Growth and properties of GaAsSb/GaAIAsSb

double heterostructure lasersC. Chaminant, J. Charil, J.-C. Bouley and E.V.K. Rao

Indexing terms: Gallium arsenide, Gallium compounds, Liquid phase epitaxial growth, p-n heterojunctions,Semiconductor junction lasers, Semiconductor growth, HI-V semiconductors

Abstract: The growth and properties of GaAs^^Sb^/Ga,. yAlyAs, _xSbx double-heterostructure (d.h.s.)lasers have been investigated. We have obtained lasers with an antimony content up tojc = 0-17 emitting atwavelengths up to 1-12 nm at room temperature for pulsed currents as low as 120 mA. The d.h.s.s were grownby liquid-phase epitaxy on GaAs substrates. The lattice mismatch problem was overcome by growing up toten ternary GaAs, -*Sbx layers of graded Sb composition. A study of quaternary GaAlAsSb layer growth hasshown the effect of initial saturation of the melt on the Al content of the resulting solid. To overcome thisproblem a special epitaxial process has been developed to grow quaternary layers with controlled aluminiumcontent. The relative amounts of Al and Sb were determined by a photoluminescence technique which alsoallowed us to evaluate the band-gap energy step between the active and confinement layers. A study of thetemperature dependence of the threshold current was also made for d.h.s having different aluminium compo-sitions. Threshold current density was found to vary as exp T/To, where To reaches 150 K for the highestaluminium content.

1 Introduction

For optical communication in the wavelength range 1 to1-6/im, the GalnAsP/InP system can be used satisfactorilyfor c.w. lasers. However, some years ago Nahory et al.1

showed that double-heterostructure (d.h.s.) c.w. lasers canalso be made from GaAsi-^Sb^/AlyGai-yAsi-^Sb^grown on a GaAs substrate. In this case the lattice mis-match between the d.h.s. and the substrate is of the orderof 10~2 and a lattice parameter adjustment must beobtained by growing several layers of GaAsi_xSbx withgraded composition. Nahory et al.2 investigated the behav-iour of such lasers with wavelengths up to 1-059/um.

Although the growth of these materials on GaAs sub-strates is more complicated than growth of GalnAsP oninP, there are several compensating advantages. The use ofGe impurity for the p-type doping gives better control ofthe p-n junction position in the double-heterostructure thanwith the commonly used zinc in the GalnAsP laser.3

Moreover, GaAs substrates are both cheaper and of betterquality than InP ones.

We report in this paper the growth and properties ofGaAsSb/GaAIAsSb d.h.s. lasers grown on GaAs substrates.We also report some problems relating to the growth ofGaAlAsSb and describe the realisation of lasers with emissionwavelength up to 1-12/im. Moreover, we have studied thethreshold current/temperature dependence with theconfinement-layer composition. A temperature coefficientTo much larger than for the quaternary GalnAsP laser canbe obtained with a suitable layer composition.

2 Double-heterostructure growth

2.1 GaAs, _ x Sbx gro wth

The growth is performed in a classical horizontal liquid-phase epitaxial apparatus at temperature ranging between750 and 800°C. The ternary GaAs^Sb* layers are

l'aper T4S2S, received 24th July and in revised form 4th September1979The authors are with the Centre National D'Etudes des Telecom-munications, 196 rue de Paris, 92220 Bagneux, France

grown on (100) oriented GaAs substrate using an equilib-rium cooling technique with a constant 0-6 deg C/mincooling rate. The liquidus temperature is precisely deter-mined by the method of direct visual observation of themelt.4 Fig. 1 shows the X-ray reflection topograph of twodifferent composition single layers. In Fig. la the Sbcontent x is 0-03 and the calculated corresponding misfitparameter is +2-4x 10~3 (+refers to a grown layer incompression). In Fig. \b the Sb composition is very low,x = 0-002, and the misfit is + 1-6 x 10~4. Each topographshows a cross-hatch network aligned along the (110)direction. According to Reference 5 this network can beattributed to misfit threading dislocations lying in thegrowth plane. The largest misfit leads to the highest dislo-cation density (Fig. la). The topograph layer in \b has amisfit comparable to a Gaj.^Al^As/GaAs system with

X- ray reflectiontopograph1 layer, 5^imx=003

A£ = 2-4x10'3

X- ray reflectiontopograph

1 layer, 3pmx =0002

A° 1-6x10^

0-1cm

196

Fig. 1 X-ray reflection topographs

SOLID-STATE AND ELECTRON DEVICES, NOVEMBER 1979, Vol. 3, No. 6

0308-6968/79/060196 + 05 $01-50/0

x = 0-2, where there is no misfit dislocation formation.The ease with which misfit dislocations are observed in theGaAsj.^Sb^/GaAs system may arise from a relatively highstrain energy relaxation induced by the large differencebetween the substitutional V-atom covalent radius (15-5%),whereas for the Gaj.^Al^As system, the Hi-atoms' radiusdifference is roughly zero.

2.2 AlyGal_ growth

For the quaternary AlyGax _ y Asj _ ^Sb* material, the growthdifficulties arise from the imprecise determination of theliquidus. The small amount of the material which crystal-lises in the melt does not allow for the use of the directvisual observation technique as in the case of the ternarymelt. To overcome this difficulty quaternary layers aregrown from a GaAlAsSb melt saturated with undoped GaAssubstrate for about 2 h just before the growth. Fig. 2 showsthe Sb against Al compositions of the quaternary layersgrown by this method at different temperatures. Ourexperimental values are compared with the theoreticalphase diagram calculated by Nahory et al.5 As predicted bytheory, we observed that for an increasing aluminiumcomposition the solid Sb composition is decreased. For730°C growth, the highest Sb composition obtained foran Al composition of.y = 0 4 is 0-15. By decreasing thetemperature, it is possible to increase x slightly for a fixedy value. However, below 715°C there is an abrupt jumpin composition towards the GaSb composition.6 Thisbehaviour is a severe limitation on the wavelength range ofGaAsSb lasers. As we attempt to increase the Sb content ofthe d.h.s. to achieve longer wavelength emission, themaximum content of Al in the confinement layers isreduced, so that the optical and electrical confinementdecreases and the room temperature threshold currentdensity increases.

Fig. 3 shows other peculiarites of the quaternary-layergrowth. The aluminium composition is very sensitive to theamount of supersaturation of the quaternary melt. Adeviation of 13°C from the thermodynamic equilibriumchanges the Al content of the solid by about a half. Thesame behaviour has also been observed by Hsieh in thequaternary GalnAsP material.7 In both these materials this

Sb.0-4-

key:

73O°C

Fig. 2 Section of the Ga1. yAlyAs1. xSbx phase diagram showingthe solidus compositions x against y

The solid lines are theoretical curves from Reference 6. Points areour experimental data

effect concerns the most minority element in the melt: Alfor AlGaAsSb and P for GalnAsP. We think this effectprobably results from a composition depression in the meltnear the solid induced by the difference between thediffusion coefficients of the various elements.

(100)

10

growth solution composition:Ga:627. Sb:37%,As: 0-67. A l : 0-397.

7. At in the solid

5 10solution supercooling A . °C

15

Fig. 3 Solid Al composition of the Gat _ yAlyAs1. xSbx materialagainst the supercooling of the melt

2.3 Laser structure growth

The specific difficulties of the quaternary layer growthnecessitate the use of a nonconventional growth process.We use a two-step process method: in the first step thegrowth of step-graded layers is carried out on the GaAssubstrate, and in the second step we make the double-heterostructure growth according to the following scheme.Two pre-seed GaAs substrates are used to saturate thequaternary melts for 2h (Fig. 4). Then, the furnace tem-perature is decreased to grow the first quaternary layer onthe ternary grading layers. This method leads to a slightsupersaturation of the second quaternary melt correspondingto the growth temperature variation of the first quaternarylayer and the active region, and thus to a slight asymmetryin the Al composition of the confinement layers.

Laser structures with three or ten step-grading layers(Fig. 5a) have been grown. Their surface morphologyobserved under a phase-contrast microscope also shows across-hatched network as in the X-ray reflection topographwithout any difference in the number of step-grading layers.However, this observation does not prove that the dis-

quaternary meltsternary melt

1

GaAs saturationsubstrates

GaAsj_x Sbx layers

Fig. 4 Liquid-phase-epitaxy scheme of the d.h.s. growth

Numbers give the position of the slider during the growth of eachlayer

SOLID-STATE AND ELECTRON DEVICES, NOVEMBER 1979, Vol. 3, No. 6 197

}—d.h.s.

„ 10 layer

phase contrast photographa

Asi_xSbx (Ge)

GaAs,.xSbx(Ge)

s.e.m. photograph and e.b.i.e. curveb (x10000)

Fig. 5 Views of the d.h.s. cleaved face

location density near the active region is the same foreach structure. Fig. 5b shows the s.e.m. image of the d.h.s.Ge is used as p-doping in the active region and confinementlayer, and the p-n junction is only located at the ternary-quaternary interface, as shown from the electron-beam-induced current (e.b.i.e.) curve.

3 Photoluminescence characterisation

Photoluminescence (p.l.) is used to estimate the antimonycontent of the ternary active layer which cannot be investi-gated easily by other methods. This technique of character-isation has also helped to estimate the uniformity of the Sbcontent in the active layers and to predict the laser emissionwavelengths.

A Kr+ laser operating at 6764 A is used as the source ofp.l. excitation. The laser beam is focused to a spot of about150/im diameter on the surface of the sample placed insidea small cryostat. The infra-red radiation from the sample isanalysed by a spectrometer and detected with a Ge photo-diode coupled to a lock-in amplifier.

3.1 Single layers

As a first step to the p.l. spectral analysis of d.h.s., we haveinvestigated the dependence of the 300 and 77 K band gapson the Sb content in undoped GaAsj.^Sb^ single layers.The Sb content x of these layers is varied in the range of0 to 0-20 as has been determined by electron microprobeanalysis. The observed dependence of the band gap at300 K on x is found to be in excellent agreement with thatpreviously published by Nahory et al.* Also, as expected,from the known band gap (Ee) values of GaAs, theband-gap difference between 77 and 300 K, Eg ={Egap 77K ~Egap 300K)> is measured as 80 ± 5 meV. P.L.measurements as described above are also performed onundoped Gaj.yAlyAsj.^Sb^. single layers in which the Aland Sb contents are only varied in a limited range. Thesecompositions, in fact, correspond to the growth conditionsof p-confinement layers in dii.s. presented in this work.

3.2 Double heterostructure

During p.l. excitation of a d.h.s., if the energy of the incidentphoton is higher than that of the band gap of the topp-confinement layer, the p.l. spectrum of the p-active layer,in addition to that of the p-confinement layer, can beobtained when one and/or all of the following three pro-cesses are operating:

(/) the generation of electron-hole (e.h.) pairs in thep-active layer by a fraction of the incident laser beamunabsorbed in the p-confinement layer

'•(if) the generation of e.h. pairs in the p-active layer by thereabsorption of p.l. emission from the p-confinement layer

(Hi) the diffusion of minority electrons from the p-confinement layer across the p-p heterointerface into thep-active layer where they can recombine radiatively.

In the case of GaAs-Gaj _xAlxAs d.h.s. we have verifiedthat mechanism (Hi) alone operates when thep-confinementlayer is thick enough to absorb completely the incidentlaser beam to minimise process (/). Our experimental resultsalways showed that process (if) is never operative, and so itcan be neglected.8

Irrespective of the type of physical process, (/) or (»),operating in the p.l. study of d.h.s. presented in this work,it is interesting to note that the characterisation of bothp-Ga^yAlyAsj-^Sba- and p-GaAsj.^Sb^. layers is madepossible by p.l. spectral analysis. An example of the p.l.spectrum of a d.h.s. measured at 77 K is shown in Fig. 6. Inthis Figure the p.l. peak located at 0-886/xm (l-398eV)belongs to the p-Gaj-yAlyAsj-^Sb* layer, whereas theone located at l-002jum (l\237eV) belongs to thep-GaAsj_xSbx layer. Each of these peaks is identified asinvolving conduction band to valence band transitions inthe corresponding layer. Consequently, from the energypositions of these peaks measured in different d.h.s. wehave estimated the Al content in p-confinement layers andthe Sb content in the p-active layers. We have also estimatedthe p-p heterointerface barrier height AE which governs thecarrier confinement in d.h.s. (see Table 1).4 Laser properties

4.1 Laser fabrication

A 50/urn wide Pt-Au stripe contact is electroplated on thep-type side of the d.h.s. previously zinc overdoped at650°C for 20min. An alloyed contact of AuGeNi is usedfor the n-type side. This simple contact procedure leads to aseries resistance of 3-5 £2 for a 300;um long laser. Aftercleaving and sawing in the usual manner, laser chips are

P-GQ As,.,Sb,

09 10wavelength, pm

Fig. 6 Photoluminescence spectrum of a GcAsSb/GaAlAsSb d.h.s.at77K

198 SOLID-STATE AND ELECTRON DEVICES, NOVEMBER 1979, Vol. 3, No. 6

Table 1: Summary of the threshold temperature dependence of theinvestigated d.h.s.

% Al

161724

AE

meV160180220

To

K4050

150

Jd

kAcrrf2

1-40-60-1

Jth

kAcm"2 jum"1

-1000*- 1 0 0 *

10

•extrapolated valuesJ^ is the calculated diffusion current over the confinement barrierAE. The calculation was made from eqn. 1 with the same value of ap-GaAIAs confinement layer, i.e.:

L = 1 Mm, D = 50cm2/Vs; Nc = 6X10 l 7cnr f 3 ;

Nv = 9 X 1018 cm"3; p = 5 X 1017 cm"3

Jth is the measured or extrapolated normalised room temperaturethreshold current density

soldered p-side down onto a 1 -5 /im-thick In layer evapor-ated on a Cu heat sink.

Some 15jum wide proton-isolation stripe lasers werealso made using a 200 keV proton beam and Shipley photo-resist masking.

4.2 Laser emission characteristics

Fig. 7 shows the pulsed light/current characteristic of a50jum x 300 jum stripe laser with x = 0-17 Sb compositiondouble heterostructure. The threshold current is 1-25 Awith an active region thickness of 0-7 jum. In order tocompare the threshold current density with the GaAlAslaser, we must first normalise Jth to an L = 500jum cavitylength with the well known relation2 / $ cc \/l withm = 2-5 and take into account the effect of current spreadingunder the contact stripe induced by the low resistivity ofthe zinc-diffused GaAlAsSb layer. As in a GaAlAs laser havingthe same geometry9 we suppose that this effect increases thethreshold by a factor of about 1 -5 and thus obtain a nor-malised Jth of 6kAcm"2 jum"1, which is greater than the5 kA cm"2 jurrT1 associated with the GaAlAs laser. For1 jum emitting GaAsSb laser, Nahory et al.4 have obtained4kA cm"2 jum"1 but at the longer wavelength 1056jumthey obtained up to 7 kA cm"2 urn'1.

The spontaneous and stimulated emission spectra ofthese lasers are shown in Fig. 8. Both spectra show peaks at1-12/im which is the highest value obtained with this lasersystem. The laser emission half width is 50 A. We have notobserved a well resolved longitudinal mode structure evenat high current excitation owing to the formation of laserfilament behaviour induced by an inhomogeneous activeregion.

We have obtained relatively low threshold current withstriped lasers. For a 15 jum stripe width, the best pulsedthreshold current value is 120mA, but, nevertheless, theexternal quantum efficiency is rather low, at 10% for thetwo faces.

4.3 Threshold temperature dependence

The threshold temperature dependence is of crucial import-ance for the room-temperature c.w. operation. The lasertemperature sensitivity can be generally described by aTo parameter calculated from the relation Jth « exp T/To

between the threshold and the temperature.10 In GaAlAs/GaAs laser, To equals 150K for JC = 0-3. In quaternaryGalnAsP laser, the temperature dependence is more sensitiveand To is of the order of 40-50 K.11 Fig. 9 shows thetemperature dependence of three GaAlAsSb/GaAsSbd.h.s.s having different aluminium contents in the confine-

ment layers. Measurements were made from liquid-nitrogentemperature up to room temperature. The normalisedthreshold current density is roughly the same at liquidnitrogen temperature for each wafer, that is 1 kA cm"2 jum"1,and it then increases with temperature with different To

values. The results are summarised in Table 1 with thealuminium composition y determined by electron micro-probe analysis of the p-confinement layer and the barrierheight AE deduced from the photoluminescence measure-ments previously described.

There are several physical processes which enter into theTo parameter determination.10 The first to be considered isthe dependence of injected carrier confinement withtemperature. This effect can be estimated from the excessdiffusion current Jd crossing the AE barrier. From Refer-ence 10,

AE(1)

where D and L denote the diffusivity and diffusionlength, respectively, of the injected electron in thep-Gax _ y Aly As} _ x Sbx layer, Nc and Nv denote the conduc-tion and valence band states density and p-confinementlayer hole concentration. Calculated room temperature Jd

values are recorded in Table 1. A comparison of/d and themeasured or extrapolated room-temperature thresholddensity Jth shows that the effect of electron confinementloss is low, and thus other mechanisms must be taken intoaccount for the To parameter. However, Table 1 shows

>clearly the involvement of the Al composition of thequaternary confinement layer. Thus, we can suppose someeffect arising from the temperature-dependence of theindex step, or from optical losses of the unconfined light,or from strain-induced changes of refractive index. Furtherwork is in progress to understand the exact mechanism. Ananalogous situation can also be encountered in GalnAsPlasers, where, although AE is greater than in the GaAsSblaser studied (AZf = 240meV for X= 1-15/im), To is verylow.11

Fig. 7 Pulsed optical power against current characteristics

The inset shows the laser geometry

SOLID-STATE AND ELECTRON DEVICES, NOVEMBER 1979, Vol. 3, No. 6 199

5 Conclusion

In summary, we have studied the growth and properties ofGaAsSb/GaAlAsSb d.h.s. grown on GaAs substrates. Themain difficulty of this system is the reduction of the latticemismatch between the d.h.s. and the GaAs substrate leadingto the growth of several intermediate GaAsj_xSbx layers(up to ten). One other difficulty concerns the aluminiumincorporation in the quaternary confinement layer. It hasbeen found that the supersaturation of the quaternary melthas a great influence on the solid composition.

A photoluminescence technique has been applied toestimate the antimony content of the ternary active layerand the band gap energy difference between thep-confinement and p-active layers.

Broad contact pulsed lasers have been obtained with anormalised threshold current density of 6 kA cm"2 /im"1,and stripe lasers with pulsed thresholds as low as 120mAhave been achieved. The longest wavelength obtained is1 -12 fim, which seems close to the upper limit which can bereached with this system. Longer wavelengths have been

spontaneous /emission /

0 1A /

JJlaseremission

7A — •

GaAs, . , Sb,' \ x=O173

\

\V 600A

j \ 112pm

50A

1-2 11wavelength, pm

Fig. 8 Emission-spectrum characteristics

20

. 8

I6•a•oo0)

5 2

10

= Joexp(T/To)

i 0 100 200temperature, °K

300

Fig. 9 Normalised threshold temperature dependence of threed.h.s.s having different Al compositions

achieved but with GaAlAsSb/GaAlSb d.h.s. grown on GaSbsubstrates.12 Compared with the quaternary GalnAsPsystem, GaAsSb lasers have a better temperature stability,but this system grown on GaAs substrates is difficult togrow and does not give the range of wavelengths that can beobtained with the GalnAsP system of materials.

6 Acknowledgments

The authors would like to thank J.-P. Noblanc for hiscontinuous encouragement and discussion throughout therealisation of this work. They would also like to thankMrs. Thibierge for p.l. measurements, C. Daguet forelectron microprobe analysis, J. Burgeat for X-ray measure-,ments, J. Semo for s.e.m. measurements, P. Ged for assist-ance in laser threshold temperature measurements, andP.N. Favennec for proton bombardment experiments.

7 References

1 NAHORY, R.E., POLLACK, M.A., BEEBE, E.D., DEWINTER,J.C., and DIXON, R.W.: 'Continuous operation of lOjixm-wave-length GaAs,. ̂ Sb^/AlyGa, _ y As, _ xSbx double-heterostructureinjection lasers at room temperature', Appl. Phys. Lett., 197'6,28, pp. 19-21

2 NAHORY, R.E., POLLACK, M.A., and ABROKWAH, J.K.:'Threshold characteristics and extended wavelength operation ofGaAs,. ̂ Sb^/AlyGa, _ y As, _ ^Sb^ double-heterostructure lasers',/. Appl. Phys., 1977, 48, pp. 3988-3990

3 COLEMAN, J.J., and NASH, F.R.: 'Zinc contamination andmisplaced p-n junctions in InP-GalnAsP d.h. lasers', Electron.Lett., 1978, 14, pp. 558-559

4 NAHORY, R.E., POLLACK, M.A., DE WINTER, J.C., andWILLIAMS, K.M.: 'Growth and properties of liquid-phaseepitaxial GaAs, ^Sb*', /. Appl. Phys., 1977, 48, pp. 1607-1614

5 KRESSEL, H., and BUTLER, J.K.: 'Semiconductor lasers andheterojunction LEDs', in 'Quantum electronics: principles andapplications' (Academic Press, 1977), pp. 318-323

6 NAHORY, R.E., POLLACK, M.A., BEEBE, E.D., DE WINTER,J.C., and ILEGEMS, M.: 'The liquid phase epitaxy ofAlyGa, _ yAs, _ .xSbj. and the importance of strain effects nearthe miscibility gap', /. Electrochem. Soc, 1978, 125, pp. 1053-1058

7 HSIEH, J.J., FINN, M.C., and ROSSI, J.A.: 'Conditions forlattice matching in the LPE growth of GalnAsP layers on InPsubstrates'. Proceedings of the 6th International Symposium onGallium Arsenide and Related Compounds', IOP Conferenceseries 33b, 1977

8 RAO, E.V.K., and DUHAMEL, N.: 'Investigation of protonbombarded (AlGa) As double-heterostructure by photolumi-nescence measurements'. Proceedings of the internationalconference on defects and radiation effects in semiconductors,IOP Conference series 26, 1978

9 LAD ANY, I.: 'The influence of stripe width on the thresholdcurrent of double-heterojunction lasers', /. Appl. Phys., 1977,48, pp. 1935-1940

10 KRESSEL, H., and BUTLER, J.K.: 'Semiconductor lasers andheterojunction LEDs', in 'Quantum electronics. Principles andApplications' (Academic Press, 1977), pp. 249-280

11 NUESE, C.J., OLSEN, G.H., ENSTROM, R.E., and APPERT,J.R.: 'Vapor-grown c.w. lasers of InGaAs/InP'. Abstract of the1978 IEEE International Semiconductor Laser Conference,San Francisco, USA

12 LAW, D.H., HARRIS, J.S., Jr., WONG, K.C., and TOMASETTA,L.R.: 'GaAlAsSb/GaSb alloys: material preparation and appli-cations to optoelectronic devices'. Proceedings of the 7th Inter-national GaAs and Related Compounds Symposium. IOP.Conference series 45, 1979

200 SOLID-STATE AND ELECTRON DEVICES, NOVEMBER 1979, Vol. 3,No. 6