6950
J. Phys. Chem. B 2001, 105, 6950-6955
Formation of Amorphous Silicon by the Low-Temperature Tunneling Reaction of H Atoms with Solid Thin Film of SiH4 at 10 K Kenzo Hiraoka,* Tetsuya Sato, Shoji Sato, Shigeomi Hishiki, Katsunori Suzuki, Yukinori Takahashi, Tatsuya Yokoyama, and Shinichi Kitagawa Clean Energy Research Center, Yamanashi UniVersity, Takeda-4, Kofu 400-8511, Japan ReceiVed: February 12, 2001; In Final Form: April 25, 2001
The reaction of H atoms with SiH4 molecules deposited on the silicon substrate at 10 K led to the formation of Si2H6 as an intermediate and a thin solid product. The yields of reaction products increased with decrease of temperature from 50 to 10K. The film thickness of about 10-20 monolayers of SiH4 gave the highest yield of the solid product at 10 K. When the solid product formed at 10 K was further reacted with H atoms at 200 K, the growth of Si-Si bond network was observed. Analysis of the solid product by FT-IR and spectroscopic ellipsometry revealed that it has the structure similar to amorphous silicon (a-Si:H) formed by the plasma chemical vapor deposition.
1. Introduction Quantum tunneling of group of atoms as systems with many degrees of freedom is now generally accepted as a basic paradigm of physics and chemistry.1 In our previous papers, we studied the low-temperature tunneling reactions of H atoms with various unsaturated compounds deposited on the silicon substrate.2-8 The CO molecules were found to be slowly converted to the HCHO molecules when H atoms were sprayed over the CO film at 10 K. This result argues for the hypothesis proposed by the astrochemists9,10 that the ubiquitous HCHO found in the dark clouds may be formed on the dust grains by the consecutive tunneling reactions 1. H
H
CO 98 HCO 98 HCHO
(1)
In the recent observation of the comae of the comets Hyakutake and Hale-Bopp, no C2H4 was observed although C2H2 and C2H6 were observed with reasonable intensities.11,12 Because C2H2 is easily formed in the gas-phase reactions in the interstellar medium, the presence of C2H2 is reasonable. Despite the fact that the formation of saturated hydrocarbons cannot be explained by the gas-phase reactions, C2H6 is often observed in the interstellar medium. It is generally believed that C2H6 is formed by the consecutive hydrogenation reactions 2 taking place on the cold dust grains. H
H
H
H
C2H2 98 C2H3 98 C2H4 98 C2H5 98 C2H6
(2)
If this is the case, C2H4 must be formed as an intermediate product. To explain the paucity of C2H4 in the comae of the comets and also in the interstellar medium, we have proposed that, in reactions 2, the initial reaction, C2H2 + H ) C2H3, is the rate-controlling step and the following reactions proceed much faster than the initial one. To verify this hypothesis, we studied the reaction of H with solid C2H2 at 10 K.7,8 It was found that only C2H6 was formed as the reaction product but no other products, such as C2H4, were detected by FT-IR and * Corresponding author. E-mail:
[email protected].
by much more sensitive thermal desorption mass spectrometry. This finding argues for our proposal. It should be pointed out that the yield of the final product C2H6 was found to increase about 4 orders of magnitude when the reaction temperature was decreased from 50 to 10 K. This marked negative temperature dependence of the rate for the formation of C2H6 suggests that the rate constant for the initial-step tunneling reaction [H + C2H2 ) C2H3] increases with decrease of temperature.8 In all the reactions of H atoms with unsaturated hydrocarbons (e.g., C2H2, C2H4, and C3H6) fully saturated hydrocarbons are formed as the major products.8 Although their rates are much slower, the abstraction reactions of hydrogen atoms of the saturated hydrocarbons (except for CH4) also take place at cryogenic temperature.2,7 If hydrogenated compounds R-H whose bond energies are weaker than H2 are trapped on the dust grains, the H atom abstraction reaction, H + RH f R + H2, may take place at cryogenic temperature via tunneling reactions. Such hydrogen atom abstraction reactions may partly explain the formation of H2 in the universe. In the present work, the reaction of H with solid SiH4 deposited on the silicon substrate was studied in detail. It was found that the reaction led to the formation of solid product which was identified to have the structure similar to the amorphous silicon (a-Si:H) obtained by the plasma chemical vapor deposition (plasma-CVD). 2. Experimental Method The general experimental procedures were similar to those described previously.2-8 The cryocooler (Iwatani Plantech, type D310) and a quadrupole mass spectrometer (Leda Mass, Microvision 300D) were housed in a vacuum manifold. The vacuum chamber was evacuated by two turbomolecular pumps (ULVAC, UTM-500, 500L/s and Seiko Seiki, STP-H200L/s) connected in tandem. The base pressure of the vacuum system under the current experimental conditions was ∼5 × 10-10 Torr after baking the vacuum system. Figure 1 displays the experimental setup for the sample deposition, H atom spray over the deposited sample, and the
10.1021/jp010553b CCC: $20.00 © 2001 American Chemical Society Published on Web 06/30/2001
Formation of Amorphous Silicon
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6951 is decomposed into atoms.13 In the present experiment, the discharged H2 gas flowed through a restricted narrow bottleneck (0.2 mm internal diameter and about 2 mm long) and then passed through a 90° bent glass tubing with a 6 mm diameter and about 5 cm long before being sprayed over the solid film. From the flow rate of the H2 reagent gas and the estimated diffusion rate constant of H atoms (g100 s-1),14 the H atom flux over the solid film may be crudely estimated to be 1013-1014 atoms cm-2 s-1 under the present experimental conditions. 3. Results and Discussion
Figure 1. Schematic diagram of the experimental system (not to scale). The silicon substrate (30 × 50 × 0.5 mm3) for the deposition of the sample is firmly pressed to the cold head of the cryocooler using indium foil between the mating surfaces. The Pyrex bottleneck discharge tube for the production of H atoms is in contact with the cold head of the cryocooler through the copper sheet. Temperature of the discharge tube is about 27 K when the cold head is held at ∼10 K. The spiral Mo wire is included in the first optical trap, which confines the faint plasma leaking out from the bottleneck in the space between the spiral coil and the bottleneck. Electrons flowing out of the discharge tube are completely annihilated by applying about +50 ∼ +100 V to the spiral Mo wire. The bottleneck discharge tube is coated by thick colloidal graphite in order to prevent the UV irradiation on the sample film.
analysis of volatile products using a quadrupole mass spectrometer. The sample gas SiH4 was deposited on the silicon substrate ([100] surface with the size of 30 × 50 × 0.5 mm3) which was firmly pressed to the cold head of the cryocooler using indium foil between the mating surfaces. The sample gas SiH4 was introduced through the calibrated stainless steel capillary (internal diameter of 0.1 mm and 1 m long) onto the cooled silicon substrate in the vacuum chamber. During the gas deposition at 10 K, only a slight increase of the base pressure of the vacuum chamber was observed (5 × 10-10 f 4 × 10-9 Torr). This indicates that almost all of the gas samples introduced were deposited on the silicon substrate. After the deposition of the sample, H atoms produced by the DC discharge of H2 gas were sprayed over the sample film. The sample film was completely prevented from being bombarded by the charged particles and UV photons produced by the plasma.3 The discharge tube which was held by the copper sleeve connected to the cold head was kept at ∼27 K when the cold head was cooled to 10 K. It is known that the temperature of ions and molecules in the low-temperature glow plasma is nearly the same as that of the wall of the discharge tube. Thus, the temperature of H atoms sprayed over the sample film may be about 27 K. The real-time and in situ observation of the solid-phase reactions was made by the spectroscopic ellipsometry (SE) and Fourier transform-infrared (FT-IR) spectroscopy. The SE analysis was performed using a rotating-compensator instrument (J. A. Woolam, M-2000) that enables the measurement of ellipsometry parameters in the full range (∆ ) 0-360°; Ψ ) 0-90°). Infrared spectra of products formed from the reaction of H atoms with deposited samples were measured using an FT-IR spectrometer (Nicolet, Magna-IR 760) with a resolution of 4 cm-1 in combination with a KBr beam splitter and a liquid N2-cooled MCT (HgCdTe) detector. The quantitative analysis of the products was performed by means of the thermal desorption mass spectrometry. The measurement of the flux of H atoms sprayed over the solid film was not made in the present experiment. In the ordinary glow discharge plasma, a few percent of reagent gas
3.1. In Situ and Real-Time Observation of Products Formed from Reactions of H Atoms with Solid SiH4 Film by Means of FT-IR Spectroscopy and Spectroscopic Ellipsometry. In our previous work,2-5 product analysis was made by the thermal desorption mass spectrometry. The quantitative analysis of gaseous products using a mass spectrometer is very sensitive and highly suitable for such experiment that the absolute quantity of the reactants and products are only limited to be as low as a few monolayer (ML) thick samples condensed on the cold substrate. Strictly speaking, however, there is no guarantee that the products detected by the thermal desorption mass spectrometric method are formed at the reaction temperature but not during the warming of the sample for thermal desorption analysis. To perform in situ and real-time product analyses in the low-temperature solid-phase reactions, infrared absorption spectra of SiH4 film being reacted with H atoms were measured. Figure 2 shows the FT-IR spectra for a 4 ML thick SiH4 sample film sprayed by H atoms. The bottom spectrum is that for silicon substrate before the sample deposition. The second bottom spectrum corresponds to that for the 4 ML thick SiH4 before reaction. The spectra from the bottom to the top were measured with a time interval of 20 min. In Figure 2, the absorption due to stretching vibrations of ν1 (2187 cm-1) and ν3 (2191 cm-1) of SiH4 decreases and new broad absorption bands with lower wavenumbers grow at the expense of those with higher wavenumbers with reaction time. This clearly indicates the formation of Si-Si bond linkage in the solid film. The deformation vibration absorption band of ν2 (∼907 cm-1) and ν4 (∼890 cm-1) almost completely disappeared with reaction time of about 40 min. This indicates that a greater part of the SiH4 molecules in the 4 ML film suffered from the H atom abstraction by the reaction of H atoms in 40 min. Because the H atom abstraction reaction takes place at 10 K, we believe that the reaction proceed via tunneling processes. The formation of a thin solid product on the silicon substrate was confirmed when the silicon substrate was taken out from the vacuum system. Analysis of the solid product was made by SE. The thin solid product formed from the reaction of H with solid SiH4 at 10 K was found to be converted gradually to SiO2 when it was exposed to the ambient air. This suggests that the solid product may have the polysilane-like network structure (see Figure 2). The polysilane (SinH2n+2) is known to be readily oxidized to SiO2 in the ambient atmosphere. It was found that the oxidation reaction was largely suppressed when the solid product formed at 10 K was further reacted by H atoms at 200 K for about 1 h. With an increase of temperature from 10 to 200 K, the unreacted residual SiH4 molecules in the solid film desorbed and only the involatile solid product was left on the silicon substrate. Such a “chemical annealing” (i.e., the H atom spray at higher temperature) of the solid product apparently enhances the growth of the Si-Si bond network by the H atom abstraction reaction from the polysilane-like network as reported by Shimizu and co-workers.15,16 The chemical annealing at 200
6952 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Hiraoka et al.
Figure 2. FT-IR spectra for a 4 monolayers (ML) thick SiH4 sample film sprayed by H atoms. Reaction temperature: 10 K. The bottom spectrum is that for silicon substrate before the sample deposition. The second bottom one corresponds to that for the 4 ML thick SiH4 before reaction. The spectra from the bottom to the top were measured with a time interval of 20 min.
Figure 4. The index of refraction n and extinction coefficient k measured by the spectroscopic ellipsometry for the same sample in Figure 3. Figure 3. FT-IR spectrum of the solid product which was formed at 10 K and then further reacted with H atoms at 200 K for 1 h. The procedure [reaction of H atoms with 20 ML SiH4 film at 10 K for 1 h and the chemical annealing of the solid product at 200 K for 1 h] was repeated 20 times. The film thickness was roughly estimated to be about 500 Å by SE. The estimated film thickness should be regarded as an only approximate one because the surface roughness of the solid film was difficult to take into account in the present SE analysis.
K may have some contribution from the thermal processes in addition to tunneling ones but their relative contribution is difficult to estimate. The chemically annealed solid product at 200 K was found to be stable and did not show any structural change when it was left in the ambient atmosphere. Figure 3 shows the FT-IR spectrum of the chemically annealed solid product (formed at 10 K and annealed at 200 K). This spectrum was measured ex situ because the reference spectrum for the silicon substrate was necessary for the measurement. The absorption peak is unsymmetrical and is broader in the lower wavenumbers. The deconvoluted absorption spectrum assuming Gaussian function is found to be quite
similar to that for the a-Si:H film obtained by the conventional radio frequency plasma-enhanced chemical vapor deposition.17 A shoulder peak appearing at ∼2017 cm-1, the strong peak at ∼2081 cm-1, the broad higher-wavenumber peak at ∼2113 cm-1, and a weak peak at ∼2173 cm-1 may be assigned to the absorptions of the bulk Si-H stretching vibration, the bulk SiH2 stretching vibration, the surface SiH2,3 vibration, and SiH2(O2) vibration, respectively.17 It is likely that the polysilane network, i.e., s(SiH2)ns, formed at 10 K is largely converted to the threedimensional Si-Si bond network by the chemical annealing at 200 K. Figure 4 represents the index of refraction (n) and the extinction coefficient (k) as a function of wavelength (nm) measured by SE. The spectral features for n and k are found to be the combination of those for amorphous and crystalline silicon, being amorphous silicon rich.18 The a-Si:H synthesized by the conventional plasma CVD at the substrate temperature of around 200 °C has the n value of about 4.5 at ∼400 nm. The n value of 3.2 at ∼400 nm in Figure 4 does not necessarily
Formation of Amorphous Silicon
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6953
Figure 5. Dependence of H atom spray time on the ratio [SiH4 reacted/ SiH4 deposited] and the yield of Si2H6. Reaction temperature: 10 K. The SiH4 film thickness: 12 ML.
mean that the solid product obtained in the present experiment contains lots of voids in the film because the surface roughness was not taken into consideration in the present SE analysis. A more detailed SE analysis is now in progress in our laboratory. 3.2. Relationship between H Atom Spray Time and the Yield of Reaction Products. Figure 5 shows the dependence of the time of H atom spray on the ratio of [SiH4 reacted]/ [SiH4 deposited] and the yield of Si2H6 for 12 ML SiH4 film reacted with H atoms at 10 K. Here, the “yield” means the ratio of the amount of the product to the deposited SiH4. The reaction temperature was kept at 10 K all through the experiment. Only Si2H6 but no other SinH2n+2 with n g 2 could be detected as gaseous products by thermal desorption mass spectrometry. In Figure 5, the yield of Si2H6 increases very steeply to the maximum (0.01) and becomes nearly time-independent after 30 min. The ratio [SiH4 reacted]/[SiH4 deposited] increases more gradually than the yield of Si2H6 and reaches the plateau (0.7) with reaction time of ∼120 min. Because no gaseous products other than Si2H6 were detected, the greater part of the reacted SiH4 is likely to be converted to the involatile solid product. The more rapid increase of Si2H6 than the solid product (i.e., [SiH4 reacted]/[SiH4 deposited]) suggests that Si2H6 is the intermediate product (i.e., precursor) for the formation of the solid product. The fact that the value of [SiH4 reacted]/[SiH4 deposited] and the yield of Si2H6 do not show the steady increase but become time-independent with reaction time after ∼120 min suggests that the steady states were established for the formation and destruction of Si2H6 and also the solid product. In other words, the H atom must act not only as an activator for the formation of the Si-Si bond network but also as an etchant for the formed solid product at 10 K. The possible mechanism for the formation of polysilane (SinH2n+2) and amorphous silicon may be summarized as follows:
SiH4 + H ) SiH3 + H2 (-50)
(3)
SiH3 + H ) SiH2 + H2 (-132)
(4)
SiH2 + SiH4 ) Si2H6 (-244)
(5)
l Sin-1H2n + SiH2 ) SinH2n+2
(6)
Further reactions of H atoms with SinH2n+2 proceed as
f........f amorphous silicon
(7)
The number in parentheses represents the heat of reaction in
Figure 6. Dependence of sample SiH4 film thickness on the ratio [SiH4 reacted/SiH4 deposited] and the yield of Si2H6. Reaction temperature: 10 K. H atom spray time: 60 min.
Figure 7. Dependence of reaction temperature on the ratio [SiH4 reacted/SiH4 deposited] and the yield of Si2H6. H atom spray time: 60 min. The SiH4 film thickness: 12 ML.
kJ/mol19 (negative, exothermic). The ground state of SiH2 is singlet (X1A1) and is known to be chemically very reactive.20 It inserts to the Si-H bond of SiH4 to form the vibrationally excited Si2H6*. Because the “hot” Si2H6* is surrounded by the neighboring molecules in the solid-phase reactions, the surrounding molecules as a whole may act as an efficient heat bath to quench the “hot” Si2H6* to the stable Si2H6. Because SiH3 is unreactive toward SiH4, the SiH3 radical may be left intact until it further reacts with H to give SiH2 by reaction 4 or to regenerate SiH4 by the recombination reaction 8.
SiH3 + H ) SiH4 (-386)
(8)
Although the branching ratio of reactions 4 and 8 could not be determined in the present experiment, the steep increase in Si2H6 with time in Figure 5 indicates that reaction 4 takes place fast enough to produce Si2H6. 3.3. Film Thickness Dependence on the Yields of Reaction Products. Figure 6 shows the film thickness dependence on the ratio of [SiH4 reacted]/[SiH4 deposited] and the yield of Si2H6. The SiH4 film deposited was reacted with H atoms for 1 h at 10 K. As mentioned in the previous section, the ratio [SiH4 reacted]/[SiH4 deposited] may be regarded as the yield of solid product formed by the reaction. It should be pointed out that the ratio [SiH4 reacted]/[SiH4 deposited] increases very steeply with increase of the film thickness from 0 to 5 ML. It reaches the plateau in the range of 10-20 ML and decreases gradually with film thickness. The initial rapid growth of the yield of the solid product from 0 to 10 ML is reasonable because the presence of the neighboring SiH4 molecules is indispensable for the development of the Si-Si bond network (reactions (3)(7)). In the range of 10-20 ML, about 70% of the deposited
6954 J. Phys. Chem. B, Vol. 105, No. 29, 2001 SiH4 molecules are converted to the solid product. The gradual decrease in the yield of the solid product with >20 ML may be due to the less efficient H atom penetration into the film. In other words, the H atom is likely to diffuse rather freely in the SiH4 film thinner than 20 ML. Because H atoms are sprayed over the solid sample surface, the solid product formation must proceed from the top surface to the inside of the film and the SiH4 molecules located in the deeper region have less chance to be incorporated to the Si-Si bond network. Thus, the H atom treated film might have some inhomogeneous morphology from the top to the bottom. The thinner the sample SiH4 film, the more homogeneous solid product would be formed. The yield of Si2H6 shows a sudden increase with film thickness and reaches the plateau (0.01) already with a few ML film thickness. Because the solid product formation proceeds from the surface to inside, the intermediate product Si2H6 may be mainly formed at the interfacial region between the top-layer solid product and the unreacted underneath SiH4 layer. The nearly film-thickness independent yield of Si2H6 with equal to or greater than a few ML in Figure 6 suggests that the interfacial region between the solid product layer and the unreacted SiH4 film layer is thin at the molecular level and there may be a distinct boundary interface between the two phases. 3.4. Temperature Dependence on the Yields of Reaction Products. Figure 7 displays the ratio [SiH4 reacted]/[SiH4 deposited] and the yield of Si2H6 as a function of reaction temperature. In this experiment, the 12 ML thick SiH4 was deposited first at 10 K on the silicon substrate. After the temperature of the silicon substrate was raised to the reaction temperature, the deposited film was reacted with H atoms for 1 h. In the separate experiment, the desorption of C2H4 was observed at about 5-10 K lower than the temperature for the start of sublimation of solid C2H4 (55K) when the solid C2H4 film was sprayed by the H atoms. Thus, the measurement in Figure 7 was made up to 40 K which was about 10 K lower than the sublimation point for solid SiH4 (∼50 K). In the figure, both the ratio [SiH4 reacted]/[SiH4 deposited] and the yield of Si2H6 decrease with increase of temperature. Such a negative temperature dependence for the rates of the tunneling reactions has been observed as a general trend in our recent work.7,8 It was suggested that the observed negative temperature dependence may be due either to the increase of the steady-state concentration of H atoms on the solid film and/or to the increase of the rate constants for the tunneling reactions with decrease of the reaction temperature.7,8 In any case, the observed negative temperature dependence of the rates for low-temperature tunneling reactions is unique and an unprecedented result. Theoretically, however, the negative temperature dependence of the rate constants for tunneling reactions has been predicted. Takayanagi and Sato performed the bending-corrected rotating linear model calculations of the rate constants (k) for the two-body reactions of H + H2 and its isotopic variants at low temperatures and examined the effect of the van der Waals well.21 They found that van der Waals wells included in both potential surfaces significantly affected the calculated rate constants at temperatures lower than 10 K.21 The values of log k were found to have deep minima for the D + HD and D + DH reactions. The theoretical analysis of the many-body solid-phase reactions must treat the reactant and the substrate molecules as one system. The many-body nature of the reactant-substrate and product-substrate interaction produces a dense spectrum of vibrational energy levels. Efimov derived the existence of the dense set of levels from reactant-substrate interaction.22 The
Hiraoka et al. quasi-continuum set of vibrational levels of the reactant-substrate system acts as a dissipating channel and would promote the forward reaction. This would become more favorable at lower temperature for the coherent H atom tunneling reactions in solid phase due to the suppression of the thermal fluctuation. The present experimental findings may be the case. 4. Concluding Remarks The low-temperature tunneling reactions of H atoms with solid SiH4 were investigated. When the H atoms were sprayed over the solid SiH4 film at 10 K, the abstraction reactions of H atoms of SiH4 by H atoms took place leading to the formation of solid product. The highest yield (∼70%) was obtained for the sample SiH4 film thickness of 10-20 ML. For thicker film than 20 ML, the product yields decreased because of the less efficient penetration (diffusion) of the H atoms inside the thicker film. The solid product formed at 10 K has the polysilane-like structure. It was found that the three-dimensional Si-Si bond network was developed when the solid product formed at 10 K was further reacted by H atoms at 200 K (chemical annealing). The analyses of the chemically annealed solid product by SE and FT-IR revealed that the solid product was mainly composed of amorphous silicon containing some crystal silicon. The yield of solid product increased with decrease of temperature down to 10 K. Observed negative temperature dependence of the rate for the formation of solid product is opposite to the temperature dependence for the general chemical reactions which follow the Arrhenius equation. The observed negative temperature dependence may be due either to the increase in the steady-state H atom concentration on the film7,8 and/or to the increase of the rate constants of the tunneling reactions at lower temperature.8,21,22 The synthesis of betterquality amorphous silicon by the low-temperature tunneling reactions combined with the chemical annealing is in progress in our laboratory. References and Notes (1) Benderskii, V. A.; Makarov, D. E.; Wight, C. A. in Chemical Dynamics at Low Temperatures; John Wiley & Sons: new York, 1994. (2) Hiraoka, K.; Ohashi, N.; Kihara, Y.; Yamamoto, K.; Sato, T.; Yamashita, A. Chem. Phys. Lett. 1994, 229, 408. (3) Hiraoka, K.; Miyagoshi, T.; Takayama, T.; Yamamoto, K.; Kihara, Y. Astrophys. J. 1998, 498, 710. (4) Hiraoka, K.; Yamashita, A.; Miyagoshi, T.; Ohashi, N.; Kihara, Y.; Yamamoto, K. Astrophys. J. 1998, 508, 423. (5) Hiraoka, K.; Yamamoto, K.; Kihara, Y.; Takayama, T.; Sato, T. Astrophys. J. 1999, 514, 524. (6) Hiraoka, K.; Sato, T.; Takayama, T. J. Mass Spectrom. Soc. Jpn. 1999, 47, 382. (7) Hiraoka, K.; Takayama, T.; Euchi, A.; Handa, H.; Sato, T. Astrophys. J. 2000, 532, 1029. (8) Hiraoka, K.; Sato, T. Radiat. Phys. Chem. 2001, 60, 1. (9) Tielens, A. G. G. M.; Allamandola, L. J. In Interstellar Processes; Hollenbach, D. J., Thronson, H. A., Jr., Eds.; Reidel: Dordrecht, 1987; p 397. (10) Tielens, A. G. G. M.; Hagen, W. Astron. Astrophys. 1982, 114, 245. (11) Mumma, M. J.; DiSanti, M. A.; Russo, N. D.; Fomenkova, M.; Magee-Sauer, K.; Kaminski, C. D.; Xie, D. X. Science 1996, 272, 1310. (12) Crovisier, J. Faraday Discuss. 1998, 109, 437. (13) Cherigier, L.; Czametzki, U.; Luggenholscher, D.; Schulz-von der Gathen, V.; Dobele, H. F. J. Appl. Phys. 1999, 85, 696. (14) Payne, W. A.; Stief, L. J. J. Chem. Phys. 1976, 64, 1150. (15) Shirai, H.; Das, D.; Hanna, J.; Shimizu, I. Appl. Phys. Lett. 1991, 26, 1096. (16) Das, D.; Shirai, J.; Hanna, J.; Shimizu, I. Jpn. J. Appl. Phys., Part 2 1991, 30, L239. (17) Fujiwara, H.; Toyoshima, Y.; Kondo, M.; Matsuda, A. Phys. ReV. B 1999, 60, 13598. (18) Das, D.; Shiraishi, H.; Hanna, J.; Shimizu, I. Jpn. J. Appl. Phys.
Formation of Amorphous Silicon 1991, 30, 1239. (19) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L. Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. No. 1. (20) Gordon, M. S.; Xie, Y.; Yamaguchi, Y.; Grev, R. G.; Schaeffer,
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6955 H. F., III J. Am. Chem. Soc. 1993, 115, 1503. (21) Takayanagi, T.; Sato, S. J. Chem. Phys. 1990, 92, 2862. (22) Efimov, V. Phys. Lett. 1970, 33B, 563; Nucl. Phys. 1973, A210, 157.
