The Journal of
Physical Chemistry
0 Copyright, 1991. by the American Chemical Society
VOLUME 95, NUMBER 12 JUNE 13, 1991
LETTERS Observation of the Methyl Radlcai during the Surface Decomposition Reaction of Trimethylgaiilum Jude T. Francis, Sidney W. Benson,*vt and Theodore T. Tsotsis Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089- I21 I (Received: December 26, 1990)
We have studied the mechanism and kinetics of the decomposition reactions of trimethylgallium (TMG) in the presence of arsine. These studies were performed in a very low pressure pyrolysis (VLPP) reactor in the pressure range of 10-2-10-3 Torr. We have observed the methyl radical product cf the surface-catalyzed decomposition reactions of TMG using modulated beam mass spectrometry. The primary step in the decomposition of TMG involves the adsorption and the successive release of the methyl radicals from the surface. The apparent activation energy for the release of the methyl radical in the range of 400-575 OC is 22 f 3 kcal/mol. This apparent activation energy is much lower than the average Ga-C bond energy in the gas phase of 60 kcal/mol and can probably be accounted for by the heat of adsorption and the concerted reactions on the surface. We have suggested an overall reaction scheme to explain our observations. The introduction of arsine increases the decomposition of TMG and the methyl radical production. However, the activation energy for the production of the methyl radicals is not affected by the presence of arsine. There is no evidence for the recombination of methyl and H species on the surface. At temperatures above 600 O C there is a surprising decrease in the methyl radical signal. This has been attributed to the desorption of Ga(CH3)* species rather than methyl radicals.
1. Introduction Trimethylgallium (TMG) is used in the metal organic chemical vapor deposition (MOCVD) process to grow thin films of alloy semiconductor materials such as GaAs. MOCVD involves the copyrolysis of gaseous metal organics and hydrides. A number of different reactors have been used to grow such materials, ranging from the atmospheric pressure CVD reactor to the very low pressure ( Torr) vacuum chemical epitaxy reactor. The lower pressure reactors are favored because they provide better film uniformity over a large area and improved flow control. For CVD reactors as the overall pressure decreases, the role of the decomposition reactions in the gas phase decreases and the sur-
'Loker Hydrocarbon Research Institute, University of Southern California,
Los Angeles, CA 90089- 166 1.
0022-3654/91/2095-4583$02.50/0
face-catalyzed reactions become increasingly important. Jensen et a1.l in their experimental and modeling studies of MOCVD reactors have reported that the surface reaction kinetics and mechanism are the most sensitive factors in determining the growth rate. The role of the surface reactions in MOCVD is also evident from the mechanistic study of the CVD process by Larsen et al.2 They observed that increasing the surface area in a low-pressure reactor has a strong influence on the decomposition kinetics. Surface science studies have shed some light on the surface reactions of a TMG molecule on a GaAs surface. Squire ( 1 ) Jensen, K. F.; Mountziaris, T. J.; Fotiadis, D.I. Presented at MOCVD Session, AlChE meeting, San Francisco, CA. Oct 1989; also Moter. Res. Soc. Svmo. Proc. 1989. 145. 107. ' (i)Larsen, C. A.;Buchan. N. I.; Li, S.H.; Stringfellow, G.; Brown, D. W.J . Crystal Growth 1990, 102, 126.
0 1991 American Chemical Society
Letters
4584 The Journal of Physical Chemistry. Vol. 95, No. 12, 1991
et aL3 in their ultra-high-vacuum (UHV) studies have observed an apparent activation energy of 28 kcal/mol for the release of the methyl group from the decomposition of TMG on a GaAs surface. Creighton' in his temperature-programmed desorption studies has observed that the activation energy for the release of the methyl radical was 46 kcal/mol. The above studies were performed under ultra-high-vacuum which was far from realistic film growth conditions. In this work we have studied the surface decomposition kinetics of TMG under conditions closer to realistic growth conditions. The very low pressure pyrolysis (VLPP) technique has been previously employed to study fast gas-phase and surface reactionsa5T6 The surface processes involved in MOCVD result in the deposition of a film on the surface. To study such processes one needs to modify the VLPP technique. In this article we describe such a modified VLPP system which was used to study the methyl radical production reaction during the decomposition of TMG. The total reactor pressure was in the range of 10-3-10-2Torr, a low enough pressure so that all the gas-phase reactions were well in the falloff region. The growth of the surface film under these conditions is controlled by surface reactions, whose kinetics one can study in the presence of the growing film. The partial pressure of the metal organics utilized in our reactor is still lower than the pressures of the CVD growth conditions but closer to MOCVD than the pressure conditions of the UHV studies. One would, therefore, expect the TMG surface coverages and reaction kinetics to be more representative of typical CVD conditions than in the case of the UHV studies. The overall rate expression for TMG decomposition was reported in an earlier publication.' In this paper we focus our attention on the detailed product distribution of the surface reaction and its implications for the reaction mechanism of the MOCVD process. The motivation for our studies has been the belief that the optimal design of such MOCVD reactors involving the decomposition reactions of the metal organics and hydrides on growing semiconductor surfaces requires a better fundamental understanding of the kinetics of the individual steps and the detailed molecular mechanism of the surface reactions. 2. Experimental Section A detailed description of our VLPP system has been presented elsewhere." VLPP is a proven kinetic tool for the measurement of absolute rate constants. The VLPP system consists of a Knudsen cell flow reactor attached to an ultra-high-vacuum system. The molecular beam of products and reactants exiting the reactor is typically analyzed by modulated beam mass spectrometry. Figure 1 shows a sketch of the modified VLPP reactor used in these studies. It is made of quartz and contains an internal "hot" wall maintained at a temperature, T, and an external 'cold" wall kept at constant temperature, T,. In contrast the conventional VLPP reactor system has only one "hot" wall. During our studies, T, was usually maintained below the decomposition temperatures of TMG and arsine. At the pressure conditions of our experiments, the mean free path of the molecules in the reactor was much larger than the diameter of the vessel. Hence the number of gas-gas collisions were minimized and surface reactions predominated. The advantage that this modified VLPP reactor offered for t&e study of surface reactions is that the decomposition reaction occurred on the internal wall, while the colder external wall minimized the secondary reactions of the radical products. The internal wall was initially GaAs, quartz, or quartz coated with (3) Squire, D. W.; Dulcey, C.S.;Lin, M. C . Muter. Res. Soc. Symp. Proc. 1988, 101, 301. (4) Creighton, R. Presented at the OMVPE Worhbop, Monterey, CA, Oct 1989; Appl. Phys. Lcrr. 1990, 57(3), 279. ( 5 ) Golden, D. M.;Spokes,G. N.; Benson, S. W. Angew. Chem., Inr. Ed. Engl. 1973, 12(7), 534. ( 6 ) Robertson, R. M.; Golden, D. M.; R w i , M. J. J . Vuc. Sci. Technol. 1988, 86(6), 1632. (7) Francis, J. T.; Bcnson,S. W.; Tsotsis, T.T. J . Crysruf Gmwrh, in pres. (8) Francis, J. T. Ph.D. Thesis, University of Southern California, 1990.
1' Figure 1. Modified VLPP reactor. Legend: 1, internal heater; 2, external heater; 3, O-ring joints; 4, exit aperture. Reactor volume = 360 cm3, internal hot wall area = 40 cm2, external wall/internal wall area 9. The reactor is wrapped in Fiberfrax insulation.
a metal (such as silver). As the decomposition of TMG and arsine proceeded the initial surface changed to a growing GaAs layer, which then became the new decomposition surface. During our experiments TMG and arsine flow rates were typically in the range of 1014-1016 molecules/s. The arsine/TMG concentration ratio in the reactor was varied between 1 and 5 . The flow rates and the mass spectrometer settings were chosen accordingly, so that the observed mass peaks are large and well resolved. The reactor heating was started after the mass spectrometer signals of the reactant molecule had reached steady state. The mass spectrometer was operated with a channeltron multiplier with a gain of about 105-106. The radical products of the decomposition reaction were observed using a 20-eV electron ionization energy. The disappearance of the reactants and formation of products was followed as Ti was increased in a stepwise manner, while To remained constant. At each new temperature Ti,the reactor was allowed to reach steady state. The decomposition of TMG and arsine results in the deposition of products on the surface. The changing surface causes the reactivity of the molecules to change with time. By allowing the mass spectrometer signals to reach a steady state at any given temperature Ti one can assume that the surface reactivity has become constant with time for that temperature. In VLPP the decomposition rate constant of TMG is measured relative to the escape rate constant, which can be determined either theoretically from the Knudsen equation or e~perimentally.~ Our experimental measurements agreed with the theoretical value for all gases? with the unique exception of TMG, which appears to adsorb strongly on the reactor and mass spectrometer chamber walls. TMG desorption from the reactor and chamber walls, after the TMG flow is stopped, interferes with the measurement of the escape rate. In our studies we utilized the theoretically determined value from the Knudsen equation including the appropriate correction factor for the aperture thickness using the Clausing factor? The mean residence times of TMG and arsine in the reactor were about 2 and 1.4 s, respectively, at typical reactor conditions. 3. Decomposition of Trilnetsylgrllium Figure 2 shows the 20-eV spectrum of the exiting gases before decomposition begins at Ti = 300 O C and after decomposition at 600 O C . The main gas-phase product observed during TMG pyrolysis was the methyl radical (amu 15). Smaller amounts of ethane (amu 28,30)were also observed. The amount of methane (9) Dushman, S. Vucuum Techniques; John Wiley: New York, 1949.
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4585
Letters Ti =3cQ0c
'
'
(X0.l)
99
d e
I5
I (x0.l) 99
28
101
69
16
30
71
84
' 114 1 IC
86
d e
Figwe 2. Spectra of TMG on pyrolysis at 20 eV. hG = 2 X IW3 Torr, Te = 150 OC. Arsine/TMG ratio = I , arsine peaks are not shown for clarity. Starting substrate is silver.
produced is insignificant because there is no change in amu 16 on heating. The methyl radical product signal was obtained after subtracting the contribution of the amu 15 signal from the constant methane impurity (amu 16). At 20-eV electron ionization energy, neither TMG nor ethane show a significant mass 15 peak. Hence one can assume that the signal at amu 15 corresponds to the methyl radical species. The methyl radical and ethane signals, as indicated in Figure 2, may not reflect their true concentrations in the reactor because the sensitivity of the mass spectrometer to the methyl radical is different than that for ethane. The steady-state signal of the methyl radical species in the gas phase, as a function of the internal wall temperature, is shown in Figure 3. The methyl radical signal monotonically increases up to 575 OC. Above 575 OC the concentration of methyl radicals increases more slowly and, surprisingly, it starts to decrease above 600 OC. A regression fit to the data gives an apparent activation energy (E, f 2a) of 22 f 3 kcal/mol in the range of 45G575 OC. The T h G signals, however, continued to decline, indicating increased decomposition above 575 OC as discussed in a previous publication.' We attribute the decrease in the methyl radical signal to the enhanced desorption of Ga(CHp)xspecies from the substrate, rather than to a reduction in the TMG decomposition rate. We did not observe any Ga(CH& radicals in the gas phase but there is indirect evidence for the existence of such Ga(CH3), radicals on the surface. The evidence comes from the desorption of the external wall deposits. Figure 4 shows the 20-eV spectrum of the products desorbing from the walls after the TMG flow was shut off. To desorb these products the external wall temperature was increased from 300 to 500 OC at a rate of 50 OC/min. Up to 400 OC one observes primarily Ga(CH,), species at amu 99 and amu 101. These amu 99 and 101 peaks are not fragmented TMG peaks because the parent peak (amu 114)was not observed. The 70-eV spectrum of these products, furthermore, did not show the standard TMG fragmentation pattern. At temperatures above 400 OC the Ga(CHp), species seems to decompose to give a substantial methyl radical signal. Our explanation for not observing Ga(CH,), radicals in the molecular beam exiting the reactor during decomposition is that these species deposit on the 'cold" external wall. A surprisingly large amount of ethane is produced by the decomposition of TMG at these low-pressure conditions especially
400
500
600
700
Temperature, deg. C Figure 3. Methyl and ethane signals vs temperature Ti. ha = 2 X lV3 Torr, T,= 150 OC,arsine/TMG ratio = 1. These signals do not reflect relative concentrations because mass spectrometer sensitivity is different for the methyl radical than ethane. Starting substrate is GaAs. T,= 300-400°C 99 101
I
88
104
90 I
d e
d e
Figure 4. Spectrum of desorbing products from external wall. 20 eV; Ti was held at 625 O C during the deposition experiment. on the silver substrate. Ethane is most likely being produced by recombination of the methyl radicals. The recombination of the methyl radicals in the gas phase at these conditions is in the falloff region. The recombination rate constant is termolecular in the falloff region with a value of 1013L2/(mo12s).I0 Based on this rate constant we have estimated that the gas-phase recombination of methyl radicals accounts for less than 1% of the methyl radicals (10) CRC Handbook of Bimolecular and Termolmtlar Reactionr; Alister,
F.,Drew,R. M.,Eds.; CRC Press: Boca Raton, FL, 1987; Vol. IIIB.
4586 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
Letters
TABLE I: Distribution of Hydrocarbon Products in Two Cases decompsn mode % methane 96 ethane % methyl radical hot-cold wall 4 16 80
produce three methyl radicals. As mentioned above we believe that the initial reaction product is the methyl radical, which is then involved in secondary reactions to produce the methane and ethane products. The large amount of methane formed in the "hot-hot" wall decomposition mode is possibly due to the ab-
The production of ethane in the surface decomposition reactions has not been observed in the surface science s t ~ d i e s .Ethane ~ is observed to a larger extent with the starting silver substrate and at TMG pressures above 5 X IO4 Torr. The amount of ethane produced decreases and the quality of the GaAs film improves as the arsine/TMG ratio is increased.' Hence it is possible that the methyl radical recombination is occurring in the-gallium-rich areas of the growing film. The ethane produced, furthermore, shows the same activation energy as the methyl radical in the temperature range of 450-575 OC, indicating that the recombination step involves a negligible activation energy. The observed decomposition products of TMG (methyl radical, ethane) together with the presence of surface Ga(CH3), species suggests to us the following overall reaction mechanism:
arsine duringTMG decomposition in the external wall mode did produce an increased methane signal. This is probably by the abstraction reaction of the methyl radicals with arsine or AsH, species on the surface as shown in reaction 6. The
hot-hot wall
Ga(CH3)dg)
70
-
5
Ga(CHMs)
CH3Ga(s) CH3(g or s)
-
Ga(CH3),
- -+ CH3(g)
+ CH3W
Ga(CH3),(s)
25
+ (3 - x)CH3(g) (1)
Ga(s)
(2)
C*H&)
(3)
Ga(CH3),(g)
(4)
Reactions 1 and 2 represent the adsorption of TMG on the surface (either on an As or Ga site) and the successive elimination of the methyl groups from the adsorbed molecule on to the surface or into the gas phase. The methyl groups on the surface recombine to a small extent with methyl groups in the surface Ga(CH3), species to prcduce ethane (3). Reaction 4 represents the desorption of the Ga(CH3), surface species, which probably becomes important at temperatures above 575 OC. We do not observe substantial quantities of methane produced in our reactor by the reaction of TMG and arsine on the substrate surface due probably to the fact that CH3(s) H(s) recombination on the surface to form methane is not very favorable. The decomposition reaction is, therefore, consistent with a mechanism like reaction 1. We have also studied the products of the decomposition of TMG for the case where the external and internal walls are both "hot" (Le., T,= Ti > TMG decomposition temperature). The products of TMG decomposition observed are different from the previous case, where the external wall is "cold" (Le., T,< Ti).The gasphase products are largely methane and some methyl radicals. We believe that the methyl radicals are still the likely primary product of TMG decomposition, but the methyl radicals abstract hydrogen from the TMG(g) or TMG(s) to produce methane via a secondary reaction 5 . Typical distributions of the hydrocarbon products from decomposition of TMG from the two types of operation of the VLPP reactor are shown in Table I. The data shown are for TMG decomposition at a partial pressure of 2 X Torr. For the "hot-cold" decomposition mode we used T,= 150 OC,and T, = 575 OC, while for the "hot-hot" decomposition mode we used T, = T, = 575 OC. The fractions of methane and ethane products were estimated by calibrating their respective peaks with standard mixtures. The methyl radical product is estimated using a standard carbon balance, with each TMG molecule assumed to
+
CH,(g) + AsH,(s)
-
CH4(g) + AsH,,(s)
(6)
decomposition of arsine did not produce any observable amounts of ASH or AsH2 radicals in the gas phase. We were not able to observe any H atoms or H2 produced by the reaction because of the large background signals from the residual gases in the vacuum chamber and the small fraction of decomposition. The rate constant for arsine decomposition followed by the largest peak at amu 76 (ASH+)showed an activation energy of about 15 f 4 kcal/mol. The apparent rate constant ( k ) for these surface decomposition reactions were calculated from the mass balance for the VLPP flow reactorus This measured rate constant k is related to the surface reaction rate constant (k,) by k = k,K ,where Kq is the equilibrium constant for adsorption. Hence the%served activation energy from the Arrhenius plot for k is related to E,, the activation where AHada energy of the surface reaction by E = E, + AHada, is the heat of adsorption and always negative. 4. Conclusion
The activation energy of the surface reaction of TMG on GaAs, assuming a heat of adsorption value of about -20 kcal/mol, is in the range of 40-46 kcal/mol. This is in reasonable agreement with values reported from surface science studies3q4and theoretical ab initio calculations." This is clearly a surfacecatalyzedreaction because the average bond energy of the Ga-C bond in the TMG molecule is about 60 kcal/mol,I2 well above the observed activation energy for the release of the methyl radicals. The methyl radical signal as a function of substrate temperature shows a surprising decrease above 600 OC. We have attributed this behavior to the release of the methyl species from the surface in the form of Ga(CH3), radicals which has been observed by other workers such as C r e i g h t ~ n .These ~ radicals are not observed in the reactor exit stream. We believe that this is probably because these species adsorb on the "cold" external wall. Another surprising finding is the observation of substantial amounts of the ethane produced. Ethane is most likely being produced by the surface recombination reaction of methyl radicals. This has not been observed in prior studies3v4at UHV conditions probably due to the correspondingly lower surface coverages. We believe that the modified VLPP technique is well suited for the observation of the radical products from the surface decomposition reactions of TMG on a growing GaAs surface.
Acknowledgment. S.W.B. acknowledges assistance from the NSF (Grant CHE-8714647).We thank Jim Merritt for making
the quartz reactor and internal heater for the experiment. (1 I ) Tsuda, M.; Oikawa, S.;Morishita, M. J . Crystal Growth 1990, 99, 545. (12) Jacko, M. G.; Price, S.J. Con. J . Chem. 1963, 41, 1560.
