Experimental and theoretical study of low pressure GaAs VPE in the chloride system

332

Journal of Crystal Growth 56 (1982) 332 ~343 North-I-Iolland Publishing Company

EXPERIMENTAL AND THEORETICAL STUDY OF LOW PRESSURE GaAs VPE IN THE CHLORIDE SYSTEM J.L. GENTNER

*

Lahoratoires d ‘Electronique et de Physique AppliquCe, 3 Ar’enue Descartes, F-94450 Lirneil—BrCrannes, France

C. BERNARD Laboratoire c/c Thermodynarnique et Physico-Giinue Metallurgiques, CNRS, LWSEEG, BP 44, F-38401 St. Martin d ‘HCres, France

and R. CADORET Lahoratoire de cristallographie et de Physique des Milieux condenses, UER Sciences, Les Gézeaux, BP 45, F-631 70 Auhi/res, France

An experimental study of the chemical vapor transport of GaAs in a low pressure reactor was carried out using the AsCI

3/ GaAs/H2 method. The thermodynamic equilibrium in the Ga/As/H/Cl system was calculated by minimization of the Gibbs free energy as a function of the process parameters: temperature, total pressure, and AsCI3/H2 input ratio. The growth rate depcndances on those parameters were examined. Particularly, the total pressure dependence of the growth rate shows the relative importance of surface kinetics and mass transfer through the boundary layer in the growth rate limitation. In the pressure range of 5 mbar to 1 bar, the growth rate decreases with decreasing total pressure; nevertheless growth rates of up to 12 nm/h were achieved at 100 inbar.

I. Introduction The growth of GaAs by the arsenic trichloride method [1,21 is a well established technique. The process using an open flow reactor working at atmospheric pressure has been extensively studied (see for example ref. [29] for a review) and a large number of analysis and theoretical models have been proposed [3—5]. However, due to the increasing complexity of the device technology requirements, the process has to be optimized. For example, several authors [6,11] have achieved, with particular care, good thickness and doping homogeneities. Moreover, as demonstrated by the studies on silicon [7—9] and gallium arsenide metal organic vapor phase epitaxy [10], a low pressure growth technique presents some particu*

J.L. Gentner is a post graduate student from the University of Clermont-Ferrand, working at LIP for his PhD Thesis.

0022-0248/82/0000—0000/$02 .75 © 1982 Nortli.Holland

lar advantages. In comparison to atmospheric pro. cesses, low pressure VPE pernuts easier control of both thickness and doping homogeneity [7], of doping transitions [10], and also a reduction of the autodoping phenomenon [8—10]. Its application to

the chloride process should lead to new possibilities of this technique without loosing its specific advantage, i.e., the high purity. On a more fundamental point of view, a low pressure study will also provide worthwhile information for a more precise understanding of the growth rate limiting process and of the kinetic mechanisms. The first important stage of a vapor phase epitaxial study is a thermodynamic analysis. To reach the goals of such a study, a precise knowledge of the thermochemical data is required. Although the process has been used for more than 15 years, it is not yet well known from the thermodynamical point of view. The results presented in this work are based on a

333

J.L. Gentner eta!. /Low pressure GaAs VPE in chloride system

coherent but arbitrary set of values. The composition of the gas phase at equilibrium with a GaAs solid phase is calculated for various experimental conditions and the transport of a gas phase between source and deposit is simulated. The experimental results are then analysed and compared to the expectation of the theoretical model proposed by Cadoret [13]. Tl1e analysis as a function of the total pressure emphasizes the role of the mass transfer in the growth rate limitation, which will yield valuable information for hornogeneity studies.

The growth experiments have been carried out in the reactor shown in fig. I, using the arsenic trichloride method with a GaAs source to give a better reproducibility. A conventional reactor tube made of quartz was adapted to work under reduced pressure by connecting a rotary oil pump. To prevent oil pollution by the reaction products and oil back diffusion, a cold trap and a molecular sieve are placed between the reaction tube and the pump. A Bourdon

Case A. The source and deposition chambers are separated by a capillary (3) (a 6 mm inner diameter quartz tube plugged by a rod), the small leak controlling the flow in the reactor. The pressure is fixed over the GaAs source by the input pressure (usually, about atmospheric pressure) and over the substrate by the pressure regulation valve (2) limiting the pumping speed. Case B. The two chambers are directly connected (the quartz rod is removed) and the pressure is fixed by the pressure regulation valve (2). The input flow rate being, in that case, fixed by the flow regulation valve (1). Both processes were used and the results are analysed in section 4. In the first case, the velocity of the gas stream flowing over the source is slower than in the second case (where it increases as 1/Pat constant input flow rate measured at STP) but the pressure drop between source and deposit reduces the supersaturation (see discussion in section 3). The experimental ranges for the process parameters are: (i) 850 to 1050 K for the substrate temperature (I’d); the source temperature (T5) being 1100 K.

type manometer, protected by a PTFE membrane,is used for pressure control. The special design of the reaction chamber allows the control of both the source and deposition pressures. In practice, the growth system can be operated in two different ways:

(ii) 5 X 10~to 10_i for the input parameter represented by the PASC13/~H2 partial pressure ratio. The two pressures are controlled separately: by the bubbler temperature for AsCI3 and by the input pressure for H2.

2. Experimental

Pressure~~Gauge

Source

b1___

Deposit

________

__

___

1 In

TsTd

~

AsCI 3

A H2

2

exhaust

0

N2

trap He

N2 Iiq

pump

Schematic diagram of VPE growth system: (1) Flow regulation valve. (2) Pressure regulation valve. regulated by (1) and piessure by (2); closed: P~= 1, flow regulated by a small leak, ~d adjusted with (2). Fig. 1.

(3) Open: ~

-~‘d~ now

334

fL. Gentner eta!.

/ Low pressure

(iii) 5 X 1O~to 1 atm for the total pressure. Two dif-

ferent manometers are used to obtain an accuracy of about 1% over the whole pressure range. The growth experiments were, in all cases caned out on (001) GaAs semi-insulating substrates (grown by the LEC method at LEP) 6°disoriented towards (110), in a 40 mm inner diameter synthetic quartz tube with a total flow rate of about 5 1/h measured at STP (room temperature and atmospheric pressure). 3. Thermodynamic analysis A thermodynamic analysis is a necessary stage for understanding the process. To be carried out as completely as possible and without constraints, a very general description of the system must be chosen. In

GaAs VPE in chloride system

the Ga/As/H/Cl system, 23 gaseous species (see table 1) are expected to exist and the solution of such a multi-component problem can easily be found with the help of a computer program by minimizing the Gibbs free energy. This method was reported by several authors [14,15~jand applied in many different cases [16,171. Using this approach, it is not necessary to write a set of chemical reactions appropriate to describe the system. Species whose presence is doubtful or which are expected to be present in small quantities can easily be included for completeness. 3.1.

State ofthe art

Many thermodynamic calculations in the Ga/As! H/Cl system are available in the literature [18—251.

‘fable I Values of All?

298 (kcal/mole) SGTF a)

GaAs (solid) GaCl3 (solid) Ga (liquid) GaAs Cl Cl2 UC1 1-1 112 AsH AsH2 Ash3 As As2 As3 As4 AsC1 AsCI2 AsCl3 Ga GaCI GaC12 GaCI3 Ga2CI2 Ga2CI4 Ga2CI6

—19.5

Fengusson andGabor

1191

1181

—20.7

28.989 0 —2 2.063 52.103 0 40.4 15.9 68.5 45.58 62.481 36.7 25

—64.8 65.0 —19.54 —62.0 1331 —108.4

—237.8

Dune and Muilin

—18.5 —125

Shaw 1211

Battat et al.

Klima at al.

1221

1231

—19.8

—20.96

—21.7

1.3

1.33 84.5 ±2.9 29.082

29.082 —22.062

—22.0

—22.06

15.88

17.7

48.45

48.00

34.5

34.4

—61.8

—71.5

—19.1

—16.2

—107

b)

a) SGTE = Scientific Group Therniodata Europe. b) Our own estimation.

±0.7

34.50

—22.1

±0.1

—22.062 52.095

15.88 64.4 ±1.6 48.45 ±0.7 34.5

65.0 —19.1

—19.2

—108.4

—102.2

±1

±1.0 ±3

.

36.64 ±0.7 28±3 —15 ±4 —64 ±2 66.2 —20 ±5 [331 —62 ±6 [331 —102 ±4 [331

335

J.L. Gentner at al. /Low pressure GaAs VPE in chloride system

However, these were all performed for H2 or H2 mixed with an inert gas systems at atmospheric pressure. These theoretical treatments are often related to experimental measurements using different methods such as open flow crystal growth [18j,”entrainment” method [21,26], and closed system manometry [23]. But the rather scattered experimental results and accordingly unreliable thermochemical data suggest, as pointed out by Faktor et al. [26] and Hollan et al. [271, that a better analysis of the growth experiments could be given with a more complete and precise knowledge of the gas phase composition. The equilibrium in the Ga/As/H/Cl system is in fact rather complicated and has frequently been simplified to a single heterogeneous reaction between the major species: GaAs+HCl’~sGaC1+3As4+-~-H2

,

taking in some cases account of the reactions: As4 ~‘ 2 As2 3 GaCI+~As4 2 GaAs + GaC13 .

Although the thermodynamic properties of the system have been studied for a long time [28,291, recent spectroscopic studies [3O,44~ have unambiguously shown that, apart from GaC1 and GaCl3, the dimers Ga2CI4 and Ga2CI6 are present in this system as well. Two other molecules, GaC12 and Ga2Cl2, are expected to exist [26,31,32] and their thermochemical properties were estimated [33,341. But they have never been observed by any spectroscopic means at high temperature, perhaps because of particular sampling problems. 3.2. Thermodynamic data In table 1, the values used by several authors in their calculations are presented. (We present only the z~JI~298 values as an example of scattered data; the complete set of data can be found in the original papers.) The largest errors in the calculations are expected to arise from uncertainties in the thermochemical data for GaAs (solid phase), AsC13, and the gallium chlorides. Presently, no conclusion can be given in favor of one particular set of data. But a compilation work of the available thermochemical data, was recently completed [12].

‘Fable 2 Chemical species and data used in the thermodynamic calculation LET 1000 (cal/mole ‘ K) Condensed phases GaAs Ga As Gaseous phases Ga Cl

AH?298 (cal/mole)

Ref.

—21.197 —16.128 —11.667

—19.500 0 0

SGTE SGTE SGTE

—43.577 42.186

—65.000 28.989

SGTE SGTE

Cl2 HC1 H H2

—57.624 —48.218 —29.916 —34 .757

0 —22.063 52.103 0

SGTE SGTE SGTE SGTE

AsH2

—58.552 —44.136 —61.939 —81.493

40.400 15.900 68.500 45.580 62.481

SGTE SGTE SGTE SGTE SGTE

—8 7.974 —87.609 —61.843 —78.900 —89.056 —128.413

36.725 —64.800 —19.540 —62.000 —108.400 —237.800

SGTE SGTE SGTE 33 SGTE (a)

AsH3 As As2 As3

As4 AsCl3 GaCl GaCI2 GaCl3 Ga2Cl6

—59.043 .

a) Our own estimation.

The results are based on the set of values given in table 2. We took for the gallium dichloride the thermochemical data given by Shaulov and Mosin [33J because their values for GaCl and GaC13 were coherent with those we had chosen. The thermochemical functions for Ga2CI6 were estimated [12], all the other values being those chosen by the Scientific Group Thermodata Europe (SGTE). The results of this study may be modified by a further improvement of the thermochemical data, but we only expect a change in the constituents and the partial pressures of the species which are of minor importance. It gives a correct view of the system evolution as a function of the process parameters.

4. Results and discussion The inputs for the calculation are temperature, total pressure, and the total number of moles of each

336

fL. Gentner et al.

/ Low pressure GaAs

VPE in chloride system

element added to the system. The results are given in

GoAs/AsC~

terms of molar fractions, rather than gaseous species partial pressures, the total pressure being a parameter of the system.

As C13/H2

—

_______

4.1. Equilibrium composition

3fH2 = ]Q_2

P

=

1 atm

P

=

01 atm

2

In figs. 2, 3 and 4, the equilibrium gas phase composition is plotted as a function of the process parameters (temperature, total pressure, and input ratio AsC13/H2). The equilibrium of a gas phase, whose composition is defined by the AsCl3/H2 ratio, flowing over a GaAs solid source is considered. The results obtamed at 0.1 atm are compared with those obtained at 1 atni in figs. 3 and 4. The effects of the pressure drop on the gas phase composition are the following: (i) A simplification of the gas phase due to the decrease of GaCl2. As expected, the major species are

H2

—

GoCI ——

~

~

..—

0

As4

As~

6” —~

~.

——

H~t

~

h 8. E o

—

GoCI

GoC~ A~

-2

U —

GaC1~

.0

0.

1=1100K

2

~

2

AsH~

ASCIS/H2=10

_______________

-

H2

1.

—

hS3

—

—

~

—

~

AsH

God

900 0

—

-4

1000

1100

IIIIII:~~_.~zz__ii_I__i

Temperature (K) Fig. 3. Equilibria gas phase composition versus temperature.

GaCI, As (ii) An increase 2, As4, of 1-ICI, theand gallium ~ at chlorination, low pressure.obviously related with the decrease of FICI, GaCl2 and GaCI3, and the increase of GaC1, leads to an increase of the Asa

Ga/Cl ratio which reaches about unity for the conditions of fig. 2 at a total pressure of l0_2 atm. (iii) An increase of As2 with respect to As4, the As2 partial pressure being greater than that of As4 under 10_i atm. Thus, when the total pressure drops, the hetero-2

-1

0

log Pressure (atm) Fig. 2. Equilibria gas phase composition versus total pressure.

geneous equilibrium is shifted towards the vaporization of GaCl and As species. Only considering the reactions between the species of major importance, the equilibrium constants written as functions

of

the

J.L. Gentner at ci.

/ Low pressure GaAs

GaAs /As C1

3/H2

T

1100 K 1 atm P~ 0,1 atm

_______

2—.-—

—____ —~..

H2

— ~

VPE in chloride system

337

The plots as a function of temperature (fig. 3) and AsCl3/H2 input ratio (fig. 4) point out that the importance of GaC12 and GaC13 is maximum at low ternperature, high AsC13/H2 and high pressure conditions where these species are of major importance. In the standard conditions (T = 1100 K, P = 1 atm, AsCl3/ H2 = i0~to 10_2) our results are in good agreement cies (GaCI, As4, As2, HC1, H2) are concerned. 4.2. Chemical transport Typical transport simulation results are presented in figs. 5 and 6. Considering an (AsCl3, H2) gas mix-

~

with previously reported as far theinput majorratio speture flowing over a GaAs boat, the equilibrium gas phasethose composition, determined by asthe AsCI3/H2, the source temperature (T5), and pressure

GaC~/ /

~CI2 0

-~

~

~S3

GsAs/ AsCI3/ H2 Ts1100K Id = 90C’ K

-

2 As As

-4 _______

~

AsH a

P = 1 atm 2 AsCI3/H2=10

Ha

-3

2

-2

-0.1

——

log AsCI 3/H2

(input)

314, Kp(7) = ~ with species Fig. AsCl3/H2. 4. F) Equilibria moleKp(T~ fractions gas (x1 =composition n1/n = P1/P) versus are given input by: ratio (l~GaAs+HCl~GaCl+~ KX(T, P~phase As4+~H2, (2) As with 1, 4 2 As2 Kx(T,P)Kp(T)P Kp(T) P~ 2/P~ =

-

In these two cases the equilibrium constants increase with decreasing total pressure.

/

~,2 a

a 01

~

/

-5

HCI As~

AsHa GoOa Ma GoCla

0.01

I

2

-1

0

tog Pressure ( atm)

Fig. 5. Deposition yield and equilibria gas phase composition versus pressure over the substrate. Case 1.

338

.J.L. Gentner et a!.

/ Loss’ pressure

evaluated as 113]:

GaAsfAsCt3/H2

l

Is = 1100 K Id= 1000l~ AsCl3/ 1-42 PS

2

=

GaAs VPL’ in chloride system

io-2

÷ ‘)‘

17GaCI(PAs ‘~1 2(I5l~IC1)_1/Kp(Td), /4 4) (Ilt2)h/

where the partial pressures are those in the deposition zone. Taking the total pressure as a parameter, the supersaturation is given by:

Pd 1

Ha

K~(T 5, F5) -,

1.

GaO ~

or

—— -

o.oi

~

As,

0

~

I_I

-0.1

~‘ — —

j

.! us ~a

‘-2

o

C13

-2

-1 log Pressure (atm)

0

Fig. 6, Deposition yield and gas phase composition versus total pressure. Case 2.

~

/1~s\

~Kp(Td)~Pd)

Two cases are then possible: (1) If P~= ~d, y is only a function of the temperature difference ~T = T5 T~between the source and the aeposat —

-

(2) Ifsupersaturation P5 > ~d, Pd, ‘y decreases with thenegative. pressure ence ~sP=P~ and fact, the is may morebecome complicated to Indiffer. define because the gas phase composition changes with the total pressure. The fraction of Ga species which can be thermodynamically deposited is determined on the basis of our calculated results. The deposition efficiency p is the thermodynamic yield of the transport. and is given by: —-

p

=

(number of GaAs moles deposited)

X (number of “Ga” species moles leaving the sourcei’

(F5) is calculated. Subsequently, the concentrations of the gaseous species at equilibrium are taken as input values to be used in the calculation of the deposition equilibria, determined by the temperature (7’d) and pressure llPd) over the substrate. This coinposition at deposition conditions is plotted as a (‘tinetion of the total pressure over the substrate. Two different cases, corresponding to both the different ways the reactor can be operated (see section 2) are exammcd. Assuming equilibrium conditions and no horno.geneous reaction occurring in the gas phase between source and deposition zones, the supersaturation (‘y) which is the driving force of the reaction can be

flGa~(EflGaCli)

with i = 1, 2, 3 (dimers being negligible). It is a measure of the departure from equilibrium and is directly linked to the supersaturation. For a system working with the source chamber at 1 atm (case of fig, 5), the supersaturation created by the temperature drop ~T = T5 Td between source and deposit decreases with decreasing deposition pressure and falls rapidly to zero at about 2 X 10’2 atm. The reaction yield is obviously limited, and etching may occur. If the reactor is operated with the chainbers at the same pressure (case of fig. 6), the deposition efficiency still decreases due to the change of the gas phase composition, but no limitation is observed. —

fL. Gentner at a!.

/ Low pressure

5. Dependence of the growth rate upon experimental

GaAs VPE in chloride system

339

with AsCl3/H2. This effect is due to the supersaturation increase. (2) For AsCI3/H2 ratios ranging between iO~ and 10-2 the growth rate decreases with AsCl3/H2. In

parameters A good deal of analysis and models were published [3—51 for the atmospheric process. The model proposed by Cadoret [13], for example, was parametrized with the experimental values of Shaw [35]. This model only takes the effect of surface kinetics into account, but two rate limiting processes should be in competition: surface kinetics and mass transfer limitation by diffusion through a boundary layer

that case the saturation of the surface by adsorbed

In fig. 7 are plotted our results at low pressure and the theoretical curves deduced from the expression of the growth rate given by Cadoret [13]. The general

GaCl complexes leads to a decrease of the vacant sites coverage and to an inhibition of the growth. (3) The increase of the growth rate at high AsCl3/H2 ratios reported by 1-lollan et al. [39,40] at atmospheric pressure is also observed at low pressure. As previously pointed out [39], a transition from an adsorbed GaCl monolayer to a GaCl—GaC1 double layer, equivalent to an adsorbed GaCl2 monolayer on Ga, would increase the number of vacant sites by decreasing the number of GaCI adsorbed molecules:

behavior of the growth rate as a function of the input

two adsorbed GaCl molecules giving an adsorbed

parameter (AsCl3/H2) presents three different regimes: (1) At low AsCl3/H2 ratio, the growth rate Vincreases

GaCI—GaC1 molecule. Such a transition, similar to a roughening transition [42], could explain the increase of the growth rate at high AsCl3/H2 ratios. Further-

[36,37].

log V(pm/h) 2

A

-

TsllOOK TdIO2OK

.-

—

—...

Experimental

-

Theoretical

/

1

0.

~

1.0 atm

/i~ ~

0.2

I —1 -4

I

I

I

i

-3

-2

-1

0

log

(AsCIa/Hal

Fig. 7. Growth rate versus input ratio AsC13/H2 for different pressures (theoretical and experimental curves). At atmospheric pressure: (a) ref. [38]; (~)ref. [39]. At psessures (in mbar) of: (o) 200, (.) 100, (o) 50 and (tx) 25, all from this work.

340

J.L. Gentoer eta!.

/ Low pressure GaAs

more, some abnormal points (for AsCl3/H2

7 X

YPE in chloride system

2

lO’2 at 100 and 50 mbar show that an instability of

the surface takes place at high GaC1 coverage. A change in the adsorbed layer structure would change the lateral interaction energy and could lead to a stepped adsorption isotherm [421 for the GaC12 adsorbed layer. A better understanding of this mechanism, which may be related with a change in the gas phase composition, would need a more precise knowledge of the gaseous species partial pressure in those

ranges where GaCl2 and GaC13 are of major importance. Remembering that theoretical curves plotted in fig. 7 are deduced from the expression of the growth rate given by Cadoret [13] and that the kinetic parameters were fitted with the experimental results of Shaw [35], the shift between experimental and theoretical plots must obviously be related with a mass transfer phenomena; indeed the effect of diffusion vanishes with the pressure drop So, the kinetic parameters of the model have to be recalculated from the experimental results at low pressure to make the interpretation easier (this work will be published elsewhere~ Nevertheless, a brief analysis of the mass transfer can be given. Considering the boundary layer model described by Bloem [43], we may write that under steady conditions both the and fluxes, of adsorbedstate reactants that incorporate J~ofJ~ reactants diffusing towards the surface are equal. The general expression for the mass flux is then given by:

~

a .~‘

“.

pb =

~v

=

._

=

peq —

,

RT(O1D + l/KD) where 0 is the boundary layer thickness,D the diffusion coefficient, Kj) the surface reaction coefficient, ph the partial pressure in the bulk of the gas phase, and F~the equilibrium partial pressure. The flux and thus the growth rate are given by the driving force ~pb P~) divided by the sum of the resistances over the reaction path. In a reactor working under reduced pressure,2the boundary layer thick[43] and thus, increases ness isdecreasing proportional to P’’ with pressure. Thus, two cases are possible: —

*

Shaw’s results Iaave been obtained at atmospheric pressure on (001) substrates disoriented by 3°only.

0 ________________________________________________ 10 Reciprocal

Temperature

11

(io~iT)

Fig. 8. (001) Growth rate versus reciprocal temperature, after Shaw (351.

(i) As long as the boundary layer has not12 reached the and ö/D reactor wall, its thickness increases as P is proportional to P’1~. (ii) When the boundary layer reaches the reactor wall (radius ~),~ = 0 and olD is proportional toP. In both cases when the pressure drops, O/D becomes smaller and so does the resistance to diffusion. The results at low pressure demonstrate that the naass transfer through the boundary layer should not be neglected. It leads to a decrease of the growth rate not only at low AsCl 3/H2 ratios but also in the region of decreasing growth rate at higher AsCI3/H2 ratios, described as the kinetically limited region. Nevertheless, it appears that in both regions, the dependences of the growth rate on AsCI3/H2 and l/T(fIg. 8) are in good agreement withofthe kinetic model. fact, toa separate the effect mass transfer and Inkinetics more complicated study is necessary, i.e. we would have to take into account the limitation by all the gaseous species. Since the incorporation of GaCI is the rate limiting step of the surface reaction,we might only take GaCl into account and write the incorpo.

J.L. Gentner eta!. / Low pressure GaAs VEE in chloride system

ration flux as follows: —

~GaC1

~G~cI

341

5 Is =1093 K

.

Td=893K

the supersaturation — Db is given by: YGaCI GaCI Thus, by GaCI/’ substitution:

T

4

/neq

Ps=lbar Pdl5mbar

—

nb

GaCI -

E3

7 ~

rGaCl RT(0/D+1/KD)

(5

Finally, at low supersaturation, the flux is given by: “-

I 1

:

a2

nb rGaCI’Y GaCI

~

RT(0/D+l/K~)’

The mass transfer coefficient 0/D being nearly independent of the temperature, the mass flux towards the interface is proportional to the supersaturation y, its dependence on temperature being the same as for the surface kinetics [41] at low GaCl

/

1

/ / 0

I

102

15

2

101

AsCI3/H2 Ts~,1100K

Fig. 2

10. Growth rate versus input ratio AsC1

3/H2.

AsCI3/H210 Ps=1 bar

coverage. We should then conclude that the increase of the growth rate with 1/Tand AsC1 3/H2 ratio is due to an increase of the supersaturation and not to a mass transfer limitation.

Pd =0.1 bar

10 .

Both the rate limiting processes are competing, but

the study of the temperature dependence of the growth rate gives an argument in favor of a kinetically

limited process. However, mass transfer, even if it is not rate limiting, leads to a decrease of the growth .C

rate. Finally, in figs. 9, 10 and 11, experiments realized

5

a

with the chambers at two different pressures are presented (the source chamber was held at I atm in all cases). These results are more difficult to analyse because, as presented in section 3, the supersaturation depends strongly on the pressure drop ~.P and to compensate for this effect we had to work under high temperature difference ~T = T5 Td conditions, hence at a low deposition temperature. In all cases, the mechanism involved corresponds to the third kinetic region of increasing growth rate at high —

0 850

I

I

900 Deposition

I

950 Temperature Id )K)

Fig. 9. Growth rate versus deposition temperature.

—

1000

342

J.L. Geotner at al.

/ Low pressure GaAs

and AsCl3/H2 as input parameters. While the results of the previous studies, taking only the major species into account, are confirmed in the classical experimental conditions, we found GaCI2 and GaCl3 to be important at low temperatures and high AsCI3/U2 ratios. The main effects of the pressure reduction are an increase of the gallium chlorination to produce GaCI and also an increase of As2 with respect to As4. The results of the transport simulations point out that the supersaturation decreases with decreasing deposition pressure even if source and deposit are at the same pressure. The growth experiments at low pressure demonstrate that the deposition is feasible with a rather important growth rate. Some peculiarities of the growth rate behavior at low pressure have been discussed; the major one being the study of the relative importance of surface kinetics and mass transfer as rate limiting processes. These results provide worthwhile information permitting a better process optimization.

1G Is

1100 K

Td : 900 K AsCI3J H2 :102 Ps :1 atm

E -

(5

a U

• 0

I

0

50

VPE in chloride system

I

100

Deposition

150

200

Acknowledgments

Pressure ( mbar)

Fig. 11. Growth rate versus deposition pressure.

AsCl3/H2 ratios. Fig. 9 shows the behavior of the growth rate as a function of the deposition temperature: by decreasing Td, keeping all the other parameters constant, the supersaturation and the surface coverage increase. The rising part in fig. 10 is due to

the same effect, the increase of the supersaturation and the surface coverage, but at a higher AsCI3/H2 ratios the growth rate decreases. The variation of the growth rate as a function of the deposition pressure. is . plotted in fig. 11; in agreement with surface reaction kinetic, the growth rate increases linearly with the pressure. It should be noticed that under that con-

ditions, growth rates of up to 12 i.tm/h at a deposition pressure of 100 mbar have been measured.

6. Summary The thermodynainical study which was carried out gives a more complete description of the GaAs/AsC13/ H2 system over a wide range of temperature, pressure,

The authors wish to express their appreciation to Drs. J.P. Hallais, F. Hottier and G. Laurence for many helpful discussions and for carefully reviewing the manuscript.

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