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The adsorption of hexamethyldisilane (HMDS) and trimethylsilane (H-TMS) on graphite monolayers on Pt-. (111) surfaces was studied with thermal desorpt...
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J. Phys. Chem. B 2001, 105, 1799-1804

1799

Kinetics of the Reaction of Adsorbed Hexamethyldisilane on C/Pt(111) with D Atoms: Si-Si Bond Breaking S. Wehner,† Th. Zecho,† and J. Ku1 ppers*,†,‡ Experimentalphysik III, UniVersita¨ t Bayreuth, 95440 Bayreuth, Germany, and Max-Planck-Institut fu¨ r Plasmaphysik (EURATOM Association), 85748 Garching, Germany ReceiVed: July 12, 2000; In Final Form: October 22, 2000

The adsorption of hexamethyldisilane (HMDS) and trimethylsilane (H-TMS) on graphite monolayers on Pt(111) surfaces was studied with thermal desorption spectroscopy, and the kinetics of reactions of gaseous D atoms with adsorbed H-TMS and HMDS was investigated with reaction product detection techniques. HMDS and H-TMS adsorb and desorb molecularly in the mono- and multilayer regimes. Zero-order desorption indicates that in either regime the molecules tend to agglomerate in islands. Reaction of D atoms with adsorbed HMDS leads via insertion of D into the Si-Si bond to D-TMS products, either adsorbed or as gaseous species, depending on the substrate temperature. D impact-induced abstraction of H from the methyl groups and of methyl from HMDS is much less efficient than D insertion into the Si-Si bond, in accordance with the energetics involved. The reaction kinetics is compatible with an Eley-Rideal mechanism, and a reaction cross-section of σ ) 0.4 Å2 was obtained. Adsorbed H-TMS does not react with D.

1. Introduction Fabrication of hydrogenated amorphous silicon (a-Si:H) largearea thin films used in photovoltaic solar cells and thin film transistor substrates for active matrix liquid-crystal displays relies on plasma-enhanced chemical vapor deposition from silane or disilane gas feeds.1 The deposition, usually performed at milliTorr pressures, substrate temperatures of 500-600 K, and low-power plasma conditions, results in growth rates of 1-10 Å/s in amorphous Si films that contain about 10% hydrogen. This hydrogen is essential for saturation of dangling Si bonds in the film, which would have a negative effect on its electronic and photoconductive properties. Investigations performed to enhance the electronic properties of a-Si:H revealed that addition of hydrogen to the feed gas has a beneficial effect on the quality of the films.2,3 This feature was attributed to the suppression of higher-order silane radicals in the gas phase2,3 by which the concentration of the dominating growth species silyl gets enhanced. The addition of hydrogen to the carrier gas must also have significant consequences for the reactions that proceed on the surface of a growing a-Si:H film. Through the action of the plasma, low-energy neutral and ionic H species are generated that will induce hydrogenation and abstraction reactions upon impact on the surface. Both types of reactions are required for good film quality and fast film growth. Hydrogenation of Si dangling bonds reduces the defect density; and abstraction of H from surface silyl, silylene, and silyline groups opens the route for further film growth through bonding of silyl species from the gas phase. Despite the fact that a-Si:H thin films are mass-produced, the surface chemistry aspects of their growth have been studied rarely. SiH2 and SiH3 groups are the dominating surface species during film growth,4 and a H2 plasma can chemically etch a-Si:H films.5 * Corresponding author. E-mail: [email protected]. † Universita ¨ t Bayreuth. ‡ Max-Planck-Institut fu ¨ r Plasmaphysik.

The present study was performed to investigate Si-Si bond breaking via reactions of gaseous D atoms with a molecular disilane species. The intent was to determine the cross-sections of the involved reaction steps. Adsorbed hexamethyldisilane (HMDS) was used as a model system. This molecule was chosen because it contains three candidates for abstraction: H in the methyl groups, CH3 in the trimethylsilyl groups, and Si(CH3)3. Because methylsilanes are candidate precursors for SiC deposition,6,7 HMDS is a useful target molecule to study the relative efficiency of the various abstraction reactions that occur upon growth of a carbide film. 2. Experimental The experiments were performed in a ultrahigh vacuum system that allowed monitoring of gaseous product molecules during subjection of a surface to a D atom flux, as described previously.8 The heated tube type atom source, a modification of a published design,9 is located on the axis of a differentially pumped small vacuum chamber that is equipped with a quadrupole mass spectrometer. This chamber sticks into the main chamber and exhibits a circular front orifice that can be closed by a mechanical shutter. The D flux from the atom source was calculated from the gas throughput and the tube front temperature (2000 K) and is given here in Pt(111) monolayers per second units, 1 ML s-1 ) 1.5 · 1015 cm-2 s-1. The substrates for the this study were graphite monolayers deposited on Pt(111) surfaces by exposing clean metal surfaces to ethane at 1000 K for several minutes. Properly prepared C monolayers fully cover the Pt substrate and efficiently shield the metal.10 Any effect of the metallic carrier on the reactions can be excluded by this. With the D atom source switched off, the arrangement served for thermal desorption spectroscopy (TDS). The Pt(111) single crystal was mounted between Ta wires attached to a cryostat via Cu rods. The substrate temperature was measured and regulated through a Ni/NiCr thermocouple attached to the backside of the crystal. HMDS and trimethylsilane (H-TMS) were obtained in high purity from Fluka and

10.1021/jp002481e CCC: $20.00 © 2001 American Chemical Society Published on Web 02/14/2001

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Figure 1. Fragmentation patterns of H-TMS and HMDS according to the literature11,12 and from the present study. The signals are shown normalized to the signal of the strongest fragment, respectively.

backfilled into the chamber via leak valves. The fragmentation patterns of the silanes measured here and shown in Figure 1 are similar to the characteristics compiled in mass spectrometry data tables.11,12 The ion gauge sensitivity factors for HMDS are not known, so only uncorrected gauge readings are given below. Although the ionization cross-section for H-TMS was recently determined as 1.5 · 10-15 cm2,13 that is, 6 times the N2 crosssection, its exposures are also quoted uncorrected. For abstraction measurements, the sample with an adsorbed layer of HMDS or H-TMS was placed in front of the closed orifice. Opening of the shutter defined the zero point on the reaction time scale. The mass spectrometer was multiplexed to monitor signals in atomic mass unit (amu) channels of interest. Because the spectrometer is located in a differentially pumped enclosure, the partial pressures monitored are proportional to reaction or desorption rates. 3. Results 3.1. Adsorption of HMDS and TMS. To characterize adsorption of the silanes on the C monolayer, TD spectra were recorded after admitting increasing amounts of the adsorbates to the surface at 85 K. The spectra shown in Figure 2 illustrate that the monolayer and multilayer regimes are well separated for both adsorbates. They indicate that the adsorption energies of HMDS and H-TMS on the C monolayer are substantially bigger than their enthalpies of condensation. This is expected from the dielectric features of graphite, which makes this substrate a favorable substrate for adsorbates that exhibit a large polarizability. In the monolayer regime, HMDS is adsorbed more strongly than H-TMS, as expected for a purely physisorptive interaction between the C substrate and the admolecules. The shift of the monolayer desorption peak maxima near 143

Wehner et al.

Figure 2. Desorption spectra of H-TMS and HMDS after exposing C/Pt(111) surfaces to increasing amounts of the adsorbates at 85 K.

and 207 K with increasing coverage and the common leading edges of the desorption peaks indicate zero-order desorption for both species. This is confirmed by a linear relation between logarithmic desorption rates R and inverse temperature. Monolayer desorption energies of 68 kJ/mol (HMDS) and 46 kJ/mol (H-TMS) were deduced from the slopes of the linear segments in the log R vs T-1 curves. Desorption from multilayers, which develop after saturation of the monolayers, is also of zero order (confirmed by linear dependencies between desorption rates and inverse temperature) and exhibits maxima at 105 and 154 K for H-TMS and HMDS, respectively. Plots of the logarithmic desorption rates vs 1/T revealed desorption energies of 52 kJ/ mol (HMDS) and 37 kJ/mol (H-TMS), respectively. These numbers are in accordance with tabulated enthalpies of sublimation.14 The increase of coverage with adsorbate exposure, as shown in Figure 3, illustrates that both molecules stick with constant probability throughout the coverage regime investigated here. The scale of the ordinate in Figure 3 is normalized with respect to the desorption peak areas obtained at saturation of the monolayer signals. To confirm molecular desorption, when taking TD spectra the mass spectrometer was multiplexed to monitor the signals in the amu (m/e) channels shown in Figure 1. Because the desorbed species exhibit the same fragmentation pattern as the exposed molecules, purely molecular desorption is apparent. This corresponds to the expectation that the C monolayer should exclude any dissociation of the adsorbate. There were only features in the fragmentation pattern of desorption from multilayer HMDS that were unexpected. As seen in Figure 4, after completion of the monolayer coverage, amu 58, amu 43, and amu 15 signals occur at about 126 K, which are not accompanied by signals in the amu 146 and amu 73 channels. It is clear that the species that desorbs at 126 K cannot be HMDS. A detailed

Study of Adsorption of HMDS on C/Pt(111)

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Figure 3. Coverage increase versus exposure (uncorrected readings) of H-TMS and HMDS obtained from integrated desorption traces.

Figure 5. Segments of desorption spectra between 105 and 165 K measured in the amu 43 channel (top). Amu 43 and amu 73 signal peak positions as a function of coverage (bottom).

Figure 4. Desorption signals measured in various amu channels after admitting HMDS to C/Pt(111) surfaces at 85 K.

analysis of the low-temperature features of the amu 73 and amu 43 desorption signals is shown in Figure 5. Weak desorption signals at less than 160 K exhibit a downward shift on the temperature scale until the amu 43 signal splits into two components at saturation of the monolayer. One component, located near 150 K, tracks the amu 73 signal and originates from HMDS multilayer desorption. The occurrence of the lowtemperature component of the amu 43 signal at 126 K coincides with the completion of the HMDS monolayer. Figures 4 and 1 show that the species responsible for it exhibits a remarkably strong methyl fragment.

3.2. Reaction of D with HMDS and TMS. Reactions of D atoms with adsorbed layers of HMDS revealed singly deuterated D-TMS [DSi(CH3)3] as the only reaction product, identified through a molecular signal at amu 75 and the strongest peak due to fragmentation at amu 60. The reactions were complete, and no signal from HMDS was recorded in postreaction desorption spectra. Other reaction products were not detected. The absence of HD and CH3D illustrates that breaking of the Si-Si bond via insertion of a D is a much faster reaction than H or methyl abstraction. Reactions of D atoms with adsorbed H-TMS revealed no products on reaction time scales that were investigated here. Figure 2 shows that the reactions of D with adsorbed HMDS can be performed in various ways. At substrate temperatures of about 170 K monolayer HMDS is the target, and the D-TMS product should occur in the gas phase immediately after its formation. At 120 K the target is mono/multilayer HMDS and the product molecules can be either adsorbed or gas-phase species, depending on the coverage. At 85 K the product should remain adsorbed. The reaction kinetics data of D-TMS measured through the amu 60 signal from the DSi(CH3)2 fragment shown in Figure 6 covers these different temperature regimes. This figure shows the kinetic values of D-TMS formation that were measured at HMDS adlayers prepared at 85 K and reacted with D between 85 and 170 K. The HMDS coverage installed at 85 K corresponds to 1.3 monolayers. No gaseous product was observed at 85 K substrate temperature, because even multilayer D-TMS will remain adsorbed. At 95 K a small amount of D-TMS appeared in the gas phase; at 100 and 105 K the signals of gaseous D-TMS become significant. Note that the D-TMS signals at these temperatures grow from zero to maximum values and decrease thereafter. This behavior is expected if gas-phase

1802 J. Phys. Chem. B, Vol. 105, No. 9, 2001

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Figure 6. D-TMS reaction kinetics during admitting D atoms at HMDS-covered surfaces at various temperatures. The D flux was started at t ) 0. The amu 60 signals were smoothed for noise reduction.

Figure 7. D-TMS reaction kinetics at 120 K (top) and 170 K (bottom). The amu 60 signals were smoothed for noise reduction. The coverages indicated refer to the coverage installed at 85 K.

D-TMS originates from two consecutive reactions. In the first reaction step, adsorbed D-TMS product molecules are formed. In the second reaction they desorb because the reaction temperature is close to the desorption temperature of multilayer D-TMS. The phenomenology of the kinetics is different at the 170 K reaction temperature. The D-TMS rate assumes its maximum value immediately at reaction start and decreases at later times. This feature is expected because the target adsorbate is a HMDS monolayer and the reaction temperature is far above the desorption temperature of the D-TMS products. Accordingly, the desorption step of the product is not rate limiting and the measured rate is a rate of product formation. For a direct reaction according to an Eley-Rideal mechanism, under the given reaction conditions, the time dependence of these rates R can be written as R(t) ) Θ(0)Φσ exp(-σΦt), with Θ(0) as initial coverage of the target molecules, Φ as atom flux, σ as reaction cross-section, and t as time.15 The measured kinetics can be described very well by that rate expression by setting σ ) 0.4 Å2. Figure 7 shows the coverage dependence of the kinetics for two reaction temperatures: 120 and 170 K. At 170 K the influence of the HMDS coverage on the kinetics is as expected from the rate expression above. The initial D-TMS rates R(0) displayed in the inset are strictly proportional to the HMDS coverage present on the surface at the reaction start, Θ(0). Subsequently, the D-TMS rates decrease exponentially. The exponent σ, which controls the rate decay, does not depend on the coverage; this is confirmed by parallel lines in a plot of the logarithms of the rates. At 120 K reaction temperature the coverage dependence of the kinetics illustrates the competition of HMDS and the D-TMS product for adsorption sites. At the smallest HMDS coverage, sites for the product are available on the surface and no gaseous D-TMS is observed. Because

one HMDS molecule is transformed into two less strongly bound D-TMS molecules through reaction with D, the D-TMS species gets replaced after its formation if the total coverage exceeds a specific limit during the course of the reaction. This behavior is seen at 120 K upon increasing the HMDS coverage. If the HMDS coverage exceeds one monolayer, D-TMS appears in the gas phase immediately at the reaction start. Figure 7 also shows that at a HMDS coverage of about two monolayers the rate decrease is not exponential. During D atom exposure at HMDS layers, the mass spectrometer was multiplexed over various amu channels to allow an analysis of the fragmentation signals for a consistent identification of the products. In the monolayer regime of HMDS the fragmentation signals of the formed D-TMS product was according to the expected pattern. However, upon approaching the monolayer coverage of HMDS inconsistencies occurred in the fragmentation signals. This is illustrated in Figure 8 by the signals in the amu 60 and amu 73 channels measured during reaction of D with HMDS layers prepared at 85 K and reacted at 170 K. The amu 60 signal due to DSi(CH3)2 is a strong and unique fragmentation feature of the DSi(CH3)3 product. Its fragmentation results also in a weaker signal at amu 73 from the Si(CH3)3 fragment. (Compare Figure 1.) If the signals at amu 60 and amu 73 stem exclusively from the same product, they should exhibit the same time dependence. Because this is not the case, the amu 73 signal must contain a component that is not due to D-TMS. Using its fragmentation pattern, the D-TMS-related signal can be removed from the measured amu 73 signal to isolate the signal of the yet unidentified product. As shown in Figure 8, its contribution available through the corrected amu 73 signal exhibits intensity only close to the monolayer coverage of HMDS target molecules. Furthermore, its kinetics illustrates that its origin is depleted very quickly,

Study of Adsorption of HMDS on C/Pt(111)

Figure 8. Signals in the amu 60 and amu 73 channels measured during reaction of D with adsorbed HMDS (top). “Unexpected” contribution to the amu 73 signal. The coverages indicated refer to the coverage installed at 85 K.

within the first 50 s of reaction time. A fragmentation analysis revealed that this fast-decaying product is molecular HMDS. 4. Discussion This study demonstrates that HMDS and H-TMS adsorb and desorb molecularly on graphite-covered Pt(111) surfaces. The zero-order desorption deduced from the plots of the logarithmic desorption rates for monolayer species are in accordance with the expectation that the significant polarizabilities of these silanes and the minor influence of the energetically flat graphite substrate on the adlayer structure cause the adsorbed silanes to condense in two-dimensional islands. Initial condensation in three-dimensional islands can be excluded because the monolayer and multilayer regimes are well separated in the desorption kinetics. In line with this, the monolayer desorption energies scale with the polarizabilities of the molecules. The multilayer desorption spectra exhibit the features of three-dimensional condensation, and in agreement with this, the multilayer desorption energies were close to thermodynamic enthalpies of sublimation. The sticking coefficient of H-TMS can be estimated from its total ionization cross-section of 1.5 · 1015 cm2 as reported in ref 13. With the N2 ionization cross-section of 2.5 · 1016 cm2, the (uncorrected) exposures given in Figure 3 must be divided by 6 for correction of the H-TMS exposure. Because the monolayer is completed at 1 L (corrected) exposure, the sticking coefficient of H-TMS is about unity on C/Pt(111). Its space requirement in the monolayer is about 10 Å2, in good agreement with a flat-lying molecule. At 85 K the sticking coefficient of HMDS should be similar to that of H-TMS, but its space requirement is bigger. Therefore, the smaller slope of the HMDS uptake curve in Figure 3 suggests its smaller ionization cross-section.

J. Phys. Chem. B, Vol. 105, No. 9, 2001 1803 The TD spectra in Figure 2 exhibit the features expected for condensation at 85 K, and the series of spectra shown in Figure 4 are in line with molecular desorption. The only unexpected feature as detailed by the data shown in Figure 5 is the lowtemperature desorption peak at 126 K. It is clear from Figure 4 that the respective species is not molecular HMDS. The spectra in Figure 5 illustrate that it exists only after completion of the monolayer, and Figure 4 shows that the amount of this species on the surface grows with HMDS exposure. Its desorption temperature indicates that it is a fragment of HMDS. HMDS is stable against visible light radiation; therefore, it is suggested that dissociation was caused by stray electrons from the ion gauge that is located in a line-of-sight position relative to the sample surface. Silanes were reported as sensitive with respect to dissociation by electron impact,16 and the high ionization cross-section agrees with that. Because the weakest link in the HMDS molecule is the Si-Si bond (see below), a plausible dissociation product could be the Si(CH3)3 radical, which would lead to amu 58 and 43 signals due to methyl split off during fragmentation. The high amu 15 signal is assumed to indicate the radical nature of the desorbed species. Therefore, we interpret the 126 K desorption feature in Figures 4 and 5 through desorption of trimethylsilyl radicals formed by electron irradiation of multilayer HMDS. A direct reaction between a gas-phase H atom and an adsorbed molecule proceeds either according to the Eley-Rideal or hot-atom mechanism. The requirement for the operation of the latter mechanism is the existence of a strong attractive potential between the substrate surface and the incoming H atom.17 With the C monolayer on the Pt(111) surface the substrate-H potential is weak, and it is expected that the reactions in the present study follow the Eley-Rideal scheme, as observed previously in reactions of H with adsorbed alkyl halides.18 As already mentioned in the experimental section, the two important consequences of the operation of an Eley-Rideal mechanism are exponential product rate decay after the reaction start and proportionality between the initial rate and the coverage of the adsorbate. Both features are apparent from Figures 6 and 7 provided that the direct reaction can lead immediately to a gaseous product and transient or permanent adsorption of the product is excluded by performing the reaction at greater than the product desorption temperature. At lower temperatures desorption limitation of the product kinetics occurs, which is clearly illustrated by the kinetics measured at 120 K at increasing HMDS coverages (shown in Figure 7). The observation of only one product from the reaction of D with HMDS and no product from the interaction of D with H-TMS is surprising. According to tabulated data,19 the dissociation energies of the various bonds in HMDS are as follows: Si-Si, 3.5 eV; Si-methyl, 4.0 eV; C-H, 4.3 eV. Bonds with the following strengths are formed by reaction with D: D-TMS, 4.4 eV; D-methyl, 4.5 eV; H-D, 4.5 eV. Accordingly, abstraction of TMS is exothermal by 0.9 eV, abstraction of methyl by 0.5 eV, and abstraction of H by 0.2 eV. Only the reaction with the largest exothermicity was observed experimentally. H insertion into the Si-Si bond with an activation energy of 0.1 eV was deduced from a gas-phase study of the H/HMDS reaction.20 The authors of that work proposed that the TMS radicals generated by the reaction undergo pairing to form new HMDS. The same probably applies in the present case if the adsorbed radicals remain adsorbed. If this were not the case, a reaction of D with HMDS would leave a TMS radical on the surface which could react with a further D atom to form D-TMS.

1804 J. Phys. Chem. B, Vol. 105, No. 9, 2001 Although there would still be only one gaseous product, there would be two different pathways leading to it. It is improbable that abstraction of TMS from HMDS and deuteration of TMS exhibit the same cross-section. This would lead to a more complicated kinetics than observed here, which is very well described by only one cross-section. The dominance of D insertion into the Si-Si bond in HMDS over abstraction of H from the methyl groups is contrasted by a much bigger H abstraction rate as compared with insertion in disilane21 or polymeric Si:H films.22 The reversal of the relative magnitudes of insertion and abstraction rates is probably caused by the differences of the C-H and Si-H bond dissociation energies, 4.5 eV vs 3.7 eV.19 The influence of the HMDS coverage on the reaction kinetics is illustrated by the data in the upper panel of Figure 7. The D-TMS rate at a substrate covered with two monolayers of HMDS exhibits a clear deviation from an exponential decrease. This can only occur if the HMDS molecules underneath the second layer are not or are less easily accessible for incoming D atoms. Otherwise, all adsorbed molecules could be attacked by D in the same manner, and a pure exponential product rate decrease would result. If first-layer HMDS molecules were shielded completely by second-layer species on top of them, the D-TMS rate would stay constant until depletion of the second layer. This was not observed, allowing us to conclude that first-layer molecules are only less easy to attack by D. Accordingly, after completion of the HMDS monolayer with a few second-layer species, D-TMS that desorbs can form underneath a HMDS molecule. This event leaves the upper HMDS temporarily in a less strongly bound situation and it may desorb. It is suggested that the HMDS species extracted from the kinetic data shown in Figure 8 are produced in this way. If the reaction energy available from the formation of D-TMS were responsible for HMDS desorption, one would expect it to happen in a wide range of multilayer coverages and not only very close to completion of the monolayer. 5. Conclusions HMDS and H-TMS mono- and multilayers were prepared on Pt(111) surfaces covered by graphite monolayers. Exclusively

Wehner et al. molecular adsorption and desorption were observed. Reactions of D atoms with HMDS layers revealed D-TMS as the only product between 85 and 170 K. The reaction proceeds via an Eley-Rideal mechanism with a cross-section of σ ) 0.4 Å2. Hydrogen and methyl are not abstracted by D, in accordance with the energetics of the system. The reactions of D with adsorbed TMS exhibits a cross-section that is at least a factor of 10 smaller. References and Notes (1) Plasma Deposition of Amorphous Silicon-Based Materials; Bruno, G., Capezzuto, P., Madan, A., Eds.; Academic Press: Boston, 1995. (2) van de Sanden, M. C. M.; Severens, R. J.; Kessels, W. M. M.; Meulenbroeks, R. F. G.; Schram, D. C. J. Appl. Phys. 1998, 84, 2426. (3) Takagi, T.; Hayashi, R.; Ganguly, G.; Kondo, M.; Matsuda, A. Thin Solid Films 1999, 345, 75. (4) Miyoshi, Y.; Yoshida, Y.; Miyazaki, S.; Hirose, M. J. Non-Cryst. Solids 1996, 198-200, 1029. (5) Shirai, H.; Arai, T. J. Non-Cryst. Solids 1996, 198-200, 931. (6) Wu, C. H.; Jacob, C.; Ning, X. J.; Nishino, S.; Pirouz, P. J. Cryst. Growth 1996, 158, 480. (7) Madapura, S.; Steckl, A. J.; Loboda, M. J. Electrochem. Soc. 1999, 146, 1197. (8) Wehner, S.; Ku¨ppers, J. J. Chem. Phys. 1998, 108, 3353. (9) Bischler, U.; Bertel, E. J. Vac. Sci. Technol. 1993, A11, 458. (10) Lutterloh, C.; Biener, J.; Po¨hlmann, K.; Schenk, A.; Ku¨ppers, J. Surf. Sci. 1996, 352-354, 133. (11) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. Registry of Mass Spectral Data; John Wiley: New York, 1974. (12) NIST Chemistry Webbook: http://webbook.nist.gov/chemistry. (13) Jiao, C. Q.; Garscadden, A.; Haaland, P. D. Int. J. of Mass Spectrom. 1999, 184, 83. (14) Beilsteins Handbuch der Organischen Chemie, Bd.4/5.1, 4; Erga¨nzungswerk Springer: Berlin, 1981. Gmelin Handbuch der Organischen Chemie, Silicium Teil C System 15; Verlag Chemie: Weinheim, 1958; 8. Auflage. Suga, H.; Seti, S. Bull. Chem. Soc. Jpn. 1959, 32, 1088. (15) Wehner, S.; Ku¨ppers, J. J. Chem. Phys. 1998, 109, 294. (16) Ascherl, M. V.; Campbell, J. H.; Lozano, J.; Craig, J. H., Jr. J. Vac. Sci. Technol. 1995, A13, 2721. (17) Harris, J.; Kasemo, B. Surf. Sci. 1981, 105, L281. (18) Wehner, S.; Ku¨ppers, J. J. Chem. Phys. 1999, 111, 3209, 3218, 3225. (19) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, 1995. (20) Ellul, R.; Potzinger, P.; Reimann, B. J. Phys. Chem. 1984, 88, 2793. (21) Fabry, L.; Potzinger, P.; Reimann, B.; Ritter, A.; Steenbergen, H. P. Organometallics 1986, 5, 1231. (22) Chiang, C. M.; Gates, S. M.; Lee, S. S.; Kong, M.; Bent, S. F. J. Phys. Chem. 1997, 101, 9537.