Langmuir 2000, 16, 6541-6545
6541
Thermal Chemistry of Chlorine on Si/Cu(100) J. Han,† S. I. Gheyas,† Y. Wang,† D. R. Strongin,*,† 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 January 21, 2000. In Final Form: April 5, 2000 Photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD) studies investigated the chemistry of Cl2 on Si/Cu(100). Si deposition was carried out by exposing Cu(100) to SiH4 at 420 K. PES showed that the exposure of this Si-saturated surface to Cl2 resulted in the formation of SiCl, SiCl2, and SiCl3 species at 120 K, with the latter species becoming more prevalent at higher Cl coverages. Heating of the chlorinated surface between 120 and 500 K increased the concentration of SiCl3 species. TPD studies of Cl2/Si/Cu(100) as a function of Si coverage showed primarily SiCl3 (and probably SiCl2) desorption at the lower Si coverages and SiCl4 at the highest Si coverage. The majority of the Si was not removed from the surface as chlorosilane product, but instead diffused into the Cu bulk at temperatures above 500 K.
Introduction Research presented in this contribution is concerned with studying the reaction of Cl2 on a well-defined Cu-Si alloy surface with electron spectroscopies and thermal desorption techniques in a vacuum. While the investigation of such a system may have relevance to the many material applications of the Cu/Si system, the primary motivation of this study is to add scientific insight into the catalytic applications of the Cu/Si system. Specifically, the Direct Reaction,1 which involves the reaction of CH3Cl with a Si/Cu surface (i.e., CH3Cl + Si f (CH3)xSiCl4-x), is the major industrial route to the production of methylchlorosilanes, which are precursors to silicones. The presence of Cu is needed to catalyze this reaction, but an understanding of the role of Cu is lacking.2 Besides scientific curiosity, there is a need to develop a more complete understanding of the microscopic framework of this reaction. While the catalytic Cu/Si surface has been highly optimized over time by an empirical based approach for the production of methylchlorosilanes, there are still technological challenges that might benefit from basic studies. For example, evolving technologies require methylchlorosilanes that were once considered undesirable side products. The present day catalyst produces primarily dimethyldichlorosilane,2 but new technological applications would benefit from a higher yield of trimethylchlorosilane.3 It might be argued that a better understanding of the catalytic surface and its selectivitystructure relationship is a step in the right direction in addressing such types of issues. Results of prior research suggest that a particularly good way of understanding the microscopic controls of this relatively complex reaction with modern surface * To whom correspondence should be addressed. Tel, (215)2047119; Fax, (215)204-1532; E-mail,
[email protected]. † Temple University. ‡ Rutgers University. § Dow Corning Corporation. (1) Rochow, E. G. J. Am. Chem. Soc. 1945, 67, 963. (2) Catalyzed Direct Reactions of Silicon; Lewis, K. M., Rethwisch, D. G., Eds.; Elsevier: Amsterdam, 1993. (3) Kuivila, C. S.; Zapp, R. H.; Wilding, O. K.; Hall, C. A. In Silicon for the Chemical Industry III; Oye, H. A., Rong, H. M., Ceccaro, B., Eds.; Norwegian University of Science and Technology: Sandefjord, Norway, 1996; p 227.
science techniques is to investigate the individual and combined chemistry of CH3 and Cl on Cu/Si.4,5 Toward this end we address in this contribution the reactions of Cl on a relatively well-defined Cu-Si surface, prepared by depositing Si on a Cu(100) single crystal by decomposing SiH4. Prior scanning microscopy6,7 and diffraction studies8 have characterized the structure of Si/Cu(100) from low to high Si coverage. Briefly, at the highest coverages of Si obtainable (θSi ) 0.415) and for Si exposures at temperatures above ∼400 K, low-energy electron diffraction (LEED) shows an incommensurate (near) 5 × 3 structure,8 while STM and helium diffraction studies6 suggest that the average stoichiometry of the ordered overlayer is Cu2Si (high density Si phase). STM suggests that at the lowest Si coverages, Si is in substitutional in-plane sites, presumably surrounded by four Cu neighbors in the outermost layer (low-density Si phase).6 Only at higher coverages (θSi > ∼0.2) are the two low and high Si phases seen to coexist. The use of such a well examined surface should help to develop an understanding of the relationships between structure and reactivity in this and other Cu/Si systems. The present study utilized synchrotron-based photoelectron spectroscopy (PES) and conventional X-ray photoelectron spectroscopy (XPS) to characterize the adsorbed species on the well-defined Si/ Cu(100) surface. Temperature-programmed desorption (TPD) was used to characterize the gaseous products that formed during the thermal reactions of Cl on the Si/Cu(100) alloy surface. Experimental Section Three experimental facilities were used to obtain the data presented in this contribution. The first two, based at Temple University, provided TPD, XPS, and Auger electron spectroscopy (4) Sun, D.-H.; Gurevich, A. B.; Kaufman, L. J.; Bent, B. E.; Wright, A. P. In 11th International Congress on Catalysiss40th Anniversary; Hightower, J. W., Delgass, W. N., Iglesia, E., Eds.; Elsevier Science B. V.: 1996; Vol. 101, p 307. (5) Sun, D. H.; Bent, B. E.; Wright, A. P.; Naasz, B. M. Catal. Lett. 1997, 46, 127. (6) Graham, A. P.; Hinch, B. J.; Kochanski, G. P.; McCash, E. M.; Allison, W. Phys. Rev. B 1994, 50 (20), 304. (7) Han, J.; Gheyas, S. I.; Wang, Y.; Strongin, D. R.; Graham, A. P.; Hinch, B. J.; Wright, A. P. J. Phys. Chem. 2000, 104, 3078. (8) Dubois, L. H.; Nuzzo, R. G. Langumuir 1985, 1, 663.
10.1021/la000084p CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000
6542
Langmuir, Vol. 16, No. 16, 2000
(AES) data. The experimental chambers that housed these techniques had a typical base pressures of e5 × 10-10 Torr, and both were pumped by ion and turbomolecular pumps. One of the chambers also was equipped with LEED optics for the characterization of the sample structure. The third experimental apparatus was based at the National Synchrotron Light Source at Brookhaven National Laboratory and was the UHV end station on the U7a beam line. The end station was evacuated to 10-9 Torr with a turbomolecular and ion pump. This third chamber was used to obtain PES data. Cu(100) crystals used in this research had diameters close to 1 cm, and each had a thickness of 2 mm. Samples were mounted by wrapping Ta support wire (0.25 mm diameter) around the edges of the sample through precut slots. The temperature of the sample was measured by a chromel-alumel (type K) thermocouple, held to the back of the samples by ceramic cement (Aremco Products). Sample cooling to 120 K was achieved by attachment of the support Ta wires to a liquid nitrogen cryostat. Heating was accomplished by passing current through the Ta support wires. Samples were cleaned by repeated 500 eV Ar+ and 800 K anneal (5 min) cycles. Sharp (1 × 1) LEED patterns were obtained after cleaning. Si was deposited on Cu(100) by thermally decomposing silane (from a 0.5% SiH4/Ar gas mixture) on the sample at 420 K (resulting hydrogen desorbs as H2). Exposing Cu(100) to 15 L of the silane mixture resulted in a Si-saturated surface. This surface is referred to as Sisat/Cu(100) in this paper. The Si coverage on Sisat/Cu(100) was ∼0.4.6,7 Si coverages lower than Sisat were obtained with AES by comparing the height of the Si-90 eV AES peak to the Si-90 eV peak associated with Sisat/Cu(100). Exposure of Cu(100) and Si/Cu(100) to Cl2 in this study was carried out by two different methods. In the first method, Cl2 was introduced into the UHV chamber through a leak valve. In this scenario, which was used in the TPD and PES measurements, the Cl2 exposure is quoted in langmuirs (L, 10-6 Torr s), but the exposures are uncorrected for the sensitivity of the nude ionization gauge used to measure pressure. The second method, which was used in the XPS experiments, generated Cl2 inside the UHV chamber by electrolytically decomposing AgCl in an electrochemical cell. No measurable pressure change in the chamber was experimentally observed during dosing with the electrochemical cell. AES was used in this instance to determine how much Cl was deposited on Si/Cu(100) during dosing relative to dosing with Cl2 gas through a leak valve. Doses with the cell, therefore, are presented in langmuirs after this type of calibration. Typically only a 20 min sputter and anneal was required to clean the Cu(100) or Si/Cu(100) surface after exposure to Cl2. TPD experiments were performed with a 3 K/s heating rate, and a multiplexed mass spectrometer was used to monitor up to nine ions during a single TPD experiment. The mass spectrometer was enclosed by a gold-plated stainless steel shield with a 6.0 mm diameter aperture that was 2-3 mm from the sample during TPD. This arrangement limited the detection of extraneous species desorbing from the support wires around the sample. XPS data were obtained with Mg KR (1253.6 eV) radiation and a 100 mm hemispherical analyzer at a pass energy of 50 eV. Si 2p data obtained with XPS and presented later are difference spectra. Background intensity due to Cu features was removed in this way to better present the Si 2p feature. Synchrotronbased PES data were obtained with 400 eV radiation, and the kinetic energy of the photoelectrons was determined with a 250 mm-VSW hemispherical analyzer mounted 90° relative to the photon beam port. Cu 3p, Si 2p, and Cl 2p data were obtained with a 10 eV analyzer pass energy. Cu 3p, Si 2p, and Cl 2p data were obtained after Cl2 adsorption at 120 K, and after heating to higher temperatures. Higher temperature data were obtained by heating the sample momentarily to the desired temperature and then allowing the sample to cool back down to 120 K, where data were acquired.
Results PES Studies of Cl2/Cu(100) and Cl2/Sisat/Cu(100). The top panel of Figure 1 presents Cu 3p photoelectron data for clean Cu(100) and after exposure to Cl2. As Cu(100) was exposed to an increasing amount of Cl2 (1-3
Han et al.
Figure 1. Top panel: Cu 3p data for (a) clean Cu(100), and after exposure to (b) 1 L of Cl2 and (c) 3 L of Cl2. Bottom panel: Cu 3p data for (a) Sisat/Cu(100), (b) 1 L of Cl2/Sisat/Cu(100), and (c) 3 L of Cl2/Sisat/Cu(100).
L) there was a concomitant increase in spectral weight to the high binding energy side of the Cu 3p feature (3p3/2 and 3p1/2 contributions at 75.1 and 77.5 eV, respectively). This change is best noted by a visual inspection of the Cu 3p line shape within the binding energy window enclosed by the two vertical lines in the figure. The bottom panel of Figure 1 exhibits Cu 3p data for Sisat/Cu(100), and after this Si-saturated surface was exposed to 1 and 3 L of Cl2. Exposure of Sisat/Cu(100) to Cl2 resulted in spectral changes. Most notable was the growth of high binding energy intensity in the Cu 3p data, again which is best identified by comparing the Cu 3p intensity of the Sisat/Cu(100) and 3 L Cl2/Sisat/Cu(100) data within the two vertical broken lines. Figure 2 exhibits a series of Si 2p PES data for clean Sisat/Cu(100), 1 L of Cl2/Sisat/Cu(100), and 3 L of Cl2/Sisat/ Cu(100) at 120 K, and after stepwise heating to 250 and 500 K. Sisat/Cu(100) exhibited a Si 2p feature at a binding energy of 99.3 eV (Figure 2a). Exposure of this surface to 1 L of Cl2 (Figure 2b) resulted in a ∼50% decrease in the Si 2p signal intensity and the growth of high binding energy spectral weight in the 100-102 eV range. Further exposure of the surface so as to obtain a total exposure of 3 L of Cl2 (Figure 2c) resulted in more spectral weight appearing near 101.5 eV. After the surface was heated to 250 K (Figure 2d), a well-resolved Si 2p peak appeared near 101.6 eV, and after further heating to 500 K (Figure 2e) additional Si 2p spectral weight appeared with a peak maximum of ∼102.3. On the basis of Si 2p peak areas, it is estimated that 10% of the Si left the near surface region upon heating from 250 to 500 K. The inset to Figure 2 displays Si 2p XPS data for Sisat/ Cu(100), 3 L of Cl2/Sisat/Cu(100) at 120 K, and the latter surface after heating to 650 K. The kinetic energy of the Si 2p photoelectron is significantly higher in XPS (∼1150 eV) than in the synchrotron-based photoemission (∼300
Thermal Chemistry of Chlorine on Si/Cu(100)
Figure 2. Si 2p data for (a) Sisat/Cu(100), (b) 1 L of Cl2/Sisat/ Cu(100), and (c) 3 L of Cl2/Sisat/Cu(100) and (d) after heating 3 L of Cl2/Sisat/Cu(100) to 250 K and (e) 500 K. The vertical markers are to guide the eye to Si 2p features and to approximately mark the binding energy of the peak maxima of those features. The inset to the figure displays XPS data for (a) Sisat/Cu(100) and (b) 3 L of Cl2/Sisat/Cu(100) and (c) after heating 3 L of Cl2/Sisat/Cu(100) to 650 K.
eV) elaborated on above. These data were obtained to determine whether the reduction in the Si 2p intensity alluded to above in the synchrotron-based PES was due to the attenuation of the signal by the adsorbed Cl2 layer or to the removal of Si (i.e., etching) from the surface as gaseous product. The similarity of the peak areas of the Sisat/Cu(100) (spectrum a) and 120 K-3 L Cl2/Sisat/Cu(100) (spectrum b) Si 2p spectrum suggests that the decrease in the Si 2p signal in the PES experiments presented above was due to attenuation of the Si signal by adsorbed Cl. Furthermore, these XPS data show that the vast majority of the Si that is incorporated in the Si-Cu overlayer disappears from the near surface region at 650 K (spectrum c). Cl 2p data are presented in Figure 3 for Cl2/Cu(100) and Cl2/Sisat/Cu(100) after exposure to 3 L of Cl2 at 120 K. Exposure of Cu(100) to 3 L of Cl2 resulted in a spectrum with a Cl 2p3/2 peak maximum at 198.1 eV (Figure 3a). Exposure of Sisat/Cu(100) to 3 L of Cl2 resulted in a more complex spectrum [relative to that obtained for Cl2/Cu(100)] that was fitted with two Cl 2p contributions at 198.1 and 198.6 eV (Figure 3b). The former contribution at 198.1 eV had the same peak parameters [0.88 eV full width at half-maximum (fwhm)] used to fit the Cl2/Cu(100) spectrum. The feature at 198.6 eV was fitted by using a fwhm of 1.25 eV. The width of this fitted peak suggested that more than one Cl species probably contributed to this feature (supported by Si 2p data discussed below). TPD of Cl2/Si/Cu(100). Figure 4 exhibits 1 L Cl2/Si/ Cu(100) TPD data that were obtained as a function of Si coverage. At the lowest Si coverage (θ ) 0.2), a major desorption product was SiCl3, that desorbed in the temperature range of 200-300 K. The basis for this assignment was that m/e 168 (SiCl4+) intensity was absent, suggesting that SiCl3+ intensity was due to SiCl3 and not to the cracking of SiCl4 in the mass spectrometer. It is also possible that SiCl2 is a desorption product, but
Langmuir, Vol. 16, No. 16, 2000 6543
Figure 3. Cl 2p3/2,1/2 data for (a) 3 L of Cl2/Cu(100) at 120 K and (b) 3 L of Cl2/Sisat/Cu(100) at 120 K. Dots are raw data, dashed lines are fitted peaks, dash-dot lines are background, and solid lines are the composite peak. All Cl 2p fits were carried out with a 2p3/2-2p1/2 peak area ratio of 2:1 and an energy splitting of 1.6 eV. Fwhm values are given in the text.
Figure 4. TPD results for Cl2/Si/Cu(100) as a function of the Si relative coverage. Only SiCl3+ ion data are exhibited. SiCl3 (and probably SiCl2) was the dominant product near 200 K in the spectra associated with Si coverages of 0.2 and 0.28. At higher Si coverages (θSi ) 0.4), the dominant desorption product was SiCl4.
owing to the lack of cracking patterns for SiCl3 and SiCl2 this possibility cannot be addressed. At a Si coverage of 0.4 (saturation) on Cu(100), however, SiCl4, with a TP of 375 K, became the primary product. There is also a minor SiCl4 desorption state with a TP of 450 K. SiCl4 desorption at the higher Si coverage was based on the relative intensity ratio of m/e 168 (SiCl4+) and 133 (SiCl3+). Discussion PES results give some insight into the bonding of Cl on the Sisat/Cu(100) surface. With regard to Si-Cl bond formation, the Si 2p data are quite revealing. Significant spectral changes occur in the Si 2p region (Figure 2) upon
6544
Langmuir, Vol. 16, No. 16, 2000
Cl2 exposure at a surface temperature of 120 K, suggesting that oxidation of surface Si due to reaction with Cl has occurred. Prior research has shown that with each Cl addition to Si there is a ∼0.9-1 eV shift of the Si 2p peak to higher binding energy.9,10 The broad Si 2p spectral weight in the ∼100-102 eV range, which evolves upon exposure to Cl2 at 120 K, suggests that Si1+ (i.e., Si-Cl) and Si2+ (Si-Cl2) and possibly some Si3+ (Si-Cl3) species are present in the near surface region. This experimental observation is similar to that observed in prior studies where Si(111), for example, was exposed to Cl2.10,11 The evolution of this spectral intensity in the 100-102 eV binding energy range in the Sisat/Cu(100) circumstance also is thought to be responsible for the apparent shift of the main Si 2p feature to ∼99.6 eV (originally at 99.3 eV before exposure to Cl2). The oxidation of the Si component of Sisat/Cu(100) becomes increasingly more apparent as the surface is stepwise heated to 250 and 500 K. By 500 K the dominant chlorinated silicon species is SiCl3. The significant reduction in spectral intensity due to SiCl2 and SiCl species during heating to 500 K also is consistent with the shift of the main Si 2p peak intensity back to 99.3 eV. A significant fraction of the Cl needed to produce SiCl3 from SiCl2 and SiCl3 species may come from Cl originally bound to Cu.12 It is presumed that the formation of chlorinated Si species during heating significantly disrupts at least a portion of the Cu2Si overlayer. The formation of SiClx species at the expense of Si-Cu bonding is energetically reasonable, considering that the Cu-Si bond is weaker (52.9 kcal mol-1)13 than the Cl-Si bond (97 kcal mol-1).13 Such a disruption in the surface structure may also contribute to the broadness of the Si and Cl 2p spectra (Figure 3b) attributed to SiClx species. While SiCl3 species are favored at higher temperature (500 K) on Sisat/Cu(100), the experimental Si 2p data indicates that not all of the Si on Sisat/Cu(100) becomes chlorinated. Analysis of the Si 2p data at 500 K, for example, indicates that while a fraction of the Si overlayer undergoes reaction with Cl to form SiCl3 species, another fraction of Si remains relatively free of strong Cl bonding (for Cl2 exposures up to 10 L at 120 K). The reason for this experimental observation is not known, but it may be simply due to adsorbed Cl blocking the further dissociative adsorption of Cl2. Alternatively, it cannot be ruled out that some Si resides below the outermost surface and is unaffected by the adsorption of Cl2 at the alloy surface. Specifics concerning the bonding of Cl to the Cu component of Sisat/Cu(100) are somewhat more ambiguous than for the Si component. Cu 3p data for Cl2/Cu(100) shown in Figure 1 do suggest that exposure of Cu(100) to 1-3 L of Cl2 results in the growth of spectral weight that is attributed to the growth of CuClx species. On the basis of prior research14 of Cl2/Cu(100) it cannot be ruled out that the growth of a CuCl phase forms in the near surface region under the experimental conditions used in this study. Interpretation of the Cu 3p data for Cl2/Sisat/Cu(100) at 120 K is more speculative owing to the myriad of structural and electronic changes possible on a mul(9) Schnell, R. D.; Rieger, D.; Bogen, A.; Himpsel, F. J.; Wandelt, K.; Steinmann, W. Phys. Rev. B 1985, 32, 8057. (10) Whitman, L. J.; Joyce, S. A.; Yarmoff, J. A.; McFeely, F. R.; Terminello, L. J. Surf. Sci. 1990, 232, 297. (11) Schnell, R. D.; Rieger, D.; Bogen, A.; Himpsel, F. J.; Wandelt, K.; Steinmann, W. Phys. Rev. B 1985, 32, 8057. (12) This contention is energetically reasonable considering that the Cu-Cl and Si-Cl bond strengths are 90 and 97 kcal mol-1, respectively (ref 13). (13) CRC Handbook of Chemistry and Physics, 80th ed.; Lide, David R., Ed.; CRC Press: Boca Raton, FL, 1999. (14) Galeotti, M.; Cortigiani, B.; Torrini, M.; Bardi, U.; Andryushechkin, B.; Klimov, A.; Eltsov, K. Surf. Sci. 1996, 349, L164.
Han et al.
ticomponent with a strongly adsorbing overlayer. On a qualitative level we believe that the Cu 3p spectral changes are due to the formation of some direct Cu-Cl bonding. Supporting spectroscopic evidence for direct Cu-Cl bonding is inferred from the Cl 2p data presented in Figure 3. Exposure of Sisat/Cu(100) to 3 L of Cl2 at 120 K (Figure 3b) results in a spectrum that has at least two spectral contributions from adsorbed Cl. On the basis of a comparison between Cl 2p data for Cl2/Cu(100) and Cl2/ Sisat/Cu(100), the 198.1 eV spectral contribution is suspected to be due to Cl adsorbing directly to Cu.15 An unknown fraction of the Cl 2p intensity at 198.1 eV is proposed to be due to Cl bound to Cu in subsurface layers, which has been shown to occur in prior research for Cl2/ Cu(100)14 and Cl2/Cu(111).16 An additional Cl 2p contribution in the 120 K Cl2/Sisat/Cu(100) spectrum appears at 198.6 eV, and it is postulated that this feature is due to Cl interacting strongly with the Si component in the outermost layer. The broadness of the Si 2p spectrum (Figure 2b) at this temperature would suggest, however, that the Cl 2p feature at 198.6 eV may have contributions from at least SiCl and SiCl2 surface species. TPD of Cl2/Si/Cu(100) at Si coverage less than saturation (coverage less than 0.4 ML) results in primarily SiCl3 and probably SiCl2, but verifying the presence of the latter product is difficult owing to the lack of reference cracking patterns for SiCl3 and SiCl2. Prior studies of Si single crystals with adsorbed Cl have generally shown the desorption of SiCl2 at relatively high temperatures (∼800950 K) and the desorption of SiCl4 near 500 K at high coverages of Cl.17,18 The relatively low desorption temperature (200-300 K) of SiCl3 (and SiCl2) suggest that the bonding of at least a fraction of low coverage Si in the outermost surface of Si/Cu(100) is significantly weaker than in the pure Si circumstance (the bond energy of the Si-Cu bond is 52.9 kcal mol-1, compared to 75 kcal mol-1 for Si-Si13). This statement is mentioned with some caution, however, since only a small amount of the total Si (about 10%) desorbs as chlorinated gaseous product. Thus, whether the gaseous product results from the reaction of a minority population of weakly bound Si cannot be ruled out. Further discussion of the Cl2/Si/Cu(100) TPD results benefit from a brief description of some prior results. Recent research in our laboratory7 has shown that adsorbed CH3 on Si/Cu(100) at intermediate Si coverages [θSi ) 0.2, where θSi ) 0.4 is Sisat/Cu(100)] reacts and results in the desorption of (CH3)3SiH product at 200 and 420 K in TPD experiments. On Sisat/Cu(100), CH3 reactions result in the desorption of a single (CH3)3SiH desorption feature at 420 K. In view of STM results,7 it was concluded that the 200 and 420 K desorption features were due to the reaction of CH3 on low- and high-density Si phases, respectively. Both phases coexist on Si/Cu(100) at silicon coverages near 0.2, but only the high-density Si phase (Cu2Si stoichiometry) exists at θSi ) 0.4. One possible interpretation proposed earlier7 for the above results is that Si in the low-density phase is more weakly bound to the surface than Si in the high-density phase, the latter (15) It is a presumption that the Cl 2p feature at 198.1 eV is due to Cu-Cl bonding for both Sisat/Cu(100) and Cu(100) surfaces. This might not have been expected owing to the unique alloy structure of the Cu2Si overlayer. It may be that the Cu-Cl bond is strong enough on the alloy surface so that the character of this bond is similar to the one formed on pure Cu(100). (16) Walter, W. K.; Manolopoulos, D. E.; Jones, R. G. Surf. Sci. 1996, 347, 115. (17) Gupta, P.; Coon, P. A.; Koehler, B. G.; George, S. M. Surf. Sci. 1991, 249, 92. (18) Mendicino, M. A.; Seebauer, E. G. Appl. Surf. Sci. 1993, 68, 285.
Thermal Chemistry of Chlorine on Si/Cu(100)
of which is the strongly ordered Cu2Si phase (exhibiting the incommensurate 5 × 3 diffraction pattern). On a qualitative level, the Cl2 adsorbate TPD results are understandable by a similar argument. At the lowest Si coverage, where Si is bound most weakly, the reaction of adsorbed Cl2 results in the desorption of SiClx (x ) 2, 3) product near 200 K, while at the highest Si coverage SiCl4 desorbs near 375 K. The shift in the product distribution to SiCl4 for Cl2/Sisat/Cu(100) may be due to an increased bonding of Si in the surface (i.e., significant chlorination is needed to remove the Si from the Si/Cu lattice) compared to the lower local Si coverage circumstance. The absence of two well-defined desorption states, however, for Si containing gaseous product at a Si coverage of 0.28, where both high and low Si phases would be expected to coexist, may suggest that such an interpretation of the desorption sites is too simplistic for the chlorosilane desorption products. The very strong bonding of the Cl adsorbate to both Si and Cu can disrupt the surface structure to such an extent that there may no longer be the two clearly distinguishable regions as observed by STM on the clean Si/Cu(100) surface. Planned STM studies will investigate Si/Cu(100) with adsorbed Cl to determine the lateral extent of absorbate-induced restructuring.
Langmuir, Vol. 16, No. 16, 2000 6545
Summary Studies have investigated the thermal reaction of Cl2 with Si/Cu(100) at various Si coverages. Exposure of Sisat/ Cu(100) to Cl2 at 120 K leads to the formation of SiCl and SiCl2 species and probably results in a significant disruption of the Si-Cu bonding that characterizes clean Sisat/ Cu(100). Heating this surface to 500 K leads to the conversion of SiCl and SiCl2 species into SiCl3 surface species. Not more than 10% of the Si that characterizes Sisat/Cu(100) is removed as chlorosilane product upon heating to 500 K. Heating beyond 500 K leads to the diffusion of Si into the Cu(100) bulk. Acknowledgment. The authors greatly appreciate support from the National Science Foundation from Grant NSF CHE9732798 and partial support from Dow Corning Corporation for this research. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. LA000084P
