Mathematical and numerical models of CdTe deposition in a pre-cracking metalorganic chemical vapour deposition reactor

CRYSTAL GROWT H

Journal of Crystal Growth 133 (1993) 230—240 North-Holland

Mathematical and numerical models of CdTe deposition in a pre-cracking metalorganic chemical vapour deposition reactor T.J. Davis b

a

T. McAllister

a

v.

Maslen

a

s.w.

Wilkins

a

M. Faith

b

and P. Leech

b

CSIRO Division of Materials Science and Technology Private Bag 33, Rosebank MDC, Clayton, Victoria 3169, Australia Telecom Australia Research Laboratories, 770 Blackburn Road, Clayton, Victoria 3168, Australia

Received 5 April 1993; manuscript received in final form 14 June 1993

A mathematical model is used to calculate the deposition profile of CdTe in a horizontal pre-cracker metalorganic chemical vapour deposition (MOCVD) reactor. The model is solved numerically in two dimensions, yielding the temperature profile in the reactor and the concentrations of chemical species. The calculated deposition profiles are compared with growths of CdTe on glass. With appropriate approximations, the model is reduced to a simple form which is solved analytically. This model has enabled us to identify the cause of the non-uniformity in the deposition profile as a variation in the rate of supply of metal vapour. By optimizing the reactor temperature and the gas flow rates, high-uniformity CdTe films have been grown on GaAs substrates.

1. Introduction

reactor [4], it allows the growth of HgTe and CdTe films at temperatures much lower than the

High-quality films of Hg1 ~Cd~Te have been grown on GaAs with the MRS Quantax 226 MOCVD reactor [1] using the low temperature cell furniture designed by Czerniak and Robinson [2,3]. This reactor is designed to pre-crack the metalorganic compounds by decomposition in a heated region above the growth surface. While there has been some criticism of this type of

temperatures observed for the significant thermal decomposition of the metalorganic compounds [3,5]. Such low temperature growth produces sharp interfaces in superlattice structures and little inter-diffusion at junctions. The reactor has been used by Pain et a!. [1] to grow HgTe—CdTe superlattices. They found, however, that both the composition and thickness of the superlattice var-

y1~ ~x Metalorganic vapour port

Upper thermocouple~

Graphite

/ Susceptor /~~////////////////////////////////////~ :~j~—

___________ -—~——--—--—-~--—---———-—-‘

Hg ~

Vapour exit port block

Substrate

Molybdenum plate

Fig. 1. Sketch of the Quantax 226 low temperature MOCVD reactor and the directions of the coordinates used in the simulations. 0022-0248/93/$06.00 © 1993

—

Elsevier Science Publishers B.V. All rights reserved

T.J. Davis et cii.

/ Models of CdTe deposition in pre-cracking MOCVD reactor

led considerably with distance in the flow direction in the reactor, although there was a high degree of uniformity transverse to the flow, Recent efforts in the growth of Hg1_~Cd1Te films have been directed towards infared focal plane arrays. These arrays typically require compositional and thickness uniformity to better 2. than The 1% variation overstudy areasisgreater than the 1 cmfactors purpose of this to identify which control the uniformity so that the reactor operation may be modified to achieve high growth uniformity using dimethylcadmium (Me 2Cd) and diisopropyltellurium (iPr2Te).

2. MOCVD reactor A schematic of the MOCVD reactor is shown in fig. 1. Gaseous metalorganic compounds and mercury vapour are carried by a stream of hydrogen into a heated chamber of rectangular cross section. The upper section of the chamber is separated from the lower section by about 10 mm. and contains a graphite block heated by a radio-frequency induction furnace. The two thermocouples monitor the temperature in the cell. The thermocouple connected to the graphite susceptor forms a part of a feedback system to maintain the graphite at a constant temperature. The metalorganic gases decompose in the hotter regions of the reactor, liberating metal atoms which diffuse to a substrate placed on an alumina block where growth of HgTe or CdTe occurs. Tests of deposition and compositional uniformity in this reactor show that there is a high degreeuniformity of uniformity across flow direction but poor in the flowthe direction. The production and transport mechanisms in the reactor are therefore modelled in two dimensions rather than three, which enables fast computation to be carried out on a personal computer.

3. Numerical model The deposition process is governed by the hydrodynamics of the gas flow, the thermodynamics and the chemical reactions. To simplify the model,

231

we assume that the flow is independent of the temperature and the chemical composition and that the heat of reaction is negligible. The numerical model then consists of two parts. The first calculates the temperature profile in the reactor, which is then used in the second to calculate the steady-state concentrations fluxes of one metalorganic species and theand metal it produces on pyrolysis. The model uses X—Y coordinates with the orientation shown in fig. 1. 3.1. Simple flow model Unlike conventional horizontal reactors which suffer from thermal-induced instability [6,7], the Quantax reactor is inherently more stable because the temperature increases with height in the reaction chamber. Furthermore, the reactor is designed to promote laminar flow in the reaction cell under normal operating conditions [8]. To simplify the numerical model, the flow is assumed to be independent of temperature and density gradients and to have a parabolic profile, /

v ( y)

(1 (1) 1 ~ 1 / where Vm is the mean velocity of the gas in the cell and 1 is the distance between the lower and upper boundaries of the reaction chamber. =

6 Vm

—

—

—

~,

3.2. Temperature profile The temperature profile is calculated in two dimensions using 2T 1I~T ~T a + ~8T = K v( ~v) + (2) —

~

where T is the temperature, C~,the heat capacity per unit volume, K the thermal diffusivity, v the flow velocity and Wq the rate of heat production per unit volume. Here we have assumed that both the thermal diffusivity and the heat capacity are independent of variations in gas composition and temperature and that the flow is described by (1). To model a particular process, the thermal boundary conditions are determined by the ternperatures measured from the two thermocouples

TJ. Davis et al.

232

/ Models of CdTe deposition in pre-cracking MOCVD reactor

(fig. 1). The temperature of the upper thermocouple controls the heat supplied to the reaction chamber by the RF induction furnace, which heats the electrical conductors in the chamber to a depth characterized by the power skin depth 6e [9], (3) elecwhere f is the generator frequency, trical conductivity and ~ the magnetic permeability. In the model, thermal energy is deposited in =

1/i/4lTfO•e/.L,

0~e’the

the upper surface of the graphite susceptor and the lower surface of the molybdenum plate to a depth ~e in each conductor. The energy is deposited at a rate which maintains the region about the upper thermocouple at a constant ternperature. Heat is lost from the reaction chamber by radiative cooling from the outer surfaces and by heat conduction to the surroundings. The rate of radiative cooling is estimated using the Stefan— Boltzmann law for the rate of loss of energy per unit area from a black-body [101, 4 ~ (4) P u(T —

model, we assume that the metalorganic and metal vapours are transported only by diffusion and the flow (1) and that there are no interactions between them other than a production of metal vapour and a depletion of metalorganic vapour by chemical decomposition. The particle flux is calculated from an equation for a two-component system consisting of the hydrogen carrier gas and either the metal or the metalorganic vapour. For a gaseous system of two components with molecular masses m 1 and m2 with a total number density n and total density p, the molecular 2m flux of species 1 is given by [11] n 2 V In T J1 DV(n1/n) _DT +n1v, (5) m1 where D is the Fickian diffusion coefficient, DT is the thermal diffusion coefficient, T is the ternperature and v is the velocity of the centre of mass of the system. The thermal diffusion coefficient is related to the Fickian diffusion coefficient by a thermal diffusion factor a and the equation =

DT

—

n~m1m2 x

=

1x2aD,

=

(6)

p

where T~is the ambient temperature 2’ and K4. Stefan’s In this constant is a- 5.67 X 108 W/m approximation, no account is taken of the emissivity of the radiating surfaces or their geometry. The upper surface of the graphite susceptor and =

the lower surface of the molybdenum plate lose heat by radiation to the laboratory and there is radiative heat transfer between the inner surface of the graphite susceptor and the alumina block below it. Heat loss by thermal conduction is difficult to calculate because the thermal gradients are not known, so thermal conduction from the graphite was neglected. In the model, the heat loss from the lower boundary was adjusted to maintain the molybdenum plate at the temperature measured from the lower thermocouple in the reactor. 3.3. Deposition profile The aim is to calculate the flux of metal atoms onto the surface of the alumina block where the substrate is placed. To maintain simplicity in the

where x 1 n1/n and x2 n2/n are the mole fractions of each molecular species. We consider an approximation in which the concentrations of the metalorganic compounds and the metal atoms are very much less than the =

=

concentration of the hydrogen carrier gas. Then n2 >> n1 so that n n2 is approximately constant, and n2m2 >> n1m1 so that p n2m2, and (5) becomes J1 —D(Vn1 + an1V ln T) + n1v. (7) —

=

Using velocity profile (1) and the flux equation (7)~the concentration of species 1 is given in two dimensions by an1 ~ / 8n1 a ln T —DI + an1 —

—

~,

a ay

ax / an1

+—D~—+an1

\ay ax

ax

)

a ln T ay (8)

T.J. Davis et aL

/ Models of CdTe deposition in pre-cracking MOCVD reactor

where both the diffusion constant and the thermal diffusion parameter are functions of temperature and, therefore, functions of position, and w is the production rate due to the pyrolysis of the metalorganic compound. To solve (8), we require values for the production rate, the diffusion coefficient and the thermal diffusion factor as functions of temperature for both the metalorganic compound and the metal it yields on pyrolysis. The calculation of these parameters is discussed below. 3.3.1 Metal production rates The metal vapours (Cd(g) and Te2(g)) are produced by the pyrolysis of Me 2Cd and iPr2Te. The rate of change of the concentration of the metal vapour given by nm or the metalorganic vapour ~mo is dflm,mo(t)/dt

=

~m,mo(T)

=

±k(T)nmo(t), (9)

where the plus sign is taken for the metal and k(T) is the reaction rate which depends on ternperature. This rate factor is usually fitted by an Arrhenius form [12]

k(T) =A exp( —E/RT),

(10)

where A is a rate parameter, R is the gas constant and E is the activation energy in Joules per mole. The homogeneous pyrolysis of metalorganic compounds in hydrogen gas follows a free-radical reaction scheme [13]. For example, in a mixture of Me2Cd and Et2Te (diethyltelluride), the rate of decornposition of Et2Te depends production of radicals in the Me on the rate of 2Cd decomposilion. The presence of a metal surface, such as Te(s) or Cd(s), also affects the reaction rate by a heterogeneous mechanism. In the simulation of the deposition rates, two production rate factors are used which are based on solutions to the reaction schemes obtained from chemical kinetics simulation programs [14,151.One is a rate factor for the homogeneous reaction, obtained by fitting Arrhenius parameters to the computer simulation of the decomposition of one metalorganic compound in the presence of the other. The other is

233

a heterogeneous rate factor for the rate of decomposition when the metalorganic compound comes in contact with a metal surface, which has been estimated by observed differences between the simulated homogeneous reaction and the experimental heterogeneous reaction. 3.3.2 Diffusion coefficients The Fickian diffusion coefficient in a binary gas mixture is estimated from the molecular collision theory of gases [11]. Using a Lennard-Jones potential for the force law associated with a molecule, the diffusion coefficient is given (in 2/s) by m D c ~/T~(M1 +212M2) ~(l,l)* 12 /2M1M2 (11) PaHere C is a constant (2.628 x iO~); M~is the molecular weight of species n; P is the pressure (atm); T is the temperature (K); a-~ is the Lennard-Jones diameter (A); Q~l)* represents a normalized collision integral which depends on =

kT/ 12, where k is Boltzmann’s constant; and ,, is Lennard-Jones energy parameter n. aThe Lennard-Jones parameters forfor thespecies cross couplings between species 1 and 2 are given approximately by (see ref. [11], eqs. (8.4-8) and (8.4-9), p. 567): a-12

=

(a-1 +

o-2)/2,

12

=

~

(12)

The Lennard-Jones diameter can be estimated from the Le Bas volume of the material at its melting point according to [16] a-,, 1.18V~ 2r,~,,, 3 mol where VB is in cm

1

and a- is in

A. (13) For

organic molecules, the volume can be estimated from the structure [17]. In the absence of melting point volumes, a reasonable estimate is obtained by using twice the covalent radius, r~.For Te2, the normal gaseous form of Te, the covalent radius is twice that of Te, with a correction factor of 0.86 for the double bond [18]. The energy parameter can be estimated from the normal boiling point temperature TB [16] according to 2lkTBfl. (14) ~ l.

234

TJ. Davis et a!.

/ Models of CdTe deposition in pre-cracking MOCVD reactor

Table 1 Fit parameters for the temperature dependence of the Fickian diffusion coefficient and typical values at 350°C

_________________________________________ Species

A

p

C

9

D at 350°C (m2/s) 1.69x104

1.770

~H2H6Cd 14Te

1.91x10

~

~

Te Te2

1.38x iO~ 1° 7.55x10

1.886 1.886

on the molecular species. These are shown in table 2. 3.4. Simulations

2.57x 1.41x104 10~

D =AT”

‘

/

where A and the power p are constants for each species. These parameters are shown in table 1. Like the Fickian diffusion coefficient, the thermal diffusion factor is estimated using molecular collision theory [11]. Under the assumptions that the molecular masses of the two species satisfy M 1 >> M2 and their relative concentrations are such that x1 << 1 and x2 1, the thermal diffusion factor is given by [16] 5 =

1 VL

a-12

8

a-2

2

polynomial of the form (17) a=a +bT+cT2+dT3, where a, b, c and d are constants which depend

The normalized collision integral has been tabulated as a function of the normalized temperature [11]. Together with the above parameters, (11) is solved as a function of temperature, and to a good approximation over the range of temperatures encountered in the reactor, the diffusion coefficient can be fitted to the form

a

The thermal diffusion factors, calculated using (16) vary with temperature and can be fitted by a

~m(12)* (1,i)° O~1i2 — 12 u1~~2)*

1

—

~‘~2

2M1 (16) ‘

where the normalized collision integrals Q~ are functions of kT/11 and have been tabulated [11]. The other factors are defined above,

The temperature profile in the reactor is estimated by solving (2), as discussed in section 3.2. Typical values of the thermal conductivities and the heat capacities [19,20] are chosen near the temperature of the graphite susceptor (typically 350°C). Eq. (2) is solved using an alternating direction implicit (AD!) differencing scheme [21] until a steady state is obtained. An estimated reactor temperature profile is shown in fig. 2. !nput gases travel about 100 mm. before reaching thermal equilibrium after which the temperature profile is uniform. The length of the non-uniform region varies with the flow rate of the gas. The effect of this non-uniformity is to reduce the rate of decomposition of the metalorganic compound near the entrance. In this work, the concentration of Me2Cd always exeeds that of iPr2Te so that the deposition rate is controlled by the supply of Te. Therefore, the deposition profile is based solely on a calculation of the concentrations of iPr2Te(g) and Te2(g). Eq. (8) was solved for these concentrations, given the calculated temperature profile and the decomposition rates calculated from (9). The numerical solution is based on the Crank— Nicholson method [21] which is a combination of the explicit differencing scheme and the alternat-

Table 2 Fit parameters for the temperature dependence of the thermal diffusion factor and typical values at 350°C Species

a

C C2H6Cd Cd6H14Te Te Te 2

—0.6219 —1.0018 —0.6600 —0.6352 —1.1675

b

3 5309x10 3 7.917x10 3.378x103 2.975x103 3 5.467X10

c—6.553x10~ —9.444x106 —3.695x106 —3.018x106 —5.547X106

d2.887x10° 4.032X109 1.544x109 1.176x109 2A62X109

a0.84 at 350°C 1.24 0.38 0.33 0.61

T.J. Davis et a!.

50 0

~

/ Models of CdTe deposition in pre-cracking MOCVD reactor 50I

100 I

150

200I

235 50

Graphite Susceptor 40

380

1::

—

!-~~!!!~

40

‘~1,a:rExit

260

10

Alumina Block

-

0 0

I

I

I

50

100

150

—

10

0

200

Distance Along Flow (mm) Fig. 2. An estimated temperature profile in the reactor based on the numerical model. The isotherms are labelled in degrees 3 mm ~. Celsius. The flow rate was 4000 cm 5

4

(b)

(a) 21 00cm3 mi,,1

— -

___ -— —

4

—

__

4000cm3 mIn’ 6000cm3 miii’

390 C 370CC

I

Numerical

0

20

I 40

60

I 80 100 120 140 160 180 200

Distance (mm)

c 0

I

20

40

60

80

I

100 120 140 160 180 200

Distance (mm)

Fig. 3. (a) CdTe film thickness as a function of position in the flow direction computed for three different flow rates. The graphite susceptor temperature was 390°C.(b) CdTe film thicknesss computed at three different susceptor temperatures. The flow rate was 4000cm3 minl.

236

T.J. Davis et aL

/ Models of CdTe deposition in pre-cracking MOCVD reactor

ing direction implicit differencing scheme. The calculation is continued until a steady state is obtained. Heterogeneous reactions occurring at the boundaries are assumed to deposit metal atoms directly onto the boundary surfaces. It is further assumed that all gaseous metal atoms contacting the boundary surfaces are immediately deposited there, i.e. the metal vapour concentrations at these boundaries are zero. The flux of metal atoms onto the wafer, residing on the lower boundary, is determined from the component of the flux (7) in the y direction and adding the deposition flux due to the heterogeneous decomposition of the metalorganic compound. The deposition fluxes for a number of different flow rates and graphite susceptor temperatures are shown in figs. 3a and 3b. These fluxes were calculated for the conditions used in the experiments described below and have been converted to the equivalent film thickness for comparison with the experimental data.

4. Experiments A series of experiments were performed to determine the deposition uniformity of CdTe as a

function of flow velocity and reactor temperature. CdTe films were grown on silica glass slides placed on the alumina block along the length of the reactor. In all cases the films were grown for three hours. The CdTe film thickness was measured in the flow direction along each glass slide by Fourier transform infrared (FTIR) spectroscopy. These thicknesses are shown in figs. 4a and 4b. All the CdTe films had specular surfaces extending from near the reactor entrance to beyond the deposition maximum. In some instances, the surface quality degraded further downstream, eventually becoming matte, which is attributed to nucleation of solid metal particles in the gas phase. In particular, this occurred for the thicker films and it was not possible to collect FTIR data from these regions. It can be seen from fig. 4a that increasing the flow rate decreases the maximum film thickness, shifts the maximum downstream and broadens it. The decrease in the maximum thickness is largely due to the dilution of the metalorganic vapours as the flow rate of the hydrogen carrier gas is increased. However, the change in maximum thickness is not in direct proportion with the flow rate increase and this may be due to depletion of the

____________________

(a)

i~

—~~ P

(bl

390°C

I~’~.’ ~ ~

I:

:..:~

Experiment

0 0

I

20

40

60

80

I

I

I

I

I

100 120 140 160 180 200

Distance (mm)

‘~ 0

I

20

I

40

I

60

80

I

I

I

100 120 140 160 180 200

Distance (mm)

Fig. 4. (a) CdTe film thickness measured by FTIR spectroscopy. The CdTe films were grown at three different flow rates. The graphite susceptor temperature was 390°C. (b) CdTe thickness for films 3grown mm ~. at three different susceptor temperatures at a constant flow rate of 4000 cm

T.J. Davis et a!.

/ Models of CdTe deposition in pre-cracking MOCVD reactor

metal atoms by gas-phase nucleation. The broadening of the curve about the maximum is important because the film thickness becomes more uniform. The variation in the temperature of the graphite susceptor (fig. 4b) has little effect on the position of the deposition maximum but controls both the thickness and the breadth of the curve. A comparison between figs. 3 and 4 shows that there is a good qualitative agreement between the model and the experiments with the main features of the data being reproduced by the model. The quantitative deviations of the model from the data are to be expected given the approximations used and the lack of precise data concerning thermal boundary conditions, the reaction rates, the diffusion coefficients, etc. The deposition profile is governed by the rate of supply of Te, which is determined by the reactor temperature and the carrier gas flow rate. Lower temperatures produce a more uniform rate of decomposition of the metalorganic vapour, while higher flow rates reduce the residence time of the vapours in any one region of the reactor.

237

The interdependencies of these parameters are seen more clearly with a simple analytical model.

5. Simple analytical model In this model the production and transport of Te is described by a simple conservation equation for the flux. An approximate form of (8) is used in which we assume Fickian diffusion in the y direction only, with a constant diffusion coefficient, and a constant mean flow velocity in the x direction. The concentration of the metal is then given by

an 1, at

dn dt

/

\

)

+

( ) an

~

ax

2n ay2

a =D—+w,

where dn/dt is expressed in terms of a convective derivative. Under steady state conditions, (an/at)~ 0, so that =

t

=

(19)

X/V,,,,

5

4

(a)

____________

(bi 2100cm’ mid’ 4000cn~min’

4

(18)

370°C 390°C

I

I

- —— —

6OOOc~~l

- — —

350°C

3

Simple Analytical Model

23 -~

UI UI C

C

2 2

-

UI C --

.~ 0

-

--

•

•

-

.E I-

--

I-

‘-

-

-I •1

-~ -.

I

.1 I

‘-

—•

/

0

--

/

I

1

c

--

-

-

I 20

I 40

I I 60 80

—_ -

—

I 1 100 120 140 160 180 200

Distance (mm)

1 •~ II

-

I

0

I

20

I

40

I

60

I

80

I

I

I

100 120 140 160 180 200

Distance (mm)

Fig. 5. The deposition profiles based on eq. (23) and the reactor data used to compute (a) fig. 3a for three different flow rates and (b) fig. 3b for three different temperatures.

238

T.J. Davis et a!.

/ Models of CdTe deposition in pre-cracking MOCVD reactor

for a constant velocity. Thus (18) can be solved for n(t).The rate of production of Te is given by

n(x/vm)

=

w(y t)=k(y)n

mo

exp(—k(y)t)

(20)

where ~mO is the initial concentration of the metalorganic compound. The rate factor k is a function of temperature, which is taken as a function of y only. For this analysis, k(y) in the exponent is replaced by a constant average value Kk>, which allows the rate of production to be separated into a time dependent factor and a position dependent factor, w(y,

t) =nmok(y)

exp(—(k)t) =w~(y) w~(t). (21)

The heterogeneous reaction is not considered, Eq. (18) is solved by expanding t) and c~(y,t) as Fourier sine series with time dependent coefficients,

n(y,

n(y, t) w(y,

=

~

sin(m~ry/l),

am(t)

(22a)

m= i

~

t) =w,(t)

we,,

sin(mn’y//).

(22b)

m=

Substituting eqs. (22) into (18) yields a differential equation for am(t) in terms of 0~m The solution of this yields n(x, y), the gradient of which is the deposition flux J(y 0), J(x 0) —D—n(x ~)

the gas entrance causes the maxima of the curves to be located much closer to the reactor entrance and produces narrower curves (i.e. less uniform deposition). This demonstrates that the non-uni. improves the deposition form temperature region .

uniformity. On the basis of (23), a uniform deposition profile cannot be obtained, which indicates a fundamental problem with this type of reactor. Since the second exponent in (23) approaches zero faster than the first for all values of m, then the non-uniformity of the deposition is governed by the flow velocity and the reaction rate k(T), and hence by the temperature, i.e. the non-uniformity results from a non-constant rate of supply of metal vapour, as asserted in section 4 above. From (21), the rate of supply is constant only when X/Vm 0. This condition is approached as the flow velocity becomes large and the temperature, and hence Kk(T)~’,becomes small. This broadens the deposition profile, producing an almost uniform region beneath the maximum. The lower temperatures at the reactor entrance also reduce k(T) and therefore improve the deposition uniformity. However, a reduction in the temperature reduces the supply of metal vapour, while an increase in the flow velocity =

=

dilutes the metalorganic vapour and further reduces the supply of metal vapour, leading to low growth rates.

=

=

ay =

~ m= 1

—

CdTe

‘

—Dm~rw~/1exp D m27r2 12

exp —D

6. Improvements to deposition uniformity for

—

12

Kk) .±~. Vm

— ____

The deposition uniformity is improved by increasing the fig. flow reducing temperature. Using 3a,rate we and observe that athe flow rate of 6000 cm3 min~ and a temperature of 390°C

Vm

‘23’ “ -‘

The reactor parameters used for calculating the depositions shown in figs. 3a and 3b have been used to calculate deposition fluxes based on (23). These are shown in figs. 5a and Sb. The temperature profile was assumed to be linear. The neglect of the non-uniform temperature region at

mm beneath yields a ±1% the variation peak and in thickness a growth over rate about of 0.6 20 im h~. Although this is an acceptable uniformity, the growth rate is too low. To increase the growth rate, the iPr 2Te bubbler temperature was increased from 17 to 30°C and the flow rate through the bubbler increased its iPr maxi3 min wasThis increasestothe mum of 100 cm 2Te concentration by a factor of 4 and allows us to ~.

TJ. Davis et aL

/ Models of CdTe deposition in pre-cracking MOCVD reactor

239

3.0 (0 C 0 0

E

203040 W

50

Distance (mm)

Fig. 6. The film thickness as a function of position in the flow direction for a CdTe film grown on a GaAs 211 substrate. The reactor conditions were optimised to produce a uniform deposition.

reduce the reactor temperature to further improve uniformity. With a graphite susceptor ternperature of 370°C and the conditions described above, the numerical model predicts a maximum growth rate of about 1.5 jsm h1. Using the reactor conditions above, CdTe was grown on a GaAs (211) substrate centred 100 mm from the reactor entrance. The deposition profile was measured by FTIR spectroscopy and the results are shown in fig. 6. The error bars represent the rms uncertainties in the FTIR measurements. The characteristic maximum of the deposition curve is still evident but it has been broadened to the extent that there exists an almost uniform region beneath the maximum. There is a + 1% variation in thickness over 25 mm based on the mean thickness values obtained from the FTIR data, although the errors in the data are greater than this. The growth rate achieved for this film was 1.15 p.m h/ which is lower than expected. The film showed a highly specular surface and good crystallinity, with a double crystal rocking curve FWHM of 205 arc sec.

7. Conclusion A thorough analysis of the mechanisms affecting the deposition of CdTe in the MRS Quantax 226 MOCVD reactor has enabled us to optimise the thickness uniformity of thin films grown on GaAs. The variation with position of the rate of supply of metal vapour has been identified as the

main cause of variations in film thickness. The rate of supply depends on the temperature in the reactor and the flow velocity of the hydrogen carrier gas. Lower reactor temperatures and higher flow rates improve the thickness uniformity. Furthermore, the cooler region at the reactor entrance reduces the rates of decomposition of the metalorganic compounds leading to more uniform rates of supply of metal vapours, which improves the uniformity of deposition. An accurate control of the temperature profile in the reactor could lead to greater uniformity.

Acknowledgement The permission of the Director, Research, Telecom Australia Research Laboratories, to publish this work is acknowledged. References [1] G.N. Pain, N. Bharatuia, T.J. Elms, P. Gwynn, M. Kibel, M.S. Kwieiniak, P. Leech, N. Petkovic, C. Sandford, J. Thompson, T. Warminski, D. Gao, S.R. Glanvill, C.J. Rossouw, A.W. Stevenson and S.W. Wilkins, J. Vacuum Sci. Technol. A 8 (1990) 1067. [2] M.R. Czerniak and M.F. Robinson (to Cambridge Instruments Ltd.), German Patent No. DE3733499A1. [3] M.R. Czerniak and P.L. Anderson, S. Mr. J. Sci. 84 (1988) 645. [4] R. Triboulet, J. Crystal Growth 107 (1991) 598. [5] G.N. Pain, C. Sandford, G.K.G. Smith A.W. Stevenson D. Gao, L.S. Wielunski, S.F. Russo, G.K. Reeves and R. Elliman, J. Crystal Growth 107 (1991) 610.

240

T.J. Davis et al.

/ Models of CdTe deposition in pre-cracking MOCYD

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