Thermodynamic analysis of the MOVPE growth process

Journal of Crystal Growth 74 (1986) 181—186 North-Holland, Amsterdam

181

THERMODYNAMIC ANALYSIS OF THE MOVPE GROWTh PROCESS Akinori KOUKITU, Takeyuki SUZUKI and Hisashi SEKI Department of Industrial Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan

Received 16 July 1985; manuscript received in final form 20 October 1985

The MOVPE growth process is described by using a chemical equilibrium model at the vapor—solid interface. Variation of the growth rate with the growth parameters and the equilibrium partial pressures over Ill—V compounds and alloys are discussed. It is shown that the essential features of the growth rate reported in the literature are well explained by the equilibrium model.

1. Introduction Metalorganic vapor phase epitaxy (MOVPE) has become increasingly important in recent years. The process is developing from an exploration stage in the laboratory to the most useful production technique for the growth of Ill—V semiconductors. Surprisingly, however, very little is known about the growth mechanism. For understanding any vapor growth process, thermodynamics provides very useful information [1,21. In a previous paper, we showed that an equilibrium model is useful for predicting the composition of Ill—V alloys grown by MOVPE [3]. The purpose of this paper is to describe a thermodynamic analysis of the MOVPE growth process.

from TMG and AsH3, as an example. Under these conditions, the following 6 species are chosen as the necessary vapor species in analysing the vapor growth of GaAs: Ga, As4, As2, AsH3, H2 and CH4. The chemical reactions which connect all the species at the vapor—solid interface are: Ga(g) + ~ As4 (g) GaAs(s), As4(g) 2 As2(g), AsH1(g) ~ As2(g) + 4 H2(g). =

The equilibrium equations for these reactions are as follows: 1 4

K1 K2

=

aGaAS/PGaPA~

(5)

=

~

(6) 2P

2. Calculation procedure

(2) (3) (4)

=

=

K1

P1~’

=

2/P

(7)

.

1~

S

2

AsH2

2

From conservation constraints we have As the observed growth rate of MOVPE is mass transport limited under commonly used conditions, we assume that chemical equilibrium is obtamedtrimethylgallium at the vapor—solid interface [1,3—5].The input (TMG) is decomposed irre-

~1YPG

~

Ga

—

+PA.

Ga

— —

+PA

Ga(CH3)3(g) +4 H2(g)

—+

Ga(g)+ 3 CH4(g). (1)

Here, we describe the epitaxial growth of GaAs

0022-0248/86/$03.50

~CH

=

~

S

14P

2

+PH

+PCH 2

4’

(8)

~°AsH 2

versibly according to the following homogeneous reaction near the vapor—solid interface [6,7]:

+PAH ~2

~.

As~+

~As2

+

~AsH2

‘

9 (10)

where P~aand ~ are the input partial pressure of TMG and arsine. Eq. (8) expresses the total pressure of the system. Eq. (9) expresses that the deposition occurs in the ratio of 1 to 1 for group III and group V elements.

© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

182

A. Koukitu et al.

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Thermodynamic analysis of MOVPE growth process

The equilibrium partial pressure at the vapor—solid interface can be obtained from the solution of the above simultaneous equations. The calculations were carried out by using a method similar to that developed previously [2]. The values of the equilibrium constants were the same as

diffusion of the group III elements through the boundary layer [5]. According to Shaw [8], the growth rate under the mass transport type II or diffusion limit at constant pressure is expressed as follows: 0 r kg(P 1 11 P111), (11) =

—

those in our previous paper [3].

3. Results and discussion

where kg is the mass transfer coefficient, P~1is the inlet partial pressure of the group III element, and P111 is the equilibrium partial pressure at the 0~~Pcondisubstrate surface. Under normal growth tions (V/Ill> 1), the value of L~P(P1 1~1)is almost the same as the input partial pressure P~1, because the equilibrium partial pressures of the group III elements are very small as seen in figs. 1 and 2. In addition, we have almost the same value of ~ P for all Ill—V compounds and alloys under the same input mole fraction of group III sources. This is because all the equilibrium partial pressures of the group III elements are very small when the V/Ill ratio is larger than unity. It is well known that the growth rate of MOVPE is generally independent of temperature under commonly used conditions. In fig. 3, ~P, i.e. the driving force for the deposition, is given as a function of the deposition temperature. Although the magnitude of the driving force depends on the —

In figs. 1 and 2, the equilibrium partial pressures of the gaseous species over GaAs and InP are shown as functions of temperature. It is a remarkable feature of these systems that the equilibrium partial pressure of CH4 is very high. This shows a marked contrast to the hydride and chloride VPE systems in which no hydrocarbon species are included [2]. Another important feature found in these figures is that the partial pressure of PH3 is high in the InP system. The high equilibrium partial pressure of PH3 is generally observed in the phosphorus-containing binary, ternary, and quaternary systems and this results from the smaller equilibrium constant of eq. (7) for PH3. In the equilibrium model described above, the growth of the epitaxial layer is controlled by the

100

GaAs Vllll=1 0

5atm ~P =1 alIt

v/Ill nP =10

P~5iO

P~=5l~5atm>~Pi=1 atm

ion

H 2

H2

Co4

_______________________________

o~ As~

~

AsH3

—

PH3 _________________________________ CH4 5 ~ P _l ff 4 ~

1°

1O1°

il 600

100 Temp.(°Cl

800

Fig. 1. The equilibrium partial pressures of gaseous species over GaAs as functions of temperature.

600

100 Temp.(°C)

800

Fig. 2. The equilibrium partial pressures of gaseous species over InP as functions of temperature.

A. Koukitu et a!.

1

if4

Vllll ~ 1

/

Thermodynamic analysis ofMOVPE growth process

183

GaAs 1=700°C P~=5i~5atm ~Pi= latm

zPi = latm 100.

_______________

P~(atm)=5l01

H 2

~ As2

1° ~ili 51 ti° 600

Ternp.(°C)

A_

~~—.—_

800

Fig. 3. The driving force for the deposition as a function of the

As 2

deposition temperature. The value of L~P is the same for all Ill—V compounds and alloys.

j

~ ~2O1

As4 °

/ffl Ratio •

.

.

.

.

.

input metalorganic partial pressure, it is independent of the deposition temperature. Since the driving force is proportional to the growth rate, this explains the experimental rate—temperature relation reported in the literature [6,9—14]. The driving force for the deposition increases linearly with the increase of the input partial pressure of the group III species as shown in fig. 4. This fact predicts the linear dependence of the growth rate on the input metalorganic partial pressure, and agrees well with experimental observations [6,9,12—17].

Figs. 5 and 6 show the equilibrium partial pressures over GaAs and InP as functions of the V/Ill ratio. At V/Ill 1, a drastic change of the partial pressures is seen. This is a remarkable feature generally obtained in the MOVPE systems including the ternary and quaternary alloys as shown in figs. 7, 8 and 9. The variation of the equilibrium partial pressures in these figures is thought to be =

5atm~Pi=latm loP 100 1=600°C P~=51O H 2 p4

1=700°C V/lll~l ~Pi=latm

P~(atm)

Fig. 5. The equilibrium partial pressures over GaAs as functions of the V/Ill ratio.

~

Fig. input The driving pressure force of the for group the deposition IIIand species. as aThe function value of of ~ the P is the4.partial same for all Ill—V compounds alloys,

1Oh0~

_______

_______________________

Fig.the 0.1 6. V/Ill The equilibrium 1 1 Ratio partial . 100 pressures over InP as functions of ratio.

184

A. Koukitu et a!.

/

Thermodynamic analysis of MOVPE growth process

Al

056a05As

to

~

0

5atm

ln0740a026 As056P044 (x=1.3ii) atm >~P=1 atm 10 T=650°C P~=5lif5 82

~Pi=1atm

P~=5.1d C

C H~

84

d° -

7~—

~.

AsH

2

3

852

0-20

1 As4

~

V/ill

Ratio

Fig. 7. The equilibrium partial pressures over A1GaAs as functions of the V/Ill ratio.

related to changes of the solid stoichiometry and the carbon contamination of grown layers. In fact, the abrupt change of the conduction type from p to n with the increase of V/Ill ratio without intentional doping has been observed in the growth of GaAs and A1GaAs [18,19]. Considering the carbon contamination from hydrocarbons, the use

5atm~Pi=1alm loAs05 P05 H 1=650°C P~=51if 2 CH4

~

1 0.1

100

V/Ill Ratio

100

Fig. 9. The equilibrium partial pressures over InGaAsP (A ~sm) as functions of the V/Ill ratio.

1.3

=~

of larger values of the V/Ill ratio probably serves to decrease the contamination [20]. The larger V/Ill ratio also produces higher partial pressures of P4 and P2 species, so that in the larger V/Ill ratio we may have the same effect as the use of a cracking furnace for the phosphine flow [17]. The drastic change of the partial pressures is related to the vapor— solid distribution relation described previously [3]. For the growth of Al~Ga1_~As, the solid composition is found to be

~

i=ioo:c

~P=1atm

P&~atm)=5’l~

~

As4 0.1

1 As4

1

1081 100

1~o V/ffi Ratio

V/ill Ratio Fig. 8. The equilibrium partial pressures over InAsP as functions of the V/Ill ratio,

Fig. 10. The driving force for deposition as functions of the V/Ill ratio. The value of i.~P is the same for all Ill—V compounds and alloys.

A. Koukitu et a!.

/

Thermodynamic analysis of MOVPE growth process

a linear function of the input vapor phase composition when the V/Ill ratio is larger than unity. This can be easily understood from the variation of the equilibrium partial pressures shown in fig. 7. Since the equilibrium partial pressures of the group III elements are very small when the V/Ill ratio is larger than unity, the mole fraction x in Al rGan As alloy is given by the following equation: - .~

(‘~~ 1’Ai) (~~ZPA!) + (Pi~a —

X

=

—

—

PGa)

p0

(12)

‘~A!+ ~t~a

185

Similar equations hold for the other Ill—Ill—V and Ill—V—V systems. Furthermore, these relations hold for the growth of quaternary alloys. Fig. 10 shows /~Pas a function of the V/Ill ratio. The driving force for the deposition is independent of the V/Ill ratio when the ratio is larger than unity. This explains the observed dependence of the growth rate on arsine or phosphine mole fraction when the V/Ill ratio is much larger than unity [12,16,17,21]. In order to reduce the parasitic reactions, the low pressure system is often used. Fig. 11 shows the equilibrium partial pressures as functions of the system pressure. With the decrease of the system pressure, the equilibrium partial pressures

In a similar way, the following equation holds for

of H

the growth of InAs5 _~P,when the V/Ill ratio is smaller than unity:

2 and AsH3 decrease, whereas the partial pressure of the other species remains constant. Therefore, the driving force for the deposition is independent of the system pressure.

y

=

—

X

4P~ 2P~ —

[(P~

—

—

PPH)

4P~ 2P~ —

4~As

(i~. 0 p PP + ~ +

—

4—

—

—

4. Conclusions

PPH)

21~As 2—

PASH2)I (13)

1=700°C V/lll=10

P~r5.1O5atm

100

An analysis of the MOVPE growth process has been made assuming chemical equilibrium at the vapor—solid interface. The equilibrium model shows that under normal growth conditions (V/Ill > 1), the growth rate is independent of the deposition temperature and V/Ill ratio, and the rate increases linearly with the increase of the input partial pressures of the group III source. These conclusions explain well the experimental results reported in the literature.

As 4 As2

— - -

—

— — —

‘AsH3

Acknowledgement This work was supported by the Scientific Re-

- - -

Ga

_

search Grant-in-Aid for Special Project Research on “Alloy Semiconductor Electronics”, from the Ministry of Education, Science and Culture of Japan. —2i

100001

~ P(atm) 1

Fig. 11. The equilibrium partial pressure over GaAs as functions of the system pressure.

References [1] GB. Stringfellow, J. Crystal Growth 70 (1984) 133. [2] A. Koukitu and H. Seki, J. Crystal Growth 49 (1980) 325.

186 [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

A. Koukitu et a!.

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Thermodynamic analysis of MOVPE growth process

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