comparison was made between two group of diodes with only one difference: the composition of the cladding layers was Ga,,.&I0.S5A~in Group I and Ino.49Ga,,.51P in Group II. The strain magnitude was determined by composition of active material and was identical in both groups. Diodes were protected by aluminum-oxide coatings. The accelerated degradation test was 500 h at 140 mA and 85°C in ambient atmosphere of 95% humidity. The averaged threshold current changes over 5 samples of each group was from 22 mA to 53 mA in Group I and from 11 mA to 17 mA in Group II. The technique of high-voltage EBIC (electron-beam-induced-current) in combination with SEM imaging was used to obtain top-view and facet side-view images with a spatial resolution up to 0.1 pm. There was no < 100 > DLD observed in either Group. Instead of this, the active stripe in Group I diodes was completely darkened for about 2 pm near the output facet, while a degradation in Group II diodes was less pronounced. This observation correlates with larger increase of threshold in Group I than in Group II. Defect clusters were observed at the facet images placed across the mode spot of the laser. It was concluded that cladding composition, but not stress or partial stress-relief, plays a decisive role in observed degradation near the facet. The presence of Al-containing claddings was shown to be undesirable in laser diodes of this type. 3.1.3. Burn-in efect in uncoated devices in inert atmosphere. In the inert atmosphere the penetration of undesirable defects into active region is significantly slower, but still occurs providing a slower gradual degradation process. ~4) An unexpected result of these tests was that the operation in inert atmosphere (high-power burn-in procedure) improves the facet stability afterwards. The increase of the lifetime to the COD was observed from 6 to 750 times as compared to the operation lifetime of uncoated devices with no prior burn-in. The improvement suggests increased chemical stability of the laser facets. It is possible that the burn-in procedure makes a slow change in stoichiometry at the surface.“47’ The increased stability may be due to localized facet annealing and/or photochemical effect. It was stated

28

P. G. Eliseev

however, that lifetime improvement observed in treated uncoated laser diodes is still substantially less than that of coated lasers. M) There is no ultimate understanding of this phenomenon from a physical-chemical point of view. 3.2. Laser mirror facet heating Facet temperature during the operation is dependent on the heat sources distribution near the surface. If the temperature at the facet is higher than one averaged over the active region, it is said that the optical properties of the near-facet region differ from those of the rest volume of the active medium. Knab et a1.(‘49)noticed that facet region and lateral sides of GaAs homojunction diode are overheated due to non-radiative surface recombination. A change of output-power versus current characteristics was observed after a light etching indicating a change of the thermal balance due to decrease of surface recombination velocity (SRV). In a more recent example of high-power QW laser (proton-bombarded multistripe device) it was shown(‘43)that the facet temperature may be more than by 100 degrees higher than the heat-sink temperature whereas averaged active region temperature is only 10-15 degrees higher than the heatsink at a specific output power 7-8 mW/pm. The facet overheating AT increases at an injection current approaching a twice threshold value and then saturates until three time exceeding the threshold current at a level of AT = 100-120 degrees, accompanied by the appearance of a slower temperature gradient extending for about 150 pm inside the active region. The main overheated fraction is localized near the facet to a few micrometers which is close to the spatial resolution of the luminescence method used. As the COD occurs, the overheated region extends into the active region over 100 pm, which is much longer than the visible dark defect of the COD. An enhanced temperature supplies the acceleration of many undesirable processes in the near-facet region like the defect migration, the impurity diffusion, oxidation, etc. These processes lead to the deterioration of the region to reduce the COD power and to limit the laser action by the facet damaging. The monitoring of the facet temperature may be used to estimate a quality of the protection coating as the latter is able to reduce the temperature due to a reduction in the rate of the nonradiative surface recombination. It is useful also for the study of the failure kinetics and for working out the reliability prognosis of the laser diodes. Several techniques were used to measure the temperature at the laser diode facets, the most fruitful being: l thermal IR emission detection with a spatial resolution, 0 luminescent spectral measurements, l Raman microprobe spectroscopy, l beam-deflection or “mirage’‘-effect measurements, l thermography, l reflectance modulation.

The technique of thermal diagnostics of < 100 > DLDs in degraded LPE-grown CW The spatial resolution DH laser diodes was reported by Kobayashi et al. (‘5D-‘52). was found to be about 5 pm based on the use of an IR-detecting Thermal Plotter technique. The temperature rise in defected sites inside the active region and at the laser facet was also measured.(‘S2) The rise was found to increase the slope in DLDs at the laser threshold indicating the optical absorption to give a contribution into the local heat balance, The absorption coefficient in the DLD was estimated to be 170 cm-’ and the additional heating by laser emission was estimated as N 2.4 degrees at the injection current of 100 mA above the laser threshold. Also it was observed that the temperature rise at the cavity ends is higher by 15-20 degrees than that at the threshold and by 20-22 degrees than that in the laser regime. The temperature rise at a facet of the laser diode(“*) was found to decrease the slope versus current at the laser threshold due to high external efficiency of laser emission. This result GaAs/GaAlAs

Optical

strength

of semiconductor

laser materials

29

suggests that the rise is attributed mostly to a total active medium heating rather than specific surface heating. The temperature profile in the vertical direction becomes narrower above the threshold with a maximal magnitude of the rise * 20 degrees in the central hot spot of the facet. The spatial resolution of other optical methods could be typically about 1 pm, which is valid to measure an averaged temperature over a region including not only active layer but also adjacent layers. This means that actual temperature of active layer may be higher than measured one. As higher spatial resolution is desirable the electron beam charging thermographic technique was proposed with the resolution 0.25 pm.(‘4’)In measurements with high-power lasers, the overheated spot at the facet was identified and the influence of optical power and injection current on the surface temperature rise was established. Todoroki et ~1.“~~) measured temperature profile along the striped region in visible-emission laser diodes; the temperature rise up to 200 degrees was observed at surface at power density about 30 mW/pm. Brugger et ul.(ls4)reported some observations of noticeable surface overheating in GaAs QW lasers. Surface temperature mapping on coated and uncoated mirrors of ridge-waveguide GaAs/GaAlAs single-QW lasers were studied by spatially resolved Raman scattering in Ref. u55). A strong nonlinear dependence of temperature rise on output power was observed for cleaved uncoated mirrors achieving more than 100 degrees overheating at power density 5 mW/pm (intensity more than 1 MW/cm2). Raman line scans show hot spot regions at the facets. The facet temperatures decrease for a constant output power in the sequence for lasers with (i) cleaved, (ii) RIE-etched, (iii) 1/2-.41203 passivation, and (iv)L/4-A&O3anti-reflection (AR) coated facets. Laser mirrors with the coatings withstand up to five times the power density compared to uncoated ones without significant heating and degradation. Thus in AR-coated samples the overheating at the surface for 100 degrees occurs at power density more than 30 mW/pm. Local electroluminescence measurements along the cavity confirms the high temperature when approaching the facets and demonstrates that the temperature of the resonator bulk material does not increase significantly during operation of undegraded lasers. The appearance of disorder-activated Raman phonon modes in degraded lasers indicates strong crystal damage in the active mirror regions. Spatially-resolved light reflectance modulation (RM) measurements on mirrors of GaAs/GaAlAs lasers was reported by Epperlein. (15’) These measurements show that the change of normal-incidence reflectance AR/R increases with optical output power. Below the lasing threshold the change is due to electroreflectance caused by a reduction of the surface potential by carrier injection, whereas above the threshold it increases with a power strongly dependent on the mirror technology used. The probe beam was at wavelength of 457.9 nm and it was focused into a spot of about 1 pm in diameter. Temperature maps exhibit a localized hot region around the active layer, with the temperature dropping sharply outside within a few micrometers. The rise of temperature was estimated at the low power level of 2 mW/pm to be about 20 degrees. Compared to Raman spectroscopy, RM offers numerous advantages: it allows a continuous recording of the temperature increase as a function of the optical power P, it represents a sensitive local probe for observing the temporal development of the degradation processes, and it allows the recording of temperature maps of the hot mirror region in acting lasers. In contrast to the time-consuming point-by-point Raman measurements, the RM measurements are significantly faster, more sensitive (to about one degree rise) and provide continuous temperature records. Furthermore, RM allows an easy monitoring of the time dependence of degradation process as well as the recording of mirror temperature maps. In Ref.(“@ the RM technique was used for systematic study of the facet overheating in GaAs/GaAlAs lasers. Maximum temperature rise was about one degree per 1 mW optical power from a 10 pm wide ridge active region for a diode with an uncoated mirror. In 1/2-A1203 protected samples the rise was ten times lower (both figures at power less than 5 mW/pm). Using the temperature measurements the degradation processes have been monitored in real time. Thus, a critical temperature rise of about 120 f 10 degrees was

30

P. G. Eliseev

found for the occurrence of COD in GaAlAs material system (at power density about 18 mW/pm). A study of short-wavelength lasers was reported in Ref.(“*). Temperature rises were measured on air-cleaved, uncoated mirror facets of junction-side up mounted InGaP/InGaAlP/GaAs GRIN SCH multi-quantum-well laser diodes as a function of the injection current by Raman spectroscopy via the Stokes/anti-Stokes phonon line intensity ratio and the phonon line shift as well as by reflectance modulation as a novel application for laser mirror characterization. Lasers of ridge-waveguide type operated at 665 nm. Below the threshold current the temperature rise is due to Joule heating of the drive current across the ohmic resistor and is 35 degrees at threshold. Above threshold a significant power dependent heating caused by absorption of laser radiation is superimposed. The heating efficiency becomes six times higher than on comparable GaAlAs/GaAs mirrors. In this regime the temperature increase is considerably higher, i.e. more than 100 degrees at 4 mW for 5 pm wide ridge laser. The different measurement techniques (Raman line intensity ratio, Raman line shift and reflectance modulation) have produced consistent data. The influence of the vertical structure of visible GaInP QW lasers on the facet temperature rise was studied in Ref .(‘u . RM measurements show a sensitive dependence of the laser mirror temperatures on the number of quantum wells, the type of cladding layers, the configuration of a heat spreader layer covering the ridge waveguide, and on how the laser is mounted on a heatsink. The temperature versus injection current curves demonstrate also the contribution of laser radiation heating even in single quantum well lasers. The temperatures decay rapidly (nearly in exponential manner) from mirror into cavity within about 6 pm (l/e-point) as found from spatially resolved electroluminescence spectra detected along the laser cavity. From the view-point of obtaining the lowest facet temperature in InGaP lasers, ones of SQW type, with GaAlAs claddings and with a thick heat spreader aligned with the mirror edge provide considerable improvements (e.g. increased ramped COD level at mirror of more than 50%) over conventional InGaP lasers with InGaAlP claddings. Thus, the most significant conclusions of these studies of red-emitting lasers are(ls9):(i) the low power data show that GaAlAs claddings lead to lower facet heating (about 2.5 times) than InGaAlP cladding layers due to a lower electrical and thermal resistivity; (ii) temperature rise increases with quantum well number; (iii) the facet heating with junction-side-down mounting is lower (about twice at 15 mW) than with junction-side-up mounting. Some additional results concerning lattice disorder and facet heating were reported by Epperlein et al.““’ based on the Raman scattering technique applied to GaAs/GaAlAs SQW lasers of RW type (5 pm wide) operating at 830 nm wavelength with < 110 > mirrors formed by a chemically-assisted ion-beam (“dry”) etching and coated with 1/Zthick AhO3 passivation layers. The presence of strong structural and compositional lattice disordering was established, depending on the mirror treatment prior to coating and a presence of arsenic clusters at the dry-etched mirror surface. Both the mirror temperature rise (between 40 and 90 degrees as measured at 30 mW output power) and COD level (between 160 and 50 mW) were found to be dependent on the strength of mirror disorder, as characterized by the intensity of a specific Raman mode at 193 cm - ‘. This mode can be attributed to disorder activated longitudinal acoustic phonon scattering in addition to mode of elemental polycrystalline arsenic. Thermoreflectance measurements were reported by Epperlein.“” Surface heating has been found to depend sensitively on the mirror treatment prior to coating. The removal of any damage layer by wet etch efficiently reduces the mirror temperatures. The onset of additional heating at threshold current has been observed for all quantum well lasers investigated. This heating is due to optical absorption in the mirror region and is superimposed on the Joule heating of the drive current. Its strength increased linearly with the number of active quantum wells. A gold heat spreader lined up with the top mirror edge causes significant cooling of the facet surface.

Optical strength of semiconductor laser materials

31

Table 8. Temperature measurements of high power CW SQW laser of GaAlAa/GaM”“. Diode characteristics: 16 nm thick active QW layer of GnATAs (x = OJM), 12 x 6 pm wide stripes, L = 0.5 mm, Ala03 and AIrOr/! coatings, AR/HR = 0.15/0.95, junction-down/Cu. Threshold current 250 mA, measurements are given at 1.5, 2 and 3.2 of threshold Current, mA

Power, mW

Average midcavity temperature rise, deg.

Facet temperature rise, deg.

250, threshold 315 500 802

110 about 220 500

2 4 10

27 96 98

Degradation processes have been observed in real time by continuous monitoring of mirror temperature. Dark line defects formed during laser operation exhibit a temperature gradually increasing with time. The mirrors suffer catastrophic optical damage within seconds after having reached a critical temperature. Temperature measurement on a bent-waveguide NAM structure revealed a considerable reduction of heating, i.e. of optical power absorption. By other reports the temperature rise is not obviously attributed to optical power action at least below the COD level. A comparison of SQW and DH lasers was done in Ref.(‘6’)in respect of facet heating. In DH lasers the temperature rise at the mirrors was found to increase at the lasing threshold, indicating a substantial contribution of the laser beam to facet heating. This is in contrast to single-quantum-well lasers in which the laser emission has been shown to play a minor role. These data suggest a large difference in the facet absorption of laser photons between the two type of lasers. Consider quantitative data from these publications. As measured using the Raman spectroscopy the mirror temperature rise in 830 nm DH lasers grows up to about 200 degrees at pumping current 120 mA providing output power 50 mW. In another unit (780 nm DH laser) the same temperature rise was measured at 200 mA and 80 mW output power in CW operation mode. In both samples the temperature curve had a slope increase at the laser oscillation threshold. In SQW laser the slope change was not observed.(16*)A temperature rise was almost linear over the current range to about 80 mA and reached about 70 or 100 degrees. In both publications a quantitative uncertainty exists as a starting non-zero rise indicated in plots at zero current. It was concluded that in SQW lasers the facet heating (at least, at initial slow degradation regime) is due to the surface non-radiative recombination and is primarily determined by injection current density. The temperature rise rate was measured to be 0.4 degree/mA in 5 pm wide and 750 pm long ridge-waveguide laser diode (the rate is dependent on the diode size reaching 0.8 degree/mA in smaller units). Measurements of the temperature distribution along the cavity of 0.5 W GaAlAs SQW lasers were reported in Ref. (‘43) . The average temperature of active layer is l&l 5 degrees higher than the heat sink temperature at 0.5 W optical output. The facet temperature can exceed the average one by over 100 degrees. A current and power dependence of the temperature rise is seen in Table 8. The temperature rise does not grow in proportion to emission power, but formally the rise coefficient normalized to the power density per diode width is about 30 deg.pm/mW at double the threshold value. The highest power corresponds to a power density of about 7 mW/pm and temperature rise coefficient is equal to 12.5 deg.pm/mW, so there is no direct evidence of optically assisted facet heating. Instead, some saturation of facet temperature was found at high power before the damage. The spatial temperature profile contained a sharp increase near the output facet (about 60-70 degrees over a distance of l&l 5 pm) and a slower gradient over about 150 pm from the surface, with a temperature difference of about 20 degrees. It was shown that after surface damage of the diode, but still operating at higher current, the

32

P. G. Eliseev

overheated region (more than 100 degrees higher than a midcavity rise) appears to be about 120 pm long, which is twice as large as a visible dark line defect near the surface. The measurements of facet temperature were reported also in Ref.(‘63).The temperature rise of the facet relative to a rise inside the cavity was measured to be 30 degrees in a GaAlAs/GaAs unit of 100 pm width at optical power 250 mW and current 700 mA. In InGaAsP/GaAs units operating at the same wavelength, the temperature rise at the facet was measured to be only 15 degrees at output power 600 mW and current 2 A. Thus the facet temperature rise formally normalized to power density per width unit will be 12 deg.pm/mW in the former sample and 2.5 deg.pm/mW in the latter. Therefore the facet heating in quaternary active material appeared to be 4-5 times less than in GaAlAs at the same emission wavelength. In samples of InGaAs/GaAs operating at 980 nm, the facet heating was found in the same range as in the quaternary sample with some superlinearity versus optical power. In spite of this, the facet overheating was also measured above 100 degrees at low power (when total heating leads to thermal reducing of laser emission power). The facet temperature rise as plotted versus current density seems to be linear below a density of about 2 kA/cm*. The conclusion was made that in this range the facet overheating is due to absorption of laser emission and to non-radiative recombination of injected carriers at the mirror facets. Above 2 kA/cm* the overheating was attributed to a surface leakage current. It is also stated that in GaAlAs/GaAs lasers at a power of about 3 mW/pm (intensity more than 0.5 MW/cm*) the facet overheating reaches 150 degrees and any increase of power leads to an abrupt rise of facet temperature and consequently to optical breakdown. In AI-free lasers of 800 nm range the facet temperature does not exceed 60-70 degrees at intensity 1 MW/cm*. This allows power corresponding to 50 mW/pm without facet damage (intensity 5-10 MW/cm*). The failure mechanism in this case was electrical but not optical breakdown at current more than 2 kA/cm*. Photothermal microscopy was used for absolute temperature determination in InGaAsP/InP lasers in Ref. (142) . Facet temperature was attributed to output power by a slope coefficient of 10 deg./mW for 2 pm wide active stripes. The normalized rise coefficient is 5 deg.pm/mW. This figure claimed to be low compared to those reported for GaAlAs/GaAs and GaInP and indicates a weak nonradiative recombination process at the facets of this laser material in agreement with the higher mirror reliability of this type of laser. A technique for surface temperature measurement based on the beam deviation of the beam passing by the facet was reported in Ref. (W . There was no differentiation of the total heating of the active region and surface temperature rise. 3.3. Surface degradation processes We can conclude from the above consideration that the facet mirror degrades easily during operation if it is not properly protected against surface reactions assisted by the operation conditions. Agents of these conditions are excess carrier density in the active region, laser photon flow through the facet and active chemicals (oxygen, water vapor, etc.) attacking the facet from the ambient medium. A worse situation takes place in the ordinary unprotected edge-emitting laser diode: the active region has an exit to the open surface at the illuminated (modal) spot on the cavity mirror. Therefore the energy flow to the surface from the active region is supplied by both free carriers and photons. Under laser conditions the exit undergoes photochemical reactions and recombination-enhanced migration of atoms and defects in the near-facet volume. These erosion processes (‘.‘O* ‘46)which are not known entirely, give a gradual decrease of the optical strength so that laser operation at high output power can be considered as a preparation time to optical damage. The active exit area is very different in DH and QW lasers in accordance with the thickness of active layer(s), 0.143 pm in a typical DH laser and 5-20 nm in QW lasers. Thus the exit in QW case occupies only a small fraction of the modal spot. The difference can be partially eliminated when the waveguide and spacer layers

Optical strength of semiconductor laser materials

33

of QW laser structure are filled by excess carriers. If so, the active area increases at the facet of QW laser, and surface degradation would occur more extensively. The main process identified in GaAlAs/GaAs DH lasers was oxidation(‘4s~‘“’ supplied by ambient oxygen and humidity. The process also occurs when the dielectric layer has been deposited on the facet, but more slowly, controlled probably by the penetration of active species through the protection layer. V’ Chemical changes accompanying the facet degradation of AlGaAs/GaAs QW lasers appeared to be complicated.(‘47) Mirror facets in InGaAsP/InP DH lasers were found to be more stable chemically and the oxidation rate was significantly lower.“65’This is an advantage of these lasers for long-term reliable operation.“@ Due to the surface degradation, the temperature of facets increases during operation, eventually reaching a critical temperature, thermal runaway, and catastrophic optical damage. A study of changes in composition of the near-surface region of facets which accompany heating has been carried out in Ref. (14”for uncoated CW-operating lasers on the base of SQW GaAlAs/GaAs/GaAlAs GRIN SCH-structure. High resolution depth profiles by scanning Auger microscopy show that the laser facets can be quite variable in initial composition, and undergo pronounced stoichiometric changes even during the first few minutes of operation. At longer times a continuing out-migration of group III elements is observed. Unlike the double heterostructure lasers, facet oxidation is not pronounced and is not responsible for diffusion of Ga and Al. There are indications, however, that a slow leakage of oxygen into the crystal may occur. Spatially resolved analyses provide evidence that carrier-mediated elemental redistribution is an important factor in facet degradation. The progressive accumulation of defects which may act as non-radiative recombination centers provides a simple means of facet heating. Analyses of lasers which have suffered catastrophic damage indicate that the facets are not always melted, and that there is no typical chemical state which distinguishes them from facets of laser which are fully operational. It is interesting to compare these results to those typical for DH lasers. It has been established”67’ that after long operation (13000 h) in a “drv nitrogen” atmosphere the uncoated GaAlAs/GaAs DH laser facet was deteriorated (corroded) by oxidation supplied by interaction with trace amounts of oxygen or water vapor in the atmosphere. Auger depth profiling had shown enhanced oxygen signal in the active region to a depth of about 60 nm. It was proposed that oxide formation was promoted by high carrier and photon densities at the facet surface. Several subsequent investigations of facet oxidation have focused on the relationship between the semiconductor composition (i.e. AI content) in the active region and cladding layers, and their susceptibility to oxidation. Thermal oxidation of a DH structure and of GaAlAs films with Al fraction x ranging from 0.3-0.9 was reported to show that in all cases the presence of Al led to much thinner oxides than found for GaAs under the same oxidation conditions.“@’ It was proposed that the presence of a dense Al-rich oxide reduced oxygen permeation through the oxide layer and led to the formation of thinner oxides than would have been found for GaAs. In contrast a study of active region oxidation during DH laser operation at x from 0 to 0.17 in pure and humid nitrogen showed that the thickness of the facet oxide at constant operating time in nominally dry nitrogen increased with increasing Al content.(‘45’ Water vapor increased the oxide thickness for Al fraction of 0.17 by a factor of 100, while little effect was found for pure GaAs. The chief effect of increasing Al concentration is to facilitate initial oxidation: once a 10 nm thick oxide has been formed the rate of oxidation of all GaAlAs stoichiometries is the same, i.e. diffusion-controlled. Similar trends were observed in life-testing studies with various active layer compositions.“46’ Clearly these results are difficult to reconcile. It would appear that a number of independent factors control the oxidation rate so that trends of overall rate with increasing Al content are not monotonic. The question of exactly how the formation of an oxide on a DH laser facet might lead

34

P. G. Eliseev

to the degradation of laser performance is not entirely understood, since both the optical properties of the facet and charge carrier dynamics in the near facet region are likely to be affected by changes in the laser materials, The oxidation studies suggest that since oxidation leads to segregation of group III elements towards the surface of the facet, the generation of lattice defects is of fundamental importance to the process of facet deterioration. This was recognized in an early model which described the relationship between injected carrier densities and degradation in terms of the dependence of the nonradiative recombination rate on oxidation-generated defect concentration. (‘O)Aggregation and growth of defect-rich regions was proposed to result from energy released by nonradiative recombination. These ideas have been applied with some success to explain catastrophic failure of laser diodes, usually accompanied by massive damage to the laser facet. Measurements of facet temperature during operation indicated that the dark regions are heat sources, and that facet heating and growth of the dark region are correlated. The rising temperature enhances the defect generation rate still further, and nonradiative recombination rates increase accordingly. Thick, near stoichiometric oxide layers have been found on the facets of uncoated DH lasers after normal operation, and their formation, attributed to photo-assisted process, has been proposed to play an important role in facet degradation.“’ In spite of minor oxidation found in QW laser w’) , degradation of the facet during operation of QW laser also leads to COD. This suggests that similar degradation mechanisms are indeed operant. There are several notable differences which indicate that matters are not so simple, however.(‘47’First, time-resolved measurements show not only a monotonic increase in facet temperature as a CW QW laser is operated, but also clear evidence for thermal runaway initiated after reaching a critical temperature (120-140°C above ambient, averaged over the probe beam spot). (‘4~)Runaway of this type has been proposed but not observed in measurements of DH laser facet temperatures. Interestingly, probe beam heating of a QW laser well above the same average temperature did not trigger this process, indicating that a temperature rise is not the sole reason for failure. A second very important observation also distinguishes COD in. QW and DH lasers. In studies on coated QW laser facets, the initial gradual temperature rise was found to be correlated to the operating current density145’ indicating that facet heating due to absorption of light was unimportant in the early stages of facet degradation. This is not found to be the case for DH lasers, which showed evidence for both surface current and photo-induced heating. (‘46)This result was explained in terms of the differences in absorptivity of the active region due to differences in band structure between a 2-D QW and a 3-D DH GaAs layer. (13*) The higher transparency of the QW active region during stable laser operation minimizes photon-induced heating. Indeed, laser designs which exclude charge carriers from the facets dramatically reduce the temperatures of coated QW laser mirrors during operation. It appears that although facet changes have been implicated for both DH and QW lasers, the two types of devices may follow rather different paths to COD. Despite the recognition that their physical characteristics are critical, there is little systematic information on exactly how facets participate in laser degradation for any type of semiconductor laser. Data for coated and uncoated, pulsed and CW lasers are freely compared. Most materials studies have been post mortem, examining structure of lasers which have failed, or which have been operated for a randomly chosen period of time. Such measurements cannot provide information on the process of facet degradation, If indeed a rise in facet temperature is indicative of degradation, then measurements of QW facet temperature during operation indicate that changes in the lasers occur at the earliest times.(‘44) Identification and characterization of the chemical nature of these changes is primary motivation of the study described in Ref.(14’),with the expectation that they may provide important clues to the role of the facet in COD. The data show clearly that initial facet compositions are variable and

Optical

strength

of semiconductor

laser materials

35

far from ideal. After operation for as little as 2-10 min, the compositions of the facet regions of the active/graded index and cladding layers change markedly, but no single type of change can be linked to COD. In particular, facet oxidation is not uniform or extensive, and facets which have suffered COD are not necessarily more oxidized than those which have not. Composition changes are not limited to the facet surface, indicating that elemental redistribution during laser operation is very fast. These results suggest that the process of facet degradation is more subtle than previously realized, and that it plays a complex role in laser degradation. The following observations were made from the study of chemical changes on the facet surface of GaAlAs/GaAs QW laser materials during normal laser operation (in uncoated samples):

l

l l

l

l

l

The facets of unoperable, unbonded lasers are far from ideal in composition, showing segregation of (Al, Ga) and As. Well-defined oxides do not grow on the uncoated facets during operation and COD, The overall oxygen content of lasers fluctuates at early times, but does not differ much from that of the unbonded laser afterward. Although surface As appears to increase in the first few minutes, this is a transient effect, and the near facet region becomes significantly depleted of As and enriched in both group III elements as the laser is operated. Spatial profiles of one set of lasers shows that the largest changes in 0, As, Al and Ga concentrations occur in the graded index/QW region and extend deep beneath the surface. There is no typical state of the facet which accompanies COD. Clear evidence of melting has only been found for one laser, which was operated in pulsed mode.

The growth of stoichiometric facet oxides during operation is not primarily responsible for degradation leading to COD in uncoated QW lasers. Pronounced changes in III/V ratio of the facet to a significant depth can accompany even very brief operating time. The lack of oxide formation constitutes a third distinction between QW and DH lasers(‘47’in which oxidation is important(‘67,‘68J,to be added to the previous data showing that DH lasers do not undergo thermal runaway prior to COD while QW laser do(‘6”7’), and that DH laser facet heating depends on electron and photon fluxes during operation, while only surface currents are important for QW lasers.“62’ The influence of the cladding layer composition on the gradual facet-region degradation was demonstrated in Ref.(14’)as well as some data on advantage of InGaP claddings against The nature of this influence those of GaAlAs in respect of facet heating were reported. (‘08*Lo9) is not fully clear as the surfaces of cladding layers are suggested not to undergo recombination-enhanced degradation. It is possible to guess that the segregated species at the degraded surface is more or less mobile and in the case of the GaAIAs claddings, the excess arsenic atoms accumulate at the quantum-well surface as adjacent areas also are enriched by free As. In the case of InGaP claddings the free arsenic atoms are’ generated only at quantum-well surface and can spread into adjacent areas, so their accumulation is less pronounced. In exchange, the claddings can supply the quantum well surface with more stable In-containing compounds providing partial chemical stabilization of the surface. As a result, the surface recombination velocity grows much less quickly in diodes with InGaP claddings and, in consequence, the surface heating and darkening of the near-facet region are less pronounced.

36

P. G. Eliseev

4. PHYSICAL

PROCESSES

OF OPTICAL

DAMAGE

4.1. Preliminary comments

The optical damage seems to be a chain of physical processes, even in simplest case of direct thermal damage. Key steps of the process are energy transformations from optical energy to other forms and ultimately to a heat dissipated into ambient medium. Various ways the in order to explain transformations take place have been discussed (II-30.42-+8,5&58,'72-'82)('82-202) particular aspects of the phenomenon and to give its physical picture. The mechanisms of the energy transformation in solids under laser illumination are as follows: l

l

l l

l l

l

Linear and nonlinear (multiphoton) absorption of radiation in solids including interband absorption of photoelectric type, interband and intraband dissipative absorption, solid-state plasma absorption and lattice absorption; these mechanisms transform an optical energy into energy of internal degrees of freedom in the solids. Inhomogeneous and the mechanisms of emission heating was treated in a number of papers”7~‘72-‘98’, 179). Absorption mechanisms induced absorption were studied in Refs. (18~23~28~5658~172~175~178~ include those that occurs at electric breakdown of the medium (avalanche(‘72-‘79’or tunnelC2@ types of carrier generation, electroabsorption (“w by a mechanism of interband transitions in a high electric field,““’ etc.). Hypersonic wave generation by optical emission (‘86’due to stimulated Mandel’shtamBrillouin scattering (SMBS)(‘6~“)followed by the formation of shock wave and energy dissipation into a heat.(‘87,lE9’ Shock wave generation due to strong nonuniform heating in the sample.(2’~““~‘88’ Nonradiative recombination of excess carriers and thermalization of hot carriers; these mechanisms transform an excess electronic energy into energy of lattice vibrations and ultimately into heat. Thermoelastic stress produced by nonuniform heating of the materia1.(29~40.‘89’ Melting or evaporation of the material when the temperature rise lead up to the phase transformation points.(‘5s’9, 2o*25, 30,68’ Mechanical destruction under stresses in the medium(37~40*2”~‘88~ I893 ‘95~‘97. ‘98’,including that stress which is produced by the light pressure.(4”

The last two are actual damage mechanisms. In addition to these basic mechanisms of energy transformation we have to indicate several assisting mechanisms which can play an important role in the damage processes but are not involved in the energy transformation themselves. The assisting mechanisms are as follows: l

l

Optical wave interference: coherent emission gives rise to a standing-wave type interference pattern in front of all reflecting surfaces, including an entrance surface of the sample (with a field strength enhancement in the outside medium) and an exit surface (with a field strength enhancement inside the sample medium).(190.‘92’ Depending on the coherence length, the interference pattern is localized near the surface or propagates over the sample thickness and in the latter case the multiple pass effects have to be taken into account. Notice that in the high-quality Fabry-Perot cavity the inside intensity can be many times higher than in the incident beam. Self-focusing: the beam propagation is influenced also by nonlinear processes occurring in the bulk of the material. A self-focusing or self-defocusing (depending on the sign of the nonlinear coefficient n2) leads to a redistribution of the optical power over the beam cross-section and can cause a significant decrease of formally defined optical strength if it produces a filamentation of the emission (in the self-focusing case); in general, there is some self-action of light in solids.(‘93’

Optical

l

l

l

strength

of semiconductor

laser materials

31

Nonlinear frequency transformation of optical waves leading to a. change of absorption in the medium. Electrical breakdown of solid with multiple generation of excess free carriers (by optical absorption), Plasma formation in the outside medium leading to a partial screening of the solid surface from the direct action of optical emission and to surface heating of the solid from the outside gaseous plasma.

These assisting mechanisms influence the place and degree of optical energy localization and concentration, so they can influence the appearance and threshold of the damage. As an example, sometimes the surface damage is observed at the exit (rear) facet of the sample earlier than at the front facet. The reasons for this may be multiple. One is the above mentioned filamentation due to the self-focusing. Another cause is the interference effect providing an electromagnetic field increase by reflected waves from the rear surface. Sometimes a front surface is protected by absorption of the laser-induced plasma in air, whereas a rear side has no such protection. We do not consider illumination in the far IR where one may find bands of strong absorption, but there was no high-power experimentation. A typical method of energy transformation in semiconductors illuminated by laser beams includes the participation of free carriers followed by ultimate energy dissipation into heat. In the case of strong absorption the emission absorbed in a thin surface layer by photoelectric transitions transforms its energy to an electronic energy of free carriers which in turn transforms it into a heat by nonradiative recombination. A generation of heat occurs either in penetration depth of emission or in depth of carrier diffusion, depending which is larger. Physical pictures of optical damage and measurements of LIDT in semiconductors are quite different in the following cases: 1) strong absorption (hv > E,) which can occur in semiconductor photoreceivers, mirrors, and in devices unintentedly illuminated by short wave emission, 2) transparency region (hv < Eg) in semiconductor windows, lenses, electro-optical modulators, nonlinear converters, phase conjugation mirrors, lasers and waveguides, etc., 3) intermediate case (hv w E,) in semiconductor electroabsorption modulators. Many interesting optical applications of semiconductors are associated with light propagation near the edge of intrinsic absorption, hv = E,. Semiconductor lasers are of these applications. Damage can appear as a result of a local change of propagation conditions when the transparency is replaced by the intrinsic absorption with strong (small distance) power dissipation. It may cause a local thermal runaway provided by a property of some semiconductor compounds to decrease the energy bandgap along with an increase of temperature. So a trend exists that the heated region can absorb more strongly than the surrounding parts supplying an increased heat generation and further increase in temperature. Therefore an intermediate case between strong absorption and transparency is a rather complicated and practically important one. In further discussion the main attention will be paid to a direct thermal mechanism. It suggests the immediate dissipation of optical energy into heat by some absorption mechanisms with rapid transformation of energy from the electronic subsystem to the lattice (intraband, two-photon absorption and that photoelectric absorption which is easily followed by nonradiative recombination). Here we shall give some short comments on the mechanisms mentioned in the literature and then consider the main theoretical results. The simple temperature rise to melting point due to optical absorption has to be considered first, and this is a direct thermal mechanism of optical damage. It was seen from the above survey of experiments that the remains of material melting (surface riffles, smooth craters, solidified droplets, etc.) is the regular sign of laser-induced surface damage in both external and self-induced cases. Grinberg et ~1.~‘~)considered a thermal process of damage in semiconductors and introduced several types of photon-matter interactions which can lead

38

P. G. Eliseev

to damaging heating (to the melting point): (i) metallic type (with dominating absorption by free carriers due to intraband optical transitions), (ii) dielectric type (in the absence of free carriers, and hv c E,), and (iii) semiconductor types with free carriers produced by illumination. The “induced-metallic” case was also identified in semiconductors as a result of metallic-type absorption by excess carriers generated by the radiation. The case of hv > Eg corresponds to strong light absorption when the light penetration is small compared to the typical sample size (thickness in the range 0.1-10 pm) and very strong when the penetration depth is smaller than the diffusion length of photogenerated carriers. The latter is a surface excitation case and the power dissipation occurs not at the light penetration length but at the diffusion length. Optical absorption coefficient c1is a sharply growing function of hv above the intrinsic absorption edge, especially in direct-bandgap materials like GaAs, InP, etc. Due to this and due to participation of other optical transitions the distance of light penetration, l/a becomes in GaAs as small as 20-50 nm for blue to UV emission, which is an example of real surface-type excitation. Such very strong absorption is characteristic of second harmonic radiation of own laser emission in GaAs and similar materials. Therefore, harmonic components of the emission could not be accumulated along the cavity and corresponding photons disappear very soon after being generated, producing the power dissipation. The strong absorption case is well studied for purposes of the laser annealing (and also of the non-coherent light-induced annealing) of semiconductors widely used in industry(‘>“’ and for purposes of technology using laser-induced ablation.(“’ l****) The presence of cracks in the damaged site and of possible fragile cleavage suggests the involvement of mechanical destruction resulting from sufficiently strong elastic stress in the sample under laser illumination. This may appear as a thermoelastic stress arising because of nonuniform heating. Such a stress appears, for instance, in semiconductor windows of high-power laser at the thermal runaway conditions leading to the fragmentation of the sample.‘29)An achievement of very high temperature in the laser beam in glasses at the damage threshold was discussed by Harper”” who had shown that a mechanical damage can be produced by a high pressure appearing in “hot points” due to the presence of superheated liquid inside the material. An inhomogeneous heating in a microscale can be a cause of formation of characteristic conic pits as explained in Ref. Cl961 The fragile damage occurs along a conic face between the sample surface and the “hot point” if the point is just near the surface. The formation of gaseous bubbles and cracks in laser-damaged transparent materials was discussed in Ref.(19’)as a result of “hot point” development at heterogeneous inclusions. High-speed dynamics of nonuniform heating may be an origin of shock elastic waves as suggested in Ref.(‘*40),and one can expect the generation of shock waves in a case called of “optical detonation”. This is the situation where the optically heated region grows or moves with a supersound velocity. These types of damage may be characterized as indirect thermal mechanisms. Another possibility for elastic stress to arise is hypersound wave generation by optical waves due to stimulated Mandel’shtam-Brillouin scattering (SMBS), which was proved to appear in some solids by observation of frequency-shifted optical component in the output emission spectrum. (W Strong elastic waves can be transformed into shock waves due to the nonlinearity of the medium and the latter is known to destroy the solid. One possible mechanism for the shock wave damage is a conversion of the compression front into a tensile front at the reflection from the surface. The mechanical strength of the material to tensile stress is much lower than to compressive one, so the reflected shock front produces the material fragmentation on its path back to the bulk until the stress magnitude overcomes the material strength. The shape of the damage is a hemispherical pit in analogue to well formation by underground garnet explosion. A very probable result of SMBS process is the excess heat generation as a hypersound wave

Optical strength of semiconductor

laser materials

39

would be scattered into acoustic phonons. Therefore this nonlinear conversion of optical energy into elastic wave energy may lead to heating of the material, so it contributes to the direct thermal mechanism of damage. Finally, it is necessary to mention that the signatures of mechanical damage (cracks) can be secondary results of the direct thermal damage as a corresponding driving stress might appear at the final stage of the process when the molten material rapidly solidified and cooled. Arsen’ev et a1.c4” considered damaging by light pressure in connection with their observation of damage in CJS crystals under picosecond pulse illumination by neodymium glass mode-locked laser. They estimated the stress on the reflecting semiconductor surface at 30-100 GW/cm* in the range 10-100 atm, and stated this stress to be sufficient to explain a spallation-type damage at the rear sample facet, where the stress was of tensile character and met the lower mechanical strength of the material rather than in the case of compressive stress. In contrast, the stress by light pressure was of a compressive character at the front surface and did not produce a damage up to 150 GW/cm*. The thermal shock wave stress was also discussed in the Ref.@4’and the magnitude was estimated to reach about 100 atm at the front facet presumably due to heating by the radiation background ( w 125 MW/cm’, 200 ns) accompanying ultrashort pulses. The stress was considered to be enough to cause damage of the front surface. A possibility of SMBS-assisted damage was ruled out in the Ref.“” Dielectric type of damage has to be considered here for the sake of the role of window and dielectric protection materials in the improvements of the optical strength of semiconductor laser materials. In transparent materials the laser induced damage usually occurs at higher intensities than that in the absorption case. One obvious reason for this is that linear absorption processes are not very important, being rather small, so significant power dissipation can be supplied only by nonlinear mechanisms. The theory of optical damage and power limitation in transparent materials includes many branches reflecting the variety of nonlinear processes involved. The main branches are (i) avalanche breakdown, most typical for transparent dielectrics(C46 ‘80-‘85’, (“) u nonlinear absorption, scattering and self-action of beam”‘, ‘**25.4’,46. I”‘, and (iii) influence of defects, inhomogeneities and’*‘-17*, ‘74.ls3.j9’).Be1ow we give some resuming indications on the laser-induced processes at the surface and in the bulk of solids: 1. An avalanche breakdown mechanism suggests the generation of multiple free carriers due to the internal ionization by hot carriers, as they obtain an energy from the laser emission. Therefore the first step of energy transformation is optical absorption via intraband transitions, and initial carriers have to be supplied by one of several mechanisms: (i) equilibrium ionization, (ii) occasional ionization by external radiation, (iii) tunnel ionization, etc. Mechanisms of this type are treated in the literature for transparent dielectric materials and for high-resistance semiconductors.‘4S~‘8~‘*6’ 2. Absorption at defects and inclusions initiating the local overheating of the material inside the relatively cold matrix followed by an increase of absorption (thus the process has a positive feedback to accelerate the temperature rise). This is a sort of direct thermal mechanism applied to a localized “hot points”.“7’ 23,28, ‘72~‘97, 19*)The mechanism is closely analogous to local overheating occurring in semiconductor laser materials under the positive feedback provided by the bandgap narrowing in the hot points or planes (surface). The dependence of critical damage intensity @(At) on the pulsewidth N At”* (as shown in Ref.“*’ for external laser illumination damage and in Ref.“’ for semiconductor laser self-damage) is an indication of the heat-diffusion limited local heating. 3. Absorption in extrinsic species accumulated at the surface produces vaporation and ionization in the medium surrounding the solid. As a result, the gaseous plasma can be JPQE 2011-D

40

P. G. Eliseev

formed above the surface absorbing the laser emission and partially protecting the solid from the direct action of the laser emission. On the other hand this plasma could be aggressive to the solid producing etching and local heating. Ultimately, the plasma can produce a specific damaging at the surface which is not a genuine optical damage, but is a secondary result of laser illumination, Another possible secondary process is mechanical damage under the action of shock waves in gaseous plasma. Strong optical absorption in the plasma can be accompanied by detonation or the absorption front can generate shock waves which then attack the solid surface. 4. Linear and nonlinear absorption in the solid producing photoexcited carriers or rapidly dissipating into heat. In both cases the ultimate result is some heating of the material, very nonuniform at strong (surface-type) absorption or more uniform at bulk absorption. The heat generation can lead to a thermal runaway when temperature in some part of samples rapidly increases so some irreversible processes occurs. The surface overheating can lead to melting of material followed by rapid solidification, thermal decomposition with evaporation of volatile components of compound followed probably by deposition of products on the surface, brittle fracture in a surface layer due to nonuniform heating and corresponding thermoelastic stress producing cracks and conical pits. Massive melting at surface after solidification gives a shape of circular crater with a central peak, produced by a material segregation during the solidification to a center of the spot. This is typical for the majority of semiconductors which increase in volume at crystallization. At lower absorbed energies the heating could result in oxidation or in other chemical transformations in the surface layer. 5. SMBS of laser emission gives rise to elastic waves in solids. These waves can have a large amplitude in the case of stimulated scattering especially in a presence of high-quality resonators. The optical waveguides in semiconductor structures are also waveguides for sound waves because the velocity of sound is typically lower in media containing more heavy atoms (as in the optical waveguide core) than in media with more light atoms. This is the case if GaAs is a material in the core and AlGaAs is the material of claddings. Therefore the guiding property of the laser active region can be a cause for concentration of acoustic and hyperacoustic waves, generated by light. The intensity of such waves will be higher at low temperatures. The mechanism of destruction under the action of stimulated light scattering (SMBS) was suggested for GaAs lasers in Ref.‘34’. 4.2. Thermal mechanism, runaway and “microexplosion” 4.2.1. External illumination. The heating of semiconductor material by optical emission can lead to surface melting and accompanying thermal effects (partial ablation, chemical reactions, decomposition, etc.). If the obtaining of the melting point is the cause of damage, one can estimated a critical energy density for this kind of damaging. Let us consider the optical heating of a solid following a number of papers (see for Th e intensity in the normally incident (z-axis) optical beam example, (I,14.19.22.27,46.68.178,201,202))~ is QO(x,y,t), where x and y are transverse coordinates, This function gives both a shape to the illuminated spot, and a temporal shape to the optical pulse. The intensity evolution along the z-axis in an absorbing medium (in W/cm’) can be expressed as

@(z,t) = @(O)exp[ - [N)dY]

9

(4)

where Q(O) is the intensity at surface (z = 0) inside the material, m(O) = a0 (1 - R), where R is Fresnel reflection coefficient at wavelength of incident radiation. If absorbed light

41

Optical strength of semiconductor laser materials

dissipates to heat, the spatial density of heat sources (in W/cm3) will be as follows: W(x,y,z,t)

= (1 - R)@&,y,t)a(z)exp

[ - [aOK]

T

(5)

and maximal heat source density will be at surface, W(z = 0) = (1 - R) Cp,(x,y,t) a(0). The heating process can be treated in detail taking into account optical and transport parameters of the medium with their temperature dependencies, and also taking into account a number of effects accompanying the high intensity interactions: dynamic Burstein shift of the fundamental absorption edge, plasma effects, predominating role of nonradiative Auger-type recombination at higher temperatures, change of concentration of free carriers, their absorption ability, surface recombination, etc. Examples of more or less detailed numerical treatment of external laser induced heating in solids including some ‘78,‘79.2~202).Here we shall be limited by quantitative semiconductors are given in Refs. (‘3‘1.47. estimations in simplifying assumptions: (i) one-dimension heat flux, (ii) small diffusion length of carriers (to neglect the smoothing of the heat source distribution by carrier diffusion), (iii) using averaged temperature-dependent parameters instead of actual temperature functions, and (iv) neglecting all heat sinks except the heat conductive sink to ambient (assuming cooling by thermal radiation to be small, etc.). By such assumptions we can obtain the temperature rise rate in the form dT/dt = W(z,t)/pC ,

(6)

where p is the density and C is the specific heat capacity. Taking the temperature rise AT = (dT/dt)At to the end of a pulse of duration At, one can obtain a criterion for damage to occur in form AT > T,,, - To where T, is a melting point and To is ambient temperature of experiment. A subject for further discussion is the heated volume, or thickness L as the considered geometry is one-dimensional. For the case of strongly absorbed illumination (surface absorption) the quantity L is determined by heat spreading and carrier diffusion, so we limit a consideration by assuming a heat conduction of controlled length L = L7 = (1/2)(7~kAt)“~,where k = K/PC is the heat diffusivity and K is the heat conductivity. In the case of absorption controlled spatial distribution Eqn. (6) will be valid with L = l/a where c( is an effective optical absorption coefficient. One will have the following expressions for surface temperature rise: AT = (dT/dt)At = (1 - R)@,AtlpCL = ((1 - R)a@,AtlpC, at L = l/a, 2(1 - R)@,/(mpCAt)“*,

at L = LT)

(7/Q

The expression (8) is equivalent to one used by Grinberg et a1.“9’for the temperature rise at semiconductor surface, AT= 2(1 - R)E/(I@cA~)“~

,

(9)

where E is the energy density in the illumination pulse. The expression was used also by other aUthorS.‘27,1'8.201) The intensity at the LIDT can be expressed by substitution of critical temperature rise AT, = T, - To, where T,,, is the melting point, To is the initial (ambient) temperature of the sample. The result is @(LZDT) = pCAT,,/(l - R)aAt, at L = l/a,

(10)

@(LZDT) = pCLrATc,/(l - R)At, at L = LT.

(11)

Now we shall inspect a quantitative applicability of these and more detailed calculations to the experimentally observed LIDT data. Eqn. (11) is expected to be good approximation

42

P. G. Eliseev

to LIDT in GaAs under ruby laser illumination (wavelength is 694 nm) which is the case of strong radiation absorption. We shall use the following constants for the material: pC = 2 J/cm3, ATcr = 1216 K, k = 0.1 cm2/s, R = 0.338. Thermal parameters are taken for the middle of the temperature range between room temperature and melting point. The calculated Q(LIDT) is 6 MW/cm2 at pulsewidth 20 ns. The experimental value (see Table 4) is 8 + 2 MW/cm2. The agreement is excellent, keeping in mind the inaccuracy to factor 2 in our approximation. More detailed calculations of the LIDT intensity were performed in papers.(‘78~‘7g’ The solutions used were obtained by modeling of the heating case with two coupled equations - for free carrier density and for temperature in a one-dimensional approach using all necessary known or extrapolated parameters of the material (the absorption coefficient at given wavelength dependent on temperature, transport properties of carriers, surface recombination, Auger recombination, free-carrier absorption, etc.). The results were recognized to be quite satisfactory for the ruby laser illumination. In Table 9 we present selected comparable data. The calculated values from Ref.(‘79’are in good agreement with experiments as relative deviations are in range from - 39% to + 24%. Therefore one may conclude that in the case of strong absorption the value of @(LIDT) is explainable by the direct thermal mechanism. Now consider the case of transparency, namely, the LIDT in GaAs illuminated by neodymium lasers (wavelength is 1064 nm). Eqn. (10) for deep light penetration and negligible heat flow (“thermal diffusionless” case) is suitable for an approximation unless the temperature bandgap shrinkage changes the situation to the strong absorption case. The transition takes place at about 800 K, so the critical temperature rise is reduced to about 500 degrees; the heating to this point takes a long time, whereas further heating will occur very rapidly. The main problem is the optical absorption: the initial linear absorption (l-10 cm _ ‘) is too weak to be responsible for rapid heating, therefore the nonlinear absorption has to be taken into account. The two-photon absorption coefficient /I is known with a large inaccuracy: published values are in range from 0.023 to 5.0 cm/MW for GaAs, so estimation cannot be very accurate. Large /I were measured in early experiments, so more recent results scattered about 0.05 cm/MW. u’~*“*) The contribution of weak linear and nonlinear absorption in a total absorptivity is enhanced by free-carrier absorption as the carriers appear due to photoelectric component of the absorption. One can derive for the total absorption coefficient &or(@)= a0 + /I@ + ON(@) I

(12)

where cr is the free-carrier absorption cross-section for electron-hole pair and N is their density. Calculating the laser-induced density of free carrier and substituting it into Eqn. (12) we can obtain G,(Q) = (a, + /.I@)(1+ or@/hv) ,

(13) where 7 is the carrier lifetime. We shall use following parameters: cl0 = 10 cm-‘, 0 = 10 - I7cm’, 7 = 1 ns, hv = 1.17 eV. The calculation gives, for a 20 ns long pulse of illumination, the LIDT intensity as large as 270 MW/cm2, while the empirical result is only 35-50 MW/cm’. A discrepancy is rather large (5.5-7.7 times). The calculation in Ref.(“*) based

Table 9. Comparison of calculated and experimentalvalues of @(LIDT) for GaAs under ruby laser illumination (hv = 1.79 eV, bandgap is 1.42 eV at room temperature, the case relates to strong absorption where l/a < 1 pm) Pulsewidth, ns 20 30 40

Measured LIDT, MW/cmz 8k2 12.2 4.9

References 65 58 57

Calculated irF MW/cm2 ’ 9.4 7.4 6.1

Calculated using Eqn. (8), MW/cm* 6 4 7

Optical strength of semiconductor laser materials iOOO\

43

LIDT, MW/cm2

-. -. \ -.._

100 @

10

00

t-1064

100 Pulsewidth,

----.---nm

l( ns

Fig. 8. The dependence of LIDT intensity in GPAS illuminated by pulses of Q-switched ruby (694 nm) and neodimium-doped (1064 nm) lasers on pulsewidth. Solid curves are calculated by J. R. Meyer er al.“” for 694 nm illumination and for 1064 mn illumination with /l = 0.05 cm/MW. Dashed curves are calculated using a simplified model of Eqn. (10). Experimental points are taken from Table 5.

on more detailed description of material properties and numerically calculated temperature and carrier density profiles also gives a noticeable discrepancy as it is seen in Fig. 8, even if the largest /I from published data was used (b = 5 cm/MW). This shows that a direct heating model via nonlinear absorption is not perfectly adequate and other contributions have to be found for eventual explanation of LIDT data for “transparency” cases. Data for LIDT under 694 nm illumination are also shown in Fig. 8 and calculations are in good agreement with experimental points. This means that the direct thermal heating is quite an adequate model for the case of strongly-absorbed laser illumination. 4.2.2. Physical model for thermal runaway (microexplosion) for self-damage in laser diodes. Optical self-damage in laser is a process which is supplied by energy from radiation generated and amplified in the same material. In order to understand the mechanism of the damage one has to follow how optical energy transforms into other forms of energy to be concentrated in volume of damage defect. As the real size of minor damage is in the micrometer scale, the mechanism has to include the explanation for energy transformation at such small distances. Let us remember that residual (nonresonant) linear optical absorption leading to internal power dissipation under laser conditions is in the range 5-50 cm - ‘, therefore the absorption length of typically 0.2-2 mm for this absorption process does not give the case for strongly localized damage. A more reasonable suggestion is that the absorption of laser emission occurs in regions having extrinsic defects or inclusions. These are present in the majority of optical materials recognized as being of high quality, but not in semiconductor lasers. We had mentioned before the hypothesis of Dobson and Keeble”‘), who suggested the absorption can be provided by inhomogeneities of the material locally overheated by radiation. Due to decrease of the bandgap with temperature, the hot layer near surface can absorb laser radiation and its dissipation leads to further increases of temperature. This is suggested to be the cause of thermal runaway and damage at the surface. Another model of the catastrophic degradation of laser diodes is based on the calculation of distributed thermoelastic stress used by Kruzhilin et al.(“) The quantitative approach was developed in Ref. (‘) for temperature rise in laser medium when optical absorption is growing with temperature. The heat conduction equation was

44

P. G. Eliseev

given with distributed heat source W(T) supplied by dissipation of absorbed energy, CpdTldt = V(kVT) + W(T),

(14)

where C and p are heat capacity and density of the material, and K is the heat conductivity. This equation is known in explosion kinetics where the heat generation is temperature-dependent and supplied by exothermic chemical reaction.‘203~2@‘) Frank-Kamenetski(*03) had found a dimensionless criterion including the dependence of heat source on temperature and size parameter, which has a critical value corresponding to threshold of runaway solution for T(t). If W(T) is a linear function of the form UT, the solution of the inhomogeneous Eqn. (14) can be presented by a series, each term of which contains the cofactor exp[(a - b,)t], where b, are quantities dependent on the parameters of medium and the task geometry as well as on the number m of the term in series. (*04)The solution for temporal dependence of temperature T(f) will be rising if at least one term of the series (most likely the first one) has a > b,. This last inequality is therefore a criterion for existence of runaway behavior. When the heat generation rate is dependent on optical power density @, it will be of the form a = a(@), and a substitution of this relation into the runaway criterion gives a new inequality, @ > @,,, where Q,, is a critical intensity for a thermal runaway or “microexplosion” leading probably to thermal damage. Last quantity is dependent on medium parameters and on geometry. This approach is also valid for the stationary case. It could provide a solution for a runaway under CW operation condition. The exponential (and also some sharper than exponential) function describes the growth with a permanently increasing positive slope. The period when this slope is very low is a latent period with current process but with no visible changes. In explosion theory it is called the induction time which is simply the delay time from the start of the observable “explosion” process to its sharp occurrence. Thus the function T(f) appears to have time delay, and if the operation pulse At is shorter than this delay, the exceeding of the stationary criterion of thermal instability will not lead to a runaway. In other words, the criterion inequality would contain the critical intensity dependent on the pulse duration time At. The above considerations are true in aspect of runaway criteria, not only for the linear function W(T), but also for a wider class of rising functions.(204)In any case a general relation exists for the start of the thermal explosive evolution: @ > @&At) ,

(15)

and the next question will be whether this criterion fulfills before or later than the criterion for limitation of operation intensity in lasers provided by other means (averaged overheating, leakage of current, nonlinear saturation of laser emission power, etc.). The most probable competition comes from the total (averaged) overheating of a laser device leading to the increase of threshold current and therefore to power decrease at growing pump current. This behavior is often called a thermal saturation, and if this thermal saturation of emission power occurs before the criterion of localized thermal microexplosion is fulfilled, the optical damage may be prevented. To some extent the total overheating is a reversible failure, so it can be a tool for protection of the device against a catastrophic degradation. On the other hand, repetitive thermal cycles of whole device as well as heavy heating of soldered contacts can produce another degradation phenomena lying outside of our subject. An essence of the approach of the Ref. (‘,*O’) was to take into account in numerical treatment the intensity- and temperature-dependent heat generation rate. The dissipation of optical power was taken to be equal to local power absorption by interband (photoelectric) absorption mechanism with a fraction y going to the heat due to nonradiative recombination and thermalization of carriers. It is known that with a rise of temperature, the probability of nonradiative processes occurring becomes relatively larger, so in the runaway process the factor of growth of the nonradiative yield is also working for the positive feedback of the runaway.

Optical

strength

of semiconductor

laser materials

45

Two different cases were treated: 1. Outside illumination applicable to laser annealing and external laser-beam surface damaging, 2. Inside illumination applicable to laser self-damage and to external laser-beam damaging in the bulk and rear of the specimen facet. In both cases the light intensity at point z along the beam propagation axis was determined by calculation of the not-absorbed part. If the radiation runs from infinite z to I = 0 (the specimen surface), the intensity will be equal to Q(z) = %exp[ - ~a(CJM],

(16)

where @, is the starting incident wave intensity and a(z,T) is the coordinate- and temperature-dependent absorption coefficient. The most delicate question in this problem is the formulation of the temperature dependence of the absorption to take into account the temperature decrease of the energy bandgap in semiconductors of III-V type like GaAs, InP and their alloys. As it was mentioned before, this decrease could provide a strong increase of the absorption coefficient, converting the situation in the locally overheated region from a transparency case into case of strong absorption. The latter case supplies a sufficiently short distance for the light absorption to confine the overheating to an actual small volume as observed in experiments. The model function for the temperature-influenced absorption in Ref.(‘) was constructed using a simplified square root energy dependence of interband absorption coefficient and a linear “red” shift of the fundamental absorption edge, so the function had a form a(T) = aJ(T - T*)/T]‘/2 ,

where a,,T* and T’ are adjustable constants and to fit the GaAs characteristics. numeric calculation the relation was used a,( T’) - Ii2= 10) cm - ‘K - ‘I2 .

(17) For (18)

The quantity T* is an initial averaged temperature in the semiconductor active region which is only a little higher than an ambient temperature. It is necessary to remember that the starting state of the active medium (before a local overheating) is amplifying, but not absorbing. This is true for a mode gain, whereas a material gain is a net value which is an algebraic sum of gains provided by interband transitions between inversely populated bands and of non-resonant (residual) dissipative absorption, normally at a level from units to a few decades of reverse centimetres. Therefore in order to switch on the strong photoelectric absorption one has to supply local overheating which allows inverted population to be eliminated at working levels and to shift the absorption edge sufficiently (by several kJ’) relative to a starting photon energy hv. It was suggested that the thermal explosion process could be initiated at regions with elevated temperature where an initial local heating is provided by nonradiative recombination. The semiconductor surface is one of these regions. Internal nonraditive recombination centers and absorbing inclusions are also possible points of the process nucleation. The conditions for initial heating at the cleaved surface will be considered in more detail later. The starting overheating was assumed in Ref.(‘) to be 10-30 degrees. The heat conduction equation was transformed by introduction of dimensionless variables and parameter d: 0 = T/T’, Z = a,z, z = (yZ,a,t)/cpT’,

6 = (yZ,)/a,A,

where A is a proportionality coefficient in the simplified expression assumed for the temperature dependence of the heat conductivity of semiconductors (the lattice conductivity),

46

P. G. Ekev

14

Normalized temperature rise, 0 - 9, 17

Normalized distance, Z Fig. 9. Computed normalized temperature rise profiles along the beam axis under internal illumination in the medium with a temperature-dependent absorption coefficient, parameter of the curves is normalized time (Ref.“)).

K(T) = A/T. In GaAs this coefficient is about 110 W/cm. Coefficient y is the ratio for energy conversion to heat. These substitutions give the equation for a numeric solution: de/dz = (l/d)d/dZ[(l/O)de/dZ]

+ (6’ - &)exp [ - cv3

- e.)dx],

(19)

where 8, = T*/T’. Examples of temperature evolution at some conditions including the intensity-proportional parameter 6 are shown in Fig. 9 and Fig. 10. It is seen in Fig. 9 that a temperature peak can appear to move in the counter direction to incident light as initiated on the tail of temperature inhomogeneity assumed to exist at the starting moment (t = 0). Parameter 6 is taken to be equal to 125, the starting temperature inhomogeneity was taken

3. Normalized temperature,6 125.’ ,

lo.,’

Fig. 10. Computed surface temperature rise calculated for external (dashed curves) and internal (solid curves) illumination in the medium with a temperature-dependent absorption coefficient. The parameter of the curves is the value of d (see text)“‘.

Optical

strength

of semiconductor

laser materials

47

as 8 (Z) = 7.4 + 1.6 exp( - Z), and 0(O) = 7.4 is the average starting temperature in the bulk. In Fig. 10 it is seen that the local heating kinetics under either external or internal illumination of the solid surface, and the intensity parameter is 6. The dimensionless temperature strongly increases under external illumination. The function is steeper than the linear one expected in the adiabatic heating case. The reason for this explosion-type dependence is that the absorbing volume decreases along with heating so heat generation is localized more closely to an illuminated surface. In the case of internal illumination the temperature rise is much less and the delay time can be seen. The saturated growth is also seen to limit the maximal temperature at rather moderate values, even at y = 1. This is the result of some displacement of the point of maximal temperature in the counter direction to the optical flow. As a result the heated volume increases. In absolute values a maximal temperature 100 degrees is reached in 10 ns at a0 = 10 MW/cm*, and at lower a’,, further heating is too small to provide the melting of the material. On this base in Ref.(‘) a conclusion was made that COD occurring at intensities lower than 10 MW/cm* is a result of brittle destruction under thermoelastic stress rather than melting. The overheated region near the mirror surface produces tensile stress N lo4 N/cm* at maximal temperature rise 300400 degrees, and the destruction leads to the formation of typical small cone-shaped pits. Due to the plasticity of the material at such temperatures, the damage may be accompanied by glide, with the formation of dislocations. Signs of thermal decomposition in damaged sites were explained by the action of repetitive pulses of injection current, also after the damage. Electrical energy dissipation at the damage region is an additional source of heating, which is supplied by positive feedback because the current density can grow through the overheated region of the p-n junction due to the temperature decrease of the built-in potential barrier in the junction. Electrically-assisted heating can be taken to explain why the runaway does not stop during the laser pulse when laser action can be quenched by enhanced absorption. In experiments with optically pumped DH material (68)this electrical mechanism was not present, but typical damage intensity was about 10 MW/cm* (as measured with an accuracy of factor two). Let us consider an estimate of the power flux required to create a molten sphere 200 nm in diameter moving at velocity 300 cm/s, following Ref. c6*)The experimental condition was a pulse of 18 ns duration. To increase the temperature by 1200 K in the sphere will require a power 13.5 mW. An additional power to melt the sphere is 0.757 mW. To supply the movement of the sphere with velocity 300 cm/s the power has to be 0.31 mW larger, thus the total power to form a molten sphere 200 nm in diameter which moves at an average speed 300 cm/s is 14.6 mW. Assuming the cross-sectional area of the absorbing (overheated to more than 130°C) area to be 1.6 x 10m9cm* the average absorbed flux required is 9.1 MW/cm*. Another assumption is that the absorbed fraction of the optical flux is 0.64, so an estimation of critical optical flux forming moving molten microspheres is about 14.2 MW/cm*. This value is found to be in agreement with experiments carried out on GaAs/GaAIAs laser materials. However, it is necessary to remark that the intensity estimation was made there with some underestimation. At first, we have to notice that the main part of the runaway temperature rise occurs during 1 ns (or at maximum a few nanoseconds). Comparing this time with taken pulse duration (18 ns) one may conclude that for more rapid heating the estimated power has to be at least one magnitude order larger. Another underestimating assumption is taking the optical cross-section of the heated region to be larger than the size of the sphere to be melted. Due to heat conduction outside the overheated region the dissipated energy spreads rather than concentrates into small volume. Even in the adiabatic heating case the heated volume is at least the same as that supplied by the thermal energy, but not smaller. Neglecting the heat spreading (using an one-dimension geometry) it is possible to estimate the minimal energy needed to produce substantial melting of the material (in a 200 nm thick

48

P. G. Eliseev Table 10. Material parameters of thermal problem of COD in GnAs/GaAlAs as used in Ref.“’

Material parameter Thermal Thermal Heat of Volume

conductivity, K diffusivity, k fusion heat capacity

units

Value

W/cm.deg cm+ J/cm’ J/cm”.deg

0.12 0.58 3250 1.72 (300 K) 2.07 (900 K)

layer), which is approximately 0.1 J/cm*. Ultimately, the rough calculation leads to the conclusion that in order to produce rapid heating and melting of this portion of material during l-2 ns the heat generation rate has to be of order 60-120 MW/cm*. Thus our estimation is much higher than that mentioned above. It suggests that this rate of power dissipation cannot be supplied by an optical flux typical for COD, namely, of the order of 10 MW/cm*, if self-focusing or other energy sources are not considered. Henry et a/.@‘)considered catastrophic degradation as a process of thermal runaway in which the surface is heated to the melting point by absorbed light incident on the surface. The runaway can take place provided that the following conditions are met: The beam is sufficiently intense that if a substantial fraction of incident light is absorbed and dissipated as heat at the surface, the latter will be heated to the melting point during the pulse; The surface is sufficiently nonradiative that electron-hole recombination can take place fast enough to heat the surface to the melting point; Initially, the surface layer is sufficiently absorbing and the increase in light absorption with increasing temperature is sufficiently rapid, that the surface is able to switch from a weakly absorbing to a highly absorbing state in a time small compared with the pulse duration. The material parameters assumed in the calculations are given in Table 10. One very important step of the COD modeling in this approach is the particular modeling of the temperature dependence of the absorption coefficient starting from the initial value. Henry et a1.(6*)had used a model function based on some experimental absorption spectra of GaAs and taking into account the temperature-dependent bandgap with a coefficient about - 0.5 meV/K. This model function manifests a strong growth from initial value 140 cm-’ at zero overheating to 7500 cm - ’ at 100 degrees overheating and then increases more slowly to about 2 x lo4 cm-’ at 600 degrees overheating. Notice that the starting state is assumed to be absorbing whereas in the laser active medium the absorption can appear only if the carrier density is lower than the transparency value N,, and not earlier. Therefore the medium has some margins of heating before the absorption occurs as N, would increase and meet the actual magnitude of the carrier density. Such an optical absorption model is a very useful simplification allowing quantitative results to be obtained, but is not quite adequate and leads to some overestimation of power dissipation, especially at starting stage of the process. Namely, the initial absorption a, = 140 cm- * seems to be not well motivated, as free-carrier absorption is normally much lower and the interband absorption does not contribute at all while the population in the medium is inverted. The surface depression of the carrier density is dependent of the SRV, and may vary to some extent. In order to get the above absorption, the depression has to be rather large. The assumption in Ref.@*)that heat sources are located only at mirror surface simplifies the solution of the heat equation, as the source function W(7) in the Eqn. (14) may be transferred to the boundary conditions and this gives a homogeneous equation. The surface

Optical strength of semiconductor laser materials

temperature

49

rise is expressed as follows: AT = (2Q/~)(kAt/rc)“’ ,

(20)

where Q is the heat flux, K is the thermal conductivity, and k is the thermal diffusivity. In the runaway regime the heat diffusion distance N (kAt)li2 was estimated as only 320 nm when At is 18 ns. The size of the highly absorbing region was taken of 0.61 pm, and the absorbed fraction of incident light was 0.77. In any case, the initiation of the runaway process can occur in very thin layer near the cleaved facet. In order to calculate the power flux Q it was suggested that a heat generation is supplied by a nonradiative surface recombination which is characterized by velocity S. The recombination rate is thus s(N, x Nh)‘/2,giving at N, = Nh = N (the neutrality case) the heat generation rate Q = AFsN, where AF is the surface value of the quasi-Fermi-level difference which is close to the energy bandgap. The conceptional balance of the power dissipation at surface could be expressed as Q = AFsN = al@(l + R)/hv ,

(21)

where a and 1are the absorption coefficient and the effective thickness of absorbing layer near the surface, respectively, @ is the optical power density (intensity), and R is reflection coefficient. The right hand term in this equation is the optically supplied power income into a surface region, assumed to be converted into power of excess carriers, both left sides are heat generation rates due to the nonradiative recombination of these carriers. A complicated changeable situation during the runaway process was simplified also by the following assumptions: 1. Initially, light is absorbed by an unheated layer 400 nm thick and having an absorption coefficient 140 cm - I; 2. As the surface becomes heated the absorption coefficient of the 200 nm thick layer increases exponentially with temperature according to the model function a(AT) = a0[exp(AT/21.4”C) - l] ,

(22)

where a, is the starting value assumed to be 140 cm - I. The total absorbed power was limited to two-thirds of the incident light flux. 3. A constant thermal conductivity K = 0.19 W/cm.deg appropriate for AT = 60°C was used. With these assumptions the temperature rise function could be calculated numerically. Computed curve at power flux 15 MW/cm2 corresponds to a strong rise to melting temperatures during an 18 ns pulse. All curves contain three ranges: (a) about (b) (c) (also

low-temperature branch with a square-root-in-time temperature growth up to AT = 30 degrees; sharp runaway over more than 1000 degrees during nanosecond time, high-temperature branch with continuing temperature growth but at lower rate in proportion to a square-root of time).

At the first branch the absorbed fraction of optical flux is about 0.7%, whereas at the third branch it is up to 77%. In principle, such strong absorption can quench laser action so further transport of energy to the hot spot at surface will be stopped. The representation of absorption in the dependence of temperature is one of weak points of the model. At starting conditions it is assumed to be equal to 140 cm - ’ which, to our understanding, does not correspond to laser conditions. At higher temperature the absorption is assumed to be as high as 2 x lo4 cm - ’ with no relation to band filling which gives some absorption saturation. The self-consistent solution is very complicated, but desirable as the model is sensitive to both

50

P. G. Eliseev

important quantities: absorption in hot spot, which decreases as carrier density increases, and heat generation, which increases as carrier density increases. These quantities dominate the power dissipation in optical damage process and each of them can work as a “bottle neck” if the carrier density is too small or too large. The carrier distribution in the vicinity of the facet is a rather discussible subject of the model in Ref.(@)A proposed solution is based on neglecting the important terms of the carrier diffusion equation. As a result a rough estimation of heat generation was made using approximate values of surface carrier density. The surface recombination velocity was taken to be as large as 4 x lo5 cm/s for the active layer of GaAMs containing 8% of aluminum. In order to convert the 9.1 MW/cm2 power flux into a heat flux it is necessary to have the carrier density at the surface as large as 10” cm- 3. If one estimates the heat generation rate just at the runaway occurs (during about one nanosecond) as supplied by the surface recombination, the required excess carrier density will be larger by order of value or more which looks to be unreal. Further developments of physical models of COD processes in semiconductor lasers included working out the more detailed thermal model (206208~2’o~2’7’ for pulse regime and consideration of CW regime’2”-2’4,2’6’ in DH lasers, and then working out the thermal model for facet heating in QW lasers.(2’5’2’6’ Nakwaski’“” had treated a three-dimensional model for facet heating DH laser using the same assumptions of Ref.‘68’concerning the heat generation at surface. The temperature dependence was taken into account of thermal conductivity, and heat generation in the whole active stripe was also included. Pulse-operated laser was treated. The dependence of COD time (a permissible length of the current pulses from a the point of view of avoiding the catastrophic damage) on the amplitude of the pulses was obtained for standard stripe DH laser diode (8 x 400 pm in size, active layer thickness was 200 nm, threshold current 100 mA). Pulsewidth limits of the model were from 6.2 to 500 ns, and permissible currents for these two values were 4.5 and 0.8 A, respectively. The model was later extended by Nakwaski to high injection level, and distributed heat Both thermal conductivity and thermal diffusivity sources near the mirror are assumed. t208~209’ were assumed to be temperature-dependent. The influence of facet reflectivities was also treated numerically. For short current pulses the achievement of very high emission intensity was predicted: at current amplitude of 35 A in the same standard laser as above, the COD time was calculated to be near 3 ns and corresponding COD limitation for optical intensity was estimated at about 3 GW/cm’. By solving a three-dimensional heat conduction equation and a simplified rate equation for carriers an analytical expression has been obtained in Ref.(2’0’for the maximum optical power density as a function of a pulsewidth and of the surface recombination velocity. Calculated curves are found to be in good agreement with the experimental results for surface recombination velocity of some lo6 cm/s and for the pulsewidth between 10 ns and 1 ms. In this range the critical optical power was reverse proportional to a square root of the pulsewidth, according to Ref. (‘). The basic mechanism responsible for COD is the local heating caused by absorption of radiation near the surface and by nonradiative recombination of carriers at the facet. A model has been developed which relates the maximum optical power to the pulsewidth. Central to this model is the assumption that there is a critical temperature at which a facet is destroyed. Nonradiative recombination of carriers at the surface drastically reduces the carrier concentration at facets, and this produces the absorption of laser emission near the surface, as the gain turns into absorption in this region. The facet is assumed to act as a planar heat source which produces a heat flux. The heat spreads into the bulk material and increases the temperature of the facet, dependent on pulsewidth. The heat flux itself depends on the optical power density and the rate of the surface recombination. With the assumption of temperature-independent heat conductivity

Optical strength of semiconductor laser materials

the temperature

51

rise AT at the surface was used in the form(210):

AT = Q(4/7nc)(kAt/n)“’

“2{[1 - exp( - x2)] + J;; x erfc x)&$ , s0

(23)

where x = (kAt)-‘12 ab/2(a2sin2+ + b2cos2$)“*, Q is the heat flux density from the surface, ICis the thermal conductivity and k is the thermal diffusivity in bulk material, a and b are halfwidth and halfheight of the hot spot at surface, and At is the pulsewidth. The critical rise AT,,, was assumed (near 1000 degrees) to produce a surface damage. If so, the critical value could be calculated corresponding to temperature increase to a critical one during the Qman pulse. At short pulse approximation Q,,,,, = (n”2~/2)ATTmax(~At) - I’*,at t -0 ,

(24)

and for CW operation mode Q,,,,, = nrc[2bK(l - b2/a2)]- ‘AT,,,,,, at t -+cc ,

(25)

where K(x)is the complete elliptic integral of the first kind. Assuming the temperature dependent thermal conductivity ~(7’) = K,T, /T, which is valid for lattice thermal conductivity at high temperature, one obtained for CW mode: Qmax= nT,K,[2bK(l - b2/a2)]-‘ln(1 + AT,,,,,/T,) .

(26)

Estimation had given for Q,,,,, N 1.6 x lo6 W/cm2 for T, = 300 K. The heat flow density was determined as

Qmx= yhvF,

(27)

where y is the fraction of carriers which recombine nonradiatively, and F is carrier flow which had to be calculated from the carrier distribution. The diffusion equation D(d2N/&)

- N/t(z) - [g(z) - cr(z)]S(z) = 0 )

(28)

was used where N(z) is the concentration of excess carriers, D is their diffusion constant, z(z) is the carrier lifetime, g(z) is the gain coefficient, a(z) is the absorption coefficient due to nonresonant processes, and S(z) is photon flux. The boundary condition at the surface gives F = DdN/dzi, = sN(O), where s is the surface recombination velocity. Assuming the penetration depth of the temperature profile to be larger than an effective carrier diffusion length L near the surface it was obtained for F: F = (1 + DlsL)aLS where a is the absorption coefficient The diffusion length L is determined large currents, it was found that Q B’ = [s(0)D]‘i2. The output power

,

(29)

at surface and spatial dependence of S was neglected. by D’12[(l/z(O)) + g(O)S] - I’*.Neglecting a term 1/T at = A,!?*/[1 + B’S”2/s], where A = yhvD’!2/g(0)“2and density P is related to the photon flux S by

P = hvS(l - R)/( 1 + R) , where R is the mirror reflectivity. The interconnection was found in a form P =

(30)

between the power density P and Q

CQ'/U - (BQ2141,

(31)

where B = B’/A and C = (B/ycrD)(l - R)/( 1 + R). By substitution of Qmax(t) into this relation the maximal power Pm., could be calculated. The dependence of Pm, versus the pulsewidth could be fitted with following parameters: s = 4 x lo6 cm/s for diodes with uncoated mirrors and with sputter-cleaned mirrors and s between 1.4 x x106 cm/s and

52

P. G. Eliseev

2 x lo6 cm/s for A1303-coated diodes. Values of PmXcorresponded to optical power variations between 0.1 and 2 W from 8 pm wide stripe lasers at pulsewidths ranging from 1000 to 10 ns. The power density of COD at 100 ns was measured to be about 50 mW/pm. Calculations for the thermal model of COD in DH lasers were performed also by Kamejima and Yonezu”” for pulse operation mode. They considered the following consequence of stages of damage: 1) a thermal runaway under positive feedback from the temperature-dependent optical power absorption gives a melting at small region near the surface; 2) at the end of current pulse the locally molten region is quenched and multiple dislocation loops are generated by a climb motion to compensate for the volume change between molten state and solid state; 3) the dislocation loop can be a stronger absorption center than the surface region, so when following pulse is applied, the loops act as the initiating source for new thermal positive feedback; 4) at a high temperature even below the coefficient becomes markedly large melting point the active region absorption ( > 3 x lo4 cm - ‘) and incident light penetration depth is very small, the molten region would not increase its volume, and under asymmetric thermal condition a restricted molten region moves towards the incident light; a result is the DLD propagating to the inner part of active region from the facet surface. Temperature rise at mirror facet in laser diode was treated by Yoo et ~l.‘~“) and a relationship was derived analytically using the same model assumptions of Henry et of.@), but applied to the CW laser. Temperature rise at the surface was expressed as AT = rwTJ@anhp

,

(32)

where r = hvLsN,,,/2lcT,, w = N(0)/Nlh is the carrier density at the surface normalized to the bulk threshold value, /J = (L/2)(rcc /r~Hd)“~, d is the active layer thickness, L is a cavity length, s is the surface recombination velocity, N,,, is the carrier (hole) density at laser condition inside the diode; ICand rccare thermal conductivity of the active material and of the cladding material on the path to the heatsink, respectively, H is the length (or effective length) to the heatsink, and T,, is its temperature. The facet heating was found to be almost linearly dependent on light intensity reaching about 50 degrees at a superradiant intensity 1 MWfcm’ while about half of the rise is provided by the junction temperature rise over the active region. Yoo et ~l.(~‘*)obtained a condition for no thermal runaway for CW lasers. The criterion is expressed in terms of laser geometry, physical properties and operating conditions in such a way that a laser can be designed to have a longer lifetime. It was obtained on the basis of one-dimensional balance equations for carriers and heat. The runaway criterion is in the following form &Xt”2 + pk > r&?/Lx”’)

(33)

was expressed in dimensionless parameters p, r, 4 = (L/2)[@(1 + R)ao/hvDN#2, kl = sL/2D, a. = 2wexp[l4rw(coshp - l)/psinhp] and p = 14(1 - w2)exp [14r(coshp l)/sinhp], D is diffusion coefficient of carriers. It was used later in Ref.c2’” to describe the long-term operation lifetime restriction due to COD. The criterion contains unknown variables at both sides so it is not possible to use it until the solution is found of the equation system, so Eqn. (33) seems to be difficult to use. Lee”“) considered conditions to avoid the thermal runaway in CW double-heterostructure laser diode in respect to geometry of active region in frame of two-dimensional models for current and heat distributions. He stated that the output intensity can be increased by an order of magnitude by reducing the stripe width sufficiently, but only about a twofold increase in the output power can be realized. This conclusion is opposite to what is generally believed for high power lasers where the stripe width is made very large. The results show that the output power decreases slightly with decreasing stripe width but then increases rapidly below

53

Optical strength of semiconductor laser materials

a certain width. The other important conclusion is that the laser with a smaller stripe width can operate at higher temperature at the same output power, when compared to a laser with larger width. Analysis of facet heating leading to thermal runaway for CW DH lasers was reported by Schatz and Bethea.(2’4) They presented a steady state model consisting two parts: (i) a three-dimensional thermal model of heat flow from the surface, and (ii) an one-dimensional model for the carrier diffusion towards the facet. A thermal balance at the surface was considered in terms of the existence of a stationary solution. The thermal model could give a peak temperature rise AT = AT (Q), where Q is the heat generation rate at the surface. The carrier diffusion model allowed determination of the nonradiative recombination rate at the surface which corresponds to the heat generation rate in its dependence on temperature, Q = Q (AT). The temperature dependence of Q arises due to temperature influence on the electronic and optical transport of the pumping energy to the surface, especially to temperature bandgap shrinkage. A self-consistent solution may be found from the equation AT =

AT[QW”I,

(34)

and corresponding situations can be illustrated graphically (see Fig. 11). There both dependencies for AT and Q are plotted, and the bold line for AT(Q) is the result of thermal modeling. It is determined by material capability to conduct out the excess heat from the surface and by device configuration. The line is generally superlinear due to the temperature dependent heat conductivity of materials, and its slope is called often “thermal resistance” of the considered subject (in this case the subject is the active region edge entering to the facet surface). The bold line reaches the critical temperature rise AT,, for the damage at some heat power Q,,,, which is a maximal dissipated power of the device surface. AT,, and Q,,,O.Y both limit the allowed range of operation. If the heat generation is not temperature dependent, the solution will be found in a point A at the crossing of the bold line with the vertical line Q = Q,. This would be a stable operation point. Our actual situation is that the Q is a superlinear function of AT, and the positive feedback in the thermal balance is included in this superlinearity. With the same starting heat power Q, the dependence Q(AT) will be represented by curve 1. It crosses the bold line at the point B which is a stable state like that at point A. There is another crossing at point C which is not stable. The occasional small increase of Q from Qc moves the operational point to the right side but there is no possibility of a steady state: as it is shown AT AT c

0

Ql

9

QcQm

Q

Fig. 1I. Graphical schematics of steady states in the thermal balance of facet mirror of the CW laser at the temperature rise AT vs dissipated power Q plot. Stationary solutions are the crossing points A, B, C. Point C is unstable point (a thermal runaway is indicated by the arrows). The dashed area is the damage region which limits an allowed operation range (AT, is critical temperature rise, Q,,, is upper limit for dissipated power); Q,, QZ are initial value of Q, Q. is one at point C.

54

P. G. Eliseev Table 11. Parameters used in numerical calculations and some calculated results’*“’

Parameter Emission wavelength Temperature coefficient of the bandgap Carrier diffusion coefficient Surface recombination velocity (SRV) Thermal conductivity of - active layer - cladding layer - temperature exponent p Meltingpoint Conditions for a thermal runaway: - minimal SRV - ominimal power - minimal intensitv

unit

GaAlAs/GaAs DH

InGaAsP/InP DH

et; cm+ cm/ s

880 -5 x 104 9.6 4x 10s

1500 -3.25 x lo-” 4.0 5 x 10’

0.44

0.044

K

0.11 1.25 1510

0.6 1.4 1335

cm/s W MW/cmZ

1.3 x 106 0.5 75

2.9 x IO6 4.4 660

W/cm.K W/cm.K

schematically by arrows, the surface thermal balance goes out of the allowed range of operation, i.e. this is a runaway case leading to the damage. If the pumping current was increased, the starting heat generation rate will move to Qz, and both crossing points at curve 1 will approach each other and then will degenerate into one tangential point. As represented by curve 2, at starting value Qz there is no crossing of the bold line, therefore the system has no stable operation point and will undergo thermal runaway. Thus one can conclude that there are two possibilities for the system to happen in the damage ranges: l l

as a result of the thermal runaway following the above consideration, if the stationary solution is the crossing of curves outside the allowed range of operation.

This means, that, in principle, the stability analysis for no runaway condition is not sufficient to exclude the damage process. This schematic is not sufficent to explain the situation if the damage occurring is of an optical nature, so for this aspect one has to determine the relative contributions of different processes into the energy transport of the surface. Notice that the appearance of the thermal runaway is represented graphically as a tangent point of two curves, therefore their curvatures are of importance. Thus the runaway condition seems to be sensitive to all factors influencing the curvatures, like temperature dependencies of heat conductivity, energy bandgap, etc. Comparative calculations are given in Ref. (2’4)for both GaAlAs/GaAs DH laser and for InGaAsP/InP DH laser. Fabry-Perot type diodes were treated of the same geometry for both case: 750 pm long, 1 pm wide, 0.1 pm thick active region, the confinement factor was assumed to be 0.15, the distance to heatsink was 5 pm (the thickness of p-side of diode), slope efficiency of laser diodes was assumed to be 0.3 W/A, the spontaneous lifetime of carriers 1.5 ns, and inverse carrier lifetime dependence on output power was described by coefficient 3 x 10” J-l. Other parameters which differ for both laser materials are given in Table 11. In the Table one may see that the heat conductivity of the active layer is much smaller in 1500 nm laser whereas the same parameter in cladding layers is much smaller in the 880 nm laser. Therefore, the geometry of heat flows will be different in these diodes. Ultimately 1500 nm lasers have more effective heatsinks from the surface. Smaller temperature sensitivity of the bandgap also gives an advantage to the 1500 nm laser. As a result, the conditions of calculated thermal runaway are found at much larger powers in 1500 nm lasers than in 880 nm laser. Possible values of minimum SRV were suggested for GaAs laser material, whereas one for quaternary material was found to be not realistic, so the mechanism of SRV-supplied surface damage mechanism could be ruled out for InGaAsP/InP laser material. This is a reasonable conclusion for these long-wave lasers. Minimal powers and intensities for both laser types as calculated for the thermal runaway condition are very high. In GaAs-based material it is much higher than the experimental value, about 4 MW/cm2 for well

Optical strength of semiconductor laser materials

55

designed coated diodes and less than 1 MW/cm’ for uncoated diodes. In the case of 1500 nm laser the experimental value for CW laser diode COD is not known. The discrepancy in COD power level for GaAs-based laser is large enough to excite searches for additional sources of surface heating. Including in the heat balance the heat generation over the total stripe region, together with taking into account the temperature decrease of the heat conductivity gave in Ref.(*14)a decrease of runaway power to 22 MW/cm2, which remains much higher than empirical COD. The discrepancy problem will be also seen in the treatment of thermal balance of QW laser diodes. We shall continue the discussion below. 4.3. Quantum-well lasers: features of increased optical strength of mirror facets We had seen from comparison of Table 5 and Table 6 that in QW lasers the characteristics of COD are noticeably higher than in DH lasers. In principle, this may be accounted primarily to smaller thickness of the active region that leads to (i) a smaller area of active region entrance at facet surface, and (ii) a smaller optical confinement factor. As a result, the surface heating modeling of QW lasers treats the situation with a smaller area of heat generation at surface and a smaller effective absorption when the medium amplification would be converted into temperature dependent absorption. This gives qualitatively adequate result of higher calculated COD parameters in QW laser in comparison to DH laser, but the problem of quantitative agreement remains open. Surface heating and COD occurrence in QW laser was treated in a few papers(‘03.‘09~2’6~ *Is). Garbuzov et a1.(‘09) considered the difference in surface heating in QW laser materials on the base of GaAlAs/GaAs and InGaAsP/GaAs for the same emission wavelength (near 810 nm). They used a simplified model for thermal balance at surface and obtained the heating in proportion to confinement factor. In their model it was assumed that: 1. heat is removed only through the heatsink being at constant ambient temperature, 2. the effects associated with a finite strip width can be neglected, the parameter is emission power density p (power per unit of the diode width), 3. real geometry can be replaced near the heat source by a cylindrical symmetry, namely, the heat source was considered to be a semicylinder of radius r, the alloy layers with low heat conductivity rccare also replaced by semicylinder of radius R equal to the layer thickness. The surface temperature

rise was estimated as

AT = (W/zr)[(l/k,)ln(R/L)

+ (l/KJn(b/R)]

,

(35)

where W is heat flow from the surface (in W/cm), K, is the heat conductivity of GaAs, b is the distance to the heatsink, L is the thickness of the absorbing layer at the surface. The quantity W was represented as W = AplkL

,

(36)

where A is coefficient accounting for the ratio of internal and external optical intensity, r is optical confinement parameter, ~1,is absorption coefficient (temperature-dependent) of the depleted layer at surface, and L is its thickness. The temperature dependence of the absorption was taken from the temperature dependence of the bandgap and of the absorption coefficient on the photon energy (in a linearized form). These calculations had shown that L is assumed to be of the same value as the active layer thickness, the temperature rise appears to be rather small and saturable even at SRV as high as 10’ cm/s at the level a few degrees in the case of GaAlAs/GaAs SQW laser and a similar result was found for InGaAsP/GaAs SQW lasers in a discrepancy to observed rise by 15-30 degrees. Consequently, this thermal balance did not give an explanation of the great overheating of laser facets leading to COD. It was JF’QE 20/i -E

56

P. G. Eliseev

concluded that the size of the absorbing layer at the surface is much larger, namely, to explain the observed facet heating (30 degrees at 0.25 MWjcm’) in GaAlAa/GaAa SQW laser it was assumed L = 1 pm, and for InGaAsP/GaAs SQW it was sufficient to take L = 0.2 pm. This means in the former case the assumed increase of heat generation by two orders of value. A large quantity for L was explained as a thickness of “dead” layer with increased recombination rate. Fujii et al. (‘03)had compared the COD level in short-wavelength InGaAlP/InGaP laser to that in GaAs-based laser and concluded that higher thermal resistance in the former can be responsible for lower COD in limits of 3-5 times. In the work of Chen and Tien(215)an analytical solution of the three-dimensional heat conduction equation was presented, yielding the temperature distribution in the laser. Heat generation mechanisms were modeled through the one-dimensional rate equation. Computational results indicate that the size effects on thermal conductivity must be included. A detailed examination of the calculated results and available experiment data suggests that the thermal runaway in QW lasers is driven by absorption in confining and cladding media instead of the active region itself, in contrast to that as generally believed. For example, in QW GaAs lasers with barriers of Gao.,&.& the latter is quite transparent to laser emission at room temperature but at a temperature rise of N 400 degrees it converts to absorbing due to the temperature bandgap shrinkage. An enhanced band-to-band absorption in these barrier layers leads to optical pumping and increase of carrier concentration there. The thickness of the barrier layer is of the same range as the thickness of the active layer in DH laser. Thus the surface state above the transition temperature is closer to that in the DH laser than in QW laser. The runaway situation may be described in the same terms as one for DH laser. In Ref.(2’S)it was noticed, that the critical temperature of surface heating at the beginning of thermal runaway is regularly higher in QW lasers that in DH ones, namely, 120-140 degrees against 20-30 degrees. As these temperature are measured by averaging over a probe light spot ( m 1 pm in diameter), the local temperature rise in the active region at the runaway start may be quite close to the above mentioned transition temperature rise of 400 degrees. The calculated absolute value of the maximum temperature rise was about 5 times lower than that in experimental measurements. To explain this it was suggested that the lateral thermal conduction in the layered structure is reduced by phonon reflection and transmission at the GaAs/GaALAs interfaces. In Ref.(216)a self-consisted problem is considered for QW lasers (in comparison to DH ones) for surface heating and runaway condition, and this modeling was made taking into account the current redistribution due to enhanced carrier consumption near the surface and due to local temperature-dependent bandgap shrinkage. The details of the model are presented below. 4.3.1. A self-consistent model for facet region (2’6).The modeling for power balance in the facet region of edge-emitting laser is quite complicated. We shall consider three main aspects of the problem: (i) temperature distribution, (ii)carrier distribution, and (iii) injection current distribution taking into account the current crowding to the surface region with a redistribution of current density over the region. The whole problem has to be solved self-consistently because all the distribution profiles involved influence each other. We restrict our calculations to a simplified model of heat generation only at the surface, produced as a result of surface recombination. Therefore we assume distributed sources of heat to be of secondary importance. Free parameters of the problem were diode design parameters and pumping level expressed in terms of threshold exceeding coefficient Y = Z/Z,,,(I is operation current and Z,his threshold current). We have used a set of material parameters typical for InGaAs SQW laser diode operating in range of 980 nm. The self-consisted solutions could be found in all cases until conditions of the model are not violated. As temperature rise was

Optical

strength

of semiconductor

laser materials

51

found to be small (less than 100 degrees) both thermal conductivity and thermal diffusivity could be assumed to be constant. 1. Temperature pro$le. A 3-dimensional heat flow was treated using the stationary heat conduction equation with adequate boundary conditions at every layer interfaces involved. The calculations are similar to that described by Chen and Tien(*15)but are made for both SQW and DH structures for comparison. Stable profiles were obtained for T(z) and were expressed in terms of quantity t?(z) which is a ratio of T(z) to the heat flow density at surface. So 0(O) is the “surface thermal resistivity” and the surface temperature rise can be obtained as e(O)Q where Q is the density of heat generation rate at surface. Typical profiles are shown in Fig. 12 for GaAlAs DH and InGaAsP/InP DH (active layer thickness is 200 nm) and for InGaAs SQW (active layer thickness 7 nm). Due to the difference in the active region vertical size the thermal resistivity appears to be noticeably larger in both DH, and the difference between both DH cases is caused by different thermal conductivities of the materials involved (in the InGaAsP/InP case the cladding material is InP with kc = 0.58 W/cm.deg., whereas in GaAlAs/GaAs case cladding material is GaAlAs alloy with typical ~~ = 0.12 W/cm.deg).The surface thermal resistivity e(O) is calculated to be 2.7 x 10m4K.cm*/W for GaAlAs/GaAs DH and 1.8 x 10e5 K.cm*/W for InGaAs/GaAs SQW structures. This means that for a loo-degree rise a continuous heat generation density of 370 kW/cm* is needed in the first case and 5.6 MW/cm* in the second case. 2. Carrier concentration projile. Several types of carrier motion are affect their distribution, namely, injection through the junction, radiation transport (photon reabsorption) and diffusion along the active layer. The continuity equation has to be solved for carrier concentration over axis z, whereas the concentration seems to be unchanging along axis x due to ultra-small thickness of the active layer (being much less than the carrier diffusion length) and along axis y as uniformity is assumed in this transversal in-plane axis. Another important assumption is the neutrality condition which means that concentrations of electrons N, and holes Nh are the same, and both are equal to N. The latter is connected directly to quasi-Fermi-level separation in the active layer.

0.5

-

.InGaAs /GaAs , SQW

Fig. 12. Computed profiles of normalized temperature rise 0(z) along a normal axis to the facet mirror in GaAlAs/GaAa DH, hGaAaP/InP DH and InGnAa/GaAa SQW laser structures(n6’.

58

P. G. Eliseev

The carrier concentration in majority of active region is in the consistent state to laser action condition with well-known pinning of N near the threshold value N,h. An exception is the facet vicinity where N decreases due to enlarged carrier consumption at surface. The concentration gradient supplies the diffusion flow of carriers to the facet and there the carriers recombine non-radiatively. It is worth noting that in contrast to pure diffusion transport to the surface in uniformly-pumped medium, we have here the situation with current and radiation assistance to the carrier transport, namely: (i) in accordance to a decrease of N near the surface the injection current distributes in that manner to compensate partially the enlarged carrier consumption, (ii) if N drops below the inversion threshold NOthe medium begins to absorb the laser emission, therefore the emission acts as an optical pumping mechanism there. This leads to radiation transport of carriers from inner part of active region to the facet surface. Notice that in the limit of very high radiation intensity the carrier concentration approaches NOfrom below, but does not overcome it because of the absorption saturation. Both these mechanisms produce an increased generation rate of excess carriers and ultimately they can eliminate the diffusion limitation of the surface recombination rate. In addition to these mechanisms the temperature effect has to be taken into account. It is a result of the bandgap decrease due to temperature rise near the surface. It leads to (i) a potential well formation at surface with carrier accumulation there, (ii) a decrease of the built-in junction barrier for carrier injection which gives a local increase of the current density through the junction. Due to both these effects the surface carrier concentration has a trend to increase. 3. Current projile. An accurate calculation of the current distribution near the facet needs to take into account the variation in the consumption rate of carriers followed by the variation of carrier concentration with a decrease near the surface. As a result, the junction voltage goes down near the facet surface and gives a rise for current redistribution. The spreading (sheet) resistance of the emitter layers influences the current redistribution. If the series resistance of the passive layers could be reduced to negligible values, the junction voltage would be under an equal influence of the metallic electrode (“equipotential” model). In this case the carrier concentration will be uniform over the z-axis but current density will be nonuniform to meet spatial variation of the carrier consumption rate. The profiles of current distribution over the area of junction can be found by solution of the Poisson equation, or, in the region of electrical neutrality, the Laplace equation for passive regions surrounding the active layer. This is quite general approach to the problem of current distribution, whereas sometimes the simplified model of “vertical” current can be used with the neglecting of the current components along the axis z. In that model the diode voltage is assumed to shared locally between the junction and series resistance voltage drops. The current seems to increase where the junction voltage is reduced so voltage at series resistance is enlarged in proportion to local current density. 4.3.2. Main results and discussions. 1. Current redistribution. Surface components of current. A realistic situation along the active region near the facet corresponds to a noticeable variation of carrier concentrations and of the junction voltage due to carrier migration to the surface. According to this the injection current density would be enlarged near the surface working to compensate accelerated consumption of excess carriers. As a result the density at the surface appears to be many time higher that in the internal part of active region. Therefore the redistribution of current supplies the facet region by additional energy forming a “surface” component of current. Its magnitude is as large as the lower electrical resistivity in the emitter region. There are two surface components of current:

Optical

5.

strength

of semiconductor

laser materials

59

Carrier density, 10” cn?

4-..

Y

__.

, _ .. .. ... ._.. J

3: A

;;

2’

,y ,/,,/” ,/

_/ _I” __..._-----:-

1 OlOOl

0.01

0.1

1

z, pm

Current density. Wcrrf

Fig. 13. Computed profiles of carrier density (above) and of current density (below) in the hGah/GaAs SQW laser structure dependent on the overdriving parameter Y = I/IlP6).

one is due to enhanced recombination rate near the facet (“recombination” component) and the other is due to a temperature gradient as the facet is overheated (“temperature” component). Both are caused by variation of junction voltage: the recombination component through the effective carrier lifetime variation while the temperature component is caused through the temperature bandgap shrinkage. Thus, the usual assumption of constant current density over the active area is far from reality at least near the surface with a high recombination velocity. One important consequence of this is that ordinary diffusion limitation on a flow of carriers to the surface does not operate. Another consequence is the appearance of an additional heat source from Joule heat generation of the surface current components. In spite of current redistribution the surface carrier concentration is usually noticeably lower than that inside the active region. Such a concentration depression leads to optical absorption if the concentration drops below the inversion threshold N,. Computed profiles of current and carrier densities are shown in Fig. 13. The effect of carrier redistribution is not found to produce a strong increase of surface carrier concentration N(0) for a real set of parameters. The concentration is several times lower than that inside the active medium, N,,,. However in physical limit the effect could increase the concentration up to N(0) = Nth. It can happen in the “equipotential” case when resistance of claddings is negligibly small. 2. Radiation transport. Intense radiation in the laser cavity produces the pinning of carrier concentrations over the active region maintaining the concentration value to keep sufficient optical gain. In contrast to this near the facet one has a medium which is absorbing rather than amplifying. The laser emission can generate electron-hole pairs here due to the optical absorption so an optical pumping would work there to compensate an accelerated carrier consumption similarly to current redistribution effect. Since laser photons emitted inside the active region generate carriers near the facet, one may say that radiation transport takes place providing an additional energy supplement

60

P. G. Eliseev

to the surface. This transport also eliminates the diffusion limitation for carrier flow to the surface and leads to an increase of the surface value of carrier concentration. It is known that under optical pumping the quasi-Fermi level difference AF grows to a value which is equal to the photon energy hv and approaches the latter asymptotically in the high intensity limit. When equality AF = hv is valid, the carrier concentration is just equal to the inversion threshold, N,. In other words, an optical pumping by own laser emission can increase the carrier concentration in the absorbing region up to N, but not to a greater value. This is valid until it is possible to neglect the pair generation due to nonlinear effects of radiation (multiphoton and high-harmonics absorption). Thus, in frames of linear interaction it is excluded that the radiation transport produces anywhere a peak of carrier concentration exceeding the value inside the active region. Notice that the upper limitation on the carrier concentration for the current redistribution effect is Nth, which is higher than the upper limitation by N, for the radiation transport effect. 3. Surface concentration and recombination rate. In a basic model of facet heating it is assumed that heat generation is concentrated at a geometrical surface. The rate of generation is in proportion to the product sN(O), where N(0) is the carrier concentration at the facet surface. The latter value is reduced by surface recombination so higher s leads to lower N(0). As a result, the above product and, therefore, the surface heat generation are dependent sublinearly on surface recombination velocity. Thus in the self-consistent model the heat generation appears to be limited as the surface recombination velocity reaches its physical limit at averaged velocity of carriers (about 2 x 10’ cm/s in GaAs at room temperature in non-degenerated case). Really the heat generation remains quite moderate and not-sufficient for thermal runaway with considered set of parameters. In the above numerical example the surface rate of recombination at s = lO’cm/s and Y = 20 is about 3 x 10”cm-2 SK’ and the temperature rise is about 40 degrees. 4. Facet heating. Optical and current contributions. The temperature rise predicted by the self-consistent model is smaller than experimental facet heating and does not lead to thermal runaway. The “bottle-neck” of the problem is the recombination rate at the surface sN(0). Actually the rate is equal to the carrier flow to the surface which is dependent on the concentration gradient. To get larger flow means to increase the absolute value of the gradient dN/dz, but this means simultaneously to reduce N(0) and to reduce the recombination rate. It is valid when the carrier migration distance is not influenced. Otherwise the gradient magnitude can be increased by reducing the distance. In principle it could occur under strong action of radiation transport and current crowding. If so, the radiation transport could increase the product to value sN, and current redistribution effect does to sN,,,. In the above numerical example these quantities are 2 x 1O25and 3 x 10” cm-%, respectively. Thus a breakdown of the “bottle-neck” will give an increase of heat generation by about one order of magnitude. The corresponding temperature rise is 300400 degrees which is not the melting point of the material, but higher than the empirical threshold for thermal runaway (about 140 degrees) and is sufficient to convert the structural case from QW-type to DH-type as the waveguide region at these temperature begins to absorb the laser radiation. The calculated illustration is given in Fig. 14 for InGaAs QW laser as the dependence of facet temperature rise on the surface recombination velocity S. The curves give rise to the following self-consistent model (l-3), and to the “breakdown” case by radiation transport (4). Curve 5 corresponds to the latter case but in double-heterostructure laser. In both cases of the “breakdown” there are no steady states above some values of s, which means the thermal runaway occurrence. 5. Optical damage or optically-triggered damage? From the above calculations one can

Optical strength of semiconductor laser materials ATs,

61

K

1000

1

d.1

1

10

100

S, lO"cm/s Fig. 14. Computed dependence of the surface temperature rise on SRV in InGaAs/GaAs SQW laser structure at different Y (Y = 10, curve 1, Y = 20, curve 2, Y = 40, curve 3) and in the case of saturated absorption in the same structure (Y = 20, curve 4) and in GaAlAs/GaAa DH structure (Y = 20, curve 5)(*16).

conclude that the facet overheating and thermal runaway at the mirror facet are energetically supplied by both current crowding and radiation transport. In the numeric example considered above the contribution to the energy transport to the surface provided by current redistribution appears to be larger than that provided by radiation transport. Also the breakdown to thermal runaway can occur earlier under current-supplied energy income. Nevertheless the damage phenomena are recognized as optical damage and are primarily dependent on the optical power of the laser. We mean damage phenomena of a specific type which is distinguished from current-induced damage and is characteristic for laser diodes. A reasonable question appears: is the COD is optically supplied by energy from laser radiation or is it current-supplied by energy but optically-triggered? In the latter case a mixed damage will take place: it is localized as optically initiated but the damaged site is as large as electrically supplied by energy. There is some indication of the possibility of mixed case in QW lasers, namely (i) experimental evidence for predominating current heating of mirror surface before COD, (ii) experimental observation of current density as an invariant in COD conditions, and (iii) damaged region observation being much larger than active region. 4.4. Surface recombination and model for surface degradation 4.4.1. Surface recombination velocity. For the sake of the important role of surface processes we consider briefly the data on surface recombination velocity (SRV) and on the modeling of the processes of the surface degradation which may account for the decrease of the COD power level during the normal operation of lasers. Some studies on SRV in laser materials(z’~zz*) are of interest because the involvement of SRV in the thermal damage mechanism. Survey data on the SRV in some materials and interfaces are given in Table 12. A high SRV in GaAs and in p-InP could be attributed to the existence of a so-called “dead” The layer thickness is about 50 nrnc219). The layer in GaAs layer at the open surface (219*224*227). relates to a negative surface charge in both n- and P-types, due to occupation of some surface states. Properties of the dead layer were formulated as follows’224! (i) the luminescence is emitted only outside the layer, (ii) the recombination rate in the layer is small due to depression of the carrier concentrations, (iii) an excess hole flux to the surface is converted at the internal boundary of the layer from a diffusion one to a field-driven one, and (iv) the recombination balance corresponding to this flux is displaced from a surface to the internal boundary with a high “virtual” SRV.

62

P. G. Eliseev Table 12. A surface recombination velocity (SRV) as measured in some material surfaces and interfaces

Materials PI-GaAs

p-GaAs

Doping level, cm-3

SRV, cm/s

Comments

3 x 10’8 1.88 x 10’8 4.18 x 10” 5.1 x 10’6 2 x 10’8

( 1IO), cleaved (IOO),polished (1 lo), cleaved (1 lo), cleaved

10” 1.6 x 10” I x 10”

2.9 x 10” 1.8 x IO6 1.4 x 106 6.2 x IO5 (2 f 1) x 106 2.8 x 10’ (3 * 1) x 106 3.9 x 10’ 4 x 105 4 x 106 (1.4-2) x 106 (2 f 1) x l(r (2 + 1) x 10” 1.5 x 105

5 x 10’5 2.9 x lOI 1.7 x 10”

300* 100 350 + 100 500) 100

- 10’9

GaAlAs/GaAs DH

n-RIP

p-M Interfaces:

GamAb.ds/p-GaAs

hAb~P/GaAs Gadb.ds/GaAs Gadnd/GaAs

<900 < 210 < 1.5

References 01%

,220, (224) 020, c2-m

c225, c210, (IL) (Ill) (100)

(ZMI

,226, (221,

It is seen in Table 12 that the recombination rate at some heterojunction interfaces can be made to be “ultra-low”.(22’,226) This gives a great advantage to these interfaces above semiconductor-air faces, and this explains qualitatively the much higher COD level in window-type lasers where the active region is separated from the facet surface by a heterojunction barrier. It is known also (see(228)) that with the increase in the lattice misfit the magnitude of interface SRV can increase, for example, to N 6 x 10’ cm/s in a InGaP/GaAs interface at relative misfit N 2.5 x 10 - 2. A modern issue in the control of the surface quality is the passivation techniques allowing to reduce SRV significantly. We return to this subject in connection to a survey on methods of improvement of the COD level in semiconductor lasers. 4.4.2. Comments on the surface degradation model. SRV is sensitive to the surface contamination and extrinsic species adsorbed on it. These are responsible for the surface states introducing energy levels into the forbidden band. Physical and chemical changes at the surface during normal laser operation leads to a gradual increase in the SRV and to the possibility of a higher temperature rise at the laser facet. Certain understanding of the degradation processes was achieved in assumptions concerning an oxidation and an As segregation at the surface(‘s4*u*.~9.~~~3 230). In spite of great uncertainty in these subjects, a definite model of the surface degradation was proposed in Ref.‘23o)using data on the relationship between the segregated arsenic at the surface and photoluminescence in GaAs. ~0)The surface recombination is explained to grow in time due to the accumulation of free (segregated) atoms of As at the surface (and, possiblly, in a thin surface layer). These atoms are suggested to be nonradiative recombination centers supplying a change of SRV in time. The mechanism to produce As atoms was proposed as follows. The solid oxides of Ga and As are formed at the surface under the action of ambient oxygen. The oxidation may occur as an air-solid reaction when the surface is not passivated, or as a reaction with diffusing oxygen through the passivation layer. A sufficient amount of oxidizers may be found to be present in the passivation and protection materials, although these materials were deposited in nominally “oxygen-free” and dry atmospheres. An oxide of As interacts readily with a solid semiconductor to produce free As atoms by solid-solid reaction(? As203 + Ga = Ga203 + As,

(37)

Optical strength of semiconductor

laser materials

63

which supplies an excess of As atoms. The reaction appears to have a larger heat of formation of gallium oxide so the thermodynamic equilibrium is shifted in favor of dominance of gallium oxide. As a result, gallium atoms extract oxygen gradually from arsenic oxide, generating free arsenic adsorbed at the surface. The bottleneck of the process is a primary oxidation so the rate of As accumulation ii ruled by the income of oxygen when the surface is well protected from an open air. The model is based on comparing the surface reaction at the facet with the diffusing oxygen and solid-solid reaction type of Eqn. (31) producing recombination centers. The surface concentration of the centers and an increase of the SRV are supposed to be proportional to the density of segregated atoms. The SRV kinetics is represented by expression’230’: s(t) = s - (Sl + &>/[l + Kt(s, - s,)] ,

(38)

where so and sI are the initial and ultimate magnitudes of SRV, K is a reaction constant of gas-solid reaction modified in respect to the SRV kinetics. (230) In the case of protection coating the rate of the oxygen income is limited by diffusion through the protection film. The inhibition ratio for the rate of the arsenic accumulation by the coating is expressed as (1 + KdlD,,), where d is the protection layer thickness, D,, is the oxygen diffusivity in the layer material. The operation lifetime limitation at low power when the COD event is excluded is going due to gradual increase of the threshold current and ultimately the maximal lifetime t may be expressed quantitatively in a manner of the Arrhenius law t = Aexp(E/kJ)

,

(39)

where A is a constant which can be calculated, E is the activation energy, ks is the Boltzmann constant, T is the mirror facet temperature, but not the ambient temperature. 4.5. Discussion on damaging mechanisms

We see from the survey of theoretical works and from the comparison of their results with empirical ones, that the theory is adequate for strong absorption case at external illumination. The theory is based on the direct thermal model with an assumption of total and localized conversion of optical energy into heat. In this case the energy balance corresponds to that observed and there is no necessity for theoretical improvements, as was demonstrated on an example of GaAs illuminated by Q-switched ruby laser (pulsewidth 2MO ns). Starting from the case of the same material illuminated by neodymium laser (case of starting transparency) and coming to other transparency cases and to the case of DH and QW laser self-illumination, the theory of direct heating (using nonlinear absorption and different mechanisms localizing the energy dissipation at the laser facet mirror) is not in perfect agreement with experiments, and the actual optical strength of most studied materials appears to be noticeably lower that theoretically predicted. The feeling is that the remaining discrepancy in the limits of one order of magnitude cannot be explained by some contribution to well-known assisting mechanisms like wave interference or self-focusing, and it can not be easily reduced to the influence of material inhomogeneities. We state the problem to be open and hope to identify realistic mechanisms giving a decrease in the optical strength. As the damage under discussion is presumably the surface problem, the main attention has to be applied to the consideration of surface processes. A possible contribution to the energy dissipation at and near the surface may be come from evaporated species from the surface, namely, from extrinsic substances like water, etc., and intrinsic chemical components. The latter are shown to be evaporated from the surface at subthreshold intensities. Gases ambient in the vicinity of the surface can give rise to a sharp plasma formation (which is often seen in experiments as a visible flash accompanying the damage occurrence). The plasma can be heated by laser emission and transfer a part of this heat to the solid surface. The absorption in the gaseous plasma layer near the surface may

64

P. G. Eliseev

give a complementary contribution to the energy dissipation which is quantitatively difficulty for the direct heating model of the solid target. The decisive contribution from this mechanism is not proved ultimately, but is supported by some observations, namely: (i) regular coincidence of a visible spark (which is a sign of gaseous breakdown) at the damage threshold in many experiments with transparent semiconductors and dielectrics, (ii)influence of the surface etching on the measured LIDT in the experiments with COJaser illumination of different transparent semiconductors, and (iii) a narrow range of LIDT intensities (3&50 MW/cm*) observed in identical experiments with the CO*-laser on a number of materials. Notice that the emission of COJaser produces evaporation of water molecules adsorbed at the surface very easily and by direct absorption. As to neodymium laser emission, it does not interact so effectively with adsorbed water directly, but at a subtreshold intensity the heating of the surface can be sufficient to evaporate water and other adsorbents, so the gaseous layer is enriched by active species that can be formed as the intensity approaches the LIDT. In the case of laser diode self-damage the influence of the gaseous layer seems to be rather speculative. A strong influence of the surface passivation and protection is explained usually by the decreasing of the SRV, as is proved experimentally. The interaction with ephemere gaseous phase is noticed, however, in experiments on the influence of the ambient atmosphere on the COD and on the surface degradation leading to COD. Particularly, the COD power appears to be higher in vacuums and inert atmospheres. The “bum-in” in the inert atmosphere is found to be suitable to increase the COD-controlled operational lifetime. The latter observation has not yet been adequately explained. As shown in the calculation based on the direct heating model for COD, an energetic discrepancy exists also between theoretical and empirical optical strength. The “bottle-neck” is the heat generation at the surface in the self-consistent state. The optical transport and the current redistribution to the surface region are shown to be insufficient to give a temperature rise as observed in QW laser diodes on the base of GaAlAs/GaAs. The difference in other laser materials in the COD parameters may be an indication that the low optical strength in laser diodes of the type discussed is a material rather than an extrinsic adsorption problem. The physical situation at COD is deviating from the regular self-consistent model and the nature of this deviation (i.e. of the breakdown of the regular model) has to be understood. One probable assisting mechanism in the surface damage is the dynamic changes at the surface (possibly in the so-called “dead” layer) which provide the increase of the energy dissipation. A relief of the energetic problem is expected in the case of damage produced without the material melting, but as a result of mechanical destruction under stresses developed by nonuniform heating or by other mechanisms. As discussed in w , the laser illumination can generate elastic shock waves (“thermal shock”). Other alternative mechanisms are mentioned as follows: electrostriction, stimulated Mandel’shtam-Brilluoin scattering (SMBS), cumulating in elastic waves of thermal shock, etc.). In Ref. (40)the mechanical stress due to electrostriction at intensities of 20-30 MW/cm* is estimated to be near 0.02 kg/cm*, which is very low to play a role in the damage process. As to acoustical waves which can be generated, the numerical estimation’“’ leads to a stress magnitude of up to 30 kg/cm* whole absorbing of optical flow of density of 30 MW/cm* in GaAs. Acoustical guiding and wave cumulating can increase the magnitude. It is not the case in uniform bulky specimens but is more probable in layered diode structures. In spite of the fact that the stress level is much lower that the mechanical strength of bulky materials, in Ref. (40)the mechanical damage was recognized to operate as the local mechanical strength at surfaces which can be significantly lower than in the bulk. Mechanical strength in bulk materials like GaAs is on the level of lo3 kg/cm*. Thus no comparable stress can be generated by light in solids if its intensity corresponds to LIDT.

Optical strength of semiconductor laser materials

65

If at the surface the mechanical strength can be reduced, then the thermal shock is stated to be the most probable cause of brittle damage in Ref. @“).Estimations for thermoelastic stress in@,“) also gave a possibility of brittle damaging in COD of laser diode mirrors. Rough estimation of the quasi-static thermoelastic stress s was made by the following expression. c N arEAT,

(40)

where ar is the thermal expansion coefficient and E is the Young’s modulus. This gives a heat generation rate of about 1 J/cm2 energy dose to reach a destructive level of mechanical stress. Another possibility of mechanical damage is hypersound generation”@ which can result from stimulated scattering of an optical wave. Estimation of SMBS threshold according toczj’) gives values for non-piezoelectric solids in range 10’“-10’4W/cm2. It was mentioned above that the SMBS mechanism of damage was suggested in Ref. (34)to explain the decrease of COD power at lower temperature in epitaxial homojunction GaAs laser diodes. In Ref.(‘) it was indicated that for intense edge emission in GaAs the threshold for SMBS could be quite low due to strong dispersion, namely, in range near 0.1 MW/cm2. The acoustical confinement in heterostructure may also be a favorable condition to lower the SMBS threshold. On the other hand, when absorption of sound waves is weak, the question arises about the energy dissipation from waves to heat. For concentrated heat generation the acoustic absorption has to be strong, which leads to a high SMBS threshold. In principle, the damage by sound waves or hypersound waves can be of a not-thermal but of brittle destruction nature if the elastic vibration amplitude is large enough. In this case, the appearance of a damaged site is specific, namely, with no signs of melting. This may be the case for laser-induced damage under short pulses as described by Grasyuk and Zubarev. (40)They observed crack formation in GaAs, Si and CdSe, but in experiments specially designed to identify SMBS components proof was not found for its occurrence under illumination up to N 30 MW/cm2. Possible mechanisms of mechanical damage do not explain the obvious appearance of signs of melting accompanying numerous experiments. The only case which does not rule out the involvement of the mechanical damage in these observations is that of mechanical damage giving rise to very strong nonradiative recombination in the damage site, so further enhanced energy dissipation leads rapidly to the melting of the material. In last part of this section we shall comment on the alternative damage mechanism typical for transparent dielectrics, namely, the avalanche breakdown. This is a specific process typical of high-resistance material where the avalanche (or other type of electric breakdown) could be strongly localized (like a spark region in gaseous breakdown). Thus an occurrence of dielectric breakdown can lead easily to material damage as an optical energy would be transformed into heat in small volumes. The heating proceeds in the process by the thermalization of hot carriers and by the nonradiative recombination of thermalized ones. The energy input is controlled by the acceleration of free carriers in the radiation electromagnetic field. The rate of energy growth of a single carrier in the electric field at intensity of 30 MW/cm’ is as low as 0.01 eV/ns, and therefore is too slow for the avalanche breakdown. The electrical breakdown is a probable mechanism triggering the optical damage in dielectrics and high-resistance semiconductor as a first stage of the process supplying large density of carriers to absorb the laser radiation. In laser media the carrier concentration is rather high in the starting state, so electrical breakdown is not a necessary stage in this case. As transparent materials of protection coatings suffer from dielectric breakdown, the ultimate optical strength in these devices will be ultimately limited by the damaging of these materials, This limit is possibly very far above the actual power limitations in semiconductor lasers induced by processes in semiconductor media. Consider in conclusion of this Section the scheme of involvedprocesses in the thermal model of laser self-damage shown in Fig. 15. The main stream of energy is given in the upper row; it consists of an energy conversion chain from input electrical energy to electronic energy (of

66

P. G.

Mainenergy

Input

Eliseev

Intemal conversion Electronic e4lergy I

Degradation pfoceSSeS

(oxidation,erosion, defect formation and migration

\

COD (Overheating, thermal runaway, melting and destructionat surface)

Fig. 15. Scheme of interconnections between processes in semiconductor laser which can lead to gradual and catastrophic degradation.

electron-hole pairs in the active region), then to optical energy (of photons accumulated in the cavity) and then to output laser emission. Dissipation mechanisms are the base of degradation processes, and they are Joule losses, nonradiative recombination losses and reabsorption of photons (the latter is of non-dissipative nature excepting those part which contributes to nonradiative recombination). Intermediate factors are temperature of the active region (over the whole active region), current redistribution (especially the temperature-induced part), temperature rise at the surface and thermal bandgap shrinkage. When the surface temperature reaches a critical value for surface destruction (melting, as a particular case) the irreversible damage can occur immediately. It is shown that temperature rise (both in the whole active region and at the surface) contributes to the acceleration of gradual surface degradation which includes oxidation, erosion, defect formation and migration inward. There is a feedback from the gradual surface degradation to the total nonradiative recombination, therefore the process leading to damage would be favored. This is a basic cause of sudden failure during operation time. There are other feedback loops such as the temperature rise at surface, leading to thermal bandgap shrinkage, photon reabsorption and nonradiative recombination. This one supplies the optically induced thermal runaway with a probable damage in the end. Another is the temperature rise at the surface leading to current redistribution and nonradiative recombination. This is the path for current-induced damage which can be mixed with optical damage or acts separately.

5. IMPROVEMENT

IN OPTICAL

STRENGTH

There are several ways to improve the characteristics of optical strength of laser materials. At first we shall consider methods to improve the COD intensity by material choice, protection and passivation of mirror facets. Then we also consider methods to improve the COD specific power and the entire power from the device, mostly by special designing of the active region and laser cavity. In other words, a second group of methods (configuration designing) can provide load-off at facets keeping or increasing the optical power when the critical intensity is assumed to be invariant. The review below is not comprehensive but it gives examples of successful applications of different techniques to improve COD characteristics in laser diodes.

Optical

strength

of semiconductor

laser materials

67

5.1. Improvement of COD intensity From the above survey of empirical data on COD intensity one can conclude that in order to elevate the damage threshold of laser material it is possible to make the right choice of chemical compositions providing maximal COD intensity at desirable wavelength. Therefore the first subject is comparable studies of different materials. Then we consider possibilities to elevate COD intensity by coatings and passivation, remembering that if modification of reflectance occurs, the external COD intensity is also modified due to changes of ratio between external and internal intensities. An important type of well-protected device is lasers with “windows” or non-absorbing mirrors (NAM). 5.1.1. Material optimization. In a number of publication the Al-free alloys were used in laser structures with the aim of reducing surface problems and enlarging COD power, particularly for lasers in the ranges of optical pumping, 810 and 980 nm.(98*“2.“3~232.233) Preliminary reliability studies of strained In,,.1&.13G~.72A~quantum-well lasers operating at 8 10 nm are reported by Yellen et al. (98)InAlGaAs lasers, a possible replacement for AIGaAs lasers, have been studied with respect to three failure mechanisms. Uncoated I&.IJAk13G~.,2A~ quantum-well lasers have exhibited catastrophic optical damage limits of 1.87 MW/cm2, which is equal to that of similar AlGaAs lasers. Further, the lasers are both free of < 100 > DLD-induced sudden failures and exhibit low degradation rates even in this early stage of their development. A comparable study was reported in Ref. (8’)of Al-free laser structures of InGaAsP/InGaP system and conventional GaAIAs system both emitting near 800 nm. The single mode diodes were CW operated at room temperature. The measured COD level of a BH InGaAsP/InGaP laser with a cavity length of 470 pm was about 8.6 MW/cm2. Particularly, we could not observe COD in the long cavity InGaAsP/InGaP lasers, but they showed thermal saturation which is reversible. Also, InGaAsP lasers were found to sustain high current injection three times more strongly than AlGaAs counterparts. In Refs.“‘2~“3~233’ the version of 980 nm laser diodes were described on the base of InGaAs/InGaAsP/ InGaP strained QW heterostructures. The authors discuss the fabrication and characteristics of high-power (Pew = 430 mW) ridge waveguide lasers. In the past, high-power operation of Al-free pump lasers has been limited to 150 mW because of COD at mirror facets. This problem has been largely removed by increasing the spot size of the laser with the aid of an improved waveguide design. As a result, AI-free lasers can now achieve a maximum power comparable to the conventional InGaAs/GaAIAs-based pump lasers for 1 = 0.98 pm. Limitations of the two-dimensional passive waveguide approximation have been discussed in Refs.(“3*233’. With a closely optimized laser structure, a CW output power of 200 mW in the fundamental transverse mode was achieved. A slight change in “strength” of vertical mode confinement was shown to change significantly the onset of COD at mirror facets. In Ref.(“4’ the fabrication and comparison had been reported of buried heterostructure and ridge waveguide 980 nm lasers with strained InGaAs quantum wells, stepped InGaAsP confinement layers, and InGaP claddings. The buried heterostructure (BH) lasers exhibit superior performance with lower threshold and higher power. It demonstrated a laser action with 4.4 mA threshold current, 77% differential quantum efficiency, 196 mW of output power, and 150 K characteristic temperature. No catastrophic optical damage is observed on the laser facets, although the facets were not coated or treated. In 1.3 pm wavelength range material system of strained A1,Ga,Inl -.-,As/InP QW-type structure was tested instead of conventional Ga,In’ _ %AgP’_ ,/InP material active layer, no COD was system in Ref.(234). In spite of the aluminum-containing observed at room temperature up to about 73 mW/pm for compressive-strained five-quantum-well lasers and 34 mW/pm for tensile-strained three-quantum-well lasers

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(referenced to nominal ridge width of 3 pm). For operating the compressivestrained five-quantum-well lasers at 85°C with more than 5 mW output power, a mean-time-to-failure (MTTF) of 9.4 years is projected from a preliminary life test. These lasers are intended for uncooled, potentially low-cost applications in the subscriber loop. A self-aligned stepped substrate (S3) AlGaInP visible laser diode structure(235’had been stated to be very attractive, because the laser structure, which includes a current blocking layers and an index guiding waveguide, and is fabricated by only one-step MOVPE growth. The S3 laser structure have been improved for high power operation, and modified the facet reflectivity and optical confinement factor to increase the power level at which COD occurs. High-power operation at more than 70 mW has been achieved. The laser exhibits fundamental lateral mode oscillation up to 40 mW and stable operation at 50°C for more than 1000 h with an output power of 20 mW. Theoretical investigation has been carried out for high-power InGaAlP visible-light laser diodes in Ref. (236) . The thin active layer, essential for preventing COD of the laser facet, enhances carrier overflow and causes a deterioration in temperature characteristics. High-power transverse-mode stabilized InGaAlP lasers, operating at high temperature, were realized by employing a composition-shifted thin active layer and a highly doped p-cladding layer. High-power operation at over 100 mW was achieved. 5.1.2. Mirrorfacet protection techniques. The state and cleanness of facet mirrors affect the COD level significantly. For example, in Ref.“‘) concerning GaAlAsjGaAs DH lasers it was mentioned that the COD power of sputter-cleaned mirrors is two times as large as that of untreated mirrors. However, uncoated or unpassivated surface can easily degrade, not only under operation condition but also under storage condition. Special treatments are important to stabilize the surface for high-power and for long-term operation. Dielectric coating provides protection against aggressive chemicals and absorbing species of ambient medium. We distinguish passivation procedure from protection procedure as the former is a chemical treatment of the surface leading to the substitution of unstable compounds by stable and neutral ones which do not produce a high surface state density to semiconductor facet. The thickness of the passivation layer, for example, in the case of sulfur passivation, is as small as 1.5-2 nm. The protection coating was found to improve the COD intensity as it was mentioned above (see Chapter 2). The most widely-used materials for this purpose are A&03, SiOl, and also other oxides like Eu203, ZrO*, etc. These are substances with high optical characteristics (including LIDT), being chemically stable and having good adhesion to semiconductor surface. The absence of pores and holes is also a necessary property. A protection function of the coating is combined sometimes with a function to modify the reflectance of end mirrors of the laser cavity. The refractive index and layer thickness are of importance in the case. A single i/4n layer gives minimal reflectance, corresponding to anti-reflection (AR) coating (with the reflectance approaching zero if the layer index is equal to square root of the semiconductor index). A layer of A/2n-thickness is neutral to the reflectance keeping the magnitude of the free surface, as well as those of the thickness which is equal to an integer number of halfwaves. The effect of increased COD power by AR coating was reported.05,‘*) It was explained taking into account the change of ratio of external and internal intensity of the laser beam. Therefore this effect is of an optical nature with no indication of physical protection of the mirror surface. The effect of neutral coating on reliability and on COD was also experimentally observed(5j*5s,74* *lo) especially in the case of A&O, coating. This was a consequence of some change in the physical and chemical state of the protected surface leading to decrease of intense nonradiative recombination in a surface layer. According to Ref.(SS)the increase of the pulse COD threshold was 3 times for d/2-SiO* coating and 5.5 times

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for 2/2-A&O, coating. The advantages of gallium-oxide coating were discussed for GaAa-based optoelectronic devices in Ref.(239) 51.3. Interference coating. Stolyarov(237)considered theoretically an effect of phase-shifting multilayer coatings which can control the position of nodes and antinodes of the standing wave inside the laser cavity in respect to mirror surface. A decrease in the field strength at mirrors can be obtained if the dielectric coating supplies a negative amplitude reflectance. This means that the antinode position of the standing wave is shifted from the surface inwards by a distance of 1/4n, which is only N 60 nm in the GaAs active medium. An influence of this shifting on the COD power may be expected if the dissipation mechanism is sensitive to such a small distance. Meanwhile in Ref. (238) the COD improvement was claimed by use of multiple alternating quarter-wavelength coating of ZrO&3iOt layer pairs, which provides a negative amplitude reflectivity coefficient. A comparison was made between three groups of GaAlAs/GaAs DH mesastripe lasers by 30 units in each. The active layer was 50-70 nm thick, the mesastripe was 4-5 pm wide and the cavity was 200 pm long. Group I was of uncoated units, group II was of Si02-coated for amplitude reflectivity 26.5%, and group III was of units with multiple dielectric coating of amplitude reflectivity - 28.3%. The maximal power as limited by COD was reported to be quite different after averaging over each group and was equal to 85.8 mW in group I, 161 mW in group II and 220 mW in group III. The conclusion was made that the COD power is sensitive to phase shift at mirror reflection and usage of coating shifting the phase by x is effective to improve the power performance of lasers. Other factors which can influence the COD power in specimens with different coating materials were not considered. 5.1.4. Passivation. An important step in the preparation of stable mirror facets is surface passivation which may include cleaning, chemical treatment and coating of the surface. Appropriate passivation procedure can decisively reduce the COD occurrence in both instantaneous and long-term testing. (2b25’)The fabrication of etched mirrors for AlGaAs semiconductor lasers was described by Webb et al. (240) . The coating techniques had been presented for the passivation and reflectivity modification of the etched mirror surfaces. Measurements on coated lasers had given an excellent beam quality and satisfactory uniformity of laser characteristics across a wafer. Lasers which operate in a single transverse mode at output powers up to about 50 mW and have COD thresholds greater than 120 mW have also been demonstrated. In Ref. (24’)the full-wafer fabrication of AlGaAs lasers, which have mirrors etched by chemically assisted ion-beam etching and passivated by ion-beam sputtered AhO, had been described. The lasers had excellent beam quality with low phase-front distortion (AA/n) of less than 0.04 and high COD power. Full-wafer testing of 1400 lasers with a yield of functioning lasers of over 90%, and with good uniformity of output-power versus current characteristics over a wafer had been shown. Several studies are reported on (NH&S treatment for facet passivation.(242-244’ This was applied to the cleaved c 110 > facet of 780 nm AlGaAs high power laser diodes. The maximum achieved output power was 220 mW under CW operation with no COD, whereas untreated diode shows COD at about 120 mW. The effect of the treatment on the AlGaAs < 110 > surface was investigated by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). These analyses revealed that the (NH&S treatment drastically reduces the surface oxide of Ga, Al and As atoms, and leaves a sulfide layer of these constituent atoms. The improvement of the high power characteristics has been attributed to the sulfide passivation and to the reduction of non-radiative recombination centers between the AlGaAs and the surface oxide layer. Stable operation of more than 2000 hrs has been confirmed under 50°C at 50 mW. An increase of 70% in the COD level of AlGaInP visible laser diodes is achieved by sulfur treatment.‘*@’ The laser wavelength was 686 nm. COD power reached 30 mW in 5 pm wide stripe laser. From transmission electron microscope and energy dispersive microanalysis it

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was confirmed that most of the oxide at the mirror facets is replaced by sulfur after this

treatment. It is thought that oxide at the facets introduces surface states which cause the COD, and removal of the oxide by sulfur treatment results in the higher COD level. High-power reliability performance of strained single quantum well lasers with facet passivation layers which eliminate the COD of mirror facets in practical application . So-called E2 lasers exhibit very low gradual degradation conditions is reported in Ref. (95.191) rates and have demonstrated operational integrity as pump sources for erbium-doped fiber amplifiers. The MBE-grown lasers are vertically confined by graded-index heterostructures with Gao.dl&s cladding. The strained quantum well is a 7 nm thick hnGao.rsAs layer. Lateral confinement is achieved by a ridge waveguide structure, above 3 pm wide, etched into the top p-cladding. The E2 technology includes facet passivation followed by HR/LR facet coating. Diodes with AhO3 or Si& coatings were tested at 30, 50 or 75°C at power level 20&300 mW. None of the devices of the test population (60 units) suffered from catastrophic type of failure during 600&32000 h tests. It was stated that operational lifetimes of more than 200 kh can be extrapolated at 200 mW and 50°C. Power limitation in these lasers was imposed by thermal (reversible) saturation and roll off. 5.1.5. Window-layer lasers. An increase of COD power for 2-4 times was stated in Ref.(‘52’ in GaAlAs/GaAs DH laser diodes by usage of ZnSe window-layers deposited on mirror facet. Enlarged COD intensity was estimated to be about 12 MW/cm’ at pulsewidth of 130 ns, and COD power was N 1.5 W in 6 pm wide ridge-waveguide diodes at room temperature. ZnSe material is lattice-mismatched to GaAs only by 0.2%, so window-layers can be epitaxially grown at GaAs surface. As a result, good adhesion layers were obtained by relatively low-temperature deposition. The study in Ref.(249)was performed to compare the power stability in such GaAlAs/GaAs laser diodes of 6 pm wide ridge-waveguide type with and without ZnSe window-layers at mirror facets. Test were made at 50°C an 75°C at CW output power 10-30 mW. It was concluded that diodes with windows had a degradation rate 4-14 times lower than ordinary units. Sasaki et al.‘253’proposed WGF (“windows grown on facet”) laser structure with window layer of relative wide bandgap alloy grown on cleaved facet of GaAlAs/GaAs laser diode. Watanabe et al.C254) reported the WGF lasers of InGaAlP operating at 680 nm prepared by LP-MOCVD growth of thin Ino.5Gao.,sAlo.JaP layer on both facets of diode bar. Starting laser structure was of strained-SQW type fabricated by two-step MBE with a p-multi-quantum barrier. The kink-free maximum output of 295 mW from 4 pm ridge WGF diode was obtained which was about twice as much as that of the conventional one, where the power was limited by COD. The threshold current (about 100 mA) and the slope efficiency under 150 mW (about 0.75 W/A) were the same as those of the conventional one, which indicates that the window layers do not affect the properties. The fundamental transverse mode operation up to 150 mW was confirmed. This technique seems to be promising taking into account that InCaAlP lasers are very susceptible to facet degradation at high power operation. 5.1.6. Window-type and NAM structures. Numerous laser configuration may be related to window geometry type. The passive “window” can be made by regrowth of semiconductor material over output region or directly on facets. In other cases a “non-absorbing-mirror” (NAM) is made, or the output fraction of an active region is profiled in such a manner that supplies separation of the active region from the mode spot at the mirror facet. Simplest window-type structure was realized in form of “trough” laser where the injecting homojunction was rolled off near the facet into p-side by special selective Zn-diffusion procedure. (33)The COD power improvement of about 50% was stated in such laser structure under pulse conditions. The laser radiation penetrates the heavily doped passive n-side of junction near the surface. Optical absorption in this side was low due to the “blue’‘-shift of the intrinsic absorption edge under the n-doping. Yonezu et al.@2~75)had reported a

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Zn-diffused window-type heterojunction structure with a transparent n-type GaAIAs (x = 6%) region near the cavity end prepared also by selective diffusion during the formation of the p-side of the diode. The improvement of the pulse (100 ns) COD intensity was from IV2 MW/cm2 (5-10 mW/,um) in ordinary specimens to 10 MW/cm’ in window-type diodes ( w 40 mW/pm). In CW operation mode, 80 mW ( w 15 mW/pm) output was obtained in window lasers. A “crank” diode with transverse-junction-stripe (TJS) structure(25s)was also fabricated by selective diffusion protecting the cavity end region from junction formation.(256’The selective diffusion was used to make the junction roll-out from the facet at such a distance (10 pm) that excess carriers could not reach the facet and there was a negligible nonradiative recombination at the mirror facets. Long-term reliability (ten thousand hours) of “crank’‘-type TJS lasers was demonstrated in Ref.cz5’)with aluminum-oxide coated devices operating at 890 nm wavelength. The insertion of bent junction regions into the cavity leads to some additional optical losses due to scattering of laser emission. Therefore, devices with protection of this type may be of a higher laser threshold. In the case of crank-TJS lasers the threshold rise was not more than 10%. Diffusion technique for fabrication of window-structure was used also in Ref.(258). In structures of Epitaxial regrowth in the mirror region was used in Refs. (84.85.87.124.259). BH-LOC with a window type, a specific power of more than 130 mW/pm (at pulsewidth 75 ns) was obtained with no COD, which was 3 times higher than COD power in those diodes without windows.(84’Window transparent sections of GaAlAs (x = 0.22) were 25 pm long at both cavity ends. The output power of the window lasers was limited by uniform heating of the diode and not by mirror damage. Experiments were made to examine the effect of mirror oxidation (in boiling water) on the laser performance. Lasers without windows were found to degrade approximately four times as fast as window lasers. One possible explanation for this is that the oxide that is grown on the window sections is more stable than the oxide grown on the active layer.(84) In visible InGaAlP laser diodes there is the possibility of fabricating window regions using a property of InGaP active material to change bandgap dependency on the ordering of the alloy crystallography. An unintentional ordered (superlattice) structure of alloy has a narrower bandgap than that of disordered GaInP crystal. Thus the radiation emitted in the ordered active medium is not absorbed in the disordered window region.‘9h92)CW power of 75 mW was obtained in window-type lasers with natural superlattice disordering.“” Zn diffusion was used to produce the disordering. (92)Some data on window type lasers were also reported in Refs.(2@263*276). 5.1.7. Laser-irradiated NAM laser. Excimer-laser-irradiated nonabsorbing mirrors in GaAlAs/GaAs laser structure of VSIS (V-channeled substrate inner stripe) type were prepared by Lim et ~1.‘~~~’It was noticed that the COD power increased by up to three times in specimens treated by 60 mJ pulses at 306 nm wavelength. Increase of the irradiation dose gave no COD improvement, and the rise of oscillation threshold in irradiated diodes was within limits of a few percent. Such a technique seems to be quite simple as compared to other techniques requiring high-temperature procedures. Another example of laser-irradiation technology was given in Ref.(263). 5.1.8. NAM lasers of various types. Efficient protection of facet regions in NAM-type laser structure allowed them to demonstrate very reliable operation of CW laser diodes at enhanced levels of output power(2e27’*278)with a power-independent gradual degradation mode. In the laser structure of NAM-CDH-LOC (non-absorbing mirror constricted double-heterostructure large-optical-cavity) type a pulse power was obtained at 1.5 W (pulsewidth 100 ns) corresponding to the specific power y 200 mW/,um, 4 times higher than in samples without a NAM region and 2-3 times higher than in other window-type lasers.(8s’ JPQE 20/1--F

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The maximum achieved peak-pulsed COD level was measured at 20-30 MW/cm* (corresponding to a mode spot size of (5-7) x 1 pm). Low-noise and high-power operation in GaAlAs lasers have been developed in Ref.(268)by applying a high reflectivity facet coating approach to NAM lasers. High reflectivity coating was used to reduce feedback-induced noise coming from hopping between the diode cavity modes and between the external cavity modes. A NAM structure was used to obtain high-power operation by suppressing mirror degradation and COD effectively, even with the use of high reflectivity coating. NAM lasers with the front facet reflectivity 50% showed the relative intensity noise level to be less than - 130 dB/Hz at 3 mW under &lo% optical feedback and showed the CW output power to be as high as 50 mW in fundamental mode operation. Under 30 mW, CW operation at 50°C on the other hand, obvious degradation has not been observed over 4000 h in the NAM lasers. Thus the presented laser is useful as a low-noise source at a low-power level and also as a high-power source with high reliability. In Ref.(266)the phase locked operation of a 5-element GaAlAs diode laser array emitting at 0.6 W CW was reported. The NAM configuration was used for the suppression of COD and buried twin ridge substrate (BTRS) structure was used for the injection current confinement and for the stabilization of a transverse mode. 5.1.9. Bent-waveguide NAM laser. In Ref.(88,237) an etched-NAM structure for AlGaAs SQW GRIN ridge-waveguide lasers had been analyzed with respect to mirror coupling coefficient, threshold current penalty, and far-field pattern. The structure was made by growth on the profiled substrate providing desirable roll-out of the active and waveguide layers in the vicinity of facets. The laser beam exits mostly through wide bandgap layers of structure. Measurements of the mirror temperature showed a reduction from 50 to 20 degrees at 30 mW optical power, depending on the degree of overlap of the optical intensity with the absorbing bent-waveguide profile. Pulsed COD power levels up to 400 mW and a thermally saturated CW power of 165 mW (with single-mode operation up to 80 mW) have been achieved. Lifetime measurements at 40 mW constant optical power indicated degradation rates < 10 -5/h comparable to AlGaAs lasers with cleaved and coated mirrors. 5.1.10. Disordered NAM lasers. Impurity and defect diffusion was used to produce the compositional disordering in DH and QW laser structures. The result of disordering is creation of material with intermediate alloy composition, therefore, with a wider bandgap than that in the narrow-bandgap active layer of the laser structure. Such a material placed at the ends of the laser cavity near the facet mirror works as a window for laser emission and a barrier for electron-hole pairs. The impurity-diffusion induced disordering was observed firstly in semiconductor superlattices in Ref. (272) A review on intermixing technique in quantum wells for device technology applications is given by Marsh.‘273’ Impurities for such a disordering process can be Si, Zn, etc. Si diffusion was used by Thornton et al.(274)for the preparation of facet windows in high-power GaAlAs multistripe laser arrays. Disordering by Si diffusion from the SiOl mask was used in Ref.(277)to make window-type regions at facet mirrors in LPE-grown GaAlAs/GaAs DH lasers with 80 nm thick active layers operating at 834 nm. The length of the disordered region at each facet was 25 pm. An improvement of COD power under 100 ns pulse pumping in uncoated specimens was from 145 mW to 1580 mW and the COD intensity in the latter case was estimated to be 10.5 MW/cm’. The impurity-diffusion disordering of natural unintentionally produced superlattice in InGaP was also used to make window-regions in short-wavelength lasers as it was mentioned above.(90-92,275J 5.1.11. Non-injection region laser. The technical task exists to protect window- or NAM-regions at cavity ends from an injection current in order to avoid the presence of excess electron-hole pairs near the facet mirror. The best situation is when the regions are of high-resistivity material. In other cases the additional current-blocking insertion are need for the above purpose.““. 27F28L) Th e current-blocking layers inserted near the facets are working

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positively themselves, even though there is no NAM region. This is probably a result of the elimination of current crowding to the facet occurring in ordinary diodes. So insertion of non-injection regions (NIR) seems to be useful for COD improvement in laser diodes. The NIR laser of InGaAs/InGaAsP/GaAs operating at wavelength 980 nm was reported by Sagawa et al. (“‘) Maximal power increase was observed due to the insertion of NIR near facets from 386 to 466 mW, the former being limited by COD and the latter by thermal saturation. This improvement was attributed to the suppression of temperature rise at the facets due to the reduction of the nonradiative recombination. 5.2. Conjiguration designing 52.1. Thin-active-layer and tapered configurations. It was noticed that DH laser with thinner active layers are more resistant to COD failure, and thin-layer-lasers (TAL) were proposed for high-power operation. (2E2) The advantage of GaAlAs TAL lasers grown by metalorganic VPE technique was demonstrated in lifetests at output power 30 mW. The active region thickness was reduced to N 50 nm. This result can be understood in terms of the above discussion on the advantages of QW lasers(2B3) as compared to DH lasers, and the main cause is the thinner active region, and therefore smaller heat-generating area at the facet mirror in thin-layer case. It was noticed also that the reduced thickness is effective at the cavity end so thin-tapered-thickness (T3) configuration of DH lasers could be proposed by Murakami et aZ.‘2*4’In such lasers the active layer becomes thinner at the cavity end, whereas the major part of the region could be of ordinary thickness if the thickness could be optimized from other respects. Therefore, in such a case the mode spot at the facet is increased in size with respect to that inside the cavity. This reduces the local intensity at the same overall output power, and we call the case a “facet load-off’. In the reported example(284)the active layer thickness was 75 nm inside the cavity and 50 nm at its end. The angular divergences were 23” and 33”, respectively. Thus the increase of the mode spot leads simultaneously to improved beam collimation at the output. The improvement of COD power was reported from 30-40 mW in ordinary units to N 60 mW in T3 type units. In Ref.‘2*s’the CW power at COD was increased to 165 mW. The laser wavelength was 780 nm. Study of this type of laser was also given in Ref.(2s6) Various techniques to increase the output aperture in the laser diode near-field are interesting in the aspect of the “facet load-off’. Probably, a distributed output configurations will be suitable for such a purpose (see, for example, Ref.(*“‘). Also a promising method is the usage of bent-waveguide configurations.(237~288) Another possible way to increase the output aperture is usage of flared lateral waveguide configuration. In this case the mode spot grows up to the cavity end and fills the increasing guide cross-section due to light diffraction. This configuration was applied to high-power 1asers(289*290) and to power amplifiers. (29’,2g2) A flared waveguide end section was introduced into the ridge-waveguide GaAlAs/GaAs laser diode with high-quality etched mirrors. The maximal CW output was as high as 80 mW at ridge width 3.2 pm and mode spot width at the facet of 24 pm. Flared traveling-wave amplifiers allow us to obtain a near-diffraction-limited beam at power as high as 21 W in 300 ns long pulses from the 600 pm wide mode spot, and about 4.5 W in CW operation mode from the 450 pm wide mode spot at wavelength 860 nm.‘29’) Analysis of the flared configuration of the power amplifier was given in Ref.(292) 5.2.2. Q W optimization. The advantage by COD power of QW lasers in comparison with DH lasers were pointed out above. The quantum-well structure can be optimized with respect to the improvement of optical strength. Some experience in this direction concerning high-power AlGaAs ternary alloy quantum-well structures operating in the 780 nm wavelength region was reported. The number of QW layers and waveguide thickness in the structure influence the mode field distribution over the mode spot. A reduction of the optical power density in the direction perpendicular to the junction plane is examined to lessen facet

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degradation.(293) Investigations had been focused on the characteristic temperature and COD level. It is found that a triple quantum-well (TQW) active layer structure has superior characteristics for high-power operation. The reliability of 780 nm AlGaAs high-power laser diodes with a thin triple-quantum-well or a single-quantum-well active layer had then been investigated.(283.294) The thin active layers achieve high COD levels above 170 mW. In these laser diodes, a gradual increase of the operation current limits the lifetime of the laser diode. The gradual degradation rate is higher in the single-quantum-well laser diodes than in the triple-quantum-well laser diodes, and is also higher in shorter-cavity-length laser diodes. The power dependence of the gradual degradation rate is very small compared with the dependence on cavity length or active layer structure. The gradual degradation mainly depends on the threshold current density per active layer thickness, As a result, 60 mW reliable operation for more than 1000 hours was achieved at 50°C in 600 pm length triple-quantum-well lasers. Very high power (600 mW in a single CW mode) was obtained from three-quantum-well laser.‘27’) Monolithic stacking of up to three quantum well lasers has been demonstrated.‘295’ CW threshold current densities per well and differential quantum efficiencies are comparable to those for individual devices, and transverse far-field patterns indicate phase-locking. Catastrophic optical damage levels increase with increasing quantum-well count and are substantially higher than those achieved with a single quantum-well laser when GaAs wells are involved. Study on a possibility of increasing the maximum optical power in QW lasers by the tailoring of wide waveguide had been reported in Ref. (*‘*)An estimation was obtained of the maximum output power per unit width of the active stripe. Several variants of heterostructures with one or several quantum-well GaAs layers in an extended GaAlAs waveguide are considered. The vertical mode size a, (full width at l/e magnitude) was approximated by expression a, = C;1’lxd ,

(41)

where C is proportionality constant, x is Al mole fraction in the large-optical cavity region (extended waveguide), and d is the thickness of active layer. C is 10 nm if A and d are taken in pm. The expression is valid if it is assumed the thickness of the extended waveguide is large enough ( N 2a,) to not influence the mode spot. The COD intensity was assumed to be constant (5.5 MW/cm2 at 200 ns pulsewidth) so improvement of the COD power could be achieved by increase of the vertical mode size. It is shown that the optimal structure ensuring the maximum radiation power to - 2 kW/cm has a few (between one and three) wells located at the center of the extended waveguide approximately 3 pm wide. The maximum permissible current density above which the damage to mirrors becomes catastrophic was approximately 10 kA/cm*. 5.2.3. Non-waveguide and surface-emitting conjigurations. A possibility to increase the mode spot size at the output facet appeared in non-waveguide’296’ and vertical-cavity surface-emitting lasers (VCSELs). (297) These configurations are corresponding to window-type ones so the active region is well separated from the output facet. At typical power of 100 mW from such a device the near-field mode-spot area is about 10m6cm’ or more, thus the peak intensity at the output facet is below 1 MW/cm2. As a result, in well designed devices of such types there will be no serious problems of facet optical damage until power increased several times. In VCSEL with a buried distributed-Bragg-reflector the laser radiation intensity beyond the reflector (inside the laser cavity) is much higher than at facet. This is due to very high (close to unity) reflectivity of DBR used. The facet has no function of cavity mirror and can be AR coated. For example, at a facet intensity of 1 MW/cm* the internal intensity may by higher than several hundred MW/cm*. This may cause a new optical strength problem

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associated with damage at heterojunction interfaces of the DBR. Notice that the interfaces are separated from the active region by heterobarriers, so there is no mechanism of damage occurring in ordinary edge-emitting diodes. A theory for internal (initiated at interfaces) COD is not considered as there are no experimental data on this subject published. A subject of actual discussion is the self-focusing in VCSEL which may lower the COD level to ordinary output power. In Ref.(29*’some considerations were given on the advantage of VCSEL configuration in this respect over edge-emitting device. The advantage is based on the combination of self-focusing narrowing of the beam in the active region and diffraction spreading of the beam in the passive region. The ratio of active to passive length inside the VCSEL cavity is small, so diffraction spreading may be expected to work against the beam narrowing. It allows avoidance of the self-focusing instability in VCSELs at a power that is many time higher that the self-focusing threshold in edge-emitting diodes.

6.

CONCLUSIONS

The consideration of laser-induced damage in semiconductor materials allows us to conclude that in the simple case of strong absorption a good quantitative agreement can be found between the calculated and measured LITD values. The damage is explained by heating to the melting point, so the physical picture seems to be eventually clarified. In the “transparency” case there is no perfect quantitative agreement when known absorption mechanisms are taken into account. The discrepancy is not so large (in limit of one order of magnitude), but it appears to be exciting for searches of other than direct heating mechanism for ultimate explanation. Technical advances in the development of semiconductor lasers give a permanent increase of output power from a solitary emitting diode element. This corresponds to an increase of radiation intensity at the output facet and inside the cavity. Real intensity for a single-mode emission could reach Z&l00 MW/cm* in well-designed laser devices. Limited optical strength of semiconductor laser materials is a subject of power limitation in semiconductor laser. In addition, the self-damage appears to be an important factor of the limitation of operation lifetime producing sudden failures in high power laser diodes. Thus an understanding of optical damage processes and experience in improvements of technical performance at high output power is very real and appropriate for further progress in semiconductor lasers. The investigation of the stressed state of the facet mirror in lasers at high power operation has been the focus of recent experimental works. Laser emission absorption followed by nonradiative recombination at surface region provides overheating near the facet which leads to an acceleration of the surface reaction and ultimately gives the initiation of facet damaging. Explanation of the damage mechanism includes a positive feedback loop accelerating temperature growth in a hot region. The feedback results from a positive temperature coefficient of optical absorption. A strong absorption (explaining localization of power dissipation in submicrometer volume) can be supplied by interband absorption. In most semiconductor laser materials the increase of absorption is explainable by temperature bandgap shrinkage, i.e. negative temperature coefficient of the bandgap energy. This can make the overheated volume strongly absorbant to the laser radiation. In principle, photoelectric absorption is not immediately dissipative mechanism as electron-hole pairs can reemit absorbed energy. However with temperature increase the efficiency of radiative recombination decreases, which is also the factor of feedback for accelerated heating. It is clear that the last stage of COD is thermal “microexplosion” or runaway leading to local melting of semiconductor material. This conclusion allows us to estimate a minimal energy which has been dissipated in the destroyed volume. It is probably a small part of the

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transmitted energy, otherwise the laser action would be suppressed before damage. The dissipation mechanism is commonly assumed to be nonradiative recombination, namely at the surface in the case of facet damage. Therefore the last step of the mechanism has to be actual energy conversion into heat. This is provided by nonradiative surface recombination. The recombination occurs at surface and defects and the probability of it also increases with temperature (enhancing the positive temperature feedback). Consideration of the heat balance at surface gives runaway-type solutions in analogue to thermal explosion. According to this the primary model of the temperature runaway at surface was called “thermal microexplosion”.“) Development of the runaway model for double-heterostructure and quantum-well laser structures had lead to the conclusion that the qualitative physical picture is quite acceptable but it is not in perfect quantitative agreement with calculations based on the surface recombination model. The quantitative discrepancy is most obvious in the case of the quantum-well structure where the heat generation is reduced by a small area of the active region entering the surface as compared to the double-heterostructure. In any case the proper passivation and protection of mirror facets gives satisfactory improvement of the optical strength in GaAk/GaAs, InGaAlP/InGaP/GaAs and InGaAs/GaAs QW lasers. This means that the surface state plays an decisive role in the optical strength limitation at present stage of laser developments. This means also that one may expect other dissipative and damage mechanisms to become more important. We can indicate subjects for further studies in this field as follows: (i) optical strength at short pulses, when thermal power saturation does not work, particularly damage mechanism at picosecond pulses, (ii) power limitation mechanisms other than thermal which can work at increased power level, (iii) power limitation in “window-type” laser structure where COD can occur presumably in the bulk, but not at protected facet, (iv) optical strength at a wide range of temperatures, where, at low temperature power density can be much higher, and at high temperature - as overheating can lead to damage more easy due to higher nonradiative recombination rate and lower thermal conductivity of materials. Acknowledgements-The author is thankful to the New Energy and Industrial Technology Organization (NEDO) of Japan for partial support of this work. He is thankful also to colleagues for helpful discussions, to Dr. W. Nakwaski of Lodz Technical University, Poland, Dr. C. Harder, Dr. W. Epperlein, Dr. A. Jakubovicz, Dr. E. Latta of IBM Zurich, Switzerland, A. E. Drakin, Dr. A. Molchanov, Dr. V. Kovalev, Dr. I. Zubarev of Lebedev Physics Institute, Moscow, Russia and to I. V. Akimova of Lebedev Physics Institute, Moscow, Russia for preparation of some SEM images.

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