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GaAs crystal growth from coordination compounds using the organometallic chemical vapor deposition process for solar cells
Solar & Wuul Technology Vol. 3, No. 1, pp. 21-25, 1986 Printed in Great Britain.
0741-983X/86 $3.00+ .00 Pergamon Pre~ Ltd.
GaAs CRYSTAL GROWTH FROM COORDINATION COMPOUNDS USING THE ORGANOMETALLIC CHEMICAL VAPOR DEPOSITION PROCESS FOR SOLAR CELLS A. Z^OUK* a n d G. CONSTANTt * National Council for Scientific Research, Solar Energy Group, P.O. Box 11-8281, Beirut, Lebanon t Laboratoirc de Cristallochimie, R6activit6 et Protection des mat6daux E.R.A. 263, E.N.S.C.T., 118 Route de Norbonne, 31077--Toulouse Cedex, France
(Received 10 June 1985 ; accepted 26 July 1985) Aimtraet--GaAs thin layers have been obtained from coordination compounds (adducts) using the organometallic chemical vapor deposition process (O.M.C.V.D.). That is the first step of a study going on to realize GaAIAs/GaAssolar cells.The preparation of the compounds has shown that the bond stxength energy between the Ga and As atoms in the molecule is very important for use in a chemical vapor deposition process. The layers were p type and the use of As Et3 occurs in the chemical equilibrium of the vapour phase to obtain n type material. The crystallographic orientation of the substrates can influence the growth mechanisms of the layers.
1. INTRODUCTION
2. The use of arsine is very dangerous for the environment and its toxicity is very high, 0.2 mg c m - 3 [6]. 3. The trimethylgallium Ga(CHs)a is explosive at room temperature.
The epitaxial growth of thin I I I - V compound layers utili~ng O.M.C.V.D. technique has become an important technology realizing a wide variety of electronic devices in particular solar ceils [1]. GaAI As/GaAs heterojunctions have been realized and To overcome these problems we have used adduct efficiency in the order of 23% for AM1 conditions have molecules already bonded by coordination strength been obtained [2]. Nelson [3] and Saxena [4] has to deposit gallium arsenide layers in the aim to realize used this growth process to fabricate high efficiency GaAIAs/GaAs solar cells. concentrator ceils and much research is going on now In this paper, we present the first step of our purpose to realize cascade solar cells by this technique. It which is the elaboration of the GaAs layers using the consists of transport to the reaction zone of one or more coordination compounds (adducts) dialkylgallium of the film constituents in the form of metal alkyls and chloride-triaikylarsine and the difference in the crystal the others as hydrides. The growth of the growth of the GaAs layers between the Ga(CHs) 3 + GaA1As/Ga~s layers used trimethylgallium T . M . G a AsHs process which we call classical O.M.C.V.D. and and trimethylaluminium T.M.A1 as gallium and our method using adducts molecules. aluminium source and arsine AsH3 as arsenic source. The basic reaction for the formation of GaAs is (CHs)sGa + AsH 3
600--7000C
) GaAs + 3CH4.
2. EXPERIMENTAL
The formation of the layers occurs via the pyrolysis of these products and the subsequent recombination of the atomic or molecular species near the heated substrate. However, with this growth process, some difficulties can be observed:
2.1. Adducts preparation The use of adduct molecules to elaborate gallium arscnide layers needs to take care of the importance of the bond strength energy between the third and the fifth elements to obtain the stoichiometry in the deposit These molecules are characterized by the fact that the gallium and arsenic atoms are already bonded in the ratio 1/1 in the molecule. They were obtained following the general reaction between a Lewis acid and a Lewis
1. In some cases the metal aikyls react, at room temperature, with hydrides and other metal aikyls and lead to the formation of a polymeric adduct which influences the uniformity of the growth [5]. 21
22
A. ZAOUK and G. CONSTANT T(*C) 90
70
50
I
I
I
•
1.5
x
• CtEt2Go.AsEt3 a
ClMe2Go. AsMe 3
• CLMe2Ga. A s E t 3 0.2
D CLEt2Ga. AsMe 3 I I 2.8 2.9 IO00/T(K
0.5
I
I
3.0
3. I
-I)
Fig. 1. Vapor pressure of the coordination compounds prepared to be used in a C.V.D. process.
base (C1R2Ga)2 + 2AsR 3 ~ 2(CIR2Ga + AsR3). R = Et or Me. After distillation under low pressure, the molecules were chemically analyzed and their N.M.R.
and mass spectrums recorded. Three were liquid at room temperature, only C 1 M e 2 G a ~ A s M e 3 was white, crystallized and became liquid at 30°C. In Fig. 1 we have reported the boiling points of these molecules taken during distillation. The chemical analysis is reported in Table 1. We can observe that experimental values of the elements of C1Me2Ga,--AsEt3 correspond with the theoretical ones which is not the case for the other adducts. These were confirmed by the N.M.R. and the mass spectrums. As a matter of fact, on one hand the N.M.R. analysis has shown an excess of the Lewis acid, diluted in the molecules, very weak for CIMe2Ga ~ AsEt3 and increased for the others. Table 2 presents the percentage of this excess obtained after integration of the spectrums. On the other hand, the mass spectrums have shown that the molecular ion appears intense for C1Me2Ga ,-- AsEt3, very weak for C1Et2Ga ~ AsEt3 and does not appear at all for the two other molecules. However, we had observed the peaks of the fragment ions coming from the adducts. That is surely due to the weakness of the bond strength between the gallium and the arsenic atoms in the molecules which led us to consider C1Me2Ga ,-- AsEt 3 and CIEt2Ga ,--AsEt3 as the more stable molecules to be used in a chemical vapor deposition process. 2.2. Procedure Gallium arsenide layers were grown by the pyrolysis of the coordination compounds dialkylgalliumchloride-trialkylarsine put in a thermostated Pyrex bubbler heated at 40°C. The epitaxial reactor pre-
Table 1. Chemical analysis values of the products Elements
C
H
Cl
Ga
As
theoretical
32,43
7,09
11,99
23,31
25,33
Average experimental values
31,4
7,3
]2,]
20,9
24,6
theoretical
37,03
7,71
10,80
21,29
23,14
Average experimental values
35,5
7,9
12,9
24,4
]9,3
theoretical
23,62
5,90
13,77
27,16
29,52
values
22,9
5,4
]6,8
34,2
]3,8
theoretical
29,66
6,7]
12,51
24,61
26,48
Average experimental
27,7
6,3
18,8
30,8
]5,1
~alues
Total
CIMe2GaAsEt3
values
CIEt2GaAsEt3
values
ClMe2GaAsMe3
Average experimental
values
98,4
100
9],]
ClEt2GaAsMe3 values
98,7
23
GaAs crystal growth from coordination compounds Table 2. Percentage of the excess of the Lewis acid in the molecules reported by the N.M.R. spectrums
Coordination Compound
Percentage of the Lewis acid excess after N.M.R. integratiot
5 a
ClMe2GaAsEt3
7%
CIEt2GaAsEt3
20
~ 25 %
CIMe2GaAsMe3
35
a
40 %
CIEt2GaAsMe3
25
a
30 %
sented in a previous paper [7] is 40 cm long and 5 cm in diameter. It was a cold wall horizontal system heated by high frequency induction under atmospheric pressure. Substrates were GaAs (111), (100) orientation and Ge (111) degreased in a (1:1:1) boiling mixture of trichloroethylene, propanol, dioxan and chemically etched by (3,1,1) mixture of H2SO,, H2Oa, HzO for GaAs and CP, for Ge. The studied temperature of deposition was situated between 400 and 750°C and reported by a thermocouple inserted into the heart of graphite coated silicon carbide susceptor on which the substrates were placed. The adducts were carried by purified helium with a total flow rate in the reactor in the order of 1.3 cm s - t adjusted with helium or hydrogen flowing in a lateral drain. A second bubbler of triethylarsine at 0°C was used to study the influence of this product on the crystal growth of the layers.
substrates and increased the growth rate (3"5/~m h - ~ f°r molecule AsEts = 1 rati°) " Experiments with diethylgalliumchloride-tdethylarsine have led to the obtention of p type epitaxial layers on (100) GaAs substratcs between 450 and 600°C (Fig. 3). Above this temperature the layers were non-uniform on (100) substratcs and monocrystalline and surface relaxed on (111) face (Fig. 4). The growth rate at 450°C was very weak with a molar fraction of the order of 2-104. It increased with temperature, became constant between 500 and 600°C, and decreased above this temperature (Fig. 2). A small difference in the growth rate was observed on the (111) and the (100) substrates (Fig. 5). As a matter of fact, between 450 and 600°C the growth rate was higher on
3. RESULTS
Using dimethylgalliumchlofide-triethylarsine as starting product and helium as flow gas, polycrystalline gallium arsenide layers were obtained in the range of temperature between 550 and 750°C. When hydrogen was used as additive flow gas, we obtained p type epitaxial layers on (111) substrates and disoriented crystals on (100) face. For a molar fraction of the molecule in the order of 4"10 -4 the growth rate was about 2/an h - 1 and varied with temperature (Fig. 2). The addition oftriethylarsine in the vapor phase has led to the obtention of n type epitaxial layers on all the
2 i-
o CLMezGa.AsFt 3 • CtEIzGo.AsE~
~.5 E
i'
\
o.s
/" oI 450
~f~o=tio~ 2~1o-~ \: '\ I
550
I
T("C)
650
Fig. 2. Growth rate of GaAs layers.
\
~, ', I
750
24
A. ZAOUK and G. CONSTANT Substrate face (100) 0.7 T= 0.5
E ::k 0.3 Od-1 450
L
I
550
650
T(*C)
Fig. 5. Growth rate of GaAs layers on (111) and (100) substrates, Fig. 3. S.E.M. of GaAs layer on GaAs (100) substrate at 500°C.
(100) face but above this temperature and till 650°C it became higher on (111) substrates. The material was always p type. The hydrogen does not affect the layers but an addition of triethylarsine increased the deposition rate and allowed the obtention of n type material (Fig. 6). 4. DISCUSSION AND CONCLUSION The difference in the deposition temperature zone between the two adducts is to be correlated to the nature of the radicals bonded to the gallium and arsenic atoms in each molecule [8]. But we can see that three temperature regions identify the growth rate of the GaAs layers. The same phenomenon is observed in the Ga(CH3)3+AsH 3 process. It corresponds to a low temperature kinetic controlled regime, mid temperature mass transport limited range and high temperature
desorption limited region [9]. For the adducts the deposition at high temperature is limited by the solidity of the Ga ~ As bond in the molecule which influences the maximum temperature of deposition. On the other hand, the addition of triethylarsine in the vapor phase increases slightly the growth rate till a ratio AsEt3 molecule
1,
the role of triethylarsine in the reaction is only to diminish the dissociation of the molecule and not to incorporate arsenic atom in the layer. It occurs only in the chemical equilibrium of the vapor phase to compensate the excess of the gallium which explains the weak ratio AsEt3 molecule
4--
5-
t I
r L i
¥ E
•
I
/ ' 0 '
I
/CLMezGa.
~"o
2~" I p I typel I~ s ~ •
v
o
AsEt s
T~: 6 2 5 o c n type
~ 1
,
•
•
o _ ~_CLEt2Go.AsEt3 'L
I
Td : 550°C
I J
I
0.5
I
I
I
t5
I
2
T(°C) Fig. 6. Influence of triethylarsine on the growth rate and the
Fig. 4. S.E.M. of GaAs layer on GaAs (111) substrate at 625°C.
type of the OaAs layers.
GaAs crystal growth from coordination compounds to obtain an n type material. That is not the case for the classical process which needs a ratio AsHs -~ 10 T.M.G. [10] to pass from a p type to an n type material because the role of arsine is to give the arsenic atom to be incorporated in the layer. Another parameter can influence the growth rate of the layers. It is the crystallographic orientation of the substrates. The difference in the growth rate at low temperatures (450-600°C) between the (111) and (100) face substrates as observed in our experiments is due to the growth mechanisms controlled by the surface itself. As a matter of fact, on one hand the (111) substrates present two types of danghng bond on which could be fixed the Ga ~ As bond issued from our molecules, on the other hand, the (100) face present only one type of dangling bond either on gallium or on arsenic and the chvmisorption occurs only on one site. This will lead to a higher mobility of the chemisorbed groups and increases the growth rate of the layer on the (100) substrates. At high temperatures (above 600°C) the Ga ~- As bond is easily dissociated when bonded to only one site and that is why we obtained better layers when triethylarsine was added in the vapor phase. For (I ! 1) substrates the Ga *- As bond fixed on two sites is not easily dissociated and we obtained a smooth surface and a higher growth rate on this face. In the classical O.M.C.V.D. process, epitaxial layers have been obtained between 715 and 810°C and the most promising substrate temperature for the (111) orientation is 790-810°C [11]. This large displacement in the temperature zone is due to, when using T.M.G. + AsH 3 process, non-bonded gallium and arsenic are in the
25
vapor phase and the layer is formed by the recombination of each atom on the surface. This growth mechanism needs a higher temperature and a great quantity ofarsinein particular for (111) G a side to obtain the smooth layers. This study proved that it is possible to obtain epitaxial p or n type gallium arsenide layers using adduct molecules in a O.M.C.V.D. process at low temperatures. Efl~ciencies obtained by the classical organometallic technique encourage us to continue our studies to realize a p-n junction and in another step to elaborate (GaAI)As window for the obtainment of a solar cell device. REFERENCES
1. First InternatiOnal Conference on Metal Organic Vapor Phase Epitaxy, 4-6 May 1981, Ajaccio, France. Second International Conference on Metal Organic Vapor Phase Epitaxy, 10-12 April 1984, She~eld, England. 2. R. D. Dupuis, P. D. Dapkus, P. P. Ruth, J. J. Coleman, W. I. Simpson, J. J. Yang and S. W. Zehr, 14th I.E.E.E. Photovoltaic Specialists Conference, p. 1388 (1980). 3. N. J. Nelson, K. K. Johnson, R. L. Moon, H. A. Van der Plas and L. W. James, Appl. Phys. Left. 33, 26 (1978). 4. R. R. Saxena, V. Aebi, C. B. Cooper, M; J. Ludowise, H. A. Van der Plas, B. R. Cairns, T. J. Maloney, P. G. Borden and P. E. Gregory, J. appL Phys. 51, 4501 (1980). 5. H. M. Manasevit, F. M. Erdmann and W. I. Simpson, J. Electrochem. Soc. 118, 1864 (1971). 6. Toxic Files No. 53, Edited by the National Institute of Research and Security, Paris (1969). 7. A. Zaouk and G. Constant, J. de physique 43, C5-421 (1982). 8. A. Zaouk, E. Salvetat, J. Sakaya, F. Manry and G. Constant, J. Cryst. Growth 55, 135 (1981). 9. J. P. Duchemin, Rev. Tech. Thomson 9, 33 (1977). 10. J. P. Hallais, Acta Electronica 21, 129 (1978). 11. V. Gootschalch, W. H. Petzke and E. Butter, Krist. Tech. 9, 355 (1974).
0741-983X/86 $3.00+ .00 Pergamon Pre~ Ltd.
GaAs CRYSTAL GROWTH FROM COORDINATION COMPOUNDS USING THE ORGANOMETALLIC CHEMICAL VAPOR DEPOSITION PROCESS FOR SOLAR CELLS A. Z^OUK* a n d G. CONSTANTt * National Council for Scientific Research, Solar Energy Group, P.O. Box 11-8281, Beirut, Lebanon t Laboratoirc de Cristallochimie, R6activit6 et Protection des mat6daux E.R.A. 263, E.N.S.C.T., 118 Route de Norbonne, 31077--Toulouse Cedex, France
(Received 10 June 1985 ; accepted 26 July 1985) Aimtraet--GaAs thin layers have been obtained from coordination compounds (adducts) using the organometallic chemical vapor deposition process (O.M.C.V.D.). That is the first step of a study going on to realize GaAIAs/GaAssolar cells.The preparation of the compounds has shown that the bond stxength energy between the Ga and As atoms in the molecule is very important for use in a chemical vapor deposition process. The layers were p type and the use of As Et3 occurs in the chemical equilibrium of the vapour phase to obtain n type material. The crystallographic orientation of the substrates can influence the growth mechanisms of the layers.
1. INTRODUCTION
2. The use of arsine is very dangerous for the environment and its toxicity is very high, 0.2 mg c m - 3 [6]. 3. The trimethylgallium Ga(CHs)a is explosive at room temperature.
The epitaxial growth of thin I I I - V compound layers utili~ng O.M.C.V.D. technique has become an important technology realizing a wide variety of electronic devices in particular solar ceils [1]. GaAI As/GaAs heterojunctions have been realized and To overcome these problems we have used adduct efficiency in the order of 23% for AM1 conditions have molecules already bonded by coordination strength been obtained [2]. Nelson [3] and Saxena [4] has to deposit gallium arsenide layers in the aim to realize used this growth process to fabricate high efficiency GaAIAs/GaAs solar cells. concentrator ceils and much research is going on now In this paper, we present the first step of our purpose to realize cascade solar cells by this technique. It which is the elaboration of the GaAs layers using the consists of transport to the reaction zone of one or more coordination compounds (adducts) dialkylgallium of the film constituents in the form of metal alkyls and chloride-triaikylarsine and the difference in the crystal the others as hydrides. The growth of the growth of the GaAs layers between the Ga(CHs) 3 + GaA1As/Ga~s layers used trimethylgallium T . M . G a AsHs process which we call classical O.M.C.V.D. and and trimethylaluminium T.M.A1 as gallium and our method using adducts molecules. aluminium source and arsine AsH3 as arsenic source. The basic reaction for the formation of GaAs is (CHs)sGa + AsH 3
600--7000C
) GaAs + 3CH4.
2. EXPERIMENTAL
The formation of the layers occurs via the pyrolysis of these products and the subsequent recombination of the atomic or molecular species near the heated substrate. However, with this growth process, some difficulties can be observed:
2.1. Adducts preparation The use of adduct molecules to elaborate gallium arscnide layers needs to take care of the importance of the bond strength energy between the third and the fifth elements to obtain the stoichiometry in the deposit These molecules are characterized by the fact that the gallium and arsenic atoms are already bonded in the ratio 1/1 in the molecule. They were obtained following the general reaction between a Lewis acid and a Lewis
1. In some cases the metal aikyls react, at room temperature, with hydrides and other metal aikyls and lead to the formation of a polymeric adduct which influences the uniformity of the growth [5]. 21
22
A. ZAOUK and G. CONSTANT T(*C) 90
70
50
I
I
I
•
1.5
x
• CtEt2Go.AsEt3 a
ClMe2Go. AsMe 3
• CLMe2Ga. A s E t 3 0.2
D CLEt2Ga. AsMe 3 I I 2.8 2.9 IO00/T(K
0.5
I
I
3.0
3. I
-I)
Fig. 1. Vapor pressure of the coordination compounds prepared to be used in a C.V.D. process.
base (C1R2Ga)2 + 2AsR 3 ~ 2(CIR2Ga + AsR3). R = Et or Me. After distillation under low pressure, the molecules were chemically analyzed and their N.M.R.
and mass spectrums recorded. Three were liquid at room temperature, only C 1 M e 2 G a ~ A s M e 3 was white, crystallized and became liquid at 30°C. In Fig. 1 we have reported the boiling points of these molecules taken during distillation. The chemical analysis is reported in Table 1. We can observe that experimental values of the elements of C1Me2Ga,--AsEt3 correspond with the theoretical ones which is not the case for the other adducts. These were confirmed by the N.M.R. and the mass spectrums. As a matter of fact, on one hand the N.M.R. analysis has shown an excess of the Lewis acid, diluted in the molecules, very weak for CIMe2Ga ~ AsEt3 and increased for the others. Table 2 presents the percentage of this excess obtained after integration of the spectrums. On the other hand, the mass spectrums have shown that the molecular ion appears intense for C1Me2Ga ,-- AsEt3, very weak for C1Et2Ga ~ AsEt3 and does not appear at all for the two other molecules. However, we had observed the peaks of the fragment ions coming from the adducts. That is surely due to the weakness of the bond strength between the gallium and the arsenic atoms in the molecules which led us to consider C1Me2Ga ,-- AsEt 3 and CIEt2Ga ,--AsEt3 as the more stable molecules to be used in a chemical vapor deposition process. 2.2. Procedure Gallium arsenide layers were grown by the pyrolysis of the coordination compounds dialkylgalliumchloride-trialkylarsine put in a thermostated Pyrex bubbler heated at 40°C. The epitaxial reactor pre-
Table 1. Chemical analysis values of the products Elements
C
H
Cl
Ga
As
theoretical
32,43
7,09
11,99
23,31
25,33
Average experimental values
31,4
7,3
]2,]
20,9
24,6
theoretical
37,03
7,71
10,80
21,29
23,14
Average experimental values
35,5
7,9
12,9
24,4
]9,3
theoretical
23,62
5,90
13,77
27,16
29,52
values
22,9
5,4
]6,8
34,2
]3,8
theoretical
29,66
6,7]
12,51
24,61
26,48
Average experimental
27,7
6,3
18,8
30,8
]5,1
~alues
Total
CIMe2GaAsEt3
values
CIEt2GaAsEt3
values
ClMe2GaAsMe3
Average experimental
values
98,4
100
9],]
ClEt2GaAsMe3 values
98,7
23
GaAs crystal growth from coordination compounds Table 2. Percentage of the excess of the Lewis acid in the molecules reported by the N.M.R. spectrums
Coordination Compound
Percentage of the Lewis acid excess after N.M.R. integratiot
5 a
ClMe2GaAsEt3
7%
CIEt2GaAsEt3
20
~ 25 %
CIMe2GaAsMe3
35
a
40 %
CIEt2GaAsMe3
25
a
30 %
sented in a previous paper [7] is 40 cm long and 5 cm in diameter. It was a cold wall horizontal system heated by high frequency induction under atmospheric pressure. Substrates were GaAs (111), (100) orientation and Ge (111) degreased in a (1:1:1) boiling mixture of trichloroethylene, propanol, dioxan and chemically etched by (3,1,1) mixture of H2SO,, H2Oa, HzO for GaAs and CP, for Ge. The studied temperature of deposition was situated between 400 and 750°C and reported by a thermocouple inserted into the heart of graphite coated silicon carbide susceptor on which the substrates were placed. The adducts were carried by purified helium with a total flow rate in the reactor in the order of 1.3 cm s - t adjusted with helium or hydrogen flowing in a lateral drain. A second bubbler of triethylarsine at 0°C was used to study the influence of this product on the crystal growth of the layers.
substrates and increased the growth rate (3"5/~m h - ~ f°r molecule AsEts = 1 rati°) " Experiments with diethylgalliumchloride-tdethylarsine have led to the obtention of p type epitaxial layers on (100) GaAs substratcs between 450 and 600°C (Fig. 3). Above this temperature the layers were non-uniform on (100) substratcs and monocrystalline and surface relaxed on (111) face (Fig. 4). The growth rate at 450°C was very weak with a molar fraction of the order of 2-104. It increased with temperature, became constant between 500 and 600°C, and decreased above this temperature (Fig. 2). A small difference in the growth rate was observed on the (111) and the (100) substrates (Fig. 5). As a matter of fact, between 450 and 600°C the growth rate was higher on
3. RESULTS
Using dimethylgalliumchlofide-triethylarsine as starting product and helium as flow gas, polycrystalline gallium arsenide layers were obtained in the range of temperature between 550 and 750°C. When hydrogen was used as additive flow gas, we obtained p type epitaxial layers on (111) substrates and disoriented crystals on (100) face. For a molar fraction of the molecule in the order of 4"10 -4 the growth rate was about 2/an h - 1 and varied with temperature (Fig. 2). The addition oftriethylarsine in the vapor phase has led to the obtention of n type epitaxial layers on all the
2 i-
o CLMezGa.AsFt 3 • CtEIzGo.AsE~
~.5 E
i'
\
o.s
/" oI 450
~f~o=tio~ 2~1o-~ \: '\ I
550
I
T("C)
650
Fig. 2. Growth rate of GaAs layers.
\
~, ', I
750
24
A. ZAOUK and G. CONSTANT Substrate face (100) 0.7 T= 0.5
E ::k 0.3 Od-1 450
L
I
550
650
T(*C)
Fig. 5. Growth rate of GaAs layers on (111) and (100) substrates, Fig. 3. S.E.M. of GaAs layer on GaAs (100) substrate at 500°C.
(100) face but above this temperature and till 650°C it became higher on (111) substrates. The material was always p type. The hydrogen does not affect the layers but an addition of triethylarsine increased the deposition rate and allowed the obtention of n type material (Fig. 6). 4. DISCUSSION AND CONCLUSION The difference in the deposition temperature zone between the two adducts is to be correlated to the nature of the radicals bonded to the gallium and arsenic atoms in each molecule [8]. But we can see that three temperature regions identify the growth rate of the GaAs layers. The same phenomenon is observed in the Ga(CH3)3+AsH 3 process. It corresponds to a low temperature kinetic controlled regime, mid temperature mass transport limited range and high temperature
desorption limited region [9]. For the adducts the deposition at high temperature is limited by the solidity of the Ga ~ As bond in the molecule which influences the maximum temperature of deposition. On the other hand, the addition of triethylarsine in the vapor phase increases slightly the growth rate till a ratio AsEt3 molecule
1,
the role of triethylarsine in the reaction is only to diminish the dissociation of the molecule and not to incorporate arsenic atom in the layer. It occurs only in the chemical equilibrium of the vapor phase to compensate the excess of the gallium which explains the weak ratio AsEt3 molecule
4--
5-
t I
r L i
¥ E
•
I
/ ' 0 '
I
/CLMezGa.
~"o
2~" I p I typel I~ s ~ •
v
o
AsEt s
T~: 6 2 5 o c n type
~ 1
,
•
•
o _ ~_CLEt2Go.AsEt3 'L
I
Td : 550°C
I J
I
0.5
I
I
I
t5
I
2
T(°C) Fig. 6. Influence of triethylarsine on the growth rate and the
Fig. 4. S.E.M. of GaAs layer on GaAs (111) substrate at 625°C.
type of the OaAs layers.
GaAs crystal growth from coordination compounds to obtain an n type material. That is not the case for the classical process which needs a ratio AsHs -~ 10 T.M.G. [10] to pass from a p type to an n type material because the role of arsine is to give the arsenic atom to be incorporated in the layer. Another parameter can influence the growth rate of the layers. It is the crystallographic orientation of the substrates. The difference in the growth rate at low temperatures (450-600°C) between the (111) and (100) face substrates as observed in our experiments is due to the growth mechanisms controlled by the surface itself. As a matter of fact, on one hand the (111) substrates present two types of danghng bond on which could be fixed the Ga ~ As bond issued from our molecules, on the other hand, the (100) face present only one type of dangling bond either on gallium or on arsenic and the chvmisorption occurs only on one site. This will lead to a higher mobility of the chemisorbed groups and increases the growth rate of the layer on the (100) substrates. At high temperatures (above 600°C) the Ga ~- As bond is easily dissociated when bonded to only one site and that is why we obtained better layers when triethylarsine was added in the vapor phase. For (I ! 1) substrates the Ga *- As bond fixed on two sites is not easily dissociated and we obtained a smooth surface and a higher growth rate on this face. In the classical O.M.C.V.D. process, epitaxial layers have been obtained between 715 and 810°C and the most promising substrate temperature for the (111) orientation is 790-810°C [11]. This large displacement in the temperature zone is due to, when using T.M.G. + AsH 3 process, non-bonded gallium and arsenic are in the
25
vapor phase and the layer is formed by the recombination of each atom on the surface. This growth mechanism needs a higher temperature and a great quantity ofarsinein particular for (111) G a side to obtain the smooth layers. This study proved that it is possible to obtain epitaxial p or n type gallium arsenide layers using adduct molecules in a O.M.C.V.D. process at low temperatures. Efl~ciencies obtained by the classical organometallic technique encourage us to continue our studies to realize a p-n junction and in another step to elaborate (GaAI)As window for the obtainment of a solar cell device. REFERENCES
1. First InternatiOnal Conference on Metal Organic Vapor Phase Epitaxy, 4-6 May 1981, Ajaccio, France. Second International Conference on Metal Organic Vapor Phase Epitaxy, 10-12 April 1984, She~eld, England. 2. R. D. Dupuis, P. D. Dapkus, P. P. Ruth, J. J. Coleman, W. I. Simpson, J. J. Yang and S. W. Zehr, 14th I.E.E.E. Photovoltaic Specialists Conference, p. 1388 (1980). 3. N. J. Nelson, K. K. Johnson, R. L. Moon, H. A. Van der Plas and L. W. James, Appl. Phys. Left. 33, 26 (1978). 4. R. R. Saxena, V. Aebi, C. B. Cooper, M; J. Ludowise, H. A. Van der Plas, B. R. Cairns, T. J. Maloney, P. G. Borden and P. E. Gregory, J. appL Phys. 51, 4501 (1980). 5. H. M. Manasevit, F. M. Erdmann and W. I. Simpson, J. Electrochem. Soc. 118, 1864 (1971). 6. Toxic Files No. 53, Edited by the National Institute of Research and Security, Paris (1969). 7. A. Zaouk and G. Constant, J. de physique 43, C5-421 (1982). 8. A. Zaouk, E. Salvetat, J. Sakaya, F. Manry and G. Constant, J. Cryst. Growth 55, 135 (1981). 9. J. P. Duchemin, Rev. Tech. Thomson 9, 33 (1977). 10. J. P. Hallais, Acta Electronica 21, 129 (1978). 11. V. Gootschalch, W. H. Petzke and E. Butter, Krist. Tech. 9, 355 (1974).