Recent progress in growing reliable (AlGa)As DH lasers by molecular beam epitaxy for optical communication systems

464

Journal of Crystal Growth 56 (1982) 464

474 Nortli—l-lolland Publishing Coinpa ny

RECENT PROGRESS IN GROWING RELIABLE (A1Ga)As DH LASERS BY MOLECULAR BEAM EPITAXY FOR OPTICAL COMMUNICATION SYSTEMS * W.T. TSANG Bell Laboratories, Murray lull, New Jersey 07974, USA

The ew electro—optical elevated temperature (55—70°C) characteristics iii 5 pm shallow proton-bombarded stripe lasers. labri— cated from MB! grown DII wafers, that have Al 0 08Ga0 92Asac live layers are compared with those obtained from similar lasers hibrica ted trout high quality LPF DII wafers that are suitable for system use. The MB F lasers maintain I heir excellent cw device characteristics even at elevated temperatures. The temperature-dependence of the ew ‘th of these MIII lasers is characterized hr

‘th

exp(T/T0) with T0 150°C. Furthermore, the ew ‘th~ of these MIlL lascisar e at least as low as high quality [ l’l. lasers. The high material uniformity of MIlL waters also results in a signit)eant increase in yield ut good laseis pci MIII water. A stat istical study of the self—induced pulsation behavior of M lIt ew (AlGa) As 1)1 I pro to n—born barded stripe lasers, during accelerated aging at elevated (70°C) temperature, is made and compared with that ut snn ilar lasers grown by [PIT he 5 pitt sti ipe sha lion proton—bombarded MIlE lasers, after 100 h accelerated burn-u, Itase pulsation frequencies /“~ typically I Gl-I~n lien measured at the power levels used for optical communications. The median pulsation frequencies F~m are ‘I .6 GIlt and 1 GIlt t~rMIII and LPI. lasers, respectively. In both cases, no mirror-coatings \seie applied ti, the lasers. Long-term aging ot NI III lasers ate Icsated temperature (70°C) under constant po\ver output ot 3 niW/iniiriir has also been carried out tor lasers trout several ssaiers. LasersTrnfrootiii about two early 8500wafers Ii at 70°C are still is expected. operatingThe afterMIII 70(11) lasers Ii at have 7 0C~C. also When beenplotted evaluated, on t lie tested usual and log—nor aged ina in transmit I p i ap hi. aters median An islitere— time modulation current ‘mod of 24.1) 4.3 mA is obtained br 125 MBL laser transmitters measured at 3(lC for an extinction aged ratio of 15 to 1. For the same group of transmitters, the 20 NiIIz noise at 30’ C is (94.2 2.9) dhm. The MItE laser tiansluit ters arc still operating stably after 7.000 Ii when aged at 45 Mb/s data rate at 3t)C. The results obtained tItus tar slioss that tlie~meet the objectives for rise in 45 Mb/s Bell system 1T—3 lighrtwave transinissiun systetns and at present they ,ire being field tested in hese systems. Recent results on high through—put. high yield, and high lv reproducible I A l( ~r)As 1)11 Ia ser is at cr5 vi on ii by Ni It I at accelerated grow tIt rates as high as 1 1 .5 p nt/h a tnt others iv ill also be preset ted

1 Introduction .

Recently, room temperature low-current-threshold broad-area Fabry—Perot A1yGai~As/A1~Gai~As double-heterostructure (DH) lasers have been prepared by molecular beam epitaxy (MBE) covering the lasing emission wavelength from 8900 to 7200 A (infra-red-visible) [1—31.In this emission range, the averaged pulsed current threshold densities, ~ are at least as low as those prepared by liquid phase epitaxy (LPE) [4—6].At about 8200 A, the wavelength at which DH lasers have also been prepared by metalorganic lasers chemical (MO-CVD) the MBE havevapor lowerdeposition ~th’ The ~th ‘s of MBF.[7], grown *

Original work done in collabotation with R.L, Hartman (BTL, Murray 11111, NJ), W.R. Holbrook, P.E. Fraley (BRL, Reading, PA), M. Dixon (BTL, Allentown, PA), and A.J. Schorr (Western Electric, Reading, PA).

0022-0248/82/0000—-0000/$02.75 © 1982 North-Holland

GaAs/Al 0 3Ga07 As DII wafers have operated continuously at a constant output power of about 2 mW/ mirror at about 38°Cambient for more than 28,000 It and are still operating [8]. This first result shows that MBE laser reliability is comparable to those reported from LI~E grown stripe-geometry lasers having GaAs active layers [9,101. Recently, visible (AlGa)As single quantum well heterostructure lasers with a 200 A Al0 17Ga0 83As active layer lasing at 7520 2Ahave and been having an averaged ~th [11]. as lowSuch as 810 prepared by’ MBE ~ A/cmfor the first time close to the theoretical value falls calculated by Casey, Jr. [12] . ‘

front first principles

withoUt adjustable parameters for the thin actmve

layer regime. It has also been confirmed by photo. luntinescence and excitation spectrum line widths

LI

W. T. Tsang / Recent progress in growing reliable ~AlGa)As DH lasers by MBE

that in these quantum well heterostructures, the hetero-interfaces are abrupt and smooth to within one atomic layer [13,14]. In this paper, we report the cw elevated temperature (55—77°C) electro-opical characteristics and reliability of proton-bombarded stripe lasers fabricated from MBE grown DFI laser wafers that have A1 008Ga092As active layers. In fabricating these stripe lasers, the same processing procedures and evaluation criteria are applied to both MBE and LPE wafers. All the lasers studied in this experiment have a proton-bombardment delineated stripe width of nominally 5 pm and are shallow proton-bombarded, i.e., the proton damage does not reach the active layers [15,16]. It has been shown that the use of 5

pm shallow proton-bombarded stripe lasers instead of 12 pm deep proton-bombarded (proton-damage penetrates active layer) strip lasers resulted in significant improvement in device characteristics

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laser chips are randomly oriented with respect to the front (F) and rear (R) of the mounting stud.

2. High temperature (55—70°C)device characteristics of MBE cw (AIGa)As stripe lasers

only the cw current threshold (Ith) increases to

Since the temperature in the actual operating environment can be as high as 55°C,the stability of the device teniperatures characteristicsis ofimportant the stripeinlasers at these elevated ensuring the reli-

turn efficiency remains the same. The junction behaves ideally at both temperatures as evidenced 2 V/d12 versus I from the Id V/dI versus I and _j2 d curves [18,19]. The junction voltage stays saturated

able operation of optical communication systems. Therefore, it is important for the lasers that are going to be used in optical communication systems to be stable not only at room temperature but also at- these elevated temperatures. It is with this in mind that we present the following results on MBE proton-bombarded stripe lasers. Fig. 1 shows the light—current (L---J) characteristics from each mirror, the first and second derivative [18,191 of the current—voltage (I— V) characteristic, Id V/dJ and ~~j2d2 V/dJ2, versus I for a typical MBE (A1GaAs) 5 pm shallow proton-bombarded stripe laser under cw operation at 30 and 65°C, respectively. This test is done immediately after bonding and before any burn-in. It is seen that excellent linearity and in general excellent symmetry in L—I characteristics are obtained up to at least 4 mW/mirror and in general up to 10 mW/mirror at both temperatures. In raising the temperature from 30 to 65°C,

near lasing threshold even at 65°C. The zero-bias diode capacitance is 90 pf indicating that the protondamage indeed does not reach the active layer. We must also emphasize that the above type of device characteristic is typical of most of the MBE lasers in this study before burn-in. In fig. 2 we show a comparison of the cw ‘th ‘S at 30°Cof MBE and LPE 5 pm shallow proton-bornbarded stripe lasers used in this study. These lasers were fabricated under the same conditions. Both groups of devices were from lasers after initial screefling. There are 88 MBE diodes and 100 LPE diodes. The slightly lower MBE cw ‘th’~ exhibited in fig. 2 are probably due to better material quality and layerthickness uniformity. The linearity of the MBE data shows that the cw ‘th distribution can be described by a Gaussian distribution with a mean cw ‘th of 96 mA. It is important to point out that the 1th distribution of the MBE lasers is narrower than that of

about 15 mA, while the external differential quan-

W. T. Tsang / Recent progress in growing reliable ~‘AlGa)AsDli lasers by MBE

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LPE lasers indicating the uniformity of the MBE wafer is superior than LPE wafer. Such high degree of unifortnity will significantly increase the final yield duction of lines. the devices and ease quality control in proFig. 3 shows a comparison of the 1.—I chiaracteris-

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tics from each mirror of an MBE and LPE 5 pm shallow proton-bombarded stripe laser at 5, 30 and 55°C. Both lasers are representative and have similar thermal resistances. It is seen that the cw ‘th of the

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mirror (both front and

rear) at 5° 30°, and 55°Cof a MBL and a LPE 5 pm shallosv proton-bombarded stripe laser, respectively. Both lasers are representative and have similar thermal resistance.

at 30°C(110 mA). In figs. 4a and 4b we show the cumulative probability plots of cw ‘th’~ of 88 MBE and 100 LPE 5 pm shallow proton-bombarded stripe lasers at 30 and

TV. T. Tsang / Recent progress in growing reliable ~‘AlGa)AsDH lasers by MBE

65°C, respectively. It is seen that the width of the cw ‘ib distribution of the MBE lasers at 30°Cis unchanged even at elevated temperatures. In the case of MBE lasers, increasing the temperature from 30 to 65°Cincreases the mean of the cw ‘th distribution from 96 to 112 mA. In the case of LPE lasers, a similar increase in temperature increases the mean of the cw ‘01 from 104 to 135 mA. The corresponding T0 values calculated from ‘th exp(T/T0) using

467

of comparable normal devices which implies that temperature insensitivity is achieved at the expense of an increase in threshold. Independent data as given in more detail in ref. [24] tends to rule out a resistive shunt model as well as a mechanism thresholds

based upon the inhomogeneous incorporation of

aluminum and/or dopants in the active layer. Available evidence based on the variations of preliminary

the mean values, corrected for the average thermal rise in several chips, are 234 and 142 K for the MBE and LPE lasers, respectively. T0, measured on other MBE wafers not reported herein, confirms values in the order of 200 K for MBE wafers grown in this manner [1—3]. Furthermore, the width of the distribution at elevated temperatures increases noticeably in the case of the LPE lasers. Several studies [20—231 of GaA1As degradation have indicated an activation energy of about 0.6—0.95 eV for LPE wafers. There-

photoluminescence spectra with temperature suggests that the explanation is related to changes occurring in a normally present nonradiative mechanism. In fig. 5, the natural logarithm of the threshold pulsed current is plotted against ambient ternperature for three representative lasers from three separate MBE-grown DH ternary active wafers: (1) laser with normal temperature dependence, T0 = 170 K is computed; data points are included to emphasize exponentialty; (2) laser from a wafer, nominally similar to (1), which exhibits anomalous

fore, if all other characteristics are equal, a stronger

temperature insensitivity; (3) laser from a wafer with

dependence of the cw ‘th of a laser on temperature should lead to faster deterioration of laser characteristics and reduced functional lifetime of the laser when operated at elevated ambient temperatures.

—

/

Recently, it was found that in some ternary active

layer (0.08 AlAs mode fraction) MBE grown wafers, the pulsed current thresholds for proton-bombarded delineated stripe-geometry lasers were nearly temp-

erature independent over a temperature range about 100°C [24]. Lasers from one such wafer, pulseoperated over the practically important temperature range about room temperature exhibit only a 3 mA threshold decrease in going from —10 to +20°C and only a 3 mA increase in going from 20 to 50°C.Fitting the data over small temperature intervals using the commonly observed exponential function for the temperature dependence of threshold (I = J~ exp(T/To)) results in negative, infinite, and large positive values (~300 K) for T0, as the temperature is increased from —10 to 75°C. Lasers with such a

range of threshold insensitivity may reduce pattern dependent effects and may have significant practical

implications for the simplification of the feedback circuitry used to maintain the prebias operating current in laser transmitters. Data examined over the temperature inverval —55 to 250°Cshow that thres-

holds for the anomalous temperature insensitive devices are everywhere greater than or equal to the

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W. T Tsang / Recent progress in grossing reliable (AlGa)As 1)11 lasers by MBE

468

an Al-graded active layer in the direction perpendicular to the junction plane. Such Al-grading is also shown to be able to produce anomalous T 0 lasers. Data points were omitted for (2) and (3) for purposes of clarity. For application as light sources in optical communication system stringent criteria are imposed on the cw electro-optical characteristics of the lasers at all test points. For example, initial characteristics of the type given in fig. I are required. Furthermore, after a 100 h burn-in at either 55 or 70°C the device has to meet stringent criteria including cw device characteristics, degradation rate, and self-induced oscillation before it is considered good. It is understood that the yield at various test steps certainly depends on the various specifications imposed on the diodes and the number of tests the diodes have to pass. It is therefore difficult to compare these devices with others reported in the literature without a detailed analysis of test parameters and conditions, which is beyond the scope of this paper. However, lasers from this MBE wafer, after burn-in, had better device characteristics, lower infant mortality and better aging behavior than the LPE lasers studier! when the same test conditions were used. We feel this is in part due to the higher characteristic temperature (T0). The test results are summarized in table 1.

3. Optical self-pulsation behavior of MBE cw stripe lasers The occurrence of self-induced pulsations in the optical intensity emitted by (AlGa)AS double-heterostructure (DH) proton-bombarded stripe lasers has been observed to increase dramatically during conti-

nuous operation (cw) for short periods of time at elevated (70°C) temperature. For example, Paoli

[251found that an average of 62% (48 to 70% depending on wafer) of 103 initially nonpulsing lasers had developed some degree of sustained pulsations after cw operation at 70°Cfor as short as 50 to 60 Is. Furthermore, the pulsation frequency (I”~~~~) lowers with increasing age. This oscillation, if sufficiently low in frequency, degrades transmitter error rate performance. This phenomenon is particularly insidious in that its occurrence is apparently randomly distributed throughout the laser population. For example. Paoli [25] reported initial rates of incidence for unaged lasers ranging from 5 to 30% for four wafers, with an average rate of 20%. Thus, self-indttced pulsation becomes one of the most serious outstanding problems limiting the yield in production and the reliability in service of proton-bombarded stripe lasers intended for use in optical cotnmunication systems. In the following we report the results of a statistical study of the self-induced pulsation behavior of MBE cw (AlGa)As DH proton-bombarded stripe lasers during accelerated aging at elevated (70 and 55°C) temperatures and compare these results with those obtained for similar lasers grown by LPE. In both cases no mirror coating were used and the protonbombardment delineated stripes are nominally 5 pm instead of the usual 10 pm. Also the proton-damage did not reach the active layer. The sante standard diode processing and initial screening procedures were carried out on both MBE and LPE wafers. Then, they were subjected to accelerated aging at 70°C or 55°C for 100 h at a constant otmtput power of 2.5 mW/mirror. In the case of MBE stripe lasers, an additional group of lasers was also subjected to longer aging at 70°C for up to 450 h. After the completion

Table 1 Test results of MBE 5 pm shallow proton-bombarded stripe lasers having Al0 .0gGa0,92As active layers Lot No.

Total dioded bonded

CW FL test good (30°C)

CW EL test good (65°C)

A B

237 353

153 262

139 255

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77

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a) The maximum system operating temperature is 55°Cand some of these devices were burned in at that temperature. “Good” indicates the lasers pass the specifications for use in present optical communication systems.

W. T. Tsang / Recent progress in groiving reliable (AlGa)As Dli lasers by MIlE 1500

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LPE lasers. In fact, only two diodes tested in this group exhibited pulsation frequencies below 980 MHZ. The Foscm (median) pulsation frequency) are

AFTER BURN-IN AT 55°C I 10

shallow proton-bombarded stripe lasers over similar

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PERCENT OF POINTS WITH VALUE LESS THAN ORDINATE

Fig. 6. A comparison of the self-pulsation frequencies versus cumulative probability of MBE and LPF 5 pm shallow proton-bombarded stripe lasers after accelerated aging at 55°C for 100 h under constant output power of 2.5 mW/mirror. Diodes that show no pulsation or pulsation frequencies above 1.5 Gllz are all plotted as data points ois the 1.5 GHz line.

1 GHz and 1 .6 GHz (with slight extrapolation) for the LPE and MBE materials, respectively. Therefore, for the same geometries the MBE lasers have significantly higher F05~ than the LPE lasers. The individual u (standard) deviation of F05~ for the MBE and LPE lasers (fitting only the linear portion of the distribution in this case) are 290 and 240 MHz, respectively. Since F0~ increases

of this accelerated burn-in, the lasers were examined for self-induced pulsations and their F0~ measured at 55°C using a spectrum analyzer. In this measurement, the observation of a modulation frequency on the spectrum analyzer is used to identify the laser as a pulser even though the pulsation does not need to be fully developed.

Fig. 6 shows a comparison of F05~versus cumulative probability of MBE and LPE 5 pm shallow pro-

with

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power [25], for an output power greater than 1 .2 mW the P05~ will be certainly higher than those shown in fig. 6. It is also important to point out PROBABILITY PLOT OF OSCILLATION FREQUENCY AFTER EXTENDED ACCELERATED AGING

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ton-bombarded stripe lasers after a burn-in of 100 h at 55°C. These lasers were encapsulated and the oscillation frequencies were measured at 55°C and 1.2 mW through the fiber in the package. Since the fiber output depends on the coupling efficiency and the quantum efficiency of the lasers, the present mea-

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Fig. 7. Effect self-pulsation frequencies extendedoutaccelerated aging on (200-450 h at 70°C under ofconstant put power of 2.5 mW/mirror) on a different and snialler ensemble (18 diodes) of MBE 5 pm shallow proton-bombarded stripe lasers.

470

IV. T. Tsang / Recent progress in growing reliable /AlGa)As DH lasers by MIlE

that in a separate group of MBE lasers studied and measured under the above measuring conditions 27 out of the 58 MBE diodes examined after accelerated aging (70°C, 100 h) show no measurable modulation frequencies on the spectrum analyzer. Such results for uncoated lasers represent a very significant improvement when compared with those studies previously [25]. To study the effect of extended accelerated aging on the self-induced pulsation behavior, a different and smaller group (18 diodes) of MBE 5 pm shallow proton-bonibarded stripe lasers was subjected to 200—450 h at 70°C of continuous operation under constant power output of 2.5 mW/mirror. The oscillation frequencies of these diodes were measured at 55°C at a power level of 1 .2 mW from one face of the chip (these diodes are not packaged with coupling fibers). The results are summarized in fig. 7. It is seen that continuing accelerated aging lowers 1°’~w,However, the oscillation frequencies are the still s650 MHz at this output power when measured at 55°C. An explanation for the significantly higher ‘°~ise of the MBE 5 pm, shallow proton-bombarded stripe lasers over the corresponding LPE lasers is not selfevident. However, it is possible that the MBE lasers may be less susceptible to or may have a slower rate of development of darkening behind the mirrors [26,27] as a result of the use of different dopants (Sn and Be instead of Te and Ge) or different species of impurities. It is also very possible that the mereased stabihty of the device characteristics especially the current threshold [28] of the MBE lasers over the LPE lasers at elevated teniperatures helps in slowing down the rate of development of self-pulsation during accelerated aging.

grown DH laser wafers that have 8% AlAs in the active layer. These wafers were grown at optimized substrate temperatures and conditions [29—-32]. The laser diodes were fabricated from several different wafers (without using any detailed wafer preselection measurements) into the 5 pm wide stripegeometry structure using shallow proton irradiation. No protective facet coatings were applied to the mirrors. The aging is performed in a 70°Cdry-nitrogen anibient at a constant light output power of 3 nsW/mirror. A criterion for failure for lasers from the first wafer examined is a doubling of the initial current required to emit the constant light output at the elevated temperature; the failure criterion for lasers front the other wafers is the inability to emit 3 mW/mirror facet of stimulated emission at the elevated temperature. Lasers from the first wafer tested were screened with a 100 burn-in at 70°C, however the diodes from the other wafers were selected only on the basis of the initial roons temperature L I V characteristics. Since characteristics for MBE lasers tend to be quite uniform, this evaluation procedure is almost equivalent to a random selection. Six to ten los

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W. T. Tsang / Recent progress in growing reliable (A1Ga)As DH lasers by MBE

diodes from each wafer were put on accelerated aging. Because of the high degree of uniformity of the MBE lasers from the same wafer, only relatively few diodes from each wafer are needed in studying and characterizing the aging behavior of the wafer. This is confirmed by the fact that the MBE diodes age much more uniformly as a group when compared with LPE

diodes. This is important in achieving high quality control in production lines, Under the above criteria, fig. 8 shows the log-normal plot of the cw operating lifetime at 70°Cof two early MBE wafers with Al0 05Ga0 92As active layer

471

meet the objectives for use in Bell system 45 Mb/s FT-3 lightwave transmission systems. Of the 125 MBE laser transmitters tested in one

group, the 20 MHz noise at 30°C and 55°C are —94.2 ± 2.9 and 92.7 ± 3.5 dBm, respectively, while the modulation currents, ‘mod, needed to obtain an extinction ratio of 15 to 1 at 30°Cin the optical output power are 24.0 ± 4.3 and 27.9 ±4.8 mA at 30 and 55°C, respectively. These transmitter perfor-

mances are better than those obtained from production-line LPE laser (with uncoated mirrors) trans-

(aged up to Nov. 17, 1980). It is seen that a median

mitters. A detailed comparison of the various other transmitter performances between 125 MBE laser

lifetime

transmitters and 26 LPE laser transmitters is given

Tm

of 8500 h at 70°Cis obtained as shown

by the dashed curve [33]. The solid curve gives the aging behavior of LPE proton-bombarded stripe-geo-

in table 2. It is seen that the performance of the MBE laser transmitters is superior to the LPE laser trans-

metry lasers obtained by Hartman, Schumaker and

mitters studied in all the properties listed in table 2.

Dixon in 1977 [34]. In these LPE lasers, a 70°Cmedian lifetime of 4500 h was obtained. The present MBE proton-bombarded stripe-geometry lasers de-

This superiority is further demonstrated by the fact out of the 26 LPE lasers only 62% will satisfy a

monstrated 70°C reliability at least comparable to

those published previously for LPE proton-bombarded stripe-geometry lasers [26]. In fig. 7 the solid circles represent diodes that ceased operating under above criteria, while the open circles represent diodes that are still operating.

5. MBE laser transmitter evaluation The MBE lasers with Al0 08Ga0 92As active layers have also been assembled in 45 Mb/s transmitters [351.The results obtained thus far show that they

stricter set of specifications while 93% of the 125 MBE lasers will satisfy the same stricter set of specifi-

cations. The optical self-pulsation frequencies in these MBE lasers (uncoated mirrors) are typically above 1 GHz (measured after 100 h of accelerated aging at the elevated temperature of 70°C). The eye pattern degradation due to low frequency noise and self-pulsation effects of these MBE laser transmitters are minimal at 45 Mb/s. Five transmitters with MBE lasers have been put on test as 45 Mb/s transmitters at 30°C.All five transmitters are still operating stably both in prebias current and averaged power output after 7000 h. As in the case of the lasers put on accelerated aging at

Table 2 Properties of MBE and LPE 45 Mb/s FT3 transmitters Property

Type LPE

Number of lasers ‘mod at 30°C(mA) ‘mod at 55°C(mA) aIth/~Tat5—70°C(mA/°C) 20MHz noise at 30°C(—dBm) 20MHz noise at 55°C(—dBm) F05~at 30°C,1.1 mW (MHz) No. of fails stricter spec.

MBE

26 30.7 ± 6.4 425 ± 10.5 1~00± 0.18 87.6 ± 4.8 87.3 ± 3.9 1388

10

± 365

125 24.0 27.9 0.48 94.2 92.7 1673 9

±

4.3

4.8 0.07 ± 2.9 ± 3.5 ± 208 ± ±

W. T. Tcang

472

/ Recent progress in grossing reliable (AIGa)As 1)11 lasers bm MiliN,

r

70°C,all five MBE laser transmitters aged uniformly as a tight group. S 00

6. High through-put, high yield and highly reproducible (AIGa)As DH lasers by MilE

-

~

°°~ ~

~

~ii..i3Ol

2(7 DIODES

11230?

17

112303

IS

-~

It has generally been assumed that molecular beam epitaxy (MBE) is limited to slow growth rates (~2

400

pm/h). As a result, it is also generally assumed that MBE will not be an economical way for high throughput mass-production of epitaxial layers for optoelectronic and microwave devices. Recently, we have also shown that high throughput, high yield, and hi~~y reproducible (MGa)As DH

laser wafers can be grown by MBE with properly designed multi-chamber MBE systems [36]. This is

L

~°

~

~

200L1 I

2

L

I .~

II

91j4

I3

GROWTH RATE ~~_L 5 10

L 20

II

0. 30 40 50 60 70 80

5t~r~ /

90

95

-

98 99

PERCENT OF POINTS WITH VALUE LESS THAN ORDINATE

I-mg. 1 0. Distributions of the threshold current ~1~nsiti~s of broad—area lasers fabricated from tour DII scalers (If the saute layer structures crown It aim scoeleraicd prusvth rate ssf II .5 tn/h by MOE under the Sante gross tis conditions

demonstrated by growing two series of (AlGa)As DII laser wafers and measuring their broad-area threshold 1th’ distributions across the wafers current denisty, ~ The first series consists of four dif(3.5 cm diameter).

growth rates the qualitiesas of the DII laser10. wafers are still highly reproducible shown in fig. The low averaged ~th ‘s (~-700 A/em2 ) of the present DI-1

ferent wafers grown consecutively at growth rates of

wafers in both series also shows that the material

2.9, 4.2, 7.4 and 9.5 pm/h. The results show the 1th’~ are essentially unaffected by accelerated growth rates as shown in fIg. 9. In the second series, four DH laser

and heterojunction qualities of these wafers are as good as those previously grown at lower growth rates (~l.5 pm/h). The half-peak full-width of the f~ distributions across an area of 3 cm width of the wafers (3.5 cm diameter) are about 50-60 A/cm2. Such narrow range of distributions ensures high

wafers having the same layer structures were grown under the same conditions without interruption at 11 .5 pm/h. The results show that even at such high

device yield. 1000

7. Summary 9

eoo

~

-

9

We present the results of the cw electro-optical characteristics at elevated temperatures (55--70°C)

0

d°

~ ~~ooo Doo~og 6

eoo

0 ~ 0~

~ GROWTH DIODES RATE TESTED (~m/h)

-

102480 o v 1025801 1025603 0

400

o 200 I

L_~_L~.L 2 5 10

1025602

I.J__L..L...L I 20 30 40 50 60 70

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74

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of S pin shallow proton-bombarded stripe lasers fabricated from MBE grown DFI wafers that have Al 0 08Ga0 92As active layers, and compare them with those obtained from similar lasers fabricated from LPE grown wafers.temperatures. It is shown The thatternthe racteristics even atDII elevated

MBE lasers maintain their excellent cw device chaperature-dependence of the cw

‘tim

of MBE lasers is

98 99

PERCENT OF POINTS WITH VALUE LESS THAN ORDINATE

Fig. 9. Distributions of the threshold current densities of broad-area lasers fabricated frons four DH wafers grown at growth rates of 2.9, 4.2, 7.4 attd 9.5 pmn/h by MBI. Tlse layer structures are given in table I .

significantly less than that of the LPE lasers. Furthermore, the cw ‘th ‘s of these MBE lasers are at least as good as LPE lasers. Due to the very uniform material and layer thicknesses generated by the MBE process. the resultant slice quality is highly

W. T. Tsang / Recent progress in gross’ing reliable (‘AlGa)As DH lasers by MilE

uniform. This results in a significant increase in yield of good lasers per MBE wafer. A statistical study of the self-induced pulsation behavior of MBE cw (A1Ga)As DH (having A1 008Ga092As active layers) proton-bombarded

4’13

degree of uniformity and device yield achievable. The present uniformity can be further improved and the size of the substrate increased by continuously rotating the substrate during epitaxial growth. Such

stripe lasers during accelerated aging at elevated

commercially available multi-chamber systems 141 together with the high growth rate achievable without

(70°C) temperature is niade and compared with that of siniilar lasers grown by LPE. For the 5 pni

niaterial and heterojunction quality degradation should establish MBE as an economical way for

stripe shallow proton-bombarded lasers, those fa-

growing high quality (A1Ga)As optoelectronic and

bricated from an MBE DH wafer have significantly higher P~5c than those fabricated from LPE DH

microwave devices.

wafers. In both cases, no mirror-coatings were applied to the lasers. Long-term againg of MBE lasers at elevated temperature (70°C) under constant power output of 3 mW/mirror has also been carried out for lasers from several wafers. Lasers from two early wafers

Acknowledgement The author is indebted to R.L. Hartman, W.R.

Holbrook, P.E. Fraley, M. Dixon, A.J. Schorr, JR. Pawlik, V. Swaminathan, S. McFee, FR. Nash, B.A.

are still operating after 7000 h at 70°C. When

Dean, J.A. Ditzenberger, and many others, especially

plotted on the usual log-normal graph, a median lifetime Tm of about 8500 h at 70°Cis expected.

the laser processing and characterization groups at both Murray Hill and Reading, without whose contributions this work would be impossible.

The MBE lasers have also been evaluated, tested and

aged in transmitters. The results obtained thus far show that they meet the objectives for use in 45 Mb/s Bell system FT-3 lightwave transmission sys. teins and at present they are being field-tested in

these systems. It is shown that high through-put, high yield, and highly reproducible (A1Ga)As DH laser wafers can be grown by MBE with properly designed multi-

chamber MBE systems. This is demonstrated by growing two series of (AIGa)As DH laser wafers

and measuring their broad-area ~th distributions across the wafers. The first series consists of four different wafers grown consecutively at growth rates of 2.9, 4.2, 7.4 and 9.5 pm/h. The results show that the .J~1’s are essentially unaffected by accelerated

growth rates. In the second series, four DH laser wafers having the same layer structures were grown

under the same conditions consecutively without interruption at 11.5 pm/h. The results show that even at such high growth rates, the qualities of the

2) reproducible. of the present DH laser wafers are still highly The DH low averaged ‘~th’~series (-~‘700A/cm wafers in both also shows that the material and heterojunction qualities of these wafers are as good as those previously grown at lower growth rates (—~1.S pm/h). The narrow range of distributions of .Jth’S across the wafer (3.5 cm diameter) reflects the high

References [1] W.T. Tsang, Appl. Plsys. Letters 34 (1979) 473. 121 W.T. Tsang, AppI. Phys. Letters 36 (1980) 14. 131 w.T. Tsang, J. AppI. Phys. 51(1980) 917. 141 J.C. Dynment, FR. Nasls, C.J. Hwang, GA. Rozgonyi, R.L. 1-lartmau, 1-l.M. Marcus and S.F. I-laszko, Appi. Phys. Letters 24(1974) 481. [SI H. Kressel and ME. Ettenberg, J. App!. Phys. 47 (1976) 3533.

161

GlIB. Thompson, GD. Henshall, J.F.A. Whiteaway

and PA. Kirkby, 1. App!. Phys. 47(1976)1501. [7] R.D. Dupuis and PD. Dapkus, Appl. Phys. Letters 31 (1977) 839. 181 W.T. Tsang, R.L. Elartman, lIE. Elder and W.R Uolbrook, Appl. Phys. Letters 37(1980)141. 191 11. Kan, H. Namizaki, M. Ishi and A. Ito, AppI. Phys. Letters 27 (1975) 138. [101 A.

Thompson, IEEE J. Quantum Electron. QE-l5 (1979) 11. [11] W.T. Tsang and iA. Ditzenberger, App!. Phys. Letters [12] ll.C. Casey, Jr., J. Appi. Phys. 49(1978)3684. 39 (1981)193. [131R.C. Miller, D.A. Kleinmann, 0. Munteanu and W.T. Tsang, App!. Phys. Letters 39(1981)1. 1141 R.C. Miller and W.T. Tsang, AppI. Phys. Letters 39 (1981) 334. [15] R.W. Dixon and W.B. Joyce, IEEE J. Quantum Eleciron. OE-15 (1979) 470.

474

W. T. Tsang / Recent progress in growing reliable (AIGa)As DH lasers by MBE

[16] W.T. Tsang, J.A. Ditzenberger, W.R. Holbrook, P.E. 1raley and A.J. Schorr, to be published. [171 lW.T. Tsang, W.R. Holbrook and P.E. Fraley, App!. Phys. Letters 39 (1981) 34. [18] T.L. Paoli and P.A. Barnes, AppI. Phys. Letters 28 (1976) 714. [19] T.L. Paohi, IEEE J. Quantum Electron. QE-23 (1976) 1333. [20] R.L. !-!artman and R.W. Dixon, App!. Plsvs. Letters 26 (1975) 239. [21] K. Fujiwara, T. Fujiwara, K. lion and M. Takusagawa, App!. Phys. Letters 34 (1979) 668. 122] H. Kressel, M. Fttenberg and I. Ladany, AppI. Phys. Letters 32 (1978) 305. [23] W.B. Joyce, R.W. Dixon and RU I-lartnsan, AppI. Phys. Letters 28 (1976) 684. [24] JR. Pawhik, W.T. Tsang, FR. Nash, RE. Hartman and V. Swaminathan, App!. Phys. Letters 38 (1981) 974. [25] T.L. Pao!i, IEEE J. Quantum Electron. QE-13 (1977) 35.

[261 RE. !Iartman, R.A. Logan, L.A. Koszi and W.T. Tsang, J. App!. Phys. 50(1979)4616. [27] R.W. Dixon and W.B. Joyce, IEEE 3. Quantum Fleetron. QE-15 (1979) 470. [28J \V.T. Tsang, P.E. Fraley and W.R. Holbrook. App!. Phys. Letters 38 (1981)6. [29] \V.T. Tsang, F.K. Reinhart and iA. Ditzenberger, AppI. Phys. Letters 36(1980)118. [30] V. Ssvaminathan and W.T. Tsang, App!. PIty’s. Letters 38(1981) 347. 131] S.R. McAtee, \V.T. Tsang and D.V. Lang, J. App!. Phys., to be published. 132] W.T. Tsang and V. Ssvaminathan, Appi. Phys. Letters, to be published. [33] W.T. Tsang, R.L. Flartman, P.F. Iraley and W.R. Holbrook, App!. Phys. Letters, to he publisised. [34] R.L. 1!artman, N.E. Sehumaker and R.W. Dixon, App!. Phys. Letters 31(1977) 756. [351 W.T. Tsang, M. Dixon and BA. Dean, to be published. [36] W.T. Tsang, App!. Phys. Letters 38 (1981) 587.