STUDY OF THE GROWTH OF EPITAXIAL' LAYERS … Bound... · study of the growth of epitaxial' layers and its application to (ga,in)as composites*) by a. humbert abstract ... td=727°c

R946 Philips Res. Repts 31, .216-243, 1976

STUDY OF THE GROWTH OF EPITAXIAL' LAYERSAND ITS APPLICATION TO (Ga,In)As COMPOSITES *)

by A. HUMBERT

AbstractWe shall show that (Ga,In)As pseudo-binary composites can be pre-pared in the vapour phase by a new epitaxial synthesis method whichmakes use of the simultaneous and independent transport of indium andgallium by arsenic trichloride AsCl3 in an hydrogen atmosphere. Thecrystalline quality of the layers thus obtained is analysed and appears tobe influenced considerably by the mismatch which exists between theircrystalline parameter and that of the GaAs substrate on which they aredeposited. This dependence has been studied in detail and a method forthe progressive and controlled introduetion ofthe indium during growthhas been developed which makes it possible to optimize the crystallinequality of the deposits and therefore all their physical properties asso-ciated with this quality.

1. Introduction

The formation of pseudo-binary solid solutions, most of the physical proper-ties of which evolve monotonically with their chemical composition, makes itpossible to extend the range of application of III-V binary compounds. Elec-troluminescent devices are now made from Ga(As,P) pseudo-binary corn pounds.Likewise the (Ga,In)As solid solutions have a forbidden bandwidth which mayvary with their chemical composition and are used for devices in the ne ar-infrared region.

2. Synthesis of (Ga,ln)As pseudo-binary composites

Several methods are in use today for achievingepitaxial synthesis of (Ga,In)Aspseudo-binary compounds 1-6). They differ in the nature ofthe phase (liquid orvapour) from which growth takes place and in the nature of the respectivesources of gallium and indium (simple or compound elements, gaseous species).

The method we chose to use is based on the procedure introduced in 1964 byKnight, Effer and Evans 7) to achieve epitaxial synthesis of GaAs from thevapour phase. The elements Ga and In, arsenic trichloride AsCl3 and hydrogenare used as starting products. The degree of chemical purity with which they areavailable entitles us to expect to obtain a pure material. On the other hand, theuse of the gaseous phase offers a general flexibility in use and makes it possible,in particular, to introduce the indium progressively and regularly during thecourse of the growth.

*) This paper is a summary of A. Humbert's thesis presented to Paris VI University on13th June 1974. The author wishes to thank C. Schémali and M. T. Lesartre for theirhelp' and C. Schiller and J. Hallais for constructive discussions.

STUDY OF THE GROWTH OF EPITAXIAL LAYERS 217

2.1. Experimental technique

Investigations of the transport of gallium in the system Ga/AsCI3/H2 8) andindium in the system In/AsCI3/H2 9) show that simultaneous transport of thesetwo elements is possible under similar experimental conditions.The method used to carry out the epitaxial synthesis of (Ga,In)As pseudo-

binary composites makes use of this result: two separate sources contain,respectively, gallium and indium which are transported simultaneously, butindependently, by AsCI3•The reactions involved are

2InCI + H2 + -tAs4 = 2InAs + 2HCl,2GaCI + H2 + -tAs4 = 2GaAs + 2HCl.

The chemical composition of a deposit is, a priori, a function of all theexperimental parameters which govern the transport of the gallium, the indiumand the growth of the compound. We elected to keep constant the joint tem-perature Ts of the sources, except for certain specifications, the total rate of flowD, the temperature TD ofthe deposits, and carried out all the growths on GaAssubstrates oriented 3° off (001) towards [IÏO]. The composition x of thedeposits defined as the atomic percentage of indium substituted for gallium isthen a function solely of the following parameters:- rates of flow of carrier hydrogen dl and d2 to the gallium and indium

sources;- partial pressures of arsenic trichloride PI and P2 prescribed in the corre-

sponding circuits.

2.2. Equipment

The equipment used owes a lot to the laboratory set-ups used to carry outepitaxial synthesis of gallium arsenide 8) (fig. I), the gallium and indium sourcesof the arsenic trichloride being contained in the bubblers BA and BB, the tem-perature of which is held constant. The circuit C is used to clean the reactoror carry out cleaning "in situ" of the substrates by means of the hydrochloricacid produced by thermal reduction of the arsenic trichloride by the hydrogen.Circuit D is a doping circuit in which a low pressure of a doping element (Zn)is entrained by a hydrogen flow. Finally, a carrier flow of hydrogen is per-manently circulating in circuit E.Jn this study we shall always ensure that the growth of a programmed

deposition is preceded by the deposition of a pseudo-binary compound with avery low indium content and several microns thick during which the chemicalcompositions of the gaseous phase and the deposition will have time to sta-bilise.

2 3 4

218 A.HUMBERT

doping

H2_ FjH~~:=_=D~E~f~r2~2~2~2~;;~'~(/~2~?~?_?~'>;2G~2:~2~:2~n2;';2:;::2;:2;2?;~~?;2;2_2z2~a-=-'-=--=--=-~_~1 -*f-

I1 P22Z722222,))Z}.Z,. ,> '''''22)'' IrA B

reaction zone deposition zone

c

Hz~--thermostats

Fig. I. Diagram showing basic principle of the growth reaction vessel.

2.3. Experimental results

The influence on the chemical composition of the layers of several experimen-tal parameters which govern the growth has been established experimentally.This analysis is limited to the following parameters: rates of flow of carrierhydrogen dl and d2 to the gallium and indium sources, partial pressures ofarsenic trichloride PI and P2 prescribed in the appropriate circuits. All the otherparameters have been selected constant.

2.3.1. Influence of the molar rate of arsenic trichloride entrained tothe indium source.

Fig. 2 shows that a linear dependence has been found experimentally betweenthe chemical composition x of the deposits and the ratio a of the molar rates of

0.10

___-_-

TS=B24°CTD=742°C

x 0.30 (AsC&/Ga)=1.6·1O-5mole mn-1

i --thermodynamic model• experimental points

0.20

_a

Fig. 2. Curve showing the development of the indium content of the deposits as a function ofthe ratio a of the molar rates of arsenic trichloride added to the indium and gallium sourceswhen the rate (AsCI3/Ga) is kept constant.


arsenic trichloride to the indium and gallium sources when the rate (AsCI3/Ga)is kept constant. It can be seen that the ordinate at the origin Xo of the exper-imental straight line is not zero. Growths carried out with a zero hydrogen rateof flow to the indium source confirmed this result: the material obtained con-tains, in fact, traces of indium (ca. 0.5%) which can be detected by photo-luminescence (displacement of the acceptor peak). This residual incorporationis a result of the suction effect caused at the level of the indium source when thegallium source only is in operation.

2.3.2. Infl uence of the molar rate of the arsenic trichloride en-trained to the gallium source

Fig. 3 shows the experimental variation in the chemical composition x of the, deposits as a function of the ratio a when the molar rate (AsCI3/In) is main-tained constant. In this same figure are plotted experimental points obtained byreplacing the gallium and indium sources by GaAs and InAs solid sourceswithout changing the other experimental parameters. The major consequenceof this operation is arsenic enrichment of the vapour phase. The other partialpressures remain practically unchanged.

x 0.30

I 0.20

TS=812°CTD=727°C(AsCI3/In)=2.5'10-5mole mn-1

- thermodynamic model• In and Ga sourcestJ. InAs and GaAs

sources

0.10

2 3_0

Fig. 3. Indium content of the deposits when a is varied with (AsCI3/Ga).

4

2.3.3. Discussion of the experimental results

The linear dependence x = ka + Xo observed in fig. 2 may be comparedwith the experimental results supplied byother authors. Figure 4, for example,presents a result obtained by Enstrom et al. 6) for the growth of (Ga,In)As bythe hydride method. According to this method the gallium and indium sourcesare attacked separately by a (HCI + H2) mixture while the arsenic is suppliedindependently by the thermal decomposition of a (AsH3 + H2) mixture. Fig. 4shows that the chemical composition of the deposits is then a linear function ofthe ratio (HCI/In)/(HCl/Ga) of the molar rates of hydrochloric acid in the

220 A.HUMBERT

0.60~ according to [9JTD=727°Cx

0.20

t 0.40

O~~~~~--~~~--~~--~~o 2 4 6 8 la

-RFig. 4. Indium content of a (Ga,In)As composite obtained by the hydride method as a func-tion ofthe ratio ofthe molar rates ofhydrochloric acid added to the indium and gallium sources(according to ref. 9).

indium and gallium sources. Similarly, the synthesis of these compounds froma GaAs/lnAs/HCI system reveals a linear dependence of their chemical com-position x on the ratio (In)/(Ga) of the atomic concentrations of indium andgallium transported in the vapour phase 10). It might seem therefore that,irrespective of the method of synthesis chosen, the chemical composition x of a(Ga,In)As pseudo-binary compound obtained by epitaxial growth from thevapour phase is a linear function of the ratio (In)/(Ga) of the atomic quantitiesof indium and gallium transported. Furthermore, this law of variation hasbeen verified for different deposition temperatures (742 DCin our case, 727 DCin the case of Enstrom and Nagai). However, the different curves obtained can-not be superimposed on one another even when the growth temperature isidentical. The chemical composition of a deposit remains, therefore, a functionof this temperature and the chemical composition of the vapour phase fromwhich it is obtained. Similarly the experimental variation presented in fig. 3snows us that the linear dependence is no longer found when the ratio (In)/(Ga)is modified by a change in the atomic concentration of gallium. Thus fig. 5demonstrates a variation proportional to (Ga)-a with 0( =1= 1. In conclusion, thelinear variation x = k (In)/(Ga) observed by numerous authors expresses, infact, a linear variation x = k' (In). All the figures 2, 3 and 5 and the result ob-tained by Nagai illustrate this result. Enstrom, for his part, does not definehow he varied the ratio (HCl/ln)j(HCl/Ga) and therefore extra experimentalconfirmation is not directly possible.

The experimental results given in the above sections may be comparedwith the predictions of a theoretical model which assumes the absence of anykinetic limitation to the transport and the deposition. A quasi-chemicalmodel P) is used to describe the solid solution Ga1_xlnxAs and to obtain theactivity coefficients of GaAs and InAs. Figs 2 and 3 show that this model


1.00 (AsCI3/ln)=2.5,'D-Smole mn-1

TD= 727°Ctheoretical model

0.50x

I0.20

0.10

0.05

• experimental points

5.0

explains the experimental observations qualitatively. This result which waspreviously confirmed 10) for the system GaAs/lnAs/HCI confirms that thermo-dynamics, which does not explain the kinetic limitation of the growth rate ofthe GaAs and InAs binary compounds under the experimental conditionsselected, does make it possible, however, to predict the evolution ofthe chemicalcomposition of their solid solutions (Ga,ln)As which is an intensive parameterof the system studied *).

0.7 T.D 3.0 4.0(AsCI3/In)

a=(AsCI3/Ga)

2.3.4. Progressive and controlled introduction of indium duringgrowth

The results given in the above sections may be used to perfect a method ofprogressive and controlled introduetion of indium during the growth. To dothis it is sufficient to consider programming the molar rates (AsCI3/ln) and(AsCI3/Ga) during the deposition of the pseudo-binary compound (Ga,ln)As.These molar rates are linked to the experimental parameters PI' P2' dl and d2by the following relationships.

*) Variations between the experimental data and the model could be probably explained bysmall deviation from equilibrium at the sources.

2.0-Fig. 5. Log x = log (AsCI3/Ga) (according to fig. 3).

222 . A.HUMBERT

P2d2(AsCh/In) = _ ,

22.4

Pldl(AsCI3/Ga) =_. ,

22.4

(dl and d2 are measured at ambient temperature).

It is therefore possible a priori to use anyone of these parameters to programmethe chemical composition of a deposit.

From the practical point of view, however, these parameters are not equiv-alent. The discussion in the above section invites us, for example, to payparticular attention to the influence of the rates (AsCI3/In) and (AsCI3/Ga)on the chemical composition of the deposits. In fact, for a given growthtemperature x varies proportional to (AsCI3/In)1 while one observes a depend-ence of (AsCI3/Ga)-ex, oe=ft 1, which is generally more rapid. For a given rate ofincorporating the indium the use of this second factor therefore requires smallervariation of the chemical composition of the gaseous phase, thus making iteasy to maintain thermodynamic equilibrium at the sources.

On the other hand, an accidental fluctuation in (AsCI3/Ga) will result in arelatively larger fluctuation in x. It is advisable therefore to keep a close checkon this factor. It should be noted, however, that the calibration curves of thechemical composition of a layer as a function of the molar rates (AsCI3/In) and(AsCI3/Ga), such as those given in figs 2 and 3 which we have just discussed,have been drawn up point by point, i.e. by carrying out growths without anyarbitrary modification of the experimental parameters during the deposition.The progressive introduetion of the indium requires precisely this' variation.For each type of programming of the vapour phase, therefore, it is necessary toascertain the effect produced at the level of the deposit.

We studied a certain number of possible programmes for the vapour phase.Variation of the rates dl, d2, dl and d2, as well as the partial pressure Ps- Thedesired aim was to introduce indium linearly up to a content of x = 0.25 duringgrowth. Different values for the rate of introduetion were also considered. Thisstudy enabled us to demonstrate that it is more difficult to introduce indiumlinearly when rates dl, d2 or dl and d2 were used to programme the vapourphase. On the other hand, the choice of the partial pressure Plof arsenictrichloride transmitted to the gallium source as a programming parameter gaveexcellent results. The use of P2 has not been considered. Fig. 6 gives an examplecalibration curve of the chemical composition x of the deposits as a function ofthe reciprocal I/tl of the temperature of the arsenic trichloride, the saturationvapour pressure of which is Ps- The other growth conditions are as follows.


x

t 0.10

TS=1085KTD= 1000K(AsCt3/In) =2.2 'IO-Smote mn-I

d, = 75cm3 mn-I

0.01 L- __ !.,___--' __ --' __ -" __ ---'

3.3 3.5 3 3.7_ .J.Q/<-1

t,

Fig.6. Calibration curve for the indium content x of the deposits as a function of the reciprocalI/tl of the temperature of the arsenic trichloride added to the gallium source.

TD = 727 oeP2 = 10-2 atm,d2 = 3.0 llhdl = 4.51/h

The linear variation observed results from the relationships

log x = -ct log (AsCI3/Ga) + constant (fig. 5)

constantlog p, = constant-----

which givect

log x = + - + constant.tI

Fig. 7 gives examples of indium composition profiles determined by electronmicroprobe for programmed layers obtained by this method and one can notethe constant rate of introduetion of the indium *). In the rest of this investiga-tion this type of programming only will be used because control of the rate ofintroduetion of indium appears to be an essential parameter for the crystallinequality of the finallayers.

3. Study of the crystalline quality

Different methods of analysis are used to observe the structure defects presentin the epitaxiallayers: X-ray reflection topography, chemical etching, cathode-luminescent scanning-electron microscopy.

*) Measurements carried out by M. Klerk (philips Research Laboratories, Eindhoven, TheNetherlands).

224 A.HUMBERT

Izone of I

0.15 :progressive iIin troductionl 10pmx I of I

rindium I

I(0.8%In urn")

I0.10 I

IIIIIII

0.05II

-,Fig. 7. Indium composition profiles obtained by using different rates of programming thepartial pressure Pl during growth.

3.1. Methods of study

3.1.7. X-ray reflection topography

The set-up used enabled X-ray reflection topography to be carried outaccording to the Berg-Barrett technique 12.13).In order to knowat what depth the defects revealed by the topography are

situated it is necessary to estimate the depth of penetration of the incidentradiation. Numerous authors have tackled this problem 14-17). The differenceswhich are found with regard to their conclusions arise from the differentapproximations introduced with regard to the attenuation ofthe incident beam.In fact, if the secondary extinction phenomenon, which only occurs in the caseof mosaic-structure crystals, is disregarded, two effects of very different originmay lead to the attenuation ofthe incident beam: the primary extinction whichcharacterizes the perfect crystal and the classical photoelectric absorption whichis involved whatever the type of structure being analysed (mosaic or perfect). Inour case the epitaxiallayers examined are curved and under considerable strainand this makes it possible to eliminate the primary extinction phenomenon. Anestimate of the depth of penetration, therefore, will be obtained by consideringthe photoelectric absorption alone. For an epitaxiallayer Gao.8sIno.1sAs anda Cu KCl:1 radiation, table I gives the thickness for which the reflected intensityis only a tenth of the incident intensity for the different reflections which can beused in practice to carry out topography of a (001) face: (422), (440), (511),(531). In reality, this thickness is greater than the depth at which the contrastceases to be detectable; but the relationship which links them is not known.


TABLE I

Depth of penetratien of the reflection X-rays

reflection t (l/e) (urn) t (1/e2 ~ 0.1) (um)

115 6.0 12.0117 9.2 18.4224 1.8 3.6404 1.5 3.0026 7.5 15.0135 5.1 10.2

3.1.2. Cathode-luminescent scanning-electron microscopy

Here again it is important to know the depth at which the defects revealed aresituated. To do this, it is necessary to estimate the thickness of material at whichthe radiation recombinations take place as a function of the different param-eters which characterize the injection (acceleration voltage) and the materialbeing analysed (atomic number Z, density e, diffusion length L of the minoritycarriers created, recombination velocity of these carriers at the surface). Thedepth of material ionised by the primary electrons may be estimated fromKlein's model !") which gives the differential energy loss of the incident beamas a function of the depth of penetration. For an acceleration voltage of 20 kV,for example, it is of the order of 1 fl-min Gao.8sIno.1sAs. However, it is alsonecessary to allow for the possibility of diffusion of the electron-hole pairscreated.The corresponding diffusion equation can be successfully solved by assuming

that the phenomenon is reduced to a single dimension (depth C in the material).This quite restrictive approximation is justified to the extent that the maximuminjection depth CD approximates to the diffusion length of minority carriers(::;;;;1 lJ.m for p '" 1019cm-2) for the voltages used (::;;;;20 kV) and the surfacerecombination velocity of these carriers is high (107 cm s-1) 19).Table 11givesthe results obtained.

TABLE 11

Depth of material from which cathodoluminescence emission occurs

Eo (keV) L (urn) S/Dn (cnr-') t (urn)

15 0.5 lOs 1.520 0.5 lOs 1.8

TABLEIII

Characteristics of the samples retained for study

structuregradient of

indium contentdensityof

referenceofthe

introduetionin the

inclined

of sample of indium dislocationsdeposit (% In (.Lm-l)

deposit (%) (cm=")

116 6.4 (.LmGaAsjl5.4 (.LmF.L. direct deposit 9.3 4· lOs

117 3.7 (.LmGaAsj17.5 (.LmF.L. direct deposit 8.5 5.5' lOs

127 16.4 (.LmF.L. direct deposit 8.7 6· lOs

128 4.6 (.LmGaAs/6.4 (.LmF.L. direct deposit 4.3 2· io-numerous

175 GaAs/16.4 (.LmF.L. direct deposit 12.5 non-orienteddefects

126 Gao.963Ino.o37Asj7.4 grad1jF.L. 0.53 7.6 1.4 . lOs

125 4.6 urn GaAs/4.6 (.LmGao.972Ino.o2sAs/ {grad1 0.53 7 . 10""gradl/grad2/13.8 (.LmF.L. grad, 0.48

12

138 Gaj Asj grad dgrad2jF. L. {gradl 0.2012.5

grad; 0.45

140GaAsjGao.97Ino.o3Asjgradljgrad2jF.L. {grad1 0.25

9,7grad; 0.40

149 GaAs/Gao.97Ino.o3As/gradl/F.L. grad, 0.70 14.5

F.L.: finallayer with constant composition.grad: zone ofprogressive introduetion ofindium.

1:30\

?>a~t1l

el

STUDY OF THE GROWTH OF EPlTAXIAL LAYERS 227

3.1.3. Chemical etching

The etching agent used in this study was developed in 1965 by Abrahams andBuiocchi 20) to demonstrate the emergence of dislocations on all the simpleindex planes in GaAs. lts composition by weight is as follows: 2 ml H20,8 mg AgN03, 1 g Cr03 and 1 ml HF.

This study will show the type of correlation that can be found between theoptical images of chemically etched (Ga,In)As samples and the correspondingcathode-luminescence images.

3.2. Experimental results

Table III presents all the characteristic samples retained for investigating theinfluence of growth conditions - in particular the rate of incorporation of

ol G=300 b) G=300 E=25kV

Fig. 8. Observation of the growth surface. Cathode-luminescence image. Ca) Direct depositionwith high indium content. Cb) Direct deposition with low indium content. Cc) Programmeddeposition.

228 A. HUMBERT

indium - on the crystalline quality of the deposits.The first group of samples gives examples of direct deposits with low indium

content (x < 0.10 %). In contrast, sample no. 175 represents a direct deposit ofa composite with very high indium content (x = 0.15).

The second group of samples represents the programmed deposits.Finally, the last group represents deposits for which two rates of introduetion

of the indium have been used.

3.2.1. Study of the growth surface

Two types of morphology may be observed, the cathode-luminescence imagesof which are presented in figs 8a and 8b.

Fig. 8a shows a homogeneous distribution of disordered and high-densitydefects. This appearance is characteristic of a direct deposition with a highindium content (x > 0.10).

c) d)

a) b)

e)

STUDY OF THE GROWTH OF EPITAXIAL LAYERS 229--------------------------------------------------------

Fig. 9. Observation of the growth surface of a programmed deposit. X-ray reflection topog-raphy.

i)

g)

In fig. 8b, on the other hand, the defects are oriented in the [110] and [lIO]directions and a cross-hatched pattern occurs on the growth surface. In addi-tion, one can observe the presence of a certain number of non-luminescentpoints. This appearance is characteristic of a programmed deposit (fig. 8c) or adirect deposit with a low indium content (x < 0.10).

The cross-hatched pattern is generally visible to the unaided eye. It isassociated, therefore, with a certain relief of the growth surface. In order todefine its nature, a detailed investigation of the growth surface was undertakenby X-ray topography.

Different hkl reflections have been used with a view to studying the corre-sponding evolution of the contrast. Fig. 9 shows the result of such a study fora programmed sample into which the indium was introduced at a rate of 0.45 %

230 A.HUMBERT

In (.Lm-1. The high density of defects and the limited number of usable reflec-tions 12.15) makes it difficult to interpret them. A few remarks, however, can bemade.- The defects which are propagated along [ITO] are generally rectilinear and

of high contrast (except when the depth of penetration of the incident radia-tion limits the contrast 15)). On the other hand, those which are propagatedalong [110] are more diffuse and slightly disorientated relative to this direc-tion. These distinctions no longer occur for the growth face correspondingto the exact plane (001).

- The contrast is not uniform along a line and this suggests that it is, in fact,the image of a group of dislocations.

- Finally, the complete extinction of a line never occurs in practice when it isparallel to [ITO]. Burger's vectors of these dislocations are, therefore, ofdifferent orientation. On the other hand, the dislocations which are prop-agated along [110] seem to be mostly of the edge-shaped type.

The density of non-luminescent points is generally greater for the directdeposits with low indium content than for the programmed deposits(l:1x/I:1C < 1% In (.Lm-l). Furthermore, it appears to be a rising function oftherate of introduetion of indium in the transition zone.

3.2.2. Study of (110) cleavages

The investigation of these surfaces at right angles to the deposition surfacemakes it possible to analyse the different stages of the growth. Parallel investiga-tions are made by optical microscopy after chemical etching and by cathode-luminescence analysis. The very high density of defects makes it impossible toanalyse by X-ray reflection topography.

Fig. lOa, for example, shows us the different growth stages of a programmeddeposit (l:1x/I:1C = 0.70% In (.Lm-1). This picture has been obtained afterchemical etching of a (110) cleavage surface. The prolonged attack and thedifferent type of doping between the substrate (n R:! 1018 cm=") and the deposit(p R:! 1019 cm=") do not allow the Ga.As/substrate interface to be revealed veryclearly compared with what is normally observed for shorter durations ofattack (fig. 11). In the extreme left of the picture, however, one can make out aseries of attack pits similar to those observed by Abrahams and Buiocchi for thep-n junctions in GaAs 20). Further chemical etching of the same sample madeit possible to confirm that the position of these attack pits is at the substrate-deposit interface. The second dark line represents the GaAs-Gao.97Ino.03Asinterface as can be confirmed by the indium introduetion profile determined bythe electron microprobe (fig. 7).

The zone of progressive introduetion of indium is revealed in the form of adarker band in which, however, can be distinguished a series of parallellayersand the appearance of linear V-shaped defects which are not situated in the


0.14- X 0.03

'-G~;~;;Ï;;;;-isX =0.14-

Gao.97 InQQ3As

GaAs

aJ G=372 bJ G=1600

cJ G=1600

Fig. 10. Observation of a cleavage face (I10) of a programmed deposit. Comparison of imagesobtained by optical microscopy (a) and (c) and by cathode-luminescence scanning-electronmicroscopy (b).

G=543

Fig. I I. Observation of a cleavage face (110) of a programmed deposit. Demonstration of achange in the rate of introducing the indium by means of differential contrast optical mieros-copy.

I~-- ----- -

\ ----_------

\

232 A.HUMBERT

growth plane. The stratification occurs again in the constant-composition zone.Fig. lOb shows the cathode-luminescence image ofthe samecleavage. Thereis

a clear correlation with the optical image obtained after chemical etching(:fig. lOa). The reduction in luminescence of the constant-composition zone isassociated with the reduction in the spectral response of the photodetector andmust not be attributed to a degradation of the material. Fig. lOc gives theoptical image of a part of the transition zone after prolonged chemical attackand can be compared directly with fig. lOb. Fig. 11 represents a programmeddeposit for which two rates of introducing indium have been used:0.53% In fJ.m-\ then 0.48% In fJ.m-1. This change is clearly revealed bychemical etching without, however, there being a clearly marked interface be-tween the two areas.

3.2.3. Observa ti o ns made inside the epitaxial layer

In order to analyse the inside of an epitaxial layer the sample is bevel-cut(fig. 12) and then subjected to suitable mechanical and chemical polishing inorder to obtain an undisturbed surface, as is confirmed by examination undera scanning-electron microscope (secondary and backscattered electrons). Fig. 13shows such an example of analysis. It confirms the different growth stagesobserved in fig. 11 which gives the image of a cleavage surface of the samesample. The non-parallelism of the interfaces which appears in the cathode-luminescence image (fig. 13a) is associated with inhomogeneous polishing. Itshows that the angle a of the bevelcan vary in that region which corresponds tothe lower half of the picture. Fig. 13b shows the picture obtained by X-rayreflection topography. Interpretation is more tricky here because of the greaterdepth of penetration of the X-rays (~ 3.5 fJ.mfor the (224) Cu Ka 1 radiation)and their high sensitivity to very slight deformations in the lattice. One can note,however, the existence of a reciprocal relationship between the cathode-luminescence images and the X-ray topography pictures of the upper half of the

-- - - - -- --~--------Id - Gat_X Inx As

Xo ?!: X?!:O.03 IC_:::§E.-fI9_f[n.§.§)~....,.:>:-I------·_- jb.::!ia_~______ ia:= :u__b:~~t~ L- .__ J

(l

Fig. 12. Diagram showing how to make a bevei. (a) Side view, (b) Front view,


substrate. It confirms that it is possible to use cathode-I uminescence analysis forthe demonstration of dislocations in gallium arsenide and other III-V com-pounds 21.22). The dark band observed in the upper right-hand part of fig. 13bis interpreted as being the image of the substrate-GaAs interface through the

Fig. 13. Observation of a bevel obtained by chemical polishing of a programmed deposit.Ca) Cathode-luminescence image. Cb) Xvray reflection topography.

234 A.HUMBERT

epitaxial GaAs layer. The depth of penetration and the relatively low angle ofincidence of the Cu Kal radiation in the Bragg position for the (224) planes ofthe substrate make it possible, in fact, to present an extended image of thisinterface (fig. 13b). On the other hand, this image is no longer found for thelower half of fig. ] 3b where the substrate is outside reflection. For this regionof the sample, however, the Cu Kcx2 incident radiation is in a reflection positionrelative to the pseudo-binary composite Gao.97Ino.o3As and we find thecharacteristic image of the defects associated with the introduetion of theindium. As for the image of the zone of progressive introduetion of the indiumthis never occurs due to the very fact of its variable crystalline parameter.Fig. 14 is an enlargement of zone A marked in fig. 13. One can see (fig. 14a)

alFig. 14. Enlargement of the zone marked A in fig. 13.

b)

the different distribution of the defects in the GaAs and Gao.971no.o3As layers.In the first layer they are oriented in parallel with the [110] and [iTO] directionsand their cathode-luminescence image is an orthogonallattice. In the case of thesecond layer, however, a disorientation of the defects which are propagatedalong [110] can be seen. The same effect is observed in fig. 15 which gives the

Fig. IS. Observation of the transition zone.


image of the transition zone of the same sample. The use of different rates ofintroducing the indium has also made it possible to demonstrate the influenceof this parameter on the density of the defects observed in the transition zoneof a programmed deposit. Figs 16a and 16b show the reduction of this density

bl G.=55

Fig. 16. Observation of the zone at the end of the progressive introduetion of indium.

with the stabilisation of the crystalline parameter at the end of growth of thetransition zone. Comparison of these two figures reveals the influence of thedepth of material analysed on the apparent density of defects observed. Finally,fig. 17 gives the image of the constant-composition layer of the sample whichhas been analysed so far. As can be seen, the density of the defects here is muchlower.

al

Fig. 17. Observation of the constant-composition layer.

b}

236 A.HUMBERT

3.3. Interpretation and discussion

3.3.1. Theoretical model

Numerous investigations have been devoted to the problem of how thecrystalline parameter adapts itself between an epitaxial layer and its substrate.Frank and van der Merwe 23) in particular, and then Kuhlmann-Wilsdorf 24)

established in theory that it was favourable from the point of view of energybalance for the deposit to be elastically deformed for the crystalline parameterdisagreement to be reduced or eliminated. If, however, this deformation isinsufficient, the excess strain must be relieved by the formation of interfacialdislocations: misfit dislocations. These predictions have now been confirmedexperimentally a number of times 20.25). In 1969Abraham, Weisberg, Buiocchiand Blanc 26) were concerned more particularly with the problem of the param-eter adaptation in pseudo-binary compounds of variable chemical com-positions. According to the model the interfacial dislocations induced by theinitial substrate-layer lattice mismatch are segmented. Because of this, they giverise to inclined dislocations which are propagated outside the growth planealong (211) directions 27). When the lattice mismatch is small (less than 2%relative value) the misfit dislocations are oriented along the [110] directions ofthe growth plane (100). They are of an edge-shaped nature (Burger's vectortype lj2 (110) and situated in the growth plane) or mixed (Burger's vector ofsame type but orientated at 45° to the growth plane). The systems which con-firm Végard's law have a density nA, defined by the number of emergence pointsper unit surface of a cleavage perpendicular to the growth plane, which is pro-portional to the rate of introduetion of the substitution element, i.e. :

where }, is the inter-lattice spacing of the {110} planes of the same nature, ÀAand ÀB are the values of this distance for the binary compounds A and B ofwhich the solid solution A.B. is being studied and /).xj/).C is the gradient ofintroduetion of the substitution element.

At the beginning of the growth (C < nA- t), the misfit-dislocation density isinsufficient and new segments, i.e. new inclined dislocations, are created as thetransition zone develops (fig. l8a). Once this density is obtained, however, andwhile the rate of introduetion of the substitution element remains constant,there will be no further creation of new inclined dislocations. A misfit segmentis then produced by the curvature of an inclined dislocation along a (110)direction of the growth plane followed by a new curvature outside this plane(fig. l8b). .


misfit segment

a)

y (001)

t~J

: !

inclined distocaticns

b)

Fig. 18. Formation and propagation of inclined dislocations in the transition zone of a pro-grammed deposit. (a) ~ < /IA -t, (b) ~ > /IA - t.

In the constant-composition zone the absence of any crystalline parametervariation means the disappearance of the misfit segments and only the inclineddislocations with a density nI (nI = the number of points of emergence per unitarea of the growth surface) equal to that which exists in the transition zone willcontinue to be propagated:

(a)

where m is the mean length of a trapping segment.

3.3.2. Discussion of the experimental results

The theoretical model of Abrahams et al. 20) makes it possible to interpret acertain number ofthe experimental results previously obtained for the (Ga,In)Aslayers. Thus the cross-hatched pattern, which is systematically observed in thetransition zone of a programmed deposit, may be interpreted as being the imageof all the trapping dislocations contained in the volume of material covered bythe method of analysis (cathode-luminescence or optical microscopy afterchemical etching). In fact, if it is impossible to resolve these dislocations, theaverage orientation of the texture observed, their density and the qualitativeevolution of the latter with the rate of introduetion of the indium are thosepredicted by the model. Similarly, the "Y"s which appear on the cathode-luminescence images of the cleavage surfaces in the transition zone may beinterpreted as being the image of inclined dislocations. Fig. lOb confirms, infact, that they continue into the constant-composition zone and that the averageorientation of their branches, relative to the (001) plane, is at an angle close tothat which this plane makes with the {Ill} planes at right angles to the cleavage.Finally, the non-luminescent points observed at the growth surface may beinterpreted as the images of the points of emergence of the inclined dislocations,

238 A.HUMBERT

The examination of a bevel obtained by chemical polishing of a direct depositwith low indium content enables these interpretations to be confirmed. In fact,keeping these interpretations in mind, the morphologies observed (fig. 19) must

aJ

G=lOOO E=15keV

G=100· E=15keV

G= 1000 E=15keV

b)Fig. 19. Observation of a bevel obtained by chemical polishing of a direct deposit with lowindium content (x = 0.10). (a) Demonstration of an effect of "natural" programming. (b)Demonstration of the non-controlled nature of the introduetion of indium.


be considered to be the result of a progressive, albeit uncontrolled, introduetionof indium at the beginning of growth. This "natural" programming is demon-strated in fact by the electron microprobe analysis of the appropriate cleavages.The mean rate of introduetion of the indium which is observed in this case is arising function of the final indium content of the deposit. But it is always higherthan those which were used for the arbitrarily programmed deposits

(~; < 1% In ~m- 1 )-

This explains why a higher density of non-luminescent points is always observedat the surface of a direct deposit with low indium content (fig. 20). In fact, thedensity n) of the inclined dislocations is proportional to this rate (relationship (a)).Finally, this high value of the natural rate of incorporation of the indiumexplains why it was possible experimentally to obtain the incorporation profilespredicted by the calibration curves which give the composition x of the depositsas a function of the experimental parameters governing the vapour phase.

On the other hand, the disorientation effect of the defects propagated along[110] is more difficult to interpret. Itwill be remembered that this disorientationoccurs only when the growth surface is not the exact (001) plane and that itaffects only the defects associated with the introduetion of the indium. In orderto define precisely the part played by the orientation of the growth surface,

G=300 E=20keV

Fig. 20. Observation of the growth surface of a direct deposit. Demonstration of the highdensity of the inclined dislocations.

G=300 E=20keV

G=200 E=20keV

240 -------------------------------------------------------A.HUMBERT

depositions were made on substrates whose orientation was opposite to thatwhich had been used hitherto. There is now a displacement of 90° in the averagedirection relative to that in which the defects are disorientated. This disorien-tation is therefore insensitive to the orientation of the "cut-out" steps butappears rather to be associated with the polarity of the growth surface. Further-more, it no longer appears on the growth face parallel to the non-polar exactplane (001) (figs 9 and 21). The part played by the indium can be definedprecisely by comparing the results which have just been described with thosewhich were observed for Ga(As,P) deposits on a GaAs substrate 28). Fig. 22summarises the results obtained for the two systems according to the orientationof the substrate. Comparison of figures ex and y shows that this anisotropy effectis general and that the same distribution of defects is found when the growthsurface has a polarity of the same nature (Ga or As) as the substituted element.

Another experimental fact must be considered alongside these results. Ingeneral, an asymmetrical bending is obtained for the programmed samples(substrate-deposit), the axis with the greater curvature always being parallel tothe direction along which the defects are not disorientated (fig. 22). This pheno-menon cannot be explained by the difference between the expansion coefficientsof the substrate and its deposit, because, on the one hand, there would be noreason for it to be asymmetrical and, on the other hand, there would be anopposite curvature of the samples.

Fig. 21. Observation of the growth plane (OOI). Demonstration of the absence ofdisorientationof the defects.


{J

system disorientation of polariiy ot disorientation curvature ofsubstrate surface of defects samples

(Ga,ln)As 3° about itiol Ga j 17f1 D[1ioJ

tB(Ga,In)As 3° about [110j As D[1rO]

t£JGa (As,P) 3° about [110j As L_J[710]

y

Fig. 22. All the experimental observations which make it possible to relatea) the mean disorientation direction of the defects to the polarity of the growth surface and

to the nature of the element substituted;b) this average direction to the orientation of the axis of greater curvature.

The experiment therefore makes it possible to demonstrate the existence of anorientation relationship between the curvature anisotropy of a programmedsample and the distribution anisotropy ofthe misfit dislocations in the transitionzone.Abrahams, Wei sberg and Tietjen 27) theoretically established a quantitative

relationship between the curvature of the constant-composition layer of aprogrammed deposit, separated from its base, and the density nI of the inclineddislocations present in the transition zone

I- =k f nI,r

where k is a constant function ofthe orientation of the inclined dislocations andfis a geometrical distribution factor of these dislocations between the directions(112), introduced so as to allow only for the dislocations correctly oriented tocontribute to a curvature of given axis (fig. 23).

(001)

r (~11) (110)

, ! inclined dislocation~'

Fig. 23. Schematic representation of the curvature effect associated with the propagation ofan inclined dislocation in the constant-composition layer of a programmed deposit.

242 A.HUMBERT

To establish this relationship these authors assumed that no slip phenomenonoccurs during the growth of the constant-composition layer, i.e. its thicknessremains lower than the critical thickness at which the strains become prohibitiveand induce slip. An experimental examination of the variation in the radius ofcurvature of Ga(As,P) epitaxia1layers as a function of the rate of introduetionof phosphorus (i.e. nI) in the transition zone confirms all the theoretical pre-dictions and the latter hypothesis in particular. Separating the constant-com-position layer from its base does not therefore affect the density and the distri-bution (J= 1/2) ofthe inclined dislocations The anisotropic curvature observedin the case of our samples (deposit + substrate) can therefore be considered tobe the result of distribution anisotropy of the inclined dislocations between thedifferent (211) directions. As, moreover, the parameter mismatch in the transi-tion zones ofthe programmed deposits is closely associated with the propagationof the inclined dislocations, it is not surprising that there is also an anisotropicdistribution of the misfit segments between the (110) directions of the growthplane.

Unfortunately it has been impossible to obtain direct and meaningful exper-imental confirmation ofthis propagation anisotropy ofthe inclined dislocationsbecause of the limits to the resolution obtainable in our methods of analysis.The use of transmission microscopy might perhaps help to overcome theselimitations. The volume of material which can be analysed by this method,however, is very small and thus the observation of a propagation anisotropy,which may be present on a very large scale, is made somewhat difficult.

Laboratoires d' Electronique etde Physique appliquée

Limeil-Brévannes, May 1976

REFERENCES1) G. A. Antypas, J. Electrochem. Soc. 117, 1393,1970.2) K. Takahashi, T. Moriizumi and S. Shirose, J. Electrochem. Soc. 118, 1639, 1971.3) H. T. Minden, J. Electrochem. Soc. 112, 300, 1965.4) R. W. Conrad, P. L. Hoyt and D. D. Martin, J. Electrochem. Soc. 114,164,1967.S) H. M. Manasevit and W. I. Simps on, J. Electrochem. Soc. 120, 135, 1973.6) R. E. Enstrom, D. Richman, M. S. Abrahams, J. R. App er t, D. G. Fischer,

A. H. Sommer and B. F. Williams, Proc. 3th Int. Symp. Gallium Arsenide, Aachen,1970, p. 30.

7) J. R. Knight, D. Effer and P. R. Evans, Solid-State Electronics 8,178,1965.8) J. Hallais, LEP report.9) A. Boucher and L. Hollan, Onde Electr. 50, 165, 1970.

10) H. Nagai, T. Shibata and H. Okamoto, Jap. J. appI. Phys. 10,1337,1971.11) G. B. Stringfellow and P. E. Greene, J. Phys. Chem. Solids 30,1779,1969.12) J. B. Newkirk, Trans. AIME 215,483, 1959.13) W. H. Zachariasen, Theory of X-ray diffraction in crystals, John WHey,New York,

1945.14) E. Fabre and C. Schiller, Acta electronica 15, 55, 1972.1S) G. BIet, Revue technique THOMSON-CSF 4, 195, 1972.


16) J. K. Howard and P. J. Smith, IBM J. Res. Develop. 123, 1971.17) M. Halliwell, J. B. Childs and S. O'hara, 4th Int; Symp. on GaAs, Boulder, 1972,

J. appl. Cryst. 5, 401, 1972.18) C:A. Klein, Appl. Optics5, 1922,1966;Technical MemorandumT-651,Raytheon Com-

pany, 1965.19) D. B. Wittry and D. F. Kyser, J. Phys, Soc. Japan 21, suppl. 1966.20) M. S. Abrahams and C. J. Buiocchi, J. appl. Phys. 37, 5, 1966.21) D. W. Shawand P. R. Thornton, J. Mat. Sci. 3,507,1968.22) C. Schiller and G. Poiblaud, 4th Int. Symp. on GaAs, Boulder, 1972.23) F. C. Frank and J. H. van der Merwe, Proceedings of the Royal Society of London

SeriesA 198, 205, 1969.24) W. A. Jesser and D. Kuhlmann- Wilsdorf, Phys, Stat. sol, 19, 95, 1967.25) E. R. Thompson and K. R. Lawless, Appl. Phys. Lett, 10, 138, 1966.26) M. S. Abrahams, L. R. Weisberg, C. J. Buiocchi and J. Blanc, J. Mat. Sci. 4,223,

1969.27) M. S. Abrahams, L. R. Weisberg and J. J. Tiet jen, J. appl. Phys, 40, 3754.1969.28) C. Schiller, to be published.

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STUDY OF THE GROWTH OF EPITAXIAL' LAYERS … Bound... · study of the growth of epitaxial' layers and its application to (ga,in)as composites*) by a. humbert abstract ... td=727°c