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Formation of CH3 radicals in the decomposition of trimethyl aluminum on hot solid surfaces
Volume 116, number 6
CHEMICAL PHYSICS LETTERS
24 May 1985
FORMATION OF CH 3 RADICALS IN THE DECOMPOSITION OF TRIMETHYL ALUMINUM ON HOT SOLID SURFACES D.W. S Q U I R E 1, C.S. D U L C E Y 2 and M.C. L I N Chemistry Division, Code 6105, Naval Research Laboratory, Washington, DC 20375- 5000, USA
Received 19 February 1985
The thermal decomposition of trimethyl aluminum on different hot substrates has been studied under low-pressure conditions. The products of the decomposition reaction were analyzed mass-spectrometrically using electron-impact and resonance-enhanced multiphoton ionization. The results of this study reveal that only CH 3 appears as a gas-phase product and stable compounds such as CH 4 and C2H 6 are not observed under the conditions employed. The apparent activation energy for the production of the CH 3 radical from the Al-coated hot substrates is measured to be 11 + 2 kcal/mole.
1. Introduction Recently organometallic compounds have been used increasingly for perparation of binary or tertiary heterostructure semiconductors by means o f OMCVD (organometallic chemical vapor deposition) [ 1]. In this process, the binary or tertiary mixtures of different metal alkyls and hydrides are carried into a high-temperature reactor with inert gases such as H2, N 2 or rare gases. For the preparation of GaAs, for example, mixtures ofGa(CH3) 3 and AsH 3 have been employed as OMCVD reagents using H 2 carrier gas [2]. It is known that CH 4 and C2H 6 are the two major gaseous products formed in the deposition reaction [1,2]. However, it is not clear how these products are formed and what the detailed mechanism for the decomposition of metal alkyls is on hot solid surfaces. Mechanistically, it is possible that these stable organic products can be directly formed by surface reactions, such as [2]: Ga(CH3)x(a ) + AsHy(a) CH 4 + Ga(CH3) x _ l(a) + AsHy_ 1 , NRC/NRL Postdoctoral Research Associate. 2 Permanent address: Geo-Centers, Inc., 320 Needham Street, Newton Upper Falls, MA 02164, USA. 0 009-2614/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Ga(CH3)x(a ) -~ C2H 6 + Ga(CH3) x _ 2(a), where x, y ~< 3 and (a) represents adsorbedspecies. On the other hand, both CH 4 and C2H 6 may actually be formed in the gas phase by the well known process: CH 3 + R H ~ H C 4 + R , CH 3 + CH 3 ~ C2H 6 , where R = H, ASH2, etc. In this study we employed trimethyl aluminum (TMA), a commonly used OMCVD source for the elemental AI [1 ], as a model compound to elucidate the mechanism for the decomposition of metal alkyls on hot substrate surfaces under low-pressure conditions. The products o f this decomposition reaction were analyzed mass-spectrometrically using electron impact (for stable product detection) and resonanceenhanced multiphoton ionization (REMPI) for CH 3 probing [ 3 - 5 ] . The results of this study indicate unequivocally that the thermal decomposition of TMA on different hot substrates produces only the CH 3 radical as the initial gas phase product. Neither CH 4 nor C2H 6 was observed within the detectivity of our mass-spectrometric measurement under the low-pressure conditions, in which secondary reactions cannot readily take place. 525
Volume 116, number 6
2. Experimental The laser ionization mass spectrometer used in these experiments is that previously used in methyl radical detection [3-5], modified for studying chemical vapor deposition processes. In the new arrangement, TMA seeded in helium (1 : 1000) is leaked through a quartz orifice ,-~1 mm in diameter directly onto the surface under study. The surface, capable of being heated to 900 K, was held at 45 ° to both the direction of gas flow and the quadrupole mass spectrometer axis. Reaction products desorbing from the surface passed through a 3 mm skimmer into the ionization region of the differentially pumped quadrupole mass spectrometer (Extranuclear). The orifice/surface and surface/ionization region distances were 10 and 25 mm, respectively. Surface heating was accomplished by resistively heating tantalum wires, which conductively heated a tantalum plate on which the surfaces were mounted. Surface temperatures were measured by a thermocouple spotwelded to the backing plate. Copper foil (NRL stock) and aluminum foil (Safeway) were clamped onto the backing plate by a spot-welded tantalum ring. This surface arrangement offered the ideal combination of low thermal inertia and uniform surface temperature. Standard surface cleaning procedures were used. Helium (Matheson, ~1000 Torr) was passed through a liquid nitrogen cooled molecular sieve to remove water and other impurities, and was then bubbled through TMA held at 0°C (~1 Torr vapor pressure). The TMA (Pfaltz and Bauer) was purified by trap-totrap distillation before use. This combination minimized TMA source degradation (see section 4). The gas mixture from the bubbler was then bled through the orifice onto the surface to produce a 2.0 X 10-6 Torr rise in the ambient reaction chamber pressure. This pressure rise was over the 4 X 10-7 Torr background pressure. (This background probably produces most of the background hydrocarbon signal in figure 2.) It is important to note that an estimated TMA partial pressure of ~ 1 0 - 8 Torr at the surface gave rise to all resuits presented here. Reaction products left the surface and passed through the skimmer and into the ionization region of a quadrupole mass spectrometer. The detection system used has been previously described [3-5]. Briefly, the output of a Nd : YAG laser pumped dye laser 526
24 May 1985
CHEMICAL PHYSICS LETTERS
(Quantel, with Oxazine 720 dye) is doubled to 3 3 3 335 nm and focused through a 50 mm quartz lens into the ionization region of the mass spectrometer. Typically, laser powers were 200-300 #J per shot in the ultraviolet, more than enough for the two-photon resonant transition through the 3p 2A 2 Rydberg state in the methyl radical [5].
3. Results Mass spectrometric analysis of the products of TMA decomposition on different hot substrates by electron impact and REMPI revealed that CH 3 is the only gaseeous product formed in the decomposition reaction. The CH 3 radical was detected by its strong two-photon REMPI at 333.4 nm as mentioned'above. Fig. 1 displays the results of a wavelength scan between 332 and 335 nm with the mass spectrometer m o n itoring the CH~ ions (m/z 15) produced for two different surface temperatures. In the absence of heating (T s = 298 K), no observable resonance appears in the region of wavelength scanned. When the surface was heated above 550 K, strong REMPI signals at 333.4 nm began to appear. The upper spectrum given in fig. I shows the result obtained at T s = 838 K.
H
9
i
i
353
534
I
~,//NM
Fig. 1. Methyl radical (m/z 15) ion signalfrom a copper surface versus wavelength at 298 and 838 K. In both case~the ambient pressure is 2.4 X 10-6 Tore (Background 0.4 X 10-6 Torr.) The laser power for both these runs was about 300 ~J per laser pulse.
Volume 116, number 6
CHEMICAL PHYSICS LETTERS
24 May 1985
10 4
"
m
u
1.2
1.4
I-I 4
L ~ , ,
~ A ,
882K
1.6
....
r ....
I .... 20
r ....
I .... 30
I ....
i .... 40
i ....
I .... 50
/
3
2
I0
/~
~
~o3/T
0 ~ o 0
A
i .... 60
M/Z Fig. 2. MPI mass spectra at two surface temperatures, 296 and 882 K. Masses above m[z 60 are insignificant. Spectra were taken at 333.4 nm (~-300 #J per pulse) with an ambient pressure of 2.4 X 10-6 Torr. Note: background pressure is 4 X 10 -7 Tort, but it appears to produce almost all signal exceptm/z 12, 15 and 27. We have also recorded the mass spectra o f desorbed products at different temperatures. Fig. 2 shows the spectra covering m/z between 12 and 60 for two different substrate temperatures while the probing dye laser wavelength was fixed at 333.4 nm. At T s = 296 K, the complex spectrum displayed by fig. 2a results from the MPI o f the scattered TMA and background contaminants in the ionization region. The former was effectively decomposed when the substrate temperature was increased to 880 K. The rn/z 15 peak on the other hand grew stronger as the substrate temperature was increased, clearly indicating the facility o f the TMA decomposition and CH 3 radical desorption processes. The production o f the CH 3 radical as a function o f substrate temperature is shown in fig. 3. In this experiment the concentration o f TMA and the probing dye laser wavelength (333.4 nm) was maintained constant and the substrate temperature was varied from 300 to as high as 800 K. The intensity o f the m/z 15 peak was found to increase steeply above 550 K. The apparent activation energy for CH 3 production, E a -+ 2o = 11 + 2 kcal/mole, was derived by a least-squares fit o f the data in fig. 3 to the equation o f the form I = A + B
0 0
I 500
0 I 400
I 500
0 I 600
I 700
I 800
I 900
T/K Fig. 3. Methyl radical ion signalversustemperature for four different experiments with Arrhenius plot of same data (inset). Data sets were scaled to the intensities at 673 K. The scatter in data is attributed to laser power fluctuations accentuated in the two-photon transition. All points are taken at 333.4 rim, 200-300 #J per laser shot, and 2.0 × 10-6 Tort of the TMA : He mixture over background.
X exp(-Ea/RT). The Arrhenius plot for the changes in the m/z 15 ion signals over those detected below 500 K, which gives rise to essentially the same activation energy, is shown in the inset o f fig. 3. 4. Discussion The results o f this study clearly show that only CH 3 radicals are produced in the decomposition of TMA on hot metal substrates under low-pressure conditions. An extensive search for CH 4 and C2H 6 over a wide range o f temperature indicated that they were not present in the decomposition reaction. This result rules out the possibility o f direct formation o f these organic products when TMA is decomposed on bot substrates. At room temperature, TMA exists primarily as the dimer, A12(CH3) 6 ( A H 0 for TMA 2 ~ 2TMA is known to be 20 kcal/mole [6]). In order to test the effect o f the dimers on CH 3 production, the inlet tube was 527
Volume 116, number 6
CHEMICAL PHYSICS LETTERS
heated to 360 K. The heating was found to have no effect on the intensity of the m/z 15 REMPI signal. This indicates that the production of CH 3 radicals apparently is not controlled by the dissociation of the TMA dimers on the surface: A12(CH3) 6 + S --* 2AI(CH3)3(a ) . The observed low activation energy (11 + 2 kcal/ mole) for CH 3 production may be partly due to the formation of the new = A I - A I ( a ) surface bond: (CH3)3AI + :Al(a) (CH3) 3 _ x A I - A l ( a ) + xCH 3 . The high reactivity of TMA with various molecules is well known (e.g., ref. [7]). It can spontaneously react with 0 2, H 2 0 , alcohols and many other organic compounds through interactions with the electron deficient A1 atom in TMA. The formation of the new A I M bond on the surface may weaken at least the first A I - C H 3 bond in the TMA molecule. In the absence of direct diagnostics for the deposited surface in the present study, however, we are not sure how many CH 3 groups remain on the surface after the initial impact. In addition to the use o f the copper surface, other substrates such as household aluminum foils were also employed. These brief tests showed no noticeable change in the observed temperature effect on CH 3 production. This result is perhaps not surprising because the surface is expected to be heavily covered with A1 instances after the TMA flow is initiated. Further experiments on the decomposition reaction over semiconductors and insulators as well as the effects o f pressure and carrier gases (e.g., H 2 versus He) on the formation of CH 3 are still underway. These resuits will be reported in detail at a later date.
5. Concluding remarks In this study we have employed the very sensitive
528
24 May 1985
technique of resonance-enhanced multiphoton ionization-mass spectrometry (REMPI-MS), recently developed in this laboratory for the detection of non-fluorescing free radicals [8], to elucidate the mechanism of trimethyl aluminum decomposition on hot substrates. Our results clearly show that the commonly known organic products from metal methyl CVD reactions, CH 4 and C2H6, are not formed directly on the surface. Only CH 3 radicals were detected under the low-pressure conditions employed. The combination o f the REMPI and electron-impact mass spectrometry allows us for the first time to elucidate unequivocally the mechanism of metal alkyl decomposition on solid surfaces. We plan to employ the same technique in the near future to resolve, for example, if the following surface reaction, GaCH3(a ) + AsH(a) ~ CH 4 + GaAs is indeed the rate-controlling step in the Ga(CH3) 3 AsH 3 CVD process [2].
References [1] P.D. Dapkus, Ann. Rev. Mat. Sci. 12 (1982) 243. [2] D.H. Reep and S.K. Ghandhi, J. Electrochem. Soc. SolidState Sci. Tech. 130 (198'3) 675. [3] T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, Chem. Phys. Letters 82 (1981) 267. [4] T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Phys. Chem. 86 (1982) 36. [5] J.W. Hudgens, T.G. DiGiuseppe and M.C. Lin, J. Chem. Phys. 79 (1983) 571. [6] C.H. Hendrickson and D.P. Eymam, Inorg. Chem. 6 (1967) 1461. [7] The Use of Aluminum Alkyls in Organic Synthesis, and subsequent supplements (Ethyl Corp., 1968-1978). [8] M.C. Lin and W.A. Sanders, in: Multiphoton processes and spectroscopy, ed. S.H. Lin (World Scientific Publ., Singapore), to be published.
CHEMICAL PHYSICS LETTERS
24 May 1985
FORMATION OF CH 3 RADICALS IN THE DECOMPOSITION OF TRIMETHYL ALUMINUM ON HOT SOLID SURFACES D.W. S Q U I R E 1, C.S. D U L C E Y 2 and M.C. L I N Chemistry Division, Code 6105, Naval Research Laboratory, Washington, DC 20375- 5000, USA
Received 19 February 1985
The thermal decomposition of trimethyl aluminum on different hot substrates has been studied under low-pressure conditions. The products of the decomposition reaction were analyzed mass-spectrometrically using electron-impact and resonance-enhanced multiphoton ionization. The results of this study reveal that only CH 3 appears as a gas-phase product and stable compounds such as CH 4 and C2H 6 are not observed under the conditions employed. The apparent activation energy for the production of the CH 3 radical from the Al-coated hot substrates is measured to be 11 + 2 kcal/mole.
1. Introduction Recently organometallic compounds have been used increasingly for perparation of binary or tertiary heterostructure semiconductors by means o f OMCVD (organometallic chemical vapor deposition) [ 1]. In this process, the binary or tertiary mixtures of different metal alkyls and hydrides are carried into a high-temperature reactor with inert gases such as H2, N 2 or rare gases. For the preparation of GaAs, for example, mixtures ofGa(CH3) 3 and AsH 3 have been employed as OMCVD reagents using H 2 carrier gas [2]. It is known that CH 4 and C2H 6 are the two major gaseous products formed in the deposition reaction [1,2]. However, it is not clear how these products are formed and what the detailed mechanism for the decomposition of metal alkyls is on hot solid surfaces. Mechanistically, it is possible that these stable organic products can be directly formed by surface reactions, such as [2]: Ga(CH3)x(a ) + AsHy(a) CH 4 + Ga(CH3) x _ l(a) + AsHy_ 1 , NRC/NRL Postdoctoral Research Associate. 2 Permanent address: Geo-Centers, Inc., 320 Needham Street, Newton Upper Falls, MA 02164, USA. 0 009-2614/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Ga(CH3)x(a ) -~ C2H 6 + Ga(CH3) x _ 2(a), where x, y ~< 3 and (a) represents adsorbedspecies. On the other hand, both CH 4 and C2H 6 may actually be formed in the gas phase by the well known process: CH 3 + R H ~ H C 4 + R , CH 3 + CH 3 ~ C2H 6 , where R = H, ASH2, etc. In this study we employed trimethyl aluminum (TMA), a commonly used OMCVD source for the elemental AI [1 ], as a model compound to elucidate the mechanism for the decomposition of metal alkyls on hot substrate surfaces under low-pressure conditions. The products o f this decomposition reaction were analyzed mass-spectrometrically using electron impact (for stable product detection) and resonanceenhanced multiphoton ionization (REMPI) for CH 3 probing [ 3 - 5 ] . The results of this study indicate unequivocally that the thermal decomposition of TMA on different hot substrates produces only the CH 3 radical as the initial gas phase product. Neither CH 4 nor C2H 6 was observed within the detectivity of our mass-spectrometric measurement under the low-pressure conditions, in which secondary reactions cannot readily take place. 525
Volume 116, number 6
2. Experimental The laser ionization mass spectrometer used in these experiments is that previously used in methyl radical detection [3-5], modified for studying chemical vapor deposition processes. In the new arrangement, TMA seeded in helium (1 : 1000) is leaked through a quartz orifice ,-~1 mm in diameter directly onto the surface under study. The surface, capable of being heated to 900 K, was held at 45 ° to both the direction of gas flow and the quadrupole mass spectrometer axis. Reaction products desorbing from the surface passed through a 3 mm skimmer into the ionization region of the differentially pumped quadrupole mass spectrometer (Extranuclear). The orifice/surface and surface/ionization region distances were 10 and 25 mm, respectively. Surface heating was accomplished by resistively heating tantalum wires, which conductively heated a tantalum plate on which the surfaces were mounted. Surface temperatures were measured by a thermocouple spotwelded to the backing plate. Copper foil (NRL stock) and aluminum foil (Safeway) were clamped onto the backing plate by a spot-welded tantalum ring. This surface arrangement offered the ideal combination of low thermal inertia and uniform surface temperature. Standard surface cleaning procedures were used. Helium (Matheson, ~1000 Torr) was passed through a liquid nitrogen cooled molecular sieve to remove water and other impurities, and was then bubbled through TMA held at 0°C (~1 Torr vapor pressure). The TMA (Pfaltz and Bauer) was purified by trap-totrap distillation before use. This combination minimized TMA source degradation (see section 4). The gas mixture from the bubbler was then bled through the orifice onto the surface to produce a 2.0 X 10-6 Torr rise in the ambient reaction chamber pressure. This pressure rise was over the 4 X 10-7 Torr background pressure. (This background probably produces most of the background hydrocarbon signal in figure 2.) It is important to note that an estimated TMA partial pressure of ~ 1 0 - 8 Torr at the surface gave rise to all resuits presented here. Reaction products left the surface and passed through the skimmer and into the ionization region of a quadrupole mass spectrometer. The detection system used has been previously described [3-5]. Briefly, the output of a Nd : YAG laser pumped dye laser 526
24 May 1985
CHEMICAL PHYSICS LETTERS
(Quantel, with Oxazine 720 dye) is doubled to 3 3 3 335 nm and focused through a 50 mm quartz lens into the ionization region of the mass spectrometer. Typically, laser powers were 200-300 #J per shot in the ultraviolet, more than enough for the two-photon resonant transition through the 3p 2A 2 Rydberg state in the methyl radical [5].
3. Results Mass spectrometric analysis of the products of TMA decomposition on different hot substrates by electron impact and REMPI revealed that CH 3 is the only gaseeous product formed in the decomposition reaction. The CH 3 radical was detected by its strong two-photon REMPI at 333.4 nm as mentioned'above. Fig. 1 displays the results of a wavelength scan between 332 and 335 nm with the mass spectrometer m o n itoring the CH~ ions (m/z 15) produced for two different surface temperatures. In the absence of heating (T s = 298 K), no observable resonance appears in the region of wavelength scanned. When the surface was heated above 550 K, strong REMPI signals at 333.4 nm began to appear. The upper spectrum given in fig. I shows the result obtained at T s = 838 K.
H
9
i
i
353
534
I
~,//NM
Fig. 1. Methyl radical (m/z 15) ion signalfrom a copper surface versus wavelength at 298 and 838 K. In both case~the ambient pressure is 2.4 X 10-6 Tore (Background 0.4 X 10-6 Torr.) The laser power for both these runs was about 300 ~J per laser pulse.
Volume 116, number 6
CHEMICAL PHYSICS LETTERS
24 May 1985
10 4
"
m
u
1.2
1.4
I-I 4
L ~ , ,
~ A ,
882K
1.6
....
r ....
I .... 20
r ....
I .... 30
I ....
i .... 40
i ....
I .... 50
/
3
2
I0
/~
~
~o3/T
0 ~ o 0
A
i .... 60
M/Z Fig. 2. MPI mass spectra at two surface temperatures, 296 and 882 K. Masses above m[z 60 are insignificant. Spectra were taken at 333.4 nm (~-300 #J per pulse) with an ambient pressure of 2.4 X 10-6 Torr. Note: background pressure is 4 X 10 -7 Tort, but it appears to produce almost all signal exceptm/z 12, 15 and 27. We have also recorded the mass spectra o f desorbed products at different temperatures. Fig. 2 shows the spectra covering m/z between 12 and 60 for two different substrate temperatures while the probing dye laser wavelength was fixed at 333.4 nm. At T s = 296 K, the complex spectrum displayed by fig. 2a results from the MPI o f the scattered TMA and background contaminants in the ionization region. The former was effectively decomposed when the substrate temperature was increased to 880 K. The rn/z 15 peak on the other hand grew stronger as the substrate temperature was increased, clearly indicating the facility o f the TMA decomposition and CH 3 radical desorption processes. The production o f the CH 3 radical as a function o f substrate temperature is shown in fig. 3. In this experiment the concentration o f TMA and the probing dye laser wavelength (333.4 nm) was maintained constant and the substrate temperature was varied from 300 to as high as 800 K. The intensity o f the m/z 15 peak was found to increase steeply above 550 K. The apparent activation energy for CH 3 production, E a -+ 2o = 11 + 2 kcal/mole, was derived by a least-squares fit o f the data in fig. 3 to the equation o f the form I = A + B
0 0
I 500
0 I 400
I 500
0 I 600
I 700
I 800
I 900
T/K Fig. 3. Methyl radical ion signalversustemperature for four different experiments with Arrhenius plot of same data (inset). Data sets were scaled to the intensities at 673 K. The scatter in data is attributed to laser power fluctuations accentuated in the two-photon transition. All points are taken at 333.4 rim, 200-300 #J per laser shot, and 2.0 × 10-6 Tort of the TMA : He mixture over background.
X exp(-Ea/RT). The Arrhenius plot for the changes in the m/z 15 ion signals over those detected below 500 K, which gives rise to essentially the same activation energy, is shown in the inset o f fig. 3. 4. Discussion The results o f this study clearly show that only CH 3 radicals are produced in the decomposition of TMA on hot metal substrates under low-pressure conditions. An extensive search for CH 4 and C2H 6 over a wide range o f temperature indicated that they were not present in the decomposition reaction. This result rules out the possibility o f direct formation o f these organic products when TMA is decomposed on bot substrates. At room temperature, TMA exists primarily as the dimer, A12(CH3) 6 ( A H 0 for TMA 2 ~ 2TMA is known to be 20 kcal/mole [6]). In order to test the effect o f the dimers on CH 3 production, the inlet tube was 527
Volume 116, number 6
CHEMICAL PHYSICS LETTERS
heated to 360 K. The heating was found to have no effect on the intensity of the m/z 15 REMPI signal. This indicates that the production of CH 3 radicals apparently is not controlled by the dissociation of the TMA dimers on the surface: A12(CH3) 6 + S --* 2AI(CH3)3(a ) . The observed low activation energy (11 + 2 kcal/ mole) for CH 3 production may be partly due to the formation of the new = A I - A I ( a ) surface bond: (CH3)3AI + :Al(a) (CH3) 3 _ x A I - A l ( a ) + xCH 3 . The high reactivity of TMA with various molecules is well known (e.g., ref. [7]). It can spontaneously react with 0 2, H 2 0 , alcohols and many other organic compounds through interactions with the electron deficient A1 atom in TMA. The formation of the new A I M bond on the surface may weaken at least the first A I - C H 3 bond in the TMA molecule. In the absence of direct diagnostics for the deposited surface in the present study, however, we are not sure how many CH 3 groups remain on the surface after the initial impact. In addition to the use o f the copper surface, other substrates such as household aluminum foils were also employed. These brief tests showed no noticeable change in the observed temperature effect on CH 3 production. This result is perhaps not surprising because the surface is expected to be heavily covered with A1 instances after the TMA flow is initiated. Further experiments on the decomposition reaction over semiconductors and insulators as well as the effects o f pressure and carrier gases (e.g., H 2 versus He) on the formation of CH 3 are still underway. These resuits will be reported in detail at a later date.
5. Concluding remarks In this study we have employed the very sensitive
528
24 May 1985
technique of resonance-enhanced multiphoton ionization-mass spectrometry (REMPI-MS), recently developed in this laboratory for the detection of non-fluorescing free radicals [8], to elucidate the mechanism of trimethyl aluminum decomposition on hot substrates. Our results clearly show that the commonly known organic products from metal methyl CVD reactions, CH 4 and C2H6, are not formed directly on the surface. Only CH 3 radicals were detected under the low-pressure conditions employed. The combination o f the REMPI and electron-impact mass spectrometry allows us for the first time to elucidate unequivocally the mechanism of metal alkyl decomposition on solid surfaces. We plan to employ the same technique in the near future to resolve, for example, if the following surface reaction, GaCH3(a ) + AsH(a) ~ CH 4 + GaAs is indeed the rate-controlling step in the Ga(CH3) 3 AsH 3 CVD process [2].
References [1] P.D. Dapkus, Ann. Rev. Mat. Sci. 12 (1982) 243. [2] D.H. Reep and S.K. Ghandhi, J. Electrochem. Soc. SolidState Sci. Tech. 130 (198'3) 675. [3] T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, Chem. Phys. Letters 82 (1981) 267. [4] T.G. DiGiuseppe, J.W. Hudgens and M.C. Lin, J. Phys. Chem. 86 (1982) 36. [5] J.W. Hudgens, T.G. DiGiuseppe and M.C. Lin, J. Chem. Phys. 79 (1983) 571. [6] C.H. Hendrickson and D.P. Eymam, Inorg. Chem. 6 (1967) 1461. [7] The Use of Aluminum Alkyls in Organic Synthesis, and subsequent supplements (Ethyl Corp., 1968-1978). [8] M.C. Lin and W.A. Sanders, in: Multiphoton processes and spectroscopy, ed. S.H. Lin (World Scientific Publ., Singapore), to be published.