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Some investigations on the chemisorption and thermal heterogeneous decomposition of the MOCVD adduct ClMe2GaAsEt3
Journal of Crystal Growth 78 (1986) 185—188 North-Holland, Amsterdam
185
LETTER TO THE EDITORS SOME INVESTIGATIONS ON THE CHEMISORPTION AND THERMAL HETEROGENEOUS DECOMPOSITION OF THE MOCVD ADDUCT CIMe2GaAsEt3 F. MAURY and G. CONSTANT Laboratoire de Cristallochimie, Rèactivitè et Protection des Matériaux, Unite Associée au CNRS 445, ENSCT, 118 Route de Narbonne, F-31077 Toulouse Cèdex, France
and P. FONTAINE and J.P. BIBERIAN Unite Associêe au CNRS 794, Faculté des Sciences de Luminy, Department de Physique, Case 901, F-13288 Marseille Cédex 9, France
Received 2 October 1986; manuscript received in final form 17 July 1985
An AES study of the chemisorption of the molecule CIMe2GaAsEt3 on a Si(111) substrate and its thermal heterogeneous decomposition is proposed. The experimental results might be interpreted using a Langmuir—Hinshelwood type mechanism where the EtC1 cooperative elimination is a prominent step. This would be a new argument for our earlier assumptions on the chemical mechanism of the MOCVD GaAs growth using this adduct as a single precursor.
Adducts become increasingly important as precursors in metalorganic vapour phase epitaxy and they are especially used in three basic ways for the preparation of Ill—V compounds: (i) as single sources for both group III and group V elements [1]; (ii) formed in situ from the component molecules to avoid unwanted chemical side reactions [2]; (iii) as direct replacement for group III alkyls [3,4]. The important features in the preparation of layers, i.e. composition control, growth rate, crystal quality, purity and consequently electrical properties, are in all cases determined by the chemistry of the processes used. Thereby, since the heterogeneous decomposition of organometallic precursors is an essential part of the chemistry of this MOCVD process, its study requires much attention. In a previous work we have made assumptions on the basic chemical mechanism for the growth of Ill—V materials using adducts as single sources, which took into account the chemisorption step and differences between the strengths of the vanous bonds [1]. These deductions were issued from several studies such as analyses of growth kinetics
and composition of the pyrolysis gas versus the deposit temperature, and also correlations between various CVD results and physico-chemical properties of the starting material. The work presented in this letter was carried out after the successful use of the compounds C1R2Ga AsEt~ (R = Me, Et) [1,5,6], (C6F5)Me2Ga. AsEt3 and [C1Et2Ga AsEt2]2CH2 [7] for the GaAs epitaxial growth in order to afford a further insight into the nature of the chemical mechanism of the GaAs growth using adducts as single sources. An Auger electron spectroscopy (AES) study of the chemisorption of the adduct ClMe2Ga AsEt~on Si(111) and its thermal heterogeneous decomposition is reported. .
A stainless steel cylinder, containing the organometallic molecule C1Me2Ga AsEt~ under vacuum, was linked to an ultra high vacuum (UHV) chamber equipped with an AES spectrometer, a LEED and a mass spectrometer. A clean 7 x 7 surface of the Si(1 11) substrate was obtained by several sequences of heatings in vacuum (3 x 10— 10 Torr) to 1100°C.The starting
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
F. Maury et al.
/
Chemisorption and decomposition of MOC VD adduct ClMe,GaAsEt~
adduct was thermostated at 60°Cin order to have 1 Torr of vapour pressure in the cylinder and the introduction of the molecule in the UHV chamber was controlled with a needle valve by recording continuously the AES spectra of the Si(111) surface and the mass spectra of the gaseous phase above the substrate. The vacuum was 10~ Torr in the chamber and only argon as residual gas was observed in the mass spectra. The AES analyses of the substrate
did intensity stant about not in iO~ exhibit this of pressure theions Torr, any Si(92 element eV) the signals introduction Then, other than was at a 16), quite vacuum Siwere and conof observed The organometallic main both features on molecule the of mass the was mass spectra obtained spectra and and byof itAES. was the characteristic starting molecule ofrange. the CH~ fragmentation (m/q= of C (28), HCl~ (36), [6]: CHCC1~ (50), C2H4CI~ (63), 2H~ AsH~ (78), AsCH~(91), HAsEt~ (105) and AsEt~ (133). Although it was not possible to analyze the high energy range with our AES spectrometer, and consequently to detect with certainty the Ga(1070 eV) and As(1228 eV) transitions, evidence for the adsorption of the molecule mass given on the one hand, by the sharp decrease of the intensity of the Si(92 eV) signal when the pressure increased from 10~ Torr (due to a partial coverage of the surface) and on the other hand, by the apparition of the C(272 eV) and Cl(181 eV) signals. The detection of Ga(55 and 81 eV) and As(79 and 91 eV) in the low energy range is uncertain because of their low intensity and interference with the Si(43, 73, 79 and 92 eV) peaks. The Si surface was exposed to a partial pressure of ClMe2Ga~AsEt3 of about 10~ Torr at 20°Cduring several minutes and then the temperature of the substrate was increased in gradual steps and the AES spectra were recorded. Fig. 1 shows the variation of the Auger peak heights of Si, C and Cl with the temperature of the substrate measured with an optical pyrometer. Further hypotheses have been suggested without being fully satisfactory to account for these experimental data. One of the more plausible cxplanations that could be proposed is to assume
~
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120
,~
_________________ ~
/
-
/
/~S.\
~ ~
‘s 80
-
/ /__
~/
/
.—
i 0
/1÷101
200
~ 400 600 TEMPERATuRE
I 800 1°C)
S
I 1000
Fig. I. variation of the Auger peak heights with the substrate temperature after an exposure of the Si(111) surface to the CIMe2GaAsEt3 molecule. The inserts schematize the adsorption of the molecule on the surface of a substrate: (a) before the beginning of the decomposition; (b) after the cooperative elimination of EtCI.
that, at room temperature, molecules might be adsorbed without dissociation of the Ga—As bonds, as this is schematized in the insert (a) of fig. 1. Thus, the pseudolayer analyzed in this experiment would contain the Cl atom bonded to the Ga atom and an ethyl group borne by the As atom. By increasing the substrate temperature up to 450°C, the opposed configuration taken by the molecules would favour a cooperative elimination of these two groups which would explain the decrease of the Cl Auger peak heights. At the same time, increasing the substrate temperature up to 300°Cwould enhance the surface mobility of various adsorbed species and a better coverage of the surface of the Si substrate would be obtamed, leading to a slight decrease of the Si peak
F. Maury et a!.
/
1 87
Chemisorption and decomposition of MOC VD adduct ClMe,GaAsEt~
height as observed in fig. 1. Another consequence of this better coverage in this temperature range would be a slight increase of the C signal. Although a cooperative elimination of C2H5CI is proposed to explain the decrease of the Cl peak height, the same variation for the C signal is not observed since its intensity increases sharply from 400°Cand reaches a maximum at about 500°C when the Cl signal has almost disappeared. This discrepancy between the C and the Cl curves seems in agreement with our assumption, since it would then be probable that the carbon content in the pseudolayer analyzed after the EtC1 elimination would be larger than in the pseudolayer analyzed near the room temperature, because it would then be composed of planar species such as [Et2Ga’ AsEt2] (fig. Ib). Above 600°C, the decrease of the C peak height would correspond both to the elimination of the other alkyl radicals and to the desorption of the various species, since the Si signal increases rapidly. An interesting feature of fig. 1 is the fact that the disappearance of thetemperature Cl signal, atasabout 520°C. takes place at the same the minimal temperature of the GaAs deposition in MOCVD experiments using this precursor [5,6]. These experimental results would argue for a Langmuir—Hinshelwood type mechanism related to that of Schlyer and Ring [8] where the adduct would be adsorbed on the surface of the substrate prior to reaction in such a way that its thermal decomposition may be initiated by the EtC1 cooperative elimination and followed by the elimination of the other alkyl radicals without dissociation of the Ga—As bond, The important role of the substrate in the growth process using this adduct has been shown [5]. so that the mechanism would not be independent of the substrate. However, we have thought that for kinetics reasons the nature of the heterogeneous reaction might be compared to that reported in earlier works [1,5,6,10]. The growth rate versus the reciprocal deposition temperature for various GaAs-containing layers prepared with ClMe 2Ga AsEt3 or related adducts is shown in fig. 2. These plots exhibit the classical temperature dependent region, i.e. the low temperature kinetically controlled growth regime and the mass trans-
DEPO~IT TEM~’ERATUR~ 1°C)
7
I
~
600
~2
500
~
~ ‘1
-
~
8 0.3 0.2
.
01
-
/~~\
~‘
/ /
0.4
\
\ \
/
A
A
-
~
~
GaAsEt
b)
2
~
2
3
b)
~ CIEt2GaAsMe3 ~ CE~ GaAsEt + HPEt
I
1.0
~
/ ~
-
\
62
~ CIMe GaAsEt
.
-
I
C)
2 I
I I 121000/T
I 1.3 (K1)
1.4
11
Fig. 2. Growth rate versus reciprocal deposition temperature for GaAs (~A, K~)and GaAsP (0) layers. The starting precursors used are mentioned in the insert and the experimental data for (a), (b) and (c) are issued from refs. [5,61,[1] and 1101, respectively.
port limited growth region at higher temperature. Furthermore, according to the results of Reep and Ghandhi [9] an apparent activation energy corresponding to the heterogeneous reactions may be calculated from the slope in the low temperature range. We may note that the apparent activation energies found are in the range of 25—46 kcal moL1 each time that the adduct used contains a Cl atom and an Et group on the Ga and the As atom, respectively. The activation energies are significantly higher when Me groups are borne by the As atom (—~62 kcal mol 1), For these reasons these apparent activation energies might be related to the chloroalkyl elimination. These higher activation energies found when Me groups are bonded to As are in agreement with the fact that the As—Me bond strength is larger than the As—Et bond strength.
188
F Maury eta!.
/
Chemisorption and decomposition of MOCVD adduct CIMe,GaAsEt
References [1] A. Zaouk, F. Salvetat, J. Sakaya, F. Maury and G. Constant, J. Crystal Growth 55 (1981) 135. [2] RH. Moss and J.S. Evans, J. Crystal Growth 55 (1981) 129. [31K.W. Benz, H. Renz, J. Weidlein and M.H. Pilkuhn, J. Electron Mater. 10 (1981) 185. [41S. Minagawa, H. Nakamura and H. Sano, J. Crystal Growth 71(1985) 377 [5] A. Zaouk and G. Constant, J. Physique 43 (1982) C5—421.
3
[61 A. Zaouk, Thesis, Institut National Polytechnique, Toulouse (1983). [7] F. Maury, A. El Hammadi and G. Constant, J. Crystal Growth 68 (1984) 88. [8] D.J. Schlyer and MA. Ring, J. Electrochem. Soc. 124 (1977) 569. [91D.H. Reep and S.K. Ghandhi, J. Electrochem. Soc. 130 (1983) 675. [10] F. Maury and G. Constant, J. Crystal Growth 62 (1983) 568.
185
LETTER TO THE EDITORS SOME INVESTIGATIONS ON THE CHEMISORPTION AND THERMAL HETEROGENEOUS DECOMPOSITION OF THE MOCVD ADDUCT CIMe2GaAsEt3 F. MAURY and G. CONSTANT Laboratoire de Cristallochimie, Rèactivitè et Protection des Matériaux, Unite Associée au CNRS 445, ENSCT, 118 Route de Narbonne, F-31077 Toulouse Cèdex, France
and P. FONTAINE and J.P. BIBERIAN Unite Associêe au CNRS 794, Faculté des Sciences de Luminy, Department de Physique, Case 901, F-13288 Marseille Cédex 9, France
Received 2 October 1986; manuscript received in final form 17 July 1985
An AES study of the chemisorption of the molecule CIMe2GaAsEt3 on a Si(111) substrate and its thermal heterogeneous decomposition is proposed. The experimental results might be interpreted using a Langmuir—Hinshelwood type mechanism where the EtC1 cooperative elimination is a prominent step. This would be a new argument for our earlier assumptions on the chemical mechanism of the MOCVD GaAs growth using this adduct as a single precursor.
Adducts become increasingly important as precursors in metalorganic vapour phase epitaxy and they are especially used in three basic ways for the preparation of Ill—V compounds: (i) as single sources for both group III and group V elements [1]; (ii) formed in situ from the component molecules to avoid unwanted chemical side reactions [2]; (iii) as direct replacement for group III alkyls [3,4]. The important features in the preparation of layers, i.e. composition control, growth rate, crystal quality, purity and consequently electrical properties, are in all cases determined by the chemistry of the processes used. Thereby, since the heterogeneous decomposition of organometallic precursors is an essential part of the chemistry of this MOCVD process, its study requires much attention. In a previous work we have made assumptions on the basic chemical mechanism for the growth of Ill—V materials using adducts as single sources, which took into account the chemisorption step and differences between the strengths of the vanous bonds [1]. These deductions were issued from several studies such as analyses of growth kinetics
and composition of the pyrolysis gas versus the deposit temperature, and also correlations between various CVD results and physico-chemical properties of the starting material. The work presented in this letter was carried out after the successful use of the compounds C1R2Ga AsEt~ (R = Me, Et) [1,5,6], (C6F5)Me2Ga. AsEt3 and [C1Et2Ga AsEt2]2CH2 [7] for the GaAs epitaxial growth in order to afford a further insight into the nature of the chemical mechanism of the GaAs growth using adducts as single sources. An Auger electron spectroscopy (AES) study of the chemisorption of the adduct ClMe2Ga AsEt~on Si(111) and its thermal heterogeneous decomposition is reported. .
A stainless steel cylinder, containing the organometallic molecule C1Me2Ga AsEt~ under vacuum, was linked to an ultra high vacuum (UHV) chamber equipped with an AES spectrometer, a LEED and a mass spectrometer. A clean 7 x 7 surface of the Si(1 11) substrate was obtained by several sequences of heatings in vacuum (3 x 10— 10 Torr) to 1100°C.The starting
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
F. Maury et al.
/
Chemisorption and decomposition of MOC VD adduct ClMe,GaAsEt~
adduct was thermostated at 60°Cin order to have 1 Torr of vapour pressure in the cylinder and the introduction of the molecule in the UHV chamber was controlled with a needle valve by recording continuously the AES spectra of the Si(111) surface and the mass spectra of the gaseous phase above the substrate. The vacuum was 10~ Torr in the chamber and only argon as residual gas was observed in the mass spectra. The AES analyses of the substrate
did intensity stant about not in iO~ exhibit this of pressure theions Torr, any Si(92 element eV) the signals introduction Then, other than was at a 16), quite vacuum Siwere and conof observed The organometallic main both features on molecule the of mass the was mass spectra obtained spectra and and byof itAES. was the characteristic starting molecule ofrange. the CH~ fragmentation (m/q= of C (28), HCl~ (36), [6]: CHCC1~ (50), C2H4CI~ (63), 2H~ AsH~ (78), AsCH~(91), HAsEt~ (105) and AsEt~ (133). Although it was not possible to analyze the high energy range with our AES spectrometer, and consequently to detect with certainty the Ga(1070 eV) and As(1228 eV) transitions, evidence for the adsorption of the molecule mass given on the one hand, by the sharp decrease of the intensity of the Si(92 eV) signal when the pressure increased from 10~ Torr (due to a partial coverage of the surface) and on the other hand, by the apparition of the C(272 eV) and Cl(181 eV) signals. The detection of Ga(55 and 81 eV) and As(79 and 91 eV) in the low energy range is uncertain because of their low intensity and interference with the Si(43, 73, 79 and 92 eV) peaks. The Si surface was exposed to a partial pressure of ClMe2Ga~AsEt3 of about 10~ Torr at 20°Cduring several minutes and then the temperature of the substrate was increased in gradual steps and the AES spectra were recorded. Fig. 1 shows the variation of the Auger peak heights of Si, C and Cl with the temperature of the substrate measured with an optical pyrometer. Further hypotheses have been suggested without being fully satisfactory to account for these experimental data. One of the more plausible cxplanations that could be proposed is to assume
~
~GoA~
__________________ ~
120
,~
_________________ ~
/
-
/
/~S.\
~ ~
‘s 80
-
/ /__
~/
/
.—
i 0
/1÷101
200
~ 400 600 TEMPERATuRE
I 800 1°C)
S
I 1000
Fig. I. variation of the Auger peak heights with the substrate temperature after an exposure of the Si(111) surface to the CIMe2GaAsEt3 molecule. The inserts schematize the adsorption of the molecule on the surface of a substrate: (a) before the beginning of the decomposition; (b) after the cooperative elimination of EtCI.
that, at room temperature, molecules might be adsorbed without dissociation of the Ga—As bonds, as this is schematized in the insert (a) of fig. 1. Thus, the pseudolayer analyzed in this experiment would contain the Cl atom bonded to the Ga atom and an ethyl group borne by the As atom. By increasing the substrate temperature up to 450°C, the opposed configuration taken by the molecules would favour a cooperative elimination of these two groups which would explain the decrease of the Cl Auger peak heights. At the same time, increasing the substrate temperature up to 300°Cwould enhance the surface mobility of various adsorbed species and a better coverage of the surface of the Si substrate would be obtamed, leading to a slight decrease of the Si peak
F. Maury et a!.
/
1 87
Chemisorption and decomposition of MOC VD adduct ClMe,GaAsEt~
height as observed in fig. 1. Another consequence of this better coverage in this temperature range would be a slight increase of the C signal. Although a cooperative elimination of C2H5CI is proposed to explain the decrease of the Cl peak height, the same variation for the C signal is not observed since its intensity increases sharply from 400°Cand reaches a maximum at about 500°C when the Cl signal has almost disappeared. This discrepancy between the C and the Cl curves seems in agreement with our assumption, since it would then be probable that the carbon content in the pseudolayer analyzed after the EtC1 elimination would be larger than in the pseudolayer analyzed near the room temperature, because it would then be composed of planar species such as [Et2Ga’ AsEt2] (fig. Ib). Above 600°C, the decrease of the C peak height would correspond both to the elimination of the other alkyl radicals and to the desorption of the various species, since the Si signal increases rapidly. An interesting feature of fig. 1 is the fact that the disappearance of thetemperature Cl signal, atasabout 520°C. takes place at the same the minimal temperature of the GaAs deposition in MOCVD experiments using this precursor [5,6]. These experimental results would argue for a Langmuir—Hinshelwood type mechanism related to that of Schlyer and Ring [8] where the adduct would be adsorbed on the surface of the substrate prior to reaction in such a way that its thermal decomposition may be initiated by the EtC1 cooperative elimination and followed by the elimination of the other alkyl radicals without dissociation of the Ga—As bond, The important role of the substrate in the growth process using this adduct has been shown [5]. so that the mechanism would not be independent of the substrate. However, we have thought that for kinetics reasons the nature of the heterogeneous reaction might be compared to that reported in earlier works [1,5,6,10]. The growth rate versus the reciprocal deposition temperature for various GaAs-containing layers prepared with ClMe 2Ga AsEt3 or related adducts is shown in fig. 2. These plots exhibit the classical temperature dependent region, i.e. the low temperature kinetically controlled growth regime and the mass trans-
DEPO~IT TEM~’ERATUR~ 1°C)
7
I
~
600
~2
500
~
~ ‘1
-
~
8 0.3 0.2
.
01
-
/~~\
~‘
/ /
0.4
\
\ \
/
A
A
-
~
~
GaAsEt
b)
2
~
2
3
b)
~ CIEt2GaAsMe3 ~ CE~ GaAsEt + HPEt
I
1.0
~
/ ~
-
\
62
~ CIMe GaAsEt
.
-
I
C)
2 I
I I 121000/T
I 1.3 (K1)
1.4
11
Fig. 2. Growth rate versus reciprocal deposition temperature for GaAs (~A, K~)and GaAsP (0) layers. The starting precursors used are mentioned in the insert and the experimental data for (a), (b) and (c) are issued from refs. [5,61,[1] and 1101, respectively.
port limited growth region at higher temperature. Furthermore, according to the results of Reep and Ghandhi [9] an apparent activation energy corresponding to the heterogeneous reactions may be calculated from the slope in the low temperature range. We may note that the apparent activation energies found are in the range of 25—46 kcal moL1 each time that the adduct used contains a Cl atom and an Et group on the Ga and the As atom, respectively. The activation energies are significantly higher when Me groups are borne by the As atom (—~62 kcal mol 1), For these reasons these apparent activation energies might be related to the chloroalkyl elimination. These higher activation energies found when Me groups are bonded to As are in agreement with the fact that the As—Me bond strength is larger than the As—Et bond strength.
188
F Maury eta!.
/
Chemisorption and decomposition of MOCVD adduct CIMe,GaAsEt
References [1] A. Zaouk, F. Salvetat, J. Sakaya, F. Maury and G. Constant, J. Crystal Growth 55 (1981) 135. [2] RH. Moss and J.S. Evans, J. Crystal Growth 55 (1981) 129. [31K.W. Benz, H. Renz, J. Weidlein and M.H. Pilkuhn, J. Electron Mater. 10 (1981) 185. [41S. Minagawa, H. Nakamura and H. Sano, J. Crystal Growth 71(1985) 377 [5] A. Zaouk and G. Constant, J. Physique 43 (1982) C5—421.
3
[61 A. Zaouk, Thesis, Institut National Polytechnique, Toulouse (1983). [7] F. Maury, A. El Hammadi and G. Constant, J. Crystal Growth 68 (1984) 88. [8] D.J. Schlyer and MA. Ring, J. Electrochem. Soc. 124 (1977) 569. [91D.H. Reep and S.K. Ghandhi, J. Electrochem. Soc. 130 (1983) 675. [10] F. Maury and G. Constant, J. Crystal Growth 62 (1983) 568.