Chapter 7 Quantum Confinement Heterostructure Semiconductor Lasers

SEMICONDUflORS AND SEMIMETAU, VOL. 24

CHAPTER 7

Quantum Confinement Heterostructure Semiconductor Lasers W. T. Tsang AT&T BELL LABORATORIES HOLMDEL, NEW JERSEY 07733

I. Introduction The two-dimensional nature of electron motion in quantum well heterostructures produces several unique and important features in semiconductor lasers. For instance, these quantum-size effects shorten the emission due to the radiative transition between confined states and significantly reduce the threshold current densityg-l3and its temperature dependence14-l9 (when properly designed) as a result of the modification in the density-of-states function of the electrons. This modification is brought about by the decreased dimensionaIity of the free-electron motion from three dimensional to two dimensional.

11. Theory of Quantum Confinement Heterosbucture Lasers: Quantum Well, Quantum Wire, and Quantum Bubble Lasers 1 . DENSITY-OF-STATE FUNCTIONS

Quantum confinement of electrons or holes (charge cariers) arises from a potential well in the band edges when the well width L, is of the order of the de Broglie wavelength & of the camers. Figure la shows a schematic diagram of a conventional double-heterostructure (DH) laser, in which the active layer has all three dimensions larger than ;1, of the carriers. The corresponding density-of-state function due to electron motion in the x, y, and z directions is schematically shown in Fig. Ib and expressed as follows:

where m: is the electron effective mass, E is the energy measured from the conduction-band edge E,, and A is Planck's constant. pi3)(E)is a parabolic function. By reducing the active layer thickness I,, to the order of ;1, as

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398

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DH

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ENERGY

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(g) QB (h) FIG. 1. (a) Schematic diagram of a conventional DH laser in which the active layer has all three dimensions larger than the de Broglie wavelength A, of the camers. (b) The corresponding density-of-state function. (c) A two-dimensional quantum well laser structure. (d) The corresponding density-of-state function. (e) A quantum Wire laser structure. (f) The corresponding density-of-state function. (g) A quantum bubble laser structure. (h) The corresponding density-of-state function.

shown in Fig. lc, a two-dimensional (2D) quantum well (QW) heterostructure laser is realized. The corresponding density-of-state function due to confined electron motion in the z direct is shown schematically in Fig. Id and is given by'

where H ( E ) is a unit step function with H ( E 2 EiZ)= 1 and H(E < E&) = 0. E L denote the quantized energy levels with quantum

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

10

100 V,(h2/2mL:)

1000

399

OD

4

FIG.2. The calculated energy level of a particle in a symmetrical rectangular potential well of depth V,.

number n. In this case, the pi2)(E)becomes a step-type function. In the case of a symmetrical rectangular potential well of depth Voand width L,, E & is given by

where F takes account of the finite depth V,. Figure 2 shows the calculated energy level of a particle in a symmetrical rectangular potential well of depth VO. For all positive values of Vothere will be at least one bound state. When Vo-+ 03, F i n Eq. (3) equals 1. The electron energies due to motion in the x and y directions remain the same as in the bulk. Thus, the electron bound energy states in the conduction band are given by E'= E&

+(k: + k;) 2m: fi2

(4)

400

W. T. TSANG

where ki = nn/ai and ai is the lattice constant in the i direction. Since there are heavy and light holes in the valence band, the hole bound energy states are given by

x2

where E z and EE are given by similar equations to Eq. (3) with parameter values for holes, m& and m;"hare effective masses of heavy and light holes, respectively. For QWs with a parabolic shape, Miller et a1.*O generated these parabolic compositional profiles by alternate deposition of thin undoped layers of GaAs and Al,Gal-,As of varying thickness. Computer control was employed in the deposition. The relative thicknesses of the Al,Ga,-,As layers increased quadratically with distance from the well centers, while that of the GaAs layers decreased. An example is shown in Fig. 3a. With parabolic wells

E& = (n - +)Awm

(7)

where again n = 1, 2, 3, etc., and w, = a e / m : with k, equal to the curvature of the parabolic well. Defining the curvature k, by the potential height of the finite parabolic well at z = k LJ2, namely, Q,AE,, where AE, is the total energy-gap discontinuity between the GaAs at the bottom of the wells and the Al,Ga,-,As at the top of the wells and Q,is the fraction of A E, for the ith particle well, Eq. (7) becomes

Similar equations can be obtained for heavy and light holes. Figure 3b shows the 5 K excitation spectrum from such a parabolic quantum well. The various exciton transition peaks are indicated by Figs. 3a and c. With such parabolic wells, Miller et aL2' also show that the energy-gap discontinuity between GaAs and AlGaAs layers is evenly split between the electron and valence-band wells instead of the previously observed value of 85%15% split.2 Therefore, as a result of quantization of the particle motion normal to the film, discrete bound states will emerge, and the energy of the lowest state will be higher than the band edge of the bulk materials and increase as L, is decreased.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

401

0.1 5

-300

-200

-100

0

100

200

300

ANGSTROM

FIG.3. (a) Parabolic compositional profiles generated by alternate deposition of thin layers of GaAs and AI,Ga,-,As of varying thickness. The relative thickness of the AI,Ga,-,As layers increased quadratically with distance from the well center. The quantum levels for an electron are also shown in the parabolic well. (b) The 5 K excitation spectrum from a parabolic quantum well. (c) The quantum levels of heavy and light holes in a parabolic well.

Similarly, one can further limit the motion of the carriers in the L,, direction, as shown by the quantum wire (QWi) laser depicted in Fig. le. In this case the density-of-state function is given byI9

and shown schematically in Fig. 1f.

402

W. T. TSANG

In this case, the p f ) ( E )becomes almost like discrete spikes beginning to resemble the discrete levels in conventional gas and solid-state lasers. Such semiconductor quantum-wire lasers are expected to resemble more closely, in particular, the spectral linewidth of gas and solid-state lasers than the conventional DH and QW lasers. Finally, if one further limits the carrier motion in the L, direction, as shown by the quantum bubble (QB) laser in Fig. Ig, the density-of-state function will be given byI9 PiO’(E)=

c

1

n,l,k ( L z LY

d(E - Egz - E$ - EL),

(10)

LX)

where 6 ( E ) is a delta function, Egz, E f ; , and E L denote the quantized energy levels with quantum numbers n, 1, and k, and are given by the form of Eq. (3). In this case, the density-of-state function is truly discrete, as shown in Fig. I h, and so the QB laser should behave similarly to conventional gas and solid-state lasers, even more so than QWi lasers. 2. GAINSPECTRA OF QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

Theoretically, because of the modification of density of states from the parabolic distribution in bulk material, as in conventional DH lasers, to the staircase distribution in the QW heterostructure (Fig. 4a), the injected carrier distribution, and hence the gain spectra,22-26will be different in both cases, as depicted in Figs. 4b and c, respectively. For the laser to lase, the overall cavity losses are about the same in both the DH and QW lasers; the modification of the density of states in the QW lasers should require that fewer carriers be injected for the laser to reach threshold. This means that the threshold current for the QW laser should be lower than the conventional DH laser. Further, the spectral gain profile should be narrower. In fact, if one uses the “no k-selection rule,” the gain coefficient g ( E ) of the i-dimensional quantum confined laser for photon energy E can be formally expressed as

X [f,(E’)--f,(E’ - E ) ]dE”

where n, is the refractive index, c the velocity of light, Eg the energy gap, and Mi)a constant representing the probability of dipole transitions.f , ( E ) andf,(E) are the distribution functions of electrons and holes, respectively.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

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NEEDED TO REACH THE SAME PEAK GAIN IN ( C )

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urn Iu

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E

0

ELECTRON ENERGY

FIG.4. (a) Schematic diagrams of the density of states for bulk material and QW heterostructures. (b) The distribution of inected carriers in bulk and QW structures needed to achieve the same peak gain spectra as shown in (c).

The corresponding electron distributions and gain spectra for QWi and QB lasers are also schematically shown in Fig. 5a-d. It is seen that the threshold current should decrease with increasing degree of confinement of carrier motion provided other threshold-affecting factors were maintained the same. More importantly, the gain spectrum becomes a discrete level (&function-like) in the case of QB lasers. This indeed approached the discrete-level nature of conventional gas and solid-state lasers. also calculated the gain spectrum sensitivity to camer density in QW lasers as a function of well thickness.

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n

z 0

a

c

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w -I w

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ENERGY, E

t

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FIG.5. The electron density distributions and gain spectra for (a) QWi and (b) QB lasers are shown schematically.

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QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

405

In Fig. 6 the derivative of the peak gain with respect to electron density, dg/dn, evaluated at threshold, is plotted against well width. The value obtained for the bulk case is also shown for comparison. It is seen that the gain is much more sensitive to changes in electron density in the quantum well case than in the bulk. Indeed, the sensitivity in the extreme quantum limit (L, --* 0) is almost an order of magnitude greater than that in the bulk. This means that good optical confinement is not as critical for optimizing the threshold current for narrow quantum wells as for conventional DH structures. It is worthwhile appreciating how the difference in sensitivity of peak gain to electron concentration between quantum wells and bulk structures comes about. This difference is related to the difference dependence of the density of states on photon energy in the quantum well and the bulk. The sensitivity of the gain to electron density is determined by the relative size of the density in energy of those states contributing to the gain and the average of the density of states over the thermal electron and hole distributions. For the bulk this ratio is much smaller than in a narrow quantum well. Support for this interpretation is seen in the calculated curves. As the quantum well width increases, the fraction of carriers occupying higher quantum well subbands and hence regions of higher density of states increases, and hence the sensitivity of the peak gain to changes in electron density decreases. No such calculation has been made for QWi and QB lasers yet. But it is easy to see that dg/dn will be drastically larger in these two cases.

0

100 WELL WIDTH,

i

200

FIG.6 . The derivative of the peak gain with respect to electron density, dgldn, evaluated at threshold, is plotted against well thickness.

406

W. T. TSANG

3. TEMPERATURE DEPENDENCE OF THRESHOLD CURRENT The temperature dependence of the threshold currents of conventional DH, quantum well,'* quantum wire, and quantum bubble lasers have been theoretically calculated. Arakawa and SakakiI9 have found theoretically that the threshold current density Jthof a quatum well laser is proportional to Tln(T/const) near room temperature, whereas J* for a QB laser is independent of T. Figure 7 shows their calculated results for T near room temperature for all four types of semiconductor lasers. It is seen clearly that the temperature dependence of Jthdepends drastically on the degree of confinement of the carrier motion. If the results are expressed in terms of Jth = J, exp(T/To), To values for conventional DH, quantum well, quantum wire, and quantum bubble lasers are 104, 285, 481, and 00, respectively. Again, the quantum bubble semiconductor laser behaves like conventional gas and solid-state lasers.

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-20 0 20 TEMPERATURE ("C)

40

60

FIG.7. Numerical example of J, calculated for (a) DH laser, To= 104"C, (b) QW laser, To= 285"C, (c)QWi laser, To= 48 I T , and (d) QB laser, To= m. J, is normalized by J, at 0°C.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

407

The reason for such a dramatic increase in To can be understood as follows: for a conventional DH laser, the intrinsic (those not due to a leakage over the barrier and Auger processes) temperature dependence of Jth is ascribed to the thermal spreading of the injected carriers over a wider energy range of states, which leads to decreases of the maximum gain g(E,,) at a given injection level. Consequently, in quantum well lasers, where pi2)(E)and pL2)(E)are steplike, the effect of such thermal spreading is expected to be smaller. In the case of quantum wire lasers, one expects a further suppression of the temperature effect because pil)(E)has a spikelike structure and is a decreasing function of E. In quantum bubble lasers, the thermal spreading of carriers should vanish because the state density is d-function-like. Hence, the temperature dependence of Jth will totally disappear, as long as the electron population in the higher subbands remains regligibly small. 4.

QUANTUM NOISE AND DYNAMICS IN QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

Both the broad-band modulation and low-noise characteristics of semiconductor lasers are desirable features for use as optical communication sources. Two important parameters which have determined, at least in the past, such properties are the relaxation oscillation frequency,f,,which sets the useful direct modulation bandwidth, and the linewidth enhancement factor a,which determines the relation of AM to FM modulation indices as well as the degree to which spectral purity is degraded by amplitude phase coupling. The expressions forf, and a are given by27,28

where P,o,and z are the photon density, frequency, and passive cavity lifetime of the lasing mode; is the nonresonant value of the refractive index; n is the carrier density; and XR(n) and XI@)are the real and imaginary parts of the complex susceptibilities of the active medium. Their derivatives with respect to the carrier density are given, re~pectively,?~ by

408

W. T. TSANG

where El and T2are the lasing photon energy and the collisional broadening time due to carrier-carrier and carrier-phonon interaction, and gi(E)is the gain envelope function, which is given by Eq. (1 1). Figure 8 shows the calculated results for a andf, as a function of well width L, in GaAs. In this calculation, the maximum internal gain that is necessary for laser oscillation is assumed to be 100 cm-I. The broken lines gives the values for a conventional DH laser. In the calculation off, we have assumed z = 2.6 ps, T2= 0.2 ps, and P = 3.8 X 1013~ m - As ~ . shown in the figure, it should be possible to doublef, in a quantum well over its value in conventional DH lasers using L, < 80 A.For the range of L,, a is also reduced. This latter result was also found by Burt,= who, however, did not estimate the value of a at El. It should be noted that a also contains a free-carrier plasma dispersion contribution which was neglected in this calculation by Arakawa et a/.23 Figure 9 shows the calculated values off, and a as a function of L, (= LJ for a quantum wire laser. These results indicate thatf, can be made about three times larger than that of a DH laser and a can be substantially reduced. Thus, the calculated results suggest that a quantum wire structure should prove effective for improving quantum noise characteristics and dynamics. Of the three types of quantum confinement heterostructuresemiconductor lasers discussed above, only the QW lasers have been demonstrated so

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WELL WIDTH d) FIG.8. The calculated results for a and f, as a function of well width L, in GaAs for a quantum well laser.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

50

100 WELL WIDTH t i )

200

409

1.0

FIG.9. The calculated values forf, and a as a function of L, (L,,= L,) for a quantum wire laser.

far. This is because at present the fabrication of the other two types of lasers is still technologically difficult, even with the most advanced epitaxial growth and device processing technologies. However, the quantum wire and quantum bubble semiconductor laser structures can be effectively achieved if one places conventional DH lasers or quantum well lasers in a strong magnetic field, in which the electron motion is confined in two dimensions. However, such lasers will only be useful for investigation purposes but not as practical devices. Therefore, in the following, only quantum well heterostructure lasers, which have been most successfully prepared by molecular beam epitaxy (MBE)29 and organometallic vapor phase epitaxy, (OM-VPE)30will be presented. Up to now, most of the studies have been on the threshold current density reduction, the temperature dependence of &, the achievement of visible emission, and reliability. Rather few studies have been made on the dynamic and spectral purity properties. 111. Short-Wavelength (- 0.68 -0.85 pm) Quantum Well Heterostructure Lasers

The emission wavelength of a quantum well heterostructure laser can be varied by the well thickness, as discussed above. A calculated example is

400

-

I

I

I

I

t

I

AlAs-GaAs ENERGY BANDS

300

-

200

-

100

-

c

2

r W (z

Y w

O

400

200

0

40

80

WELL SIZE. Lz

ci,

4 20

160

FIG. 10. The lowest (n = 1 ) confined-particle energy bands for electron (e), heavy holes (hh), and light holes (Ih) as a function of well thickness L, for GaAs wells coupled by AlAs bamers of thickness L, = 20 A.

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800

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750

20 WELLS

clcl

0 0 CALCULATION

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700 -

650

1

0

5 z 0 . 4 5

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10

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20

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30

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40

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I

I

50

60

70

WELL WIDTH, L z ,

I

(i)

FIG. 1 1 . Plot of emission wavelength as a function of well width. The crosses are the calculated wavelengths of n = 1 (e-hh) transitions for each sample.

7. QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

411

shown in Fig. 10 for the lowest ( n = 1) confined-particle energy bands (“minibands”) for electrons, heavy holes, and light holes as a function of well thickness L, for GaAs quantum wells coupled by AlAs barriers of size LB= 20 A.31Indeed, Woodridge et al.,32by using MBE, have prepared current-injection lasers with multiple quantum wells as thin as 13 A and obtained lasing at the shortest wavelength of 7040 A (300 K). Their results of lasing wavelength as a function of well thickness are given in Fig. 11 for multiquantum well lasers with 80 A AL.MG~.MAs barrier and cladding layers. Also plotted is the calculated wavelength for n = l(e-hh) transitions for each sample. It can be seen that for a given well width the measured emission wavelength is longer than the calculated transition

-

1-

L

A

FIG. 12. Schematic diagram showing the layer structure and doping levels of the MQW lasers. The multilayers were unintentionallydoped. The SEM photograph is of the cleaved cross-sectional view of the actualMQW laser structure at high magnification. There are 14 GaAs quantum wells each - 136 A thick and I3 Ab.2,G%,7fisbarriers each 130 A thick.

-

412

W. T. TSANG

-

wavelength by 150-200 A over the whole range of well widths studied and that this cannot be accounted for by uncertainty in the well thickness. This difference is larger than the exciton binding energy and varies from sample to sample, so that Woodridge et al.32concluded that the participation of LO phonons in the emission process is also unlikely. Reabsorption in the cavity or effective gap shrinkage at high injection may account for these observations.

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T O T A L G a A s ACTIVE M A T E R I A L ( p m ) FIG.13. Summary of the distribution, as represented by the shaded region, of the Ja’s of all the MQW wafers grown by MBE during a period of about one and half years. The (0)and (A) represent Ja’s of two systematic consecutiveseries of MQW wafers. (-) represents the best average J,,, of standard DH lasers having A&,,,G%.,As cladding layers grown also by MBE.

7.

413

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

5. DEVICE CHARACTERISTICS OF CONVENTIONAL MULTIQUANTUM WELLHETEROSTRUCTURE LASERS The ability of MBE to prepare ultrathin (5200 A) GaAs and Al,Ga,-,As layers with the latter free of alloy clusters33resulted in the preparation of high-quality multi-quantum well (MQW) heterostructure lasers3-9,14,34,35(Fig. 12). In these conventional MQW laser^,^,'^*'^ the barriers and the cladding layers have the same AlAs composition, x 2 0.3. With these MQW lasers, an extensive study has been made on the device characteristic^.^ Wafers with different numbers of wells and different well and barrier thicknesses have been investigated. These results showed that threshold current densities Jthas low as the lowest J, (800 A cm-2) obtained from standard DH lasers36with approximately the same AlAs composition in the cladding layers were obtained despite the reduced optical confinement factor r and the increased number of interfaces (Fig. 13). Significant beam-width reduction in the direction perpendicular to the junction plane was obtained. Half-power full widths as narow as 15 were measured for some MQW wafer^.^ Extensive studiesNon spectral behavior have been carried out by Holonyak and co-workers on MQW lasers prepared by OM-VPE by Dupuise and Dapkins. O

6 . DEVICE CHARACTERISTICS OF MODIFIED MULTIQUANTUM WELLHETEROSTRUCTURE LASERS

a. Laser Layer Structure and Threshold Current Density Theoretically, because of the modification of the density of states in quantum well lasers, the Jtbof quantum well lasers should be lower than that obtainable with DH lasers. However, the experimental results shown This has been found by Tsang in Fig. 13 do not reflect such impr~vement.~ to be related to the injection efficiency of the carriers over the various barriers in MQW lasers! In order to determine the optimal barrier height of the Al,Ga,-,As barrier layers for obtaining low Jtb,a series of eight-well MQW laser wafers with Al,Ga,-,As (0.3 5 y 5 0.35) was grown. In this series, all the layer structures were maintained approximately the same, whereas only the AlAs composition x in the Al,Ga,-,As barrier layers was varied (Fig. 14). It is seen that indeed the averaged Jthdoes vary with the barrier height of the Al,Ga,-,As barrier layers, as shown in Fig, 15, in which the average Jth of each wafer is plotted against the AlAs composition x (and the barrier height) of the Al,Ga,-,As barrier layers of that wafer. As the AlAs composition x increases from 0.08, the Jthdecreases first significantly to a minimum at about x = 0.2 (the cross over point of the two dashed lines) and then increases with increasing x for x greater than -0.2. Such behavior can

414

W. T. TSANG

A t , Ga +x As EARRIERS

J ' -

\\! t

GaAs WELLS FIG.14. Schematicenergy-band diagram of the modified MQW laser.

be understood in the following manner. The Jthdecreases with increasing x at first because of two possible reasons: ( 1 ) As the barrier height of the Al,Ga,-,As barrier layers increases, the modification of the density of states becomes increasingly significant. Specifically, the density of states increases with increasing depth of the wells. This increased density of states leads to a corresponding lowering of the threshold needed for achieving population inversion. This effect is expected to continue for all x but gradually saturates for large x. (2) As observed by P e t r ~ f f in , ~ ~contrast to regular DH, the MQW structure shows that the dislocations are not behaving as nonradiative BARRIER H E I G H T OF T H E GaAS/A!2xGa1-xAS M U L T I L A Y E R S (mev)

-

0

2.0

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2 00

300

400

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0.2

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AeAS M O L E F R A C T I O N X I N AJ2,Gal-,As BARRIER LAYERS

FIG. 15. Shows the variation of the average J,,, of several wafers as a function of their respective AlAs composition x (and barrier height) in the AI,Ga, -,As barrier layers.

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QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

415

centers. This effect is believed to be related to the two-dimensional nature of the carrier ~onfinement.~~ As the well depth is increased by increasing x, the increased two dimensionality due to camer confinement decreases the effectiveness of any dislocations present as nonradiative centers. The resultant lowering of Jtbwith increasing x will be faster, will gradually slow down after a certain x, but will continue to decrease. However, the present results show a turnover at x of about 0.2. The increase of Jtbwith increasing x beyond 0.2 can be understood as follows, As the barrier height becomes too high, it becomes increasingly difficult for the carriers to pass over the barrier and be injected into the next well. This decreasing carrier-injection efficiency with increasing x results in increasing Jtb.It is interesting and important to note that the turnover point occurs at x 0.2, a lower limit of AlAs composition in the cladding layers above which serious carrier leakage over the barrier into the cladding layer is avoided in regular DH lasers when operating near room temperature. This observation provides strong support for the previously described model.

-

By further optimizing the barrier and well thicknesses and increasing the AlAs mole fraction in the cladding layer to y - 0.45, an extremely low Jtb of 250 A cmb2 (average value) for broad-area Fabry-Perot diodes of 200 X 380 pm2 was 0btained.4'~ Such an extremely low Jtbis to be compared with - 800 A cm2 for the previous conventional MQW lasers3 and for otherwise similar-geometry DH lasers.36 Gain-guided proton-bombarded stripe-geometry lasers fabricated from these MMQW wafers have a cw threshold current of - 30 mA instead of 80 mA,38 compared with typical conventional MQW and nonoptimized DH laser wafers also prepared by MBE.3,36Such a cw threshold still represents a very significant reduction even when compared with the median value of 70 mA of the best LPE and MBE DH laser wafers.39Since these lasers are shallow protonbombarded gain-guided stripe-geometry lasers, the component of threshold current due to lateral current spreading in the cladding layers and camer out-diffusion in the active layers is expected to be about the same in all three types of lasers. This constant component makes the threshold reduction appear smaller in stripe-geometry lasers than in broad-area lasers. The net optical gain and carrier lifetime at threshold as a function of injection current and temperature are also measured for single-quantum well (SQW) and modified MQW (MMQW) heterostructure lasersm Figure 16 shows such an example. It is seen that the rates of change of net gain with respect to injection current are significantly enhanced in QW heterostructure lasers (10 cm-' mA-' for MMQW lasers and 3.8 cm-I mA-' for SQW lasers) compared to the DH laser, which is -2 cm-' mA-'. These

-

416

W. T. TSANG

O

-E

-20 -

I

MMQW

+--

V

Z a

C

-40-

0

+ W 2

-60 -

-80 20

30

iocm-1 mA-f

DH LASER

40

50

60

70

CURRENT (mA)

FIG. 16. The net optical gain as a function of injection current for a single quantum well and a modified multiquantum well heterostructure laser.

p - G a As P-GaQ55A'0.45 A s n -GaAs p -G a0.55A I 0.45As MQW LAYER n-Gaa55 A 1 o . d ~ n-GaAs SUB

"&-I

51 nm

(

0 0.2 0.45 FIG. 17. A schematic diagram of an MMQW guided-index GaAs/AlGaAs visible (7800 A) laser.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

- 4 0 -20

0

20

417

40

0 50400450 -40 -20 0 20 4 0 ( 0 1 CURRENT(mA) (b) ANGLE(DEG) FIG. 18. (a) cw light-current characteristics for a visible MMQW laser. (b) Far-field patterns for the same laser.

enhanced rates of QW lasers over DH lasers are consistent with the reduced threshold currents of the former and theoretical calculations of BurtzZas depicted in Fig. 6. High-quality MMQW lasers have also been prepared by OM-VPE for visible and high-power ~ p e r a t i o n .Figure ~ ~ ~ ~17 ' shows a schematic diagram of an MMQW index-guided visible (7800 A) GaAsIAlGaAslaser prepared by OM-VPE. Low threshold current (35 mA) and high output power (up to 40 mW cw) in the fundamental transverse mode as shown in Fig. 18 were ~btained.'~

PROTON IMPLANT

W ACTIVE LAYER

\ h-0% 6A'0.4AS b-GoAs

FIG.19. Schematic of a coupled multiple-stripe MMQW laser.

418

W. T. TSANG

Because of the excellent material uniformity prepared by OM-VPE and MBE, coupled multiple-stripe MMQW lasers were fabricated by Scifres et uL4' from OM-VPE-grown wafers, as shown in Fig. 19 for very high output power operations. Cw output power up to 400 mW and pulsed (75 ns) output power of 2.1 W from an uncoated mirror facet have been obtained. The threshold currents are of the order of -300-330 mA pulsed and 320 - 350 mA cw, corresponding to Jtb of 1.2- 1.3 kA/cm2 over a 250 100,urn area.

-

2 .o

:::I 0.3 0

,

I

20

80

60

40

,

,

100

120

HEAT-SINK TEMPERATURE ( " C )

4.0

(b)

-f

?- 3.0

-

-

b)

W

9 0

2.0 I 0

-

n

r r u

-

0

I

To= 2 2 OK

I

I

I

20

40 TEMPERATURE ( " C )

60

I

80

FIG.20. Threshold-temperature dependence of a proton-bombarded stripe-geometry laser (0,MQW 18 18-2; A, MQW 18 18-4) and (b) (0)buried MQW laser and (0)buried DH laser under cw operation.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

419

b. TemperatureDependence of Threshold Current With A1,Ga,~,As/A1,Gal~,As QW heterostructure lasers it is also generally observed that the threshold temperature dependence is less sensitive than in conventional DH lasers in both broad-area and stripe-geometry A Toin the range 1 7 0 - 2 5 0 K is quite typical. Figure 20 shows the threshold temperature dependence of a porton-bombarded stripegeomtry laser,16 a buried MQW laser,42and a buried DH laser under cw operation. The increased Toobserved in broad-area diodes is also preserved regardless of the stripe geometry used. Such an improvement in Toappears to be consistent with expectations from the modification of the density-ofstate function for quantum well lasers. However, a To as low as 80 K has also been observed in otherwise low-threshold MQW laser wafers. It appears that the value of To may depend to some extent on the layer structures, as suggested by theoretical treatment^.^^ In fact, it has been found experimentally that Todid indeed depend on the well thickness.

c. Reliability of Quantum Well Lasers Preliminary cw accelerated aging results (in dry nitrogen, 70°Cambient, at a constant power output of 3 mW per mirror) on 5 pm shallow protonbombarded uncoated stripe-geometry lasers fabricated from conventional MQW wafers prepared with GaAs wells are shown in Fig. 2 1. Even though the MQW lasers have pure GaAs wells and more interfaces, a median lasing lifetime of - 5000 h at 70°C was obtained.29 Proton-bombarded stripe-geometry GaAs/AslGaAs laser diodes with MMQW active layers grown by OM-VPE have been operated continuously at 5 mW/facet for over 1 8 0 0 h at 70°C with an average degradation rate of 3.5Yo/kh and over 1 1 0 0 h at 100°Cwith an average degradation rate of - 13%/kh.44Uomi et all3 also reported more than 1100 h of constant power operation of 20 mW at 70°C with their index-guided MMQW lasers prepared by OM-VPE emitting at 7800 A. 7. DEVICE CHARACTERISTICS OF GRADED-INDEX

WAVEGUIDE SEPARATE CONFINEMENT HETEROSTRUCTURE QUANTUM WELLLASERS

As was first demonstrated by T ~ a n g , ~the . ~ability , ~ ~ to profile the AlAs composition of the epilayers by MBE also made possible the preparation of a heterostructure semiconducting laser with graded-index waveguide and separate carrier and optical confinements (GRIN-SCH),S*6,39 as shown in Fig. 22. Such structure not only provides separate confinement of light and carriers to provide further optimization possibilities for Jth, but also an arbitrarily graded-index profile outside the camer-confinement region.

420

W. T. TSANG

POWER LIFETIME OF-0.87,um LASERS

z

W

t

w

100---A10.08G00.92 A s ACTIVE LAYER

!k

A K W

(MBE)

]

A FAILED CONVENTIONAL

v)

a

A ALIVE

-I

10 -

L

I

I 1

,

l

l

I

,

J

,

MOWLASERS

(MBE)

, , ,,

39

FIG. 21. Log-normal plot of 70°C cw aging results of MBE-grown conventional MQW

lasers.

The combination of graded index and the use of very thin camer-confinement regions, even into the quantum-well regime, produces the GRINSCH structure and permits the .I tot continue h decreasing with decreasing d even for d 5 700 A.It has been shown that the threshold current density Jth of broad-area Fabry-Perot DH lasers can be described by

with the gain -current relation assuming the linear form

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

U N2 Nz-yPx py-2

421

N i.PiwN ,P- AL;Ga;-iAS

pz

A"B

n , p a n,p- GOAS

(b)

FIG.22. Energy-band diagram of a GRIN-SCH laser: Nipi stands for N,P - Al,Ga,-iAs; n,p stands for n,pGaAs; CB, conduction band; VB,valence band.

In Eqs. (16) and (17), d is the active layer thickness in micrometers, q the internal quantum efficiency at threshold; aiincludes all the internal optical losses; r is the optical confinement factor; L the cavity length; R the power reflectance of the mirror (assumed identical for both end mirrors);,,g the gain coefficient; p the gain factor; Jnomthe nominal current density for a 1 pm thick active layer and unity quantum efficiency; and Jo the value of Jnomat which g,, is linearly extrapolated to zero. It is interesting to compare the optical confinement factor achieved in these GRIN-SCH single-quantum well laser structures with conventional single-quantum well structures. For a parabolically graded-index profile, the optical field is approximately Gaussian. With proper normalization factors, the confinement factor can be shown to be

r=

(d/w,)

(18)

where d is the active region width and Wois the Gaussian beam radius. For d = 100 A and W, = 2336 A,we find = 0.034. By comparison, the confinement factor in a conventional single quantum well is given by

r = 100 X d2/G

(19)

422

W. T. TSANG

where x is the composition of the barrier AlxGa,-,As and A,, is the lasing frequency. In that case, we find for d = 100 A, r = 0.0026. Thus, the use of the graded-index separate confinement structure represents a 13-times improvement in optical confinement over the conventional single-quantum well structure. From Eq. (1 6) it is seen that the Jth of a laser is due to three different contributions. The first term is the intrinsic term. The second term is the internal loss term, with aigiven by

+ +

ai= Tar, + (1 - T)afc,, a, a, 7 X 10-'*p arc (cm-') = 3 X lo-%

+

(20) (21)

In Eqs. (20) and (21), arcis the free-carrier absorption loss in the active layer and at threshold is 10 cm-'; arc,,is the free-carrier loss in the adjacent AlxGa,-,As cladding layers and for the usual doping concentrations (- lo1*~ m - ~is )- 10 crn-'; asis the optical scattering loss due to irregularities at the heterointerfaces or within the waveguide region (measured losses of - 12 cm-l can be accounted for by a roughness amplitude of only 100 A in conventional LPE-grown DH lasers); and a, is the coupling loss when the optical field spreads beyond the AlxGa1-,As cladding layers and is usually negligible when the AlXGal-,As cladding layers are thick (-2 pm). Thus, the measured ol, so far is typically 10-20 cm-' in LPE-grown lasers. The third term is the mirror loss term, which is -30 cm-' for L = 380 pm and R = 0.32. The values of Jo/q and l / $ ?that best fit the experimental results, especially when d 2 1000 A, are 4500 and 20, respectively, as suggested by case^.^' Using these values and an aiof 10 cm-', the relative importance of the three terms in Eq. ( 16) is shown in Fig. 23 by the solid curves as a function of GaAs active layer thickness d for DH lasers with Ab,3Gh.,A~cladding layers. It is seen that for the usually used active layer thickness of k 1000 A, the main contribution to Julcomes from the intrinsic linear term. Both the internal loss and mirror loss terms remain relatively unchanged and unimportant in this regime. However, for d 5 700 A,the contribution to Jth due to the mirror loss and internal loss terms becomes dominant and increases rapidly with decreasing d as a result of decreasing optical confinement I'. The effect on nonradiative recombination velocity at the interfaces are neglected.* Included in Fig. 23, as shown by the dashed curves, are the mirror loss and the internal loss terms calculated for a graded-index waveguide separate confinement heterostructure (GRIN-SCH) laser with the minimum beam width W, = pm. The inset depicts the energy-band diagram of the GRIN-SCH laser. The same parameter values as those used in the previous DH laser except r, which is calculated for the parabolically graded waveguide, are used in obtaining the previous curves. The intrinsic term remains the same. For

-

-

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

423

ACTIVE LAYER THICKNESS d ( p )

FIG.23. Relative importance of the contributions to Jta by the intrinsic, the internal loss, and the mirror loss terms: (-), calculated for regular DH lasers; (- - -), GRIN-SCH lasers. Both use previously determined parameter values as described in the text. ( * . * ), calculated for GRIN-SCH lasers using the parameter values determined in this experiment. The symbols I, A, and M refer to the first (intrinsic), second (internal loss), and third (mirror loss) terms, respectively, of Eq. (16).

d 5 700 A, although both the mirror loss and internal loss terms remain dominant over the intrinsic term, they are significantly reduced from those of DH lasers and stay almost constant with decreasing d. These result from an increased r and the feature that d/T remains almost constant in the GRIN-SCH lasers in the very thin d regime. By comparison of the DH

424

W. T. TSANG

lasers, the present calculation shows that (1) a reduction in Jthis obtained only when d is thinner than certain value d, depending on the W, of the GRIN-SCH laser; and (2) for the same W, the Jtbof the GRIN-SCH lasers should continue to decrease with decreasing d even when d 5 d,. Indeed, both features have been confirmed by experimental results. Had a superlinear gain - current been assumed in Eqs. ( 16) and ( 17), the decrease of Jth with decreasing d would have been even more drastic. In fact, the presence of the quantum size effect in such a thin active layer regime, as discussed in Section 11, will significantly increase the gain coefficient G. Kasemset et aL4’ have calculated the gain-current relation for QW heterostructure lasers. For a typical well width of 100 ,h, the gaincurrent relation is shown in Fig. 24. Presented also in the figure for comparison is the gain - current relation for normal heterostructure lasers (i.e., one which does not show quantum size effects). It can be seen that the use of the quantum well structure results in a significant enhancement of the optical gain at any particular injection level. This is due primarily to the increased density of states at the lasing energy achieved by quantization, as discussed in Section 11. Such an effect has not been included in the calculation of the intrinsic term in Fig. 23. The effect of an enhanced carrier transport to, and confinement in, the quantum well, due to electrons “funneling” by the graded composition

’

/’ 0 Zx?03

I

4x?O3 (,,,J

6x403

8x1~1~

404

A /cm2.pm)

FIG.24. Optical gain as a function of normalized current density in a 100 A quantum well laser and that of the normal double heterostructure laser. (-), calculated values for the 100 A quantum well; (- - -), calculated values for normal DH lasers, both of which assume parabolic bands with the k selection rule.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

425

regions has also not been taken into account in calculating the curves in Fig. 23. This effect may actually be important in reducing the threshold current and increasing the Toof the GRIN-SCH quantum well lasers. Further reduction in the Jthof the GRIN-SCH lasers can also come from optimization of the mirror loss and/or the internal loss terms. The mirror loss term can be reduced by having long L and reflective mirror coatings; however, these are only external structural variations. Extremely lowthreshold GRIN-SCH lasers with single and double active layers (layer thickness 200-400 A) were prepared first by MBE.6 As a result of an increased optical confinement, a significant reduction in the internal loss cui is 5 3 cm-I, and the gain constant p is 0.08-0.12. The internal loss a; is reduced by having the p and n-Al,Ga,-,As cladding layers doped to - 10'' ~ r n -and ~ both the active layer and the graded-index waveguide layer undoped (- 1014- 1015 ~ m - ~ The ) . quantity d/T is increased due to the use of GRIN structure. The present measured values for j? of 0.8 -0.12 is also larger than those estimated by Stem (1973) in his calculations. This can be the combined result of the quantum size effect and increased efficiency in utilizing the injected carriers due to the built-in graded bandgap layers on both sides of the active layer, which essentially funnels the camers into the active layer. The increased p as discussed earlier in relation to Eq. (16) and Fig. 23 is particularly advantageous for lasers with very thin active layers. Plotted in Fig. 23, as given by the dotted curves, is the relative importance of three different contributionsto J& using the various parameter values determined experimentally and to Jthusing the various parameter values determined experimentally and with L = 380 pm. As a result of reduced ai and increased gain constant p, the internal loss term is negligible. Although the mirror loss term remains dominant, its magnitude is also significantly reduced due to increased /I. Threshold current densities similar to those obtained here, -250 Acm-2, have also been obtained by Yamakoshi et U I . ' ~with ~ MBE, as shown in Fig. 25, in which the threshold current density of GRIN-SCH lasers is plotted as a function of L, for different AlGaAs cladding layers. Similar results have also been obtained by several groups with OMVPE.'0,47,4s The results obtained by Hersee et a1.'O are shown in Fig. 26. It is also shown that there is a significant reduction in Jthof GRIN-SCH lasers over the conventional SCH structure. They also show that the temperature dependence of Jtb,To decreases with decreasng L, and is substantially higher for GRIN-SCH than for conventional SCH lasers, as shown in Fig. 27. Recently Fuji et ~ 1by. incorporating ~ a superlattice buffer layer below the GRIN-SCH layers, as shown in Fig. 28, have obtained an average Jthof 190 A/cm2 for broad-area Fabry-Perot lasers with a cavity length of 450

426

W. T. TSANG

r

1500 r

II

67 67 E \

-2

a > f 1000 z

-

W

n II-

2 W

a a

3

0

0 -1

500

-

0 I v)

w

a

I II

--v--EC 1

II

1

L vEV

--IF-

L= 4 0 0 p m

0’

I

I

-

1

pm and an internal quantum efficiency of - 95%. Such a Jtbrepresents the lowest value obtained thus far in any laser structure. Figure 29 gives the variation of Jtbwith cavity length for DH, conventional multi-quantum well, GRIN-SCH, and superlattice-buffer-layered GRIN-SCH AlGaAs/ GaAs lasers.

::I ; 0 10

,

20

I

40 60 100

,

,

, , ,~

200 4000600

QUANTUM WELL THICKNESS L, (A)

FIG.26. Threshold current density as a function of well width in GRIN-SCHe(X,x = 0.4; A,x = 0.6)and SCH (0,x = 0.4) lasers prepared by OM-VPE. L = 4000-5000 A; x, = 0.18.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

427

200

-

150

Y

c

1

100

50 B xc'0.4 I 100

I 200

GRIN-SCH

I

I

300

400

L,d)

'

FIG.27. Variation of Towith well width for SCH and GRIN-SCH lasers.

Since the layer thicknesses, materials, and heterointerface qualities of MBE-grown lasers are so uniform, lasing-power distribution across the entire width of the broad-area lasers is always extremely uniform and not of isolated filaments. As a result, a very high output power (pulsed) of 4 W per facet from 20 X 380 fim2 broad-area lasers has been obtained with

-

-GaAs (0.5pm. 1 x l 0 ~ ~ c m - ~ ) -AIxGal-,As ( 1 . 3 p m , Ix101ecm-3) -AIGaAs (0.2p m , 3 x 1 0 1 7 c m - 3 ) NDOPED GaAs (Lz : 6 n m ) -AIGaAs -AI,Ga+,As

(0.2prn, 3 X 1 0 1 7 c m - 3 ) (1.3 p m , 1 X 1 0 1 8 c m - 3 )

a A s (15 n m l - A I G a A s ( 1 5 n m ) ( 5 + 5 ) -GaAs (3.0pm, 2 ~ l O ~ ~ c m - ~ )

AI,Gal-,As

( X -0.7)

A '0.lBGaO.Bz As GaAs

FIG. 28. A schematic diagram of a GaAs/AIGaAs GRIN-SCH laser structure with a superlattice buffer layer and the corresponding energy-band diagram.

428

W. T. TSANG

300

I

GRIN-SCH

2 2 00

W P

I-

Z W

a a: 2

“

4

00

0 TSANG

A HERSEE e t . a l . 0

0

10

20

MO-CVD

30

I / L ( l n l / R ) cm-’ FIG.29. The variation of J& with cavity length. An average Ju, is plotted as a function of (l/L)ln( I/R). GRIN-SCH lasers with superlattice (0)and without a superlattice buffer layer Heme et a/.lo (A). (O),

a),

SCH lasers.49What is more important is that all this power is concentrated in a single spot of 30” (0,) by 5” (el,),as shown in Fig. 30. Although high output powers are obtained in array lasers of both gain- and index-guided type^,^,^^ the far-field patterns tend to be double lobed. For gain-guided array lasers under high injection levels for high output powers, the lateral current spreading and carrier out-diffusion processesSZare expected to smear out the individual laser stripes, resulting, in actuality, in a broadarea laser. Thus, for the purpose of high output powers, one can employ MBE- or OM-VPE-grown laser wafers and simply form broad-area (250 pm) stripe lasers. This will yield a single-spotted far-field pattern with narrow beam divergence. The GRIN-SCH laser wafers just described were also processed into cw stripe-geometry lasers. The cw electrooptical characteristics of proton-deheated stripe-geometry lasers fabricated from GRIN-SCH wafers were studied39and compared with similar geometry lasers fabricated from LPEand MBE-grown DH wafers with standard composition. Lasers were fabricated into a 5 pm wide stripe geometry using shallow proton irradiation and 380 pm long optical cavities. The same processing and characterizing procedures were employed for both GRIN-SCH and DH lasers. The cw light-current (L-I) characteristic from each mirror was linear up to 10 mW per mirror. In addition, the “tracking” of the light outputs from both

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

I

A

2.65 A

429

(c)

0.63

x

1.0

0.55 0.52

-

0.47

L

-20"

0"

20'

I

-40'

1 -ZOO

I

Oo

I

20

I

40"

FIG. 30. Far-field intensity distributions at current levels near and above lasing in (a), (b) parallel and (c), (d) perpendicular directions to the junction plane of a typical symmetric SCH laser 200 prn wide and 380 p m long prepared by MBE; the threshold current was 0.42 A.

mirrors up to 10 mW cw per mirror were significantly better than that of similar geometry LPE- or MBE-grown regular DH lasers fabricated during the same time period. In Fig. 3 1, the cw threshold distribution at 30°C of GRIN-SCH protonbombarded stripe-geometry lasers is compared with those of the lowest threshold wafers of MBE- and LPE-grown DH lasers. It is seen from Fig. 3 1

110

I

I

I

I

I

I

I

I

I

I

I

I

I

I

1

0

g 100 (u

z

70-

w

5

(L

60-

V

n J

0

50-

I

cn

40I-

0.01

I

I

1 2

I

I

I

I

I

I

I

I

I

1

1

1

1

5 10 203040506070 80 90 95 9899 CUMULATIVE ('10)

FIG.31. The cw threshold distribution at 30°C of GRIN-SCH proton-bombardedstripgeometry lasers compared with those of the lowest threshold wafers of MBE- and LPE-grown DH lasers.

UNDOPED

FIG. 32. Schematic diagram of a GRIN-SCH DH laser prepared by hybrid-crystal growth.

J

7.

*

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

431

PULSED CURRENT ( m a )

2c

20

30

s

c

A

-3E

I

& 0

n 0 a

n

u_

u fa

z

\

n W

g 1c

z

w

a

a

I3

n

b-

3

I-

2

b-

a

0 0

2 0

w

3

v,

-1

3

a

d c CURRENT (mA)

FIG. 33. Light-current characteristics of a GRIN-SCH BH laser under cw and pulsed conditions.

that there is a significant reduction, by - 25%,to 52 mA dc in the median cw threshold current of the GRIN-SCH stripe lasers. This occurs because the current components (carrier out-diffusion and lateral current spreading) not contributing directly to the lasing are to first order fixed by the structure. The percentage reduction in the current component producing the lasing threshold should be even larger for the GRIN-SCH wafers. This larger reduction can be made more readily apparent by fabricating buriedGRIN-SCH lasers, as is shown next. In addition, these lower thresolds were achieved without compromising the improved distribution (a = 3.4 mA dc) available using MBE growth procedures. That is, even though the active layers are 200-400 A and the structure far more complex, the similarity of the 0’s indicates that the same degree of control and reproducibility in material quality and layer thickness uniformity is still well main-

432

W. T. TSANG

P

LASER

_K Ql

'G2

UNDOPED GaAs

1

'n+-GaAs

/

S.I.GoAs SUB.

RIDGE WAVEGUIDE

n-AIGaAs n+-GaAs

-x 0 0.18 0.45

--+

FIG.34. (a) Equivalent circuit of a monolithic laser driver. (b) Cross-sectional structure of GRIN-SCH laser/MESFET driver circuit monolithically integrated on a semi-insulating substrate.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

433

tained. As mentioned before, wafer uniformity can be further improved by having continuous substrate rotation during growth. Buried-heterostructure (BH) lasers operating in the fundamental transverse mode fabricated from GRIN-SCH wafers have cw thresholds as low as 2.5 mA (Fig. 32). This is the lowest ever reported for any laser. The active stripe is zs 3 pm in width and 250 pm long. An external differential quantum efficiency as high as 80% was obtained. Compared with regular three-layer BH the threshold currents have been significantly reduced, and more important, output powers of 20 mW per mirror (Fig. 33) have been obtained, a considerable increase over regular three-layer BH lasers of similar stripe widths ( 5 mW per mirror). The output powers are similar to those obtained from large optical waveguide BH laser^,^^-^^ but the threshold current of the present GRIN-SCH BH lasers is significantly lower. (Threshold is - 10 mA for W = 1 pm; 30 mA for W = 3 pm; a-

-

300K

-

7-

t-

6-

W

u

i? \ z

2

5 -

E

4-

t-

3 0

5(3

32-

-

1-

-

J

0 -3

-2

-1

0

1

2

3

values of FIG.35. Relationship of light output as a function of input voltage V,, at various values ofL VG3.The bias current of the laser is set at onehalf of the threshold current by Q, .

I

434

W. T. TSANG

L = 300 pm for four-layer buried optical waveguide (BOG) lasers,54and -23 mA for five-layer BOG laserP with W- 2 ,um and L = 380 pm.) Because of the extremely low threshold currents achievable with GRINSCH lasers, this laser structure proves to be particularly suitable for integration with other optoelectronic devices. As a demonstration, Sanada et have achieved monolithic integration of a GRIN-SCH laser with a driver circuit on a GaAs substrate. Figure 34a shows the equivalent circuit of the monolithic laser driver, while Fig. 34b shows the cross-sectional structure of GRIN-SCH laser/MESFET driver circuit monolithically integrated on a semi-insulating substrate. The measured relationship of light output is a function of input voltage V,, at various values of V,, .The bias current of the laser is set at one-half of the threshold current by Q1and is shown in Fig. 35. Using the GRIN-SCH single-quantum well (6 nm thick) structure, the integrated laser has exhibited room-temperature cw operation characteristics with an extremely low threshold current of 15 mA as well as a high quantum efficiency of 50%. Measurements have also shown the conversion ratio of laser output power to input gate voltage of 4.3 mW/V, and the turn-on and turn-off time of the light output of 400 and 900 ps, respectively, demonstrating high-sensitivity and fast-response performance of the present monolithic laser driver. IV. Long-Wavelength ( A - 1.3 - 1.6 pm) Quantum Well Heterostructure Lasers 8. IQ.,,G%.~~As-I~P QUANTUM WELLS Quantum well structures of I Q . ~ , G % . ~ ~ A s with / I ~ PL, as small as 25 A have been prepared by Razeghi and Hirtz5*using low-pressure OM-VPE Figure 36 shows the Auger spectrum of a chemically etched level which cuts all four 1~.~,Ga,,,~As layers of the four IQ,,G%,~,As well separated by InP barrier layers. The 2 K photoluminescence spectrum of the sample from the four different wells as excited with a Nd-YAg laser at 1 170 meV is shown in Fig. 37. Such quantum wells have also been prepared by hydride vapor phase e p i t a ~ y . ~ ~ Tsanga has also prepared current-injection IQ.~~G%.~, As/InP quantum well lasers using MBE. A scanning electron micrograph (SEM) of a G%,47 As/InP MQW heterostructure (layers chemically delineated to enhance the heterointerfaces) is shown in Fig. 38. The Ga,-,4,1q.s3As wells are - 250 A,and the InP barriers are - 330 A for this wafer. Lasers with such well thicknesses did not show any significant upward energy shift at room temperature. Thus, results from a wafer with a well thickness of

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

435

K

w

(3

3

a

0

100

200

300

400

500

(pm)

FIG. 36. Auger spectrum of a chemically etched level which cuts all four Ga,,In,,,As layers of the four-well G%.47 In,,,,As/InP sample.

- 70 A are presented in the following discussion. Although the layers of the wafer shown in Fig. 38 are too thick to produce an observable quantum size effect, it is seen that they are very smooth and uniform is thickness. Figure 39a shows the L-I curves of a laser diode at various heat-sink temperatures fabricated from a MQW laser wafer having four Gq,, Iq,s3Aswells of 70 A and InP bamers of - 150 A.These thickness were estimated from growth-rate measurements. The room-temperature (24°C)threshold is about 2.7 kA cm-*, which is about 15%lower than that of AlGaInAs/InP DH lased1emitting at 1.5 pm also prepared by MBE. In the temperature range 10- 75 "C, the threshold temperature dependence can be described very closely by a single dependence with T0-45 K, as shown in Fig. 39b. The usually observed breaking point in threshold temperature dependence?' i.e., different Tofor the low- and high-tempera-

-

436

W. T. TSANG I

I

I

I

I

I

Ga0471n0.53As-1np

T= 2K C 598

700

800 900 PHOTON ENERGY ( m e V )

1000

FIG.37. Photoluminescence spectrum of a Ga.4,1n,,s3As/InP sample measured at 2 K with excitation at 1170 meV (Nd:YAG laser, 20 mW focused beam).Well assignments are indicated above each peak. Note that the peaks assoCiated with the 25 and 50 I\ wells are clearly multicomponent i? nature. The full width at half-maximum of the lowest energy peak (associated with the 200 A well) is 8.3 meV. The inset schematically illustrates the sample structure.

FIG. 38. An SEM p h o t o p p h of a G~,4,1n,,s,N/InP MQW heterostructure with four Ga,,471q,53A~ wells of 250 A and three InP barriers of 330 A. The cladding layers are InP the structure is grown by MBE.

-

-

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

a

437

7

10

-

2 1 G00.47In0,~3As(~70%)/InP(-1 5 0 % )

; i 7D

-I

0

6 - MQW LASER 5 -

x 4 -

v)

w

P

I

3-

I-

&w

2-

P

[L

3 V

1

I

I

I

I

I

I

I

ture regions, was not observed or at least was not as obvious in these MQW lasers. However, the Tomeasured is not higher than for AlGaInAs/InP DH where To- 40 K for temperatures between 10 and 45"C, also prepared by MBE and emitting at 1.5 pm. It has been suggested from theoretical s t ~ d i e sthat ~ ~ the , ~ ~Toof the 1.3 - 1.5 pm MQW lasers should be

438

W. T. TSANG

significantly increased as a result of the reduced-phase space for Auger recombination processes. However, the present initial results with G%.471q,53As/InPMQW lasers do not show such improvement. One obvious reason is that the present MQW lasers, as indicated by the still high-threshold current density (2.7 kA cm-2), instead of less than 1 kA cm-2, is still not perfect enough; another reason is that the layer structures are not of the right design to reveal such predicted improvement. Theoretical studies by S ~ g i m u r indicate a ~ ~ that the Auger component of the threshold current and its temperature dependence strongly depend on QW structure. The other explanation comes from a theoretical investigation by B ~ r twhose , ~ preliminary prediction indicates that the ratio of the Auger recombination rates in bulk (DH lasers) to that in two-dimensional confined structures (QW lasers) may actually be proportional to (EJkT)'/2, where E, is the activation energy of the Auger process involved in the bulk, k the Boltzmann constant, and T the temperature. It is seen that if E, is comparable to kT (-24 meV at room temperature), then (EJkT)'/2is approximately unity, and no significant improvement in To can be expected for QW lasers. The question of To in 1.3- 1.6 pm QW lasers is therefore still quite complex and unclear both theoretically and experimentally. Current injection InGaAsP/InP quantum well lasers have also been prepared by LPE recently.65(See Appendix for further discussion.) 9. II~o.~~G~,,,,AsIn,,,2Ab,4,As QUANTUMWELLS

Temkin et ~ 1 investigated . ~ ~ the properties of MBE-grown I Q . ~ ~ GA %s./~I ~ , ~ ~ A ~ , ,multi-quantum As well lasers. These devices, operating at room temperature in the 1.5- 1.6 pm range, have well thick-

p -1nGoAs p-In P

p - I n AlAs n-InAIAs n -1nP

n - I n P SUB

FIG.40. Structure of MBE-grown InGaAs/InGaA1As/InAls/InPMMQW laser.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

439

nesses as low as 80-90 A and barrier thicknesses as low as 30 A. In the broad-area devices with a total active layer thickness of 0.14 pm, they have observed threshold current density as low as 2.4 kA/cm2. Kawamura et al.,67using MBE, also obtained current-injectionInGaAs/ InGaAlAs/InAlAs modified multi-quantum well lasers operating at 1.57pm. This MMQW laser is composed of InGaAs wells. The InGaAlAs quaternary barriers and InAlAs and InP cladding layers are as shown in Fig. 40. Photoluminescence (4 K) as short as 9668 A,equivalent to 0.474 eV above the band gap of I%.53Ga,,.47As,has been obtained by Welch et a1.68 from a single quantum well of 15 A prepared by MBE with I%.szAb,48As cladding layers. In addition to InGaAs/InAlAs quantum wells, optically pumped GaSb/ Al,,6Ga,,4As MQW lasers operating at - 1.5 pm have also been obtained by Temkin and T ~ a n gusing ~ ~ MBE. Superlattices of GaSb/AlSb have also been investigated.

-

V. Very-Long-Wavelength ( A 2.5-30 pm) Quantum Well Heterostructure Lasers

Lead-salt diode lasers provide tunable laser sources in the 2.5 - 30 pm wavelength range. The entire wavelength range can be covered with PbSnSe, PbSSe, and PbCdS diodes. Alternatively, PbSnTe/PbSnYbTe can be used for wavelengths in the 6- 30 pm range, and a new material, PbEuSeTe, can be used to cover the 2.6-6.6pm wavelength range.70*71 Molecular beam epitaxial (MBE) growth of Pb,-,EuxSeYTel-, lattices matched to PbTe substrates has been used to fabricate double-heterojunction diode lasers with - 1.5 pm wide active regions operating up to 147 K cw (180 K pulsed). This is the highest cw operating temperature ever achieved with lead-salt diode lasers.'l Recently, Partin7* prepared single-quantum well lead-chalcogenide lasers by MBE. The dopant and composition (x) profiles of a Pbl-,Eu, Se,,Te,-,, diode laser are shown in Figs. 41a and b, respectively. The selenium concentration was adjusted to obtain lattice matching between the PbEuSeTe layers and the PbTe substrate. This laser structure has a PbTe single-quantum-well active region of thickness I,= 300 A. The Pb1-,EuxSeyTe,-, confinement layers have x = 0.018 near the active region, yielding an increase in energy band gap of 99 meV at 80 K. The europium concentration was increased further from the active region to form a separate optical cavity structure, since the index of refraction of PbEuSeTe decreases with increasing europium concentration. Mesa stripegeometry diode lasers were fabricated as previously reported using an

440

W. T. TSANG

0 (a)

5 DEPTH ( p m ) I

(b)

10

I

DEPTH ( pm)

FIG. 41. (a) Dopant profde an4 (b) europium concentrations versus depth for a single quantum well laser with L, = 300 A.

anodic oxide for electrical in~ulation.’~ The stripe widths for these lasers were 16-22 pm, and the cleaved cavity lengths were 325-450 pm. The threshold current for a 300 A quantum well active region is shown as a function of temperature in Fig. 42. Pulsed ( 1 ps, 1 kHz) and cw data are shown for transitions between the n = 1 states in the conduction and valence bands. Below about 130 K, a mode with much higher photon energy (corresponding to transitions between n = 2 states) was observed at a higher “threshold” current. This n = 2 threshold current decreases with increasing temperature until it becomes approximately equal to the n = 1 threshold at 140 K (pulsed). Above this temperature, the n = 2 threshold current increased rapidly, and the n = 1 transition was not observed. The PbTe quantum well width L, was varied in the sequence 300,600, 1200, 2500 A in a series of otherwise similar growths. The high-temperature performance improved up to L, = 1200 A. The threshold of a laser with this value of L, is shown as a function of temperature in Fig. 43. The

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

441

10'

Lz = 300A E p , PULSED

--a I0

0

W

K K

3 V

D

10'

0

I00

200

TEMPERATURE (K)

FIG.42. Threshold current versus temperature for a laser with L, = 300 A.

cw and pulsed curves have a kink at about 100 K, apparently caused by a switch from laser operation between n = 1 states at low temperature to operation between n = 2 states at high temperature. However, laser operation was observed from 13 K (at 6.45 pm wavelength) up to 174 K cw (at 4.4 1 pm) and to 24 I K pulsed (at 4.0 1 pm). These are the highest operating temperatures ever observed for lead-salt diode lasers. The operating temperature of the 300 A quantum well lasers was probably limited by leakage current out of the well. Pulsed operation at temperatures as high as 235 K has also been obtained with a GRIN-SCH Pb,-,Eu,Se/Pb,-,Sn,Se laser, as shown in Fig. 44, by Norton et ~21.'~These lasers were prepared by MBE. The quantum well is 1000 A.

-

442

W. T. TSANG I

I

0

I

I

I

I

100

I

I

200

I

I

300

TEMPERATURE ( K ) FIG.43. Threshold current versus temperature for a laser with L, = 1200 A.

A

'"8/;;;....

Pb+xEu,Se

Pb+,Eu,Se

EgPb4-ySnySe

FIG.44. A GRIN-SCH Pb,-,Eu,Se/Pb,-,Sn,Se

EFFECTS

laser prepared by MBE.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

443

VI. Summary The theoretical analyses of the density-of-state functions, the spectral gain, the temperature dependence of threshold currents, the quantum noise and dynamic characteristics of quantum confinement heterostructure lasers, as well as quantum well, quantum wire, and quantum bubble lasers, were reviewed. Because of the decreased dimensionality of the carrier motion from three dimensional to two dimensional (quantum well laser) to one dimensional (quantum wire laser) to zero dimensional (quantum bubble laser), successive significant modifications result in the density-of-state function as the dimensionality decreases. This modification results in a shortened emission wavelength due to radiative between confined states, and significantly reduces the width of the gain spectrum, the threshold current density, and its temperature dependence, and improves the quantum noise and dynamic characteristics. In the case of quantum bubble laser, the performance characteristics should almost completely resemble those of a conventional gas or solid-state laser due to the fact that the density-of-state function becomes 6-function-like. Experimental results from short-wavelength (A 0.68-0.85 pm), longwavelength (A - 1.3- 1.6 pm), and very-long-wavelength (A 2.5 -30 pm) quantum well heterostructure lasers were reivewed. These include quantum well lasers from AlGaAs/GaAs, InGaAs/InP, InGaAs/InAlAs, AlGaSb/GaSb, PbEuSeTe/PbTe, and PbEuSe/PbSnSe heterostructures. Of the various types of quantum well laser structures, the modified multiquantum well heterostructure and in particular the GRIN-SCH quantumwell laser have been established widely to give the lowest threshold current density ever achieved with semiconductor lasers. For instance, J,,, of 190 A/cmZhas been obtained in broad-area Fabry - Perot GRIN-SCH diodes of AlGaAs/GaAs with a cavity length of 450 pm, and a threshold current as low as 2.5 mA has been obtained with GRIN-SCH buried heterostructure lasers. An internal quantum efficiency as high as 95% has been obtained with GRIN-SCH AlGaAs/GaAs lasers. Though theoretical analysis shows that further significant improvement in laser performance can be expected with quantum wire and quantum bubble semiconductor lasers, at present no such laser structures have been constructed due to a lack of suitable material preparation and fabrication technologies.

-

-

Appendix

Very recently, high-quality Ga,,47 Iq,,,As/InP quantum wells have also been prepared by a new epitaxial technique, chemical-beam epitaxy (CBE).* Results obtained on Gao.47I%,,As/InP current-injection lasers showed that there was a definite improvement in To.

444

W. T. TSANG

In all kinds of chemical vapor deposition (CVD), because the pressure inside the reactor is typically greater than torr and up to atmospheric, the flow of the gaseous reactants is viscous. If, however, the pressure is sufficiently reduced (down to < tom) so that the mean-free paths between molecular collisions becomes longer than the source inlet and substrate distance, the gas transport becomes a molecular beam. Such thin-film deposition process is called chemical beam deposition or chemical-beam epitaxy7’ if the thin film is an epitaxial layer. Thus, CBE is the newest development in epitaxial growth technology. It combines many important advantages of molecular beam epitaxy (MBE)76and organomet a l k chemical vapor deposition (OM-CVD),’7 both of which were first developed in 1968. And, therefore, it promises to advance the epitaxial technology beyond both techniques. In CBE, unlike MBE, which employs atomic beams (e.g., Al, Ga, and In) evaporated at high temperature from elemental sources, all the sources are gaseous at room temperature. They can be organometallic or inorganometallic compounds. For 111-V semiconductors the Al, Ga, and In are derived by the pyrolysis of their organometallic compounds, e.g., trimethylaluminum, triethylgallium, and trimethylindium, at the heated substrate surface. The As2 and P2 are obtained by thermal decomposition of their hydrides passing through a heated baffled cell. The use of hydrides was first introduced into the MBE process in 1974 by Moms and Fukui7*and later applied to the growth of GaAs and InGaAsP by Calawa and P a n i ~ h . ~ ~ Unlike OM-CVD, in which the chemicals reach the substrate surface by diffusing through a stagnant gas boundary layer above the substrate, the chemicals in CBE are admitted into the high-vacuum growth chamber in the form of a beam. Further, in OM-CVD, most of the pyrolysis of the organometallics is believed to occur in the gas phase, while in CBE there is no gas-phase reaction. Therefore, comparing with MBE, the main advantages include: (1) the use of room-temperature gaseous group-I11 organometallic sources, which simplifies multiwafer scale-up; (2) semi-infinite source supply and precision electronic flow control with instant flux response (which is suitable for the production environment); (3) a single goup-I11 beam that guarantees material composition uniformity; (4) no oval defects even at high growth rates (important for integrated-circuit applications); and ( 5 ) high growth rates if desired. Comparing with OMCVD, these include: (1) no flow pattern problem encountered in multiwafer scale-up; (2) the beam nature produces very abrupt heterointerfaces and ultrathin layers conveniently; (3) clean growth environment; (4) easy implementation of in situ diagnostic instrumentation, e.g., RHEED and RGA; (5) compatible with other high-vacuum thin-film processing techniques, e.g., metal evaporation, ion-beam milling, and ion implantation.

-

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

445

A gas-handling system75 similar to that employed in organometallic chemical-vapor deposition (OM-CVD) with precision electronic mass flow controllers was used for controlling the flow rates of the various gases admitted into the growth chamber, as shown in Fig. 45. Hydrogen was used as the carrier gas for transporting the low-vapor-pressure group-111 alkyls. Separate gas inlets were used for group111 organometallics and groupV hydrides. A low-pressure arsine (ASH,) and phosphine (PH,) ~ r a c k e rwith ~ ~ a, ~reduced ~ input pressure of - 200 torr (in fact, - 40 torr is sufficient) maintained on the high-pressure side of the electronic mass flow controller was used. The cracking temperature was 920°C. Complete decomposition of arsine and phosphine into arsenic, phosphorous, and hydrogen was routinely achieved, as observed by the absence of arsine and phosphine peaks inside the growth chamber with an in situ residue gas analyzer. Triethylgallium (Et,Ga) maintained at 30 "C, triemethylindium (MetJn) at 3 7 T , and trimethylaluminum (Met,Al) at 25°C were used. The Et,Ga, Met31n,and Met,Al flows were combined to form a single emerging beam,

-

RHEED GUN LIQUID NITROGEN COOLED SHROUDS

/

+H2

CONVENTIONAL MBE OVEN

0PRECISION ELECTRONIC MASS @

FLOW METER VALVE

RHEEDSCREEN

I

RESIDUAL GAS ANALYZER

FIG.45. Gas-handling system and growth chamber with in situ surface diagnostic capabilities incorporated into a CBE system and atomic beams of Be and Sn for p and n-type dopings, respectively.

446

W. T. TSANG

impinging by line of sight onto the heated substrate surface. This automatically guarantees composition uniformity.80The typical growth rates were 3.65 pm/h for GaInAs and 1.5-2.5 pm/h for InP, although even higher rates have been achieved. Such growth rates are higher than those typically used in MBE. The growth temperatures were usually - 550- 580°C. Elemental Be and Sn were used as the p- and n-type dopant, respectively. Note that the use of CBE allows the use of evaporated atomic beams as dopants. Gas source dopants can also be used. Continuous growth was employed at the interfaces by switching out and in the appropriate gas components. Before the growth of GaInAs/InP double-heterostructure and quantum well laser wafers, the technique was first studied by investigating its ability to grow high-quality InP and G%.47In,,,,As epilayedl and quantum well structures**lattice matched to InP substrates. Excellent material quality and heterointerfaces were obtained. Typical 2 pm thick undoped InP layers were n-type - 5 X lo1,- 1 X 10l6 cm-, with a 300 K mobility of -4500 cm2/V s and a 77 K mobility of - 30,000 cm2 V-’ s-l. Typical 2 - 5 pm thick G%.47I%,,As epilayers with no two-dimensional electron-gas effect have mobilities of 10,000- 12,000 and 40,000- 57,000 cm2/V s at 300 and 77 K with n = 5 X 1014-5 X lo1, cmb3. Bulk G%.471%,,,Asepilayers also show a very intence efficient luminescence exciton peak with linewidths as narrow as 1.2 meV, which is equivalent to the calculated intrinsic (full width at half-maximum, FWHM) alloy broadening for (1.3 meV). Such a linewidth is the narrowest ever measured G%.471q,53A~ for any alloy semiconductors, including AIxGal-,As with x > 0.1. One extreme way of testing the technique is to evaluate the quality of the quantum well (QW) heterostructures grown by it. High-quality QWs should have smooth and abrupt (“squareness” of the QW) interfaces, few background impurities, and a high PL efficiency. In order to facilitate the study of more than one quantum well simultaneously, multilayer structures consisting of a 0.5pm InP buffer, a 0.2pm G%.471~.,3As control layer, plus G%.47III,,~~ASquantum wells of different thicknesses alternated with 700 A InP barriers were grown on InP(Fe) substrates. The 0.2 pm thick G%.47I%.,,As control layer (behaving as a bulk material) served as a reference wavelength in the photoluminescence (PL) spectrum from which the upshifts of the quantum wells can be calculated precisely. The quantum well thicknss was determined from transmission electron microscopy (TEM) measurements and from the steady-state growth rate. In CBE, this latter process was found to be very reliable and reproducible from run to run. Photoluminescence measurements were made at 2 K using the 647.1 nm line of an Kr ion laser as the optical pump. The pumping power used typically ranged from 0.1 to 10 pW over a pumping area of 50 pm in diameter. Very sharp intense efficient luminescence peaks due to excitonic

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS I

I .I

1.2

I

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447

I

Ga47InwAs/InP QUANTUMWELLS T =2K

1.3 1.4 1.5 WAVELENGTH X (pm)

1.6

FIG. 46. A typical photoluminesce.nce spectrum from a stack of quantum wells with different thicknesses separated by 700 A InP barriers at 2 K. The pumping power is 1 pW and the pumping area is of - 50 Grn diameter (Ref. 82).

transitions in the quantum wells were obtained, as shown by a typical example in Fig. 46.With the 10 A well, the emission peak has been shifted from 1.57 pm (the bulk Ga,-,471n,-,,3As reference) to 1.17 pm. Note that, with a growth rate of 3.6 pm/h employed in the present experiment, the 10 A well required a growth time of only 1 s, yet extremely sharp and intense luminescence was obtained, indicating that the heterointerfaces were extremely uniform and smooth. Separate samples with 6 A wells have also been successful grown with similar luminescence quality and with an emission peak at 1.09 pm. Further, such results have been reproduced from run to run. It should be pointed out that all the CBE-grown wells exhibited a single sharp peak due to excitonic transitions. No extrinsic transition peaks were observed, indicating the purity of the material. Figure 47 represents a comparison of PL linewidths (FWHM) as a function of quantum well thickness for the best published G~.4,1~.5,As/InP83-87 quantum wells grown by either OM-CVD or MBE. It is clear that the present quantum wells have significantly narrower PL linewidths than any of the previous G%.471q,3As quantum wells ever reported at all well thicknesses. Such high-quality quantum wells achieved with CBE indisputably demonstrated the superiority of this technique over OM-CVD and MBE in producing highquality G%.47 I%.,,As/InP quantum wells. The low-temperature PL linewidth for Ga,,47 1q,53Assingle quantum wells are

448

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FIG. 47. Represents a compilation of PL linewidths (FWHM) as a function of weU thickness for the best published Gq,,,In,,,,As/InP quantum wells grown by OM-CVD and MBE together with present results grown by CBE. (- - -), calculated broadening due to band filling impurities. A sheet carrier density of 2 X 10" cm-2 was used. (. * .), calculated broadening due to "effective" interface roughness, L,, of 4 2 , assuming finite-height barriers (Ref. 82).

determined by three major contributions,i.e., alloy broadening,88broadening due to geometric well-width fluctuation^,*^^^^-^^ and broadening due to an equilibrium concentration of carriers (band-filling effects) and defects associated with the heterointerfaces.88In G~.471q,,,As/InP quantum wells, alloy broadening dominates for well thicknesses of L, Z 50 A and

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

449

amounts to about 1.3 meV. Alloy broadening becomes reduced though not negligible in narrow wells because the electronic wave function spreads more into the InP cladding layer. We have measured a value of 1.3 meV in I Q . ~ ~ Abulk S layers.8* The dashed curve extremely high-quality G%.47 shown in Fig. 47 was calculated by Welch et dg8 for broadening due to an equilibrium number of carriers produced from impurities in the well and/ or the barrier layers. A sheet carrier density of 2 X 10” cm-2 was used. The dotted curve was the calculated linewidth broadening, A E, due to a total (both heterointerfaces) geometric well-width fluctuation, L,, of one monolayer (a,,/2 = 2.93 A) using the relationship AE = [d(AE,,)/dL,]AL,. Elh is the energy upshift due to quantum size effect in wells with finite-height barriers. For narrow wells (550 A) broadening due to well-width fluctuation becomes very severe and is the dominant contribution to PL linewidths. Our linewidths are far significantly narrower than the calculated broadening. Neglecting contributions due to alloy broadening in narrow wells, we estimated an “effective” interface roughness of 0.124, which can be interpreted as showing that the quantum well was largely consisting of a big domain of the same thickness L, perforated with a small fraction of . the above discussions, we conclude small domains of (L, ~ , / 2 ) From that our quantum wells have extremely flat heterointerfaces, no band filling due to background impurities, and a minimal alloy broadening of 1.3 meV for wells 250 A. This is further supported by the results of excitation ~pectroscopy~~ obtained from GaAs/AlGaAs quantum wells also grown with the same technique, which show an abrupt heterointerface smooth to within one monolayer. In Fig. 48, we show the measured PL energy upshifts, A Elh, of five different Ga,,471%.,3As/InP quantum well samples, each having stacks of quantum wells of different thicknesses as thin as 6 A as a function of well thicknesses. The three solid curves were calculated with different ratios of conduction-band edge differences to valance-band edge differences,A EJ A E,. For the first time, experimental values agree well with the theoretical curves. Further, the extreme consistency and well-behaved nature of the various different samples prove the reliability of the data and the reproducibility of the growth technique. Based on the present data alone, it is difficult to determine the actual A EJA E, ratio. From the above results on bulk G%.471~,,As and InP epilayers and Ga,,471%.,3As/InP quantum wells, we are confident that CBE is capable of producing extremely high-quality materials and heterointerfaces. We shall next present our results on G%.47I%,,As/InP DH and QW lasers. Quantum-well lasers, although more complex than DH lasers, are of great interest because they offer emission wavelength tunability through well thickness adjustment^:^ lower threshold:’ reduced threshold-temperature dependen~e,9~.~~ narrower gain spectra,99and an enhanced rate of

+

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THICKNESS OF QUANTUM WELL L,

(2)

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FIG.48. The measured PL energy upshifts of five different Ga,,,,Iq,,,As/InP quantumwell samples (symbols) each having stacks of quantum wells of different thicknesses as a were calculated with different ratios of function of well thickness. The three (-) A E J A E,. The G ~ o .III,,~~AS ~, dispersion relation was taken to be of parabolic shape (Ref. 82).

change of peak gain with respect to changes in injected carrier density.lOO.'O1 a1 these unique properties have been well studied and confirmed to a large extent in GaAs/Al,Ga,-,As MQW laser^.^^-^^* On the other or InP hand, MQW lasers with Gao.471q,,3Aswells having Ab.481~,52As barriers prepared by MBE,'08-111atmospheric or low-pressure OMCVD, * L 2 ~ 1 1and 3 hydride vapor-phase epitaxy114have scarcely been studied. Their threshold current densities were typically at least two to three times larger than in DH lasers, and no improvement in threshold-temperature dependence has been reported to date except in those prepared by liquidphase epitaxy (LPE).1'5*'16 Note that in these LPE MQW lasers, GaInAsP quantum wells were employed in order to avoid melt-back problems during the growth of the InP barriers. Here, we shall show that low-threshold

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

451

G%.471%.53As/InP MQW lasers can be grown by CBE and that there is a definite improvement in the threshold-temperature dependence when compared with similarly grown G%.47I%,53As/InPDH lasers. Both DH and MQW laser wafers were grown under similar conditions. For DH wafers, the materials employed were four-layer epitaxial structures confinement , layer), - 0.1 -- 0.3 pm of 2.0 pm n-InP (- 1 X loL8~ r n - ~ undoped G%.471%,53As (- 1 X loi5 ~ m - ~ active , layer), -3.0 pm p-InP (-8 X lOI7 ~ m - ~confinement , layer) and -0.1 pm P + - G % , ~ ~ I % . ~ ~ A S ~ , layer). For MQW laser wafers, the layer structures (-5 X loi8~ m - cap were the same except the active layer was replaced with an active region comprising typically 4 - 8 G%.47IQ,,~As wells of 70 - 150 A thick separated by InP barriers of 150 A.Note that, unlike LPE, no anti-melt-back GaInAsP layer was needed above the G%.471%,53As active layer. Figure 49 shows the Auger depth profile of a DN sample. It is clear that the composition switching at the heterointerfaceswas abrupt to within the resolution of the Auger profiling limit and there was no composition transients. The dc drift in signal is due to a drifl in the ion current collecting system. For current threshold density (&) evaluation, broad-area lasers were fabricated from each wafer. The area of the diodes was 375 X 200pm2with two cleaved mirrors and two scribed sidewalls in order to avoid internal circulating modes. The current pulses were - 100 ns- 1 p s and lo3 pulses per second. Figure 50a shows the light output versus pulsed current amplitude for a typical DH laser at different heat-sink temperatures. A plot of the threshold

-

-

0

7 14 21 28 35 42 49 56 SPUTTER TIME (min ), 750%/min

63 70

FIG.49. The Auger depth profile of a DH laser wafer. The slight dc drift in signal is due to

a drift in the ion current collecting system.

452

W. T. TSANG

2' 612518 25 '

TEMP.('C)=

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1 2 PULSED CURRENT (AMP)

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HEAT-SINK TEMPERATURE ("C)

FIG.50. (a) The light output versus pulsed current amplitude for a typical Ga.4,1n,,53As/ InP DH laser at different heat-sink temperatures. (b) A plot of the threshold current versus heat-sink temperature.

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS

4

453

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FIG.51. (a) The light output versus pulsed current amplitude for a typical Ga,,,.,,Iq,53As! InP MQW laser at different heat-sink temperatures. This wafer has 8 quantum wells of 70 A separated by 150 A InP barriers. (b) A plot of the threshold current as a function of heat-sink temperature. (- - - ), replotted from Fig. 5 I b for a DH laser.

454

W. T. TSANG

current, I,, versus heat-sink temperature is given in Fig. 50b. It is seen that in the temperature range of 2 -6O"C, the threshold-temperature dependence can be exactly described by a single dependence relation, a exp(T/To), with To= 45 K. For G%.471q,,3As/InP DH laser wafers, the emission wavelengths ranged from 1.68 to 1.72 ,urn, depending on the degree of lattice matching. The best wafers have an average threshold current density Jthof 1.3 kA/cm2 at 25 "C for an active layer thickness of - 0.3 pm. We believe that this Jth is the lowest value reported thus far for G%.47I%.,,As/InP DH lasers. The differential quantum efficiency was as high as 18%per facet. Multi-quantum well laser wafers were evaluated in the same manner. Figure 51a shows the light output versus pulsed current amplitude for a typical MQW laser at different heat-sink temperatures. This laser wafer has 8 quantum wells of 70 A separated by 150 A barriers resulting in an emission of 1.47 pm.This represents an energy upshift of 100 meV due to quantum size effects, in reasonable agreement with low-temperature photoluminescence measurements on single quantum wells.82A plot of the threshold-temperaturedependence is given in Fig. 5 1b. In the temperature range of 2-80°C, a To of 80 K was obtained. The dashed curve is the threshold-temperature dependence of a G%,4,1n0.,~As/InPDH laser replotted for convenience of comparison. At 25"C, the averaged J, was as low as 1.5 kA/cm2, and the differential quantum efficiency was - 18% per facet, Again, we believe this J, to be the lowest reported for Gh,47 I%,,,As/ InP MQW lasers grown by any technique. Table I lists some of our better DH and MQW laser wafers grown during this period. It is quite evident that the T,'s for MQW lasers are in general about 1.5 -2 times higher than for DH lasers. This represent the first conclusive comparison performed.

-

-

TABLE I SOMEOF THE BETTER DH AND MQW LASERWAFERS BY CBE AND THEIR PERFORMANCE GROWN CHARACTERISTICS~ Laser type

DH DH MQW MQW MQW MQW a

No. of QWs

-

8

5 6 4

QW or active thickness(& 1000

3000 70

80 100 150

Jth (kA/cm2) 1.6

1.3

1.5 2. I 1.75 1.6

To (K)

Lasing (pm)

37

1.68

45 80

75

75 65

-

The InP barriers are 150 A in all NQW laser wafers.

1.72 1.47 1.50 1.54 1.60

7.

QUANTUM CONFINEMENT HETEROSTRUCTURE LASERS r

I

I

.

U

I

I

455

r

Ga0,471n0,53 As/InP

-t

LASERS

MOW LASER I- 4 . 2 X I ~ n

z a

m

-a* P

Irr v) z

w

+f t

3

n I-

3 0

17,160

17,250

17,330

WAVELENGTH t i )

J

14,650 14

'50

WAVELENGTH t i )

FIG.52. A comparison of the lasing spectra at room temperature under pulsed operation at

- 50 Zthfor (a) DH and (b) MQW lasers.

The Jth's are also substantially lower than for previously reported MQW lasers. Previously, by MBE growth, Tsang1Ioobtained a Jthof -2.7 ka/cm2, Temkin et al.'O* obtained -2.4 kA/cm2, Asahi et all@'obtained -3.5 kA/cm2, and Panish et al."' obtained 3.5 kA/cm2. By OM-CVD, Nelson et al.lI3reported a Jth of 7.5 kA/cm2. A comparison of the lasing spectra at room temperature under pulsed operation at 1.2 Ithis given in Fig. 52a and b for a DH and a MQW laser, respectively. Such spectra were rather characteristic for each type. As previously observed,110.'13 it is seen that the MQW laser spectrum (broadarea diode) shows a substantial reduction in spectral envelope width when compared with that of the DH laser. This narrowing of emission spectral envelope is believed to be related to the gain narrowing due to the modification of the density of state in quantum well structures.

-

-

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456

W. T. TSANG

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