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J. Phys. Chem. B 2000, 104, 3078-3084
Thermal Chemistry of CH3 on Si/Cu(100)† J. Han,‡ S. I. Gheyas,‡ Y. Wang,‡ D. R. Strongin,*,‡ A. P. Graham,§,⊥ B. J. Hinch,§ and A. P. Wright| Department of Chemistry, Temple UniVersity, Philadelphia, PennsylVania 19122, Department of Chemistry, Rutgers UniVersity, Piscataway, New Jersey 08855, and Dow Corning Corporation, Midland, Michigan 48686-0995 ReceiVed: August 16, 1999; In Final Form: NoVember 8, 1999
Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH3 (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH4 at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu2Si structure. These Cu2Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH3)3SiH desorbed near 200 K from CH3/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH3)3SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH3)3SiH at 200 and 420 K were believed to be due to the thermal reaction of CH3 with the low Si density and high Si density (i.e., Cu2Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu2Si phase covered the surface, only the 420 K peak was present during CH3/Si/Cu(100) TPD. Results also suggested that (CH3)Si and possibly some (CH3)2Si intermediates predominated on the surface below room temperature, and (CH3)3Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH3)3SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).
1 Introduction Many studies have investigated the structure of Cu/Si-based alloy surfaces and those interfaces consisting of these elements. This effort has been in large part due to the importance of this system in the semiconductor industry. Investigations that have addressed the chemical reactivity of Cu/Si-based surfaces have been less apparent, even though the Cu/Si system is at the heart of a billion-dollar silicone industry. The Rochow1 or Direct Synthesis,2 which is used in industry to produce chloromethylsilanes (precursors to silicones), relies on the reaction of methyl chloride with silicon containing catalytic amounts of Cu: Cu
CH3Cl + Si 98 (CH3)xSiCl4-x A typical catalytic system also contains impurity concentrations of tin, zinc, and aluminum that are required to form dimethyldichlorosilane,3,4,5 which is the desired product, with high selectivity (usually approaching yields higher than 90%). The active catalyst is thought to be composed of crystallites of Cu3Si stoichiometry. With regard to the structure and reactivity of this catalytic surface, however, little is known.2,6,7 The specifics of the surface reactions that proceed chloromethylsilane formation and the structure of the active site from which this product †
Part of the special issue “Gabor Somorjai Festschrift”. * To whom correspondence should be addressed. Telephone (215) 2047119; Fax (215) 204-1532; E-mail
[email protected] ‡ Temple University. § Rutgers University. | Dow Corning Corporation. ⊥ Present address: Max-Planck Institute fu ¨ r Stro¨mungsforschung, Bunsenstrasse 10, D-37073 Go¨ttingen, Germany.
forms are not well understood. A microscopic understanding of how Cu8,9 facilitates the reaction and how promoters3,5 increase its catalytic activity is severely lacking.10 Presumably, studying this reaction in the ultrahigh vacuum environment (UHV) with modern surface spectroscopies,11-13 could help to provide a microscopic framework by which to understand the surface structure and reactivity of the working Direct Synthesis catalyst. Such an experimental approach, however, is complicated by the fact that CH3Cl does not dissociate on a Cu/Si surface in UHV. Sun et al.14,15 have shown, however, that the reactivity and selectivity of this reaction can be accurately modeled in UHV by individually adsorbing CH3 and Cl2 on polycrystalline Cu3Si. The reactions of the surface monolayer formed in this manner yield a product distribution of chloromethylsilanes that is similar to that obtained under industrial conditions (i.e., high pressure of CH3Cl). Our goal in the present research is to further model the reaction in UHV, but on well-defined Cu/Si surfaces so that vacuum-based surface analytical techniques can be fully exploited. Our presumption is that molecular level details determined in UHV on model single-crystal Si/Cu surfaces can be used to understand the working surface of the high-pressure industrial catalyst. The well-defined Cu/Si surface used in this study is prepared by depositing Si on Cu(100) [via the dissociative adsorption of SiH4]. Prior structural studies have shown that under well-prescribed conditions this procedure results in the formation of a copper-silicide covered surface with approximately a 5 × 3 superstructure.16,17 The 2-D stoichiometry in this layer is Cu2Si and is thought to be similar to a Si-rich basal plane in Cu3Si bulk crystals. Furthermore, this Cu2Si
10.1021/jp9928863 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
Thermal Chemistry of CH3 on Si/Cu(100) overlayer is incommensurate with the square symmetry of the Cu(100) substrate. The STM has suggested that, at low Si coverages, each Si atom sits in the topmost plane and is surrounded by four neighboring coplanar Cu atoms (a net Cu-rich environment).16 Only at higher coverages does the Cu2Si phase form, through a lateral segregation of silicon density on the Cu(100) substrate. The individual Si atoms, within the high Si coverage Cu2Si phase, actually have six in-plane neighboring Cu atoms. Thus an “increasing local Cu coordination” is observed as the Si concentration increases (which is perhaps contrary to expectations). This increasing coordination is achieved through a fundamental symmetry change in the surface plane. The clean Cu(001) starts out with a 4-fold, or square, symmetry. The Cu2Si phase at high SiH4 exposure is associated with a near perfect 6-fold, or hexagonal symmetry. The symmetry change also is accompanied by an atomic density change in the surface plane. If clean Cu(001) has one monolayer (ML) of exposed copper atoms, an exact 5 × 3 Cu2Si phase has 0.4 ML of exposed Si atoms as well as 0.8 ML of Cu. (A Cu(001) 5 × 3 cell has 15 atoms/unit cell. In contrast, an exact 5 × 3-Cu2Si has 6 surface Si atoms/unit cell and 12 surface Cu atoms/unit cell.) Hence, there would be a net 20% fractional increase of surface-exposed atoms in going from a clean Cu(001) surface to the idealized 5 × 3 Cu2Si overlayer on the same substrate. However, the 20% figure is approximate as the Cu2Si phase is not strictly a commensurate phase. Earlier helium atom diffraction studies16 have shown that the true Cu2Si incommensurate unit cell is still more closely represented by a 5 × 13 unit cell. This larger unit cell results from a further 3.8% compression of the 5 × 3 structure. The final best estimate of the overall atomic density in the Cu2Si phases is 24.7% higher than on the unreconstructed surface.18 Note also, the overlayer does still maintain the correct 2:1 stoichiometry. Last, Si surface coverages cannot be maintained at substrate temperatures above 600 K. At these temperatures Si is believed to begin diffusion into the bulk of the copper substrate. In this contribution, research is presented that was concerned with addressing chemistry relevant to the Direct Reaction on these well-defined model Si/Cu(100) surfaces. Specifically, the thermal chemistry of CH3 on Si/Cu(100), having varying surface concentrations of Si, was studied with temperature programmed desorption (TPD), photoelectron spectroscopy (PES), and scanning tunneling microscopy (STM). TPD was used to characterize the kinetics and gaseous product distribution associated with the surface chemistry of CH3 on Si/Cu(100). The kinetics of the methylsilane product, which resulted from this chemistry, were sensitive to the surface coverage of Si. Based on scanning tunneling microscopy (STM) and photoelectron spectroscopy (PES) results, a simple model, related to local Si densities and Si environments, was proposed to explain this dependence of methylsilane desorption kinetics with Si coverage. Finally, the nature of the surface species that proceeded methylsilane desorption was inferred from TPD experiments. 2 Experimental Section The experimental data presented here were obtained in three separate experimental facilities. The first facility, at Temple University, was used to obtain temperature programmed desorption (TPD) and Auger electron spectroscopy (AES) results. The experimental chamber that housed these techniques had a typical base pressure of 3 × 10-10 Torr and was pumped by ion and turbomolecular pumps. The chamber also was equipped with low energy electron diffraction (LEED) optics for sample
J. Phys. Chem. B, Vol. 104, No. 14, 2000 3079 geometric characterization and an ion gun for sample cleaning. The second experimental apparatus was based at the National Synchrotron Light Source at Brookhaven National Laboratory and was the UHV end-station on the U7a beamline. The endstation was evacuated to 10-9 Torr with a turbomolecular and ion pump. This second chamber was used to obtain the photoelectron spectroscopy (PES) data. A third UHV chamber, at Rutgers University, was equipped with a scanning tunneling microscope (STM), LEED optics, sample transfer, and an ion gun. This chamber was pumped by ion pumps and a turbomolecular pump, had an operating pressure of 1 × 10-10 Torr, and was used primarily to determine the topography of the Si/ Cu(100) surface. Cu(100) crystals used in this study were polished by standard metallurgical methods and were within 1° of the specified orientation. In general, copper samples showed substantial carbon and oxygen contamination when introduced into the experimental chamber. Both contaminants were removed, by repeated 500 eV Ar+ (for TPD and PES studies) or 1 keV Ne+ (for STM study), and 700-800 K anneal cycles. Sharp (1 × 1) LEED patterns were observed after these cleaning procedures. Si/Cu(100) surfaces were prepared by exposing Cu(100) to a 0.5%SiH4/Ar mixture. Samples were kept at 420 K during the deposition process, which was shown in prior studies to result in the deposition of Si and the desorption of H2.17 The Si-saturated surface, which we refer to as Sisat/Cu(100) in this paper, was prepared by exposing the Cu single crystal to about 10 L of SiH4. Prior investigations16 of this surface showed that Sisat/Cu(100) had a Si coverage 0.4 ML (see Introduction). The Si coverages quoted in TPD data were obtained with AES by using the height of the Si-90 eV peak for Sisat/Cu(100) as reference. CH3 was generated by the thermal pyrolysis of azomethane, CH3N2CH3. In this particular study azomethane was passed through a vacuum compatible leak valve into a quartz tube at a temperature of about 1023 K. The sample was held about 1.5 cm from the tube exit during exposure. Preparation of methyl radicals in this way for UHV studies has been described by Stair and co-workers.19 By using AES (monitoring the C-272 eV peak), we determined that a “5 L” exposure (1 L ) 10-6 Torr‚s) of CH3 (total exposure to pyrolysis products of CH3N2CH3) at temperatures in the 120-170 K range produced a Si/ Cu(100) surface that was saturated with CH3. Higher CH3 exposures did not lead to an increase in the C-272 eV intensity. Samples were approximately 1 cm diameter disks with a thickness of 2 mm. Samples used for TPD and PES studies had slots around their perimeters and Ta support wire (0.25 mm diameter) was wrapped around the sample. The ends of the wires were spot-welded to Ta tabs that were mechanically attached to a liquid nitrogen cryostat with cooling capability to 120 K. Heating of the samples was accomplished by passing current through the Ta wires. A chromel-alumel (type K) thermocouple was held to the back of the samples by ceramic cement. TPD experiments were performed by heating the sample at a heating rate of 3 ( 1 K/s. Gases desorbing from the surface were analyzed by a multiplexed quadropole mass spectrometer, capable of simultaneously measuring up to nine ions during a single TPD experiment. The mass spectrometer was housed in a gold plated stainless steel shield having a 6.0 mm diameter aperture. During TPD experiments the sample was placed within 2-3 mm of this aperture hole, thereby limiting the detection of gases evolving from the support wires. Reference cracking patterns were obtained for all of the different products discussed in this paper.
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Han et al.
Synchrotron data at the NSLS were obtained with a 250 mm VSW hemispherical analyzer mounted 90° relative to the photon beamport. Cu 3p and Si 2p spectra were obtained with 250 and 400 eV photon energies, respectively (analyzer pass energy of 22 and 10 eV for Cu 3p and Si 2p data, respectively). Adsorption of CH3 in these photoemission studies was carried out at 120 K. Data obtained at higher temperatures were obtained by heating the sample at a rate of 5 K/s to the desired temperature and then cooling to 120 K. For STM experiments the Cu sample was held on a Mo plate with Ta clamps. The Mo plate and sample could be transferred between different parts of the apparatus. For this ease of motion, a thermocouple could not be mounted on the sample. For sample annealing, the temperature was measured using a calibrated, lowtemperature, optical pyrometer. In the temperature range 300500 K, for silane dosing, temperature control was established through adjustments of power input to a calibrated, radiativeheating tungsten filament. After silane exposures the sample was allowed to cool for longer than 30 min in order to reach the chamber ambient temperature (room temperature) prior to placing the sample in the STM. All STM experiments were carried out at room temperature. The tungsten STM tip was cleaned by field emission to a test Pt sample (1 µA at 200300 V) before Si/Cu sample imaging. Images were taken with sample biases in the range +5 to -5 V, and with tunnel current typically between 0.1 and 1 nA. 3 Results 3.1 Scanning Tunneling Microscopy of Si/Cu(100). Figure 1a exhibits a relatively large area topographic scan of a Si/Cu(100) prepared by exposing Cu(100) to SiH4 (∼8.6 L) at 420 K, which resulted in a Si coverage that was believed to exceed 0.33 ML, but was less than 0.4 ML. Prevalent in the image are domains that consist of well ordered “stripes” that are separated by about 5 copper lattice spacings (∼13 Å). These domains were previously shown to have an average stoichiometry of Cu2Si.16 Coexisting with the “striped” domains on Si/Cu(100) were regions that could not be better resolved in this study. In the latter regions a dynamic “noise” dominated the signal, and this noise was present at all sample biases (see for example Figure 1b). While these noisy regions were not resolvable they may represent regions containing highly mobile Cu adatoms that were displaced during the deposition of Si. The images certainly are not consistent with a highly mobile adsorbate present on the tip, or any other tip instability. At silane exposures below ∼6 L the striped Cu2Si domains were not observed. In contrast, at Si saturation, the STM has demonstrated that the entire surface is filled with the striped Cu2Si phase.16 For saturation exposures at 420 K, the final domain size typically exceeded 1000 Å in length. (In contrast, a typical dimension of the Cu2Si domains was as low as 65 Å for exposures at room temperature.20) However, once the domains are formed their size appears very stable until the temperature exceeds approximately 600 K, at which point diffusion into the bulk occurs. 3.2 CH3/Si/Cu(100). 3.2.1. Thermal Chemistry of CH3 on Si/Cu(100). Figure 2 exhibits CH3/Si/Cu(100) TPD as a function of Si coverage on Cu(100). The primary desorbing product desorbing from the surface was trimethylsilane, (CH3)3SiH. This assignment was based on close agreement between the cracking pattern of the desorbing product and a reference (CH3)3SiH cracking pattern obtained with our mass spectrometer. A relatively small amount of (CH3)2SiH2 (not more than 5% of the methylsilane yield) was also thought to be in the product
Figure 1. STM images of Si/Cu(100). (a) 8.6L SiH4 exposure at 420 K. Image size approximately 1000 Å wide by 1100 Å high. Tunnel current ) 0.27 nA, sample bias ) 3 V. (b) 10.3L SiH4 exposure at 420 K. Image size approximately 380 Å wide by 500 Å high. Tunnel current ) 1 nA, sample bias ) 0.5 V. Both images are displayed in “derivative” mode, mimicking illumination from the left-hand side.
distribution. (CH3)3SiH was the primary methylsilane desorption product at all Si coverages, although the desorption behavior was sensitive to the Si coverage. Specifically, at the lowest Si coverage used in our study (∼0.1 ML), the temperature of the peak desorption rate, TP, of this product was 200 K. By a Si coverage of 0.2, a second desorption feature appeared with a TP near 380 K. Increasing the Si coverage toward saturation resulted in a concomitant increase in the second TP, until a saturation Si coverage was achieved, at which point that TP was equal to 420 K. Furthermore, at a saturation coverage of Si, the 200 K desorption feature was no longer present and the 420 K desorption feature dominated the desorption profile of (CH3)3SiH. TPD data exhibited in the inset to Figure 2 show that ethane, C2H6, was also produced at low Si coverage, and this product was characterized by a TP in the 300-400 K range. [While not shown, CH4 also desorbed in a similar temperature range from clean Cu(100).] The yield of C2H6 rapidly decreased as Si was deposited on the Cu(100) surface. At a Si coverage of 0.08 ML
Thermal Chemistry of CH3 on Si/Cu(100)
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Figure 3. Cu 3p data for (a) clean Cu(100), (b) Sisat/Cu(100), and (c) CH3/Sisat/Cu(100) at 120 K.
Figure 2. TPD of CH3/Si/Cu(100) as a function of Si coverage. (CH3)3SiH is the dominant desorption product at all Si coverages. The inset exhibits data concerned with the desorption of C2H6 from CH3/Si/Cu(100) as a function of Si coverage.
the yield of this product (along with CH4) had decreased by over an order of magnitude and by a Si coverage of 0.2 ML; C2H6 was absent from the product distribution (which at this point included only methylsilanes). 3.2.2 PES Studies. Figures 3 and 4 exhibit Cu 3p and Si 2p photoelectron spectra, respectively, after a saturation coverage of CH3 was placed on Sisat/Cu(100). Cu 3p data presented in Figure 3 show that the incorporation of Si into the Cu(100) lattice to form the Cu2Si overlayer at 420 K resulted in spectral changes. Specifically, the formation of alloy surface resulted in a small (∼0.3 eV) shift of the Cu 3p peak maximum from the position that characterized Cu(100).21 Adsorption of a saturation coverage of CH3 on Sisat/Cu(100) did not result in any further observable change in the Cu 3p peak position, relative to clean Sisat/Cu(100). Si 2p data presented in Figure 4 indicated that the adsorption of CH3, however, did induce a small chemical shift (∼0.3 eV) of the Si 2p peak from 99.3 to 99.6 eV. Heating CH3/Sisat/Cu(100) to 600 K resulted in a reduction in the Si 2p intensity and a shift of the peak maximum back to 99.3 eV. Control PES experiments were also carried out that investigated the Cu 3p core level after clean Cu(100) was exposed to CH3. The binding energy position of the Cu 3p peak was found to be insensitive to the presence of CH3, within our experimental resolution. Figure 5 displays C 1s data that were obtained for a saturation coverage of CH3 on Si(0.3 ML)/Cu(100) at 120 K and after the stepwise heating of this surface to 300 and 600 K. Adsorption of CH3 at 120 K resulted in a C 1s feature with an intensity maximum at a binding energy of 283.7 eV. After heating to 300 K, which resulted in the desorption of methylsilane product, the C 1s feature lost high binding energy spectral weight relative
Figure 4. Si 2p data for (a) Sisat/Cu(100), (b) after adsorption of CH3 at 180 K, and (c) after heating CH3/Sisat/Cu(100) to 600 K.
to the 120 K spectrum. The difference spectrum presented in the inset, obtained by subtracting the 300 K data from the 120 K data, emphasizes the location of this spectral weight loss. Based on an analysis of the 120 K and 300 K C 1s data, it was estimated that 20% of the C 1s intensity was lost upon heating to 300 K. Heating to 600 K resulted in a 80% loss of C 1s signal relative to the 120 K data, due primarily to the desorption of (CH3)3SiH. 3.2.3 Hydrogen Atom Post-Dosing of CH3/Sisat/Cu(100). TPD results that are shown in Figure 6 were obtained in three different
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Figure 5. C 1s data for (a) CH3/Si(0.35 ML)/Cu(100) at 120 K (b) after heating CH3/Si/Cu(100) to 300 K, and (c) 600 K. (d) C 1s data for clean Cu(100). The inset displays a difference spectrum obtained by subtracting the 300 K data from 120 K data.
ways. In the first (Figure 6a), CH3 was adsorbed on Sisat/Cu(100) at 150 K and the surface was immediately exposed to atomic H, which was generated by passing H2 over a hot tungsten filament. In this circumstance, the (CH3)SiH3 product desorbed at 160 K and a smaller feature attributed to (CH3)2SiH2 product exhibited desorption near 150 K. Also, the desorption of (CH3)3SiH, which was the dominant product without H atom exposure (see Figure 2), was eliminated and a TPD feature assigned to (CH3)2SiH2 appeared instead at 380 K. Second, CH3 was adsorbed on Sisat/Cu(100) at 170 K, but in this circumstance the surface was momentarily heated to 390 K, cooled back to 170 K, and then exposed to atomic hydrogen (Figure 6b). After such a surface preparation, (CH3)3SiH desorbed from the surface near 200 K. (CH3)2SiH2 also desorbed near 400 K in this circumstance. While the data is not shown, it was experimentally confirmed that heating CH3/Sisat/Cu(100) to 250 K (instead of 390 K) and then cooling the surface to 180 K prior to H atom exposure resulted in a similar TPD product distribution as was shown in Figure 6a. No (CH3)3SiH desorption was observed under these experimental conditions. In a control experiment (Figure 6c), CH3 was adsorbed on Sisat/Cu(100) at 170 K, the surface was momentarily heated to 390 K and cooled back to 170 K, prior to TPD. This scenario resulted in (CH3)3SiH desorption near 420 K, very similar to its desorption behavior without the preheating treatment (see Figure 2). 4 Discussion It is suspected that the initial adsorption of CH3 does not perturb the Cu-Si bonding in the Cu2Si overlayer to a significant degree. This contention is primarily based on PES data that show that the Cu 3p binding energy associated with
Han et al.
Figure 6. TPD from (a) CH3/Sisat/Cu(100) after exposure to H atoms. (b) TPD of CH3/Sisat/Cu(100) after heating to 390 K, cooling back to 150 K, and subsequent exposure to H atoms. (c) TPD from CH3/Sisat/ Cu(100) after heating to 390 K and cooling back to 170 K. M/e 45 and 59 data presented in the figure correspond to (CH3)SiH2+ and (CH3)2SiH+, respectively. The major methylsilane product that has been associated with each desorption feature (based on cracking patterns) is indicated.
the Cu-Si alloy is not affected by CH3 adsorption (Figure 3), and that the Si 2p level exhibits only a small chemical shift (0.3 eV). The presence of the Si 2p shift suggests that at least a fraction of CH3 that comprises the monolayer probably binds directly to the Si component of the Sisat/Cu(100) surface. Whether the well-ordered “striped domains” observed by STM remain structurally intact after CH3 adsorption, however, cannot be addressed by the present data, but this issue is the subject of ongoing experiments. Results suggest that the predominant organo-Si species on CH3/Sisat/Cu(100) at 120 K is CH3Si. This statement receives strong support from atomic hydrogen post-dosing TPD experiments that showed that (CH3)SiH3 was the dominant desorption product (TP ) 200 K) after a saturation layer of CH3 on Sisat/ Cu(100) was exposed to atomic hydrogen. A relatively small amount of (CH3)2SiH2 was also experimentally observed during these experiments, suggesting that (CH3)2Si groups were a minority species on Sisat/Cu(100). A presumption in the interpretation of these experiments is that the surface became saturated with H during exposure to atomic hydrogen, allowing (CH3)xSi intermediates to be removed as gaseous (CH3)xSiH4-x product during TPD. While the initial organo-Si intermediates appear to contain one, or possibly two, methyl groups at 120 K, additional methyl groups become bound to a fraction of the surface Si atoms upon heating and desorb as (CH3)3SiH. Based on hydrogen postdosing data (Figure 6) it is hypothesized that those (CH3)3Si surface species that ultimately lead to (CH3)3SiH production
Thermal Chemistry of CH3 on Si/Cu(100) on Sisat/Cu(100) form in the 250-390 K range on Sisat/Cu(100). The subsequent production of (CH3)3SiH near 420 K is rate limited only by the availability of surface hydrogen from dissociating CH3 above 390 K. The present results allow us to estimate the amount of Si that is removed from the Sisat/Cu(100) surface as methylsilane product. First, analysis of C 1s and Si 2p X-ray photoelectron spectroscopy (XPS) data (not shown)22 for a Sisat/Cu(100) surface with a saturated monolayer of CH3 allows us to estimate the C:Si concentration ratio to be 1.0 ( 0.2. Second, C 1s data (Figure 5) suggests that approximately 0.8 of the initial CH3 monolayer, which is adsorbed at low temperature, is removed as methylsilane product upon heating to 600 K. Assuming that (CH3)3SiH is the sole methylsilane product, it is estimated that 22% of the Si in the Cu2Si layer is removed as gaseous product. It is mentioned that analysis of our Si 2p data (comparison of Figure 4a and c) could have been interpreted to suggest that over ∼50% of the Si is removed as gaseous product. The Si 2p data obtained after heating to 600 K, however, is not only reduced by the desorption of Si-containing product, but also by Si diffusion into the Cu bulk and attenuation of the Si 2p photoelectrons by decomposition fragments. (CH3)3SiH remains the primary product desorbing from CH3/ Si/Cu(100) at all Si coverages less than saturation. The details of (CH3)3SiH production, however, are sensitive to Si coverage. At the lowest Si coverage used in this study (i.e., 0.08 ML), (CH3)3SiH desorbs in a single peak with a TP of 200 K, over 200 K lower than at saturation Si coverage. At higher Si coverages (but lower than saturation), (CH3)3SiH exhibits two desorption features at 200 and ∼420 K. The chemical reason for the almost 220 K difference in desorption temperature for (CH3)3SiH cannot be unambiguously ascertained from our experiments, but all our results taken together offer some possibilities. STM shows that Si/Cu(100) at Si coverages which exceed 0.2 ML exhibit two types of structural domains. One part of the surface is composed of a structure that does not produce a well resolved STM image and it is hypothesized that this is due to highly mobile Cu that is displaced by the addition of Si. The remaining fraction of the surface exhibits well ordered “stripes” of Cu2Si stoichiometry. At a saturation coverage of Si, prior research has shown that the surface is covered in its entirety by the “striped” regions.16 Results presented in this contribution show that (CH3)3SiH desorbs in a single state at 420 K from CH3/Sisat/Cu(100), and based on STM results it is surmised that the chemistry that proceeds this product occurs on the Cu2Si domains. At 0.2 ML of Si, TPD shows (CH3)3SiH desorption near 380 K, and this desorption state, while not exactly at 420 K, is still associated with the interior of Cu2Si domains observed by STM. The origin of the low-temperature desorption state at 200 K is somewhat more uncertain. Our favored interpretation of the STM image at Si coverages less than saturation is that Cu2Si domains coexist with domains of a lower Si density (and of the 4-fold symmetry.) We believe that the majority of the chemistry that proceeds (CH3)3SiH desorption at 200 K occurs primarily in the low Si density (dynamically disordered) regions. This statement implies that this low Si density region can dissociate CH3 (by 200 K) at a lower temperature than the Sisat/Cu(100) surface (∼390 K), but we cannot offer a reason for this at this point in time. Reduced bonding of Si to the lattice, however, in the low Si density region (relative to the Cu2Si phase) may ultimately play a role in the low desorption temperature of (CH3)3SiH from this surface. An additional contribution to the reactivity of Si(250 K, but they form on surfaces with
