Coupling versus Surface-Etching Reactions of Alkyl Halides on GaAs

Neil T. Kemp, Nathan J. Paris, Deborah Giveen, and Nagindar K. Singh*. School of Chemistry, The University of New South Wales, Sydney 2052, Australia...
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Langmuir 2006, 22, 9554-9565

Coupling versus Surface-Etching Reactions of Alkyl Halides on GaAs(100): I. CF3CH2I Reactions Neil T. Kemp, Nathan J. Paris, Deborah Giveen, and Nagindar K. Singh* School of Chemistry, The UniVersity of New South Wales, Sydney 2052, Australia ReceiVed May 1, 2006. In Final Form: August 22, 2006 We report the rich surface chemistry exhibited by the reactions of 1,1,1-trifluoroethyl iodide (CF3CH2I) adsorbed onto gallium-rich GaAs(100)-(4 × 1), studied by temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED) studies and X-ray photoelectron spectroscopy (XPS). CF3CH2I adsorbs molecularly at 150 K but dissociates, below room temperature, to form a chemisorbed monolayer of CF3CH2 and I species. Recombinative desorption of molecular CF3CH2I competes with the further reactions of the CF3CH2 and I chemisorbed species. The CF3CH2 species can either undergo β-fluoride elimination to yield gaseous CF2dCH2 or it can undergo self-coupling to form the corresponding higher alkane, CF3CH2CH2CF3. A second coupling product, CF3CH2CHdCF2, is also evolved, and it is postulated that migratory insertion of the liberated CF2dCH2 into the surface-carbon bond of the chemisorbed CF3CH2 is responsible for its formation. The iodines, formed by C-I scission in the chemisorbed CF3CH2I, and the fluorines, derived from β-fluoride elimination in CF3CH2, react with the surface gallium dimers, and Ga-As back-bonds to generate five etch products (GaF, AsF, GaI, AsI, and As2) that desorb in the temperature range of 420 to >600 K. XPS data reveal that the surface stoichiometry remains constant throughout the entire annealing temperature range because of the desorption of both gallium- and arsenic-containing etch products, which occur sequentially. In this article, plausible mechanisms by which all products form and the binding sites of these reactions in the (4 × 1) reconstruction are discussed. Factors that control the rate constants of etch product versus hydrocarbon product formation and in particular how they impact on the respective desorption temperatures will be discussed.

1. Introduction

* Corresponding author. E-mail: [email protected]. Tel: +61 2 9385 4687. Fax: +61 2 9385 6141.

that there be a high selectivity in the product distribution, where an extremely large range (C1 to >C100) of products including alkanes, alkenes, and oxygenates8 can result. The numerous surface investigations of alkyls on transition metals (see, for example, reviews by Bent5 and Zaera6,7) have provided invaluable insights into the more complex mechanisms in industrial processes. In contrast, alkyl surface chemistry on semiconductor surfaces has not been studied as extensively, although studies of halogen etching reactions on both silicon (see, for example, the review by Winters and Coburn9) and GaAs1-3 do exist. A few investigations from our group on alkyl reactions on GaAs(100)10-12 have shown that this surface can facilitate alkyl coupling reactions to form the higher hydrocarbons as well as the corresponding alkene, an alkane, and hydrogen via disproportionation reactions,13 with the latter being similar to alkyl disproportionation reactions observed on Au(111).14 With ethanethiol and ethyl iodide,10 it was shown that the ethyl coupling product butene formed irrespective of whether the ethyl groups were derived from surface-adsorbed ethanethiolate (in which case the coadsorbed species was sulfur) or from ethyl iodide dissociation (in which case the coadsorbed species was iodine). We have continued the investigations of coupling versus etching reactions of a range of alkyl halides on GaAs, with the aim being to understand the alkyl surface chemistry on GaAs in order to determine the surface reactivity of this material in comparison with that of transition metals and the factors that control product

(1) Hung, W.-H.; Wu S.-L.; Chang, C.-C. J. Phys. Chem. B 1998, 102, 11411148. (2) Singh, N. K.; Bolzan, A. Surf. Sci. 1996, 357-358, 656-662. (3) Su, C.; Dai Z-G.; Luo, W.; Sun, D.-H.; Vernon, M. F.; Bent, B. E. Surf. Sci. 1994, 312, 181-197. (4) Schmid, J. H.; Mar, R.; Tiedje, T. Appl. Phys. Lett. 2003, 82, 4549-4551. (5) Bent, B. E. Chem. ReV. 1996, 96, 1361-1390. (6) Zaera, F. Mol. Phys. 2002, 100, 3065-3073. (7) Zaera, F. Chem. ReV., 1995, 95, 2651; Zaera, F. Prog. Surf. Sci. 2001, 69, 1-98.

(8) Shi, B.; Davis, B. H. Catal. Today 2005, 106, 129-131. (9) Winters, H. F.; Coburn, J. W. Surf. Sci. Rep. 1992, 14, 161-269. (10) Singh, N. K.; Doran, D. C. Surf. Sci. 1999, 422, 50-64. (11) Singh, N. K.; Kemp, N. T.; Paris, N.; Balan, V. J. Vac. Sci. Technol., A 2004, 22, 1659-1666. (12) Kemp, N. T.; Singh, N. K. Chem. Commun. 2005, 4348-4350. (13) Singh, N. K.; Bolzan, A.; Foord, J. S.; Wright, H. Surf. Sci. 1998, 409, 272-282. (14) Paul, A.; Yang, M. X.; Bent, B. E. Surf. Sci. 1993, 297, 327-344.

Previous investigations of alkyl halides on transition-metal and semiconductor surfaces have shown that these surfaces have the ability to facilitate dehalogenation reactions to release alkyls species and halogens. The benefits of this are numerous and can be exploited in many industrial processes. In microelectronics, the reactions can be used in dry etching processes of III-V semiconductors (e.g., GaAs1-4) because the dissociated halogen atoms form volatile compounds by reaction with the surface atoms that can desorb immediately on formation. The reactions of the adsorbed alkyl species show interesting surface chemistry both in terms of the products formed and the mechanisms by which they form, which can vary from surface to surface and can also be dependent on surface crystallography.5-7 Most metals are efficient at promoting β-hydride elimination in alkyls to form alkenes, and the process is easier on earlier transition metals (e.g., nickel). The late transition metals (e.g., silver and gold) tend to favor reductive elimination to form the corresponding higher alkanes, and in the case of silver, it is known to occur with 100% efficiency. The branching ratios of these reactions are important in industrial processes such as petroleum reforming, Zieglar-Natta polymerization of olefins, and Fischer-Tropsch (FT) synthesis of long-chain hydrocarbons from CO/H2 mixtures, among others. In the FT synthesis, for example, it is desirable

10.1021/la061207u CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006

Coupling Versus Surface-Etching Reactions

distribution on this surface. This article on CF3CH2I is an extension of a preliminary report11 that discussed the coupling reactions based on thermal desorption data, whereas in this article we summarize the results presented in that paper and provide complementary XPS and LEED data, which further support hypotheses postulated previously to derive a more conclusive molecular-level picture of the surface reactions involved. This article places greater emphasis on the mechanisms by which gallium and arsenic etch products form based on the XPS data and in light of the more recent structure proposed for the galliumrich GaAs(100)-(4 × 1)15 and previous investigations of surface etching of GaAs with halogens, iodine, and fluorine. CF3CH2I surface reactions have not been studied to any great extent previously. On both Ag(111)16,17 and Si(100),18 dissociative adsorption of CF3CH2I was observed, and on both surfaces, the adsorbed CF3CH2 underwent β-fluoride elimination to yield CF2d CH2. Although this result on Si(100) was not unexpected, because coupling reactions have not been reported on silicon previously, the results were unexpected from Ag(111) because alkyl groups on this surface typically selectively couple to form the corresponding higher alkanes.19-22 In another study on Ag(111), the surface CF3CH2 species generated by methylene (CH2) insertion into the adsorbed CF3 moiety again underwent β-fluoride elimination only. Gellman and co-workers,17 using transitionstate theory, proposed that the fluorination at the β position as in CF3CH2I and CF3CF2CH2I causes the destabilization of the transition state (or the stabilization of the initial state), which then leads to a net increase in the reaction activation barrier and hence a decrease in the coupling rate. Huang and White16 recently confirmed that there is an increased activation barrier to CF3CH2 coupling on Ag(111), but they proposed that this was related to the tilted orientation of the CF3CH2 with respect to the surface normal, compared to the upright orientation that CH3CH2 adopts on the same surface. The tilted geometry, arising from the strong interactions between the silver surface and at least one of the fluorines in the adsorbed CF3CH2, decreases the surface mobility and thus suppresses the migratory coupling reaction. Consequently, the CF3CH2 species selectively undergoes β-fluoride elimination. Interestingly, on GaAs(100), unlike Ag(111), CF3CH2I (this study) exhibits both β-fluoride elimination and coupling reactions similar to CH3CH2I reactions10 on GaAs(100), which suggests that the effects, if any, of fluorine-surface interactions on GaAs and the concomitant reduced mobility are minimal. In addition, the halogen species (iodine and fluorine) liberated from the initial C-I scission when CF3CH2I adsorbs dissociatively and subsequent β-fluoride elimination in the adsorbed CF3CH2 form a number of etch products that do not appear to influence the alkyl surface chemistry, and at the same time, their evolution assists in maintaining the surface stoichiometry at a nearly constant value over the entire temperature range that the reactions occur. This latter observation is important for applications in microelectronics processing where iodine and fluorine can be used to remove the surface layers of GaAs in a controlled manner, whereas the complete removal of the adsorbed CF3CH2 species as gaseous hydrocarbon products will ensure that no carbonaceous species remain on the surface during the etching process. In this respect, the surface chemistry of CF3CH2I possesses the advantage of (15) Lee, S. M.; Lee, S.-H.; Scheffler, M. Phys. ReV. B 2004, 69, 125317. Lee, S.-H.; Moritz, W.; Scheffler, M. Phys. ReV. Lett. 2000, 85, 3890. (16) Huang, W. X.; White, J. M. J. Phys. Chem. B 2004, 108, 7911-7916. (17) Paul, A.; Gellman, A. J. Langmuir 1995, 11, 4433-4439. (18) Lin, J.-L.; Yates, J. T., Jr. J. Vac. Sci. Technol., A 1995, 13, 178. (19) Zhou, X. L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1992, 219, 294. (20) Zhou, X. L.; White, J. M. Catal. Lett. 1989, 2, 375. (21) Zhou, X. L.; White, J. M. Surf. Sci. 1992, 297, 327. (22) Zhou, X. L.; White, J. M. J. Phys. Chem. 1991, 95, 5575.

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“clean” surface-etching reactions that are exhibited by halogens on GaAs(100), such as chlorine gas,3 but it does not suffer from the disadvantage of being as corrosive as halogens and hence is less problematic as far as expensive pumping gear is concerned. An additional advantage of using CF3CH2I is that, being a liquid, it will allow accidental spills to be more easily contained than gaseous chlorine or fluorine. These two advantages can make alkyl halides such as CF3CH2I more desirable as semiconductor etchants than chlorine gas. This article is a two-part work studying the influence of the chemical identity of the alkyl halide on the surface-etching versus coupling reactions on gallium-rich GaAs(100)-(4 × 1). In paper I, we discuss the rich chemistry exhibited by CF3CH2I where both gallium and arsenic fluoride and iodide etch products and hydrocarbon products are observed to form. We postulate that the formation of a wide range of etch products is initiated by the reaction of the fluorines, formed by β-fluoride elimination in the adsorbed CF3CH2 species, with the Ga-As back-bonds in the surface. In paper II, the reactions of CH2I2 are presented. Unlike CF3CH2I, CH2I2 reactions do not yield gallium or arsenic etch products whereas a plethora of hydrocarbon products comprising methane, higher alkenes from ethene through to butene, methyl iodide, and two higher alkyl iodidessethyl and propyl iodidess are formed from the surface reactions of the adsorbed CH2 and I species. The absence of gallium and arsenic iodide etch products in the CH2I2 case is attributed to the stability of the methyl species that persists on the surface up to high temperatures when gallium iodide is expected to evolve. The iodines in this case form the alkyl iodides instead. A similarity between CF3CH2I and CH2I2 reactions is that higher hydrocarbons are formed. In the case of CF3CH2I, the mechanism by which this occurs follows the olefin insertion mechanism, whereas in the case of CH2I2 reactions the higher alkenes are formed by the methylene insertion mechanism. These two mechanisms have been postulated previously for FT synthesis of higher hydrocarbons.23 2. Experimental Section All experiments were carried out in two stainless steel ultrahigh vacuum instruments that were ion and titanium sublimation pumped to base pressures below 5 × 10-10 mbar. One of the instruments is equipped with facilities for temperature-programmed desorption (TPD) experiments and has four-grid rear view optics (VG Microtech) for low-energy electron diffraction (LEED) studies and Auger electron spectroscopy (AES). The second instrument is equipped with an unmonochromated Mg/Al dual anode X-ray source (PSP Instruments), a concentric hemispherical analyzer (VG100AX), and an electron gun (VG LEG63) for X-ray photoelectron and Auger electron spectroscopy. Both instruments are equipped with facilities for residual gas analysis using a quadrupole mass spectrometer (UTI, 100C) and sample cleaning by argon ion bombardment, and the sample arrangement allowed sample cooling to liquid-nitrogen temperature and heating to >850 K. The GaAs samples of approximate dimensions 1 cm × 2 cm were cut from polished GaAs(100) wafers and were mounted (with no prior chemical etching) onto a Ta backing plate before attachment to the sample manipulators. The sample could be heated to 850 K by conduction from the resistively heated Ta backing plate. After system bake-out, the samples underwent repetitive cycles of argon ion bombardment (500 eV Ar+, 2.5 µA current) and annealing at 770 K until no carbon or oxygen could be detected on the surface by AES and the surface displayed a (4 × 1) LEED pattern. All experiments were performed on this well-defined gallium-rich reconstruction of GaAs (100) unless otherwise indicated. This Garich (4 × 1) surface is a less-ordered form of the (4 × 2) structure (23) Overett, M. J.; Hill, R. O.; Moss, J. R. Coord. Chem. ReV. 2000, 206207, 581.

9556 Langmuir, Vol. 22, No. 23, 2006 with imperfect order along the [110] direction.24 The out-of-phase arrangement of the (4 × 2) structures by half of the surface cell in the [110] direction gives rise to c(8 × 2) periodicity. Using densityfunctional theory, Scheffler et al.15 have shown that this galliumrich structure has eight subsurface Ga dimers that are covered by a nearly planar surface layer. The surface layer consists of 12 Ga atoms including 2 in-surface dimers and 16 As atoms per c(8 × 2) cell. They have also shown that the As atoms in this surface layer move toward the vacuum and the surface Ga dimers move slightly below the surface, thus forming nearly planar bonded sp2 bonded geometry.25 A consequence of this structure is that three distinct arsenic sites are exposed and two of these have dangling bonds directed toward the vacuum. Hence this structure allows adsorption at both the gallium and arsenic atoms. Trifluoroethyl iodide (99%, Aldrich Chemicals) was purified by several freeze-pump-thaw cycles prior to its first use and thereafter prior to the start of experiments. The purity of the dosing vapor was verified in situ by mass spectrometry. Sample dosing was performed in both UHV chambers with the use of a high-precision leak valve (with a sapphire pad) attached to 1/4-in.-diameter stainless steel tubing that could be positioned to within 1 cm of the front face of the sample surface. All exposures of CF3H2I are reported in L (langmuirs), where 1 L ) 1 × 10-6 Torr s, and the pressure values used for the exposure calculations are uncorrected for ion gauge sensitivity for CF3CH2I relative to nitrogen (set at 25). Note that the fluence of CF3CH2I at the surface was much higher than that measured at the ion gauge head and the reported values have been corrected by multiplication by an enhancement factor, which was experimentally determined to be 60 in these set of experiments. This enhancement factor, due to the dosing geometry used in the experiments, was determined by calculating the ratio of the desorption peak area of the CF2dCH2 species (see below) following (i) back-filling of the chamber with CF3CH2I and (ii) using the geometry noted above at a fixed pressure for a fixed time. Hence, the exposure values reported in L should be taken as proportional to the actual fluence at the surface. For thermal desorption studies, the mass spectrometer utilized was differentially pumped, and the dosed samples were placed 2 mm in front of a selective-area probe (6 mm diameter) during data acquisition so that only desorbing molecules from the center of the GaAs(100) surface were sampled by the mass spectrometer. Control experiments conducted on the dosed surfaces to investigate the effects (if any) of the incident electron flux from the mass spectrometer showed that no significant electron-induced damage occurred in the adlayer during the time span of the data acquisition (40-50 s). Linear heating rates in the range of 10-15 K s-1 were employed during data acquisition. Efforts to use lower heating rates resulted in readsorption of the desorbing species, particularly in the lowtemperature regime. Unless otherwise stated, temperature-programmed desorption (TPD) experiments involved dosing the GaAs substrates with CF3CH2I at 150 K and then monitoring the reaction products desorbing from the surface. X-ray photoelectron spectra were acquired with the X-ray source operating at 500 W power, and the electron-energy analyzer was set at a constant pass energy of 60 eV. Control experiments conducted to investigate damage caused in the adsorbed layer by X-ray exposure did not show any detectable damage. The binding energy in the XP spectra was corrected by assigning a value of 40.8 eV of binding energy for the As 3d photoemission feature.26 This assignment results in a binding energy of 19.1 eV for the Ga 3d photoemission feature, and this value falls in the range of 18.5-19.8 eV reported for GaAs in the NIST database.27 (24) Frankel, D. J.; Yu, C.; Harbison, J. P.; Farrell, H. H. J. Vac. Sci. Technol., B 1980, 5, 354. Biegelsen, D. K.; Bringans, R. D.; Northrup, J. E.; Swartz, L.-E. Phys. ReV. B 1990, 41, 5701. Kamiya, L.; Aspnes, D. E.; Florez, L. T.; Harbison, J. P. Phys. ReV. B 1992, 46, 15894. (25) Xue, Q.; Hashizume, T.; Zhou, J. M.; Sakata, T.; Ohno, T.; Sakurai, T. Phys. ReV. Lett. 1995, 74, 3177. (26) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992. (27) http://srdata.nist.gov/xps

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Figure 1. XP spectra monitoring the region covering the As 3d and I 4d photoemission peaks following annealing of the dosed surface at the temperatures indicated. The 120 K spectrum was acquired following CF3CH2I exposure of 20 L.

3. Results Section X-ray photoelectron spectroscopy was used to study the bonding of the adsorbed CF3CH2I and the change in the bonding characteristics as the adsorbed layer is heated, in increments, up to 720 K. Thermal desorption studies were conducted to monitor desorbing species and in particular to identify reaction products that form as the adsorbed CH3CH2I layer is thermally activated; detailed thermal desorption data are presented in a previous report on this work.11 LEED was used to observe changes in the surface structure following CF3H2I exposure and following thermal activation of the adsorbed layers so as to link structural changes to XPS and thermal desorption data. 3.1. X-ray Photoelectron Spectroscopy Studies of CF3CH2I Adsorption and Thermal Dissociation on GaAs(100)-(4 × 1). Following the preparation of clean and well-ordered GaAs(100)(4 × 1), XP spectra were taken over regions of the As 3d, Ga 3d, I 3d, I 4d, C 1s, and F 1s photoemission features. To study the thermal decomposition of CF3CH2I, the surface was exposed to 20 L of CF3H2I at 120 K. XP spectra were taken over the regions noted above, immediately after the exposure and then following a series of heating experiments, involving heating to progressively higher temperatures in increments and allowing the sample to cool back to 120 K prior to data acquisition. Data were acquired for heating experiments up to 720 K. The clean-surface XP spectra (not shown) showed intense As 3d and Ga 3d peaks at 40.8 and 19.1 eV, respectively, consistent with typical values for these peaks in bulk GaAs.26-28 Figures1 and 2 show the XP spectra taken over the As 3d/I 4d and Ga 3d regions, respectively, following CF3CH2I exposure at 120 K. The bottom traces in each figure show that both the As 3d and Ga 3d peaks are no longer observable, signaling the formation of physisorbed multilayers. The total attenuation of these features with an average kinetic energy of 1447 eV implies that the multilayer thickness must be comparable to the mean free path of electrons through the condensed CF3CH2I with that kinetic energy. This equates to a thickness of approximately 20 Å and most likely corresponds to more than five layers.29 Heating to 720 K returned both the As 3d and Ga 3d peaks to almost their (28) Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S. P.; Shirley, D. A. Phys. ReV. B 1974, 9, 600. (29) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. M.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211. Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley and Sons: New York, 1990; Vol. 1.

Coupling Versus Surface-Etching Reactions

Figure 2. XP spectra monitoring the Ga 3d photoemission peak following annealing of the dosed surface at the temperatures indicated. The 120 K spectrum was acquired following CF3CH2I exposure of 20 L.

preadsorption intensities, indicating complete removal of the surface species by that temperature. No observable bindingenergy shifts or changes in peak profiles were observed during the heating process for these two features. This was not unexpected because the low resolution of the spectrometer employed in the study would not have been able to resolve the chemically shifted peak induced by monolayer coverages of dissociated iodine and fluorine. It is known from a previous study that a binding-energy shift of 0.35 eV was observed in the Ga 3d feature for iodinecovered GaAs[110]30 when a small amount of GaI was observed to form, and this shift could be detected only by using synchrotron soft X-ray photoelectron spectroscopy. Thermal activation of the adsorbed species, which we show below, results in the loss of fluorine species to the substrate atoms in the temperature region of 360-450 K. However, no distinct chemically shifted peaks in the Ga 3d region expected at ∼20.8 eV or As 3d expected at ∼41.6 eV31 appeared in that temperature range, possibly because of the low concentrations of fluorine liberated, which were not sufficiently high to give rise to new photoemission features. However, basic line shape analysis comprising the determination of the fwhm of the As 3d and Ga 3d peaks (discussed in detail below) in the annealed samples reveal that fwhm changes do occur at the different annealing temperatures, suggesting that the spectra contain chemically shifted components that are not resolvable by the spectrometer either because of its limited resolution or because of the low coverage of the surface fluorine atoms. Figure 1 also shows unresolved I 4d5/2 and 4d3/2 spin-orbit split peaks that appear following CF3CH2I exposure. Deconvolution of the feature, using numerical curve fitting routines, yielded values of 52.0 and 50.3 eV for the 4d3/2 and 4d5/2 peaks, respectively. The binding-energy values and energy difference in the spin-orbit split peaks (1.7 eV) are the same as those observed for iodine adsorbed on InAs(001).25 On heating from 120 to 180 K, there is an ∼1.3 eV shift to a lower binding energy, (30) Varekemp, P. R.; Harkansson, M. C.; Kanski, J.; Shuh, D. K.; Bjorkqvist, M.; Gothelid, M.; Simpson, W. C.; Karlsson, U. O.; Yarmoff, J. A. Phys. ReV. B 1996, 54, 2101. Varekemp, P. R.; Harkansson, M. C.; Kanski, J.; Bjorkqvist, M.; Gothelid, M.; Kowalski, B. J.; He, Z. Q.; Shuh, D. K.; Yarmoff, J. A.; Karlsson, U. O.; Yarmoff, J. A. Phys. ReV. B 1996, 54, 2114. (31) Williston, L. R.; Bello, I.; Lau, W. M. J. Vac. Sci. Technol., A 1993, 11, 1242. Barriere, A. S.; Desbat, B.; Guegan, H.; Lozano, L.; Seguelong, T.; Tressaud, A.; Alnot, P. Thin Solid Films 1989, 170, 259.

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Figure 3. X-ray photoelectron spectra monitoring the spin-orbit split I 3d3/2 and I 3d5/2 photoemission peaks following annealing of the dosed surface at the temperatures indicated. The 120 K spectrum was acquired following CF3CH2I exposure of 20 L.

which is indicative of the partial exposure of the chemisorbed layer from underneath the physisorbed layers that desorb in this temperature range. At these temperatures, C-I dissociation must commence to form CF3CH2 and I species. Further heating to 210 K shows a large reduction in the peak intensity, with no further shift in binding energy, due to the complete removal of the physisorbed layer. Between 370 and 470 K (not shown), there was a further reduction in intensity that, as we will show below in the thermal desorption studies, coincides with the desorption of molecular recombinative CF3CH2I. Between 470 and 570 K, the peaks lose most of their intensity, indicating further removal of iodine species as etch products confirmed in the thermal desorption experiments discussed below. By 720 K, almost all iodine has been removed, with less than 5% remaining on the surface at this temperature, which could be removed from the surface only by argon bombardment. The iodine 3 d5/2 and 3 d3/2 spin-orbit split peaks, observed at 631.4 and 619.9 eV, respectively, are shown in Figure 3. They show the same trends in peak-intensity changes and bindingenergy shifts as exhibited by the I 4d feature. It should be noted that decreases in the peak intensities of the iodine 3d and 4d peaks are accompanied by the emergence of the Ga 3d and As 3d peaks as the substrate becomes exposed as a result of the removal of the multilayers. Figure 4 shows the C 1s spectra taken following CF3CH2I exposure at 120 K and then subsequent heating to higher temperatures between 120 and 210 K. The two main peaks at 285.0 and 291.8 eV are attributed to the R and β carbons, respectively, in the molecularly adsorbed CF3CH2I. With thermal activation, both of these peaks lose intensity with a major reduction in intensity occurring for temperatures greater than 180 K as a result of the removal of the multilayers. This decrease in intensity is accompanied by an apparent chemical shift of 0.6 eV to lower binding energies for both carbon peaks. Shifts in binding energies were also seen in I 3d and I 4d spectra in this temperature range and can be attributed to the removal of the physisorbed layers to expose the chemisorbed monolayer. The binding energies for the R and β carbons in the monolayer are 284.4 and 291.2 eV, respectively. Above 210 K, the carbon peaks are difficult to detect above the background noise because of the emerging As Auger peak at 278 eV and also because carbon in submonolayer coverages is not easily detectable using the laboratory X-ray source that has low photon energy and intensity and hence a low photoionization cross section for the C 1s shell. However, we know that carbon-containing species exist on the surface at

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Figure 4. XP spectra monitoring the C 1s photoemission peak following annealing of the dosed surface at the temperatures indicated. The 120 K spectrum was acquired following CF3CH2I exposure of 20 L.

Figure 5. XP spectra monitoring the F 1s photoemission peak following annealing of the dosed surface at the temperatures indicated. The 120 K spectrum was acquired following CF3CH2I exposure of 20 L.

temperatures up to 550 K because hydrocarbon species are detected desorbing from the surface in the thermal desorption experiments discussed below. The F 1s spectra, taken following CF3CH2I exposure and following heating to higher temperatures similar to that for the C 1s and I 3d and 4d spectra, are shown in Figure 5. At 120 K, the F 1s binding energy is 687 eV, which is close to the 687.4 eV value obtained for fluorine layers on GaAs.31 With heating to higher temperatures, the peak center remains constant while the intensity undergoes a large reduction with the removal of the molecular multilayers above 165 K. For temperatures higher than 240 K, the signal-to-noise ratio in the spectra were not of sufficiently high caliber to be presented here and hence are not shown in Figure 5. Although the X-ray photoionization cross section for F 1s is high, the low peak intensity at temperatures greater than 240 K is due to the low surface concentration of fluorine at these temperatures, which could not be detected by XPS. For this reason, peak-fitting procedures to determine the proportion of fluorines adsorbed at arsenic and gallium sites were not conducted. To determine whether β-fluoride elimination has occurred at these low temperatures, we determined areas of the C 1s peak at 291.8 eV and the F 1s peak, and using the photoionization cross section ratio29 of F 1s to C 1s (1:0.20), the ratio of the carbon to fluorine peak area is calculated to be 1:3.1 ( 0.2, which is consistent with the expected 1:3 ratio for the CF3

Figure 6. (a) Normalized peak intensities of Ga 3d, As 3d, and I 3d photoemission features plotted as a function of the substrate annealing temperature of the 20 CF3CH2I dosed surface. (b) Absolute peak intensities of Ga 3d and As 3d plotted as a function of the annealing temperature (left-hand y scale) and Ga 3d/As 3d ratios plotted as a function of the annealing temperature (right-hand y scale).

moiety. This ratio suggests that CF3CH2 species exists on the surface at these temperatures. It must be stressed, however, that we do not rule out the possibility that some β-fluoride elimination may have commenced at these low temperatures, as observed on Ag(111)16,17 for the same molecule, although this fraction would be expected to be reasonably small. Normalized intensities of the Ga 3d, As 3d, and I 3d peaks are presented in Figure 6a and show that (i) the Ga 3d and As 3d peak intensities increase rapidly from low temperatures up to 240 K, after which they increase only steadily and (ii) for the same temperature range the I 3d peak intensity drops rapidly, after which the decrease is more gradual, with 400 K is due to the desorption of recombinative molecular species and AsI and GaI etch products. Absolute Ga 3d and As 3d peak intensities plotted as a function of temperature are shown in Figure 6b. Also plotted in Figure 6b is the Ga 3d/As 3d peak intensity ratio for the temperature range of 240-720 K, calculated using a photoionization cross section value of 0.53 for As 3d and 0.31 for Ga 3d.29 The ratios for temperatures below 240 K have not been plotted because the presence of physisorbed layers at these temperatures cause the peak intensities to be very sensitive to small variations in surface coverage, which then introduces large errors in the ratios. Thus, the ratios at these temperatures may not be accurate. The average Ga 3d/As 3d

Coupling Versus Surface-Etching Reactions

Figure 7. (a). Full-width at half-maximum (fwhm) values of the Ga 3d photoemission feature plotted as a function of the substrate annealing temperature of the 20 L CF3CH2I dosed surface and (b) fwhm values of the As 3d photoemission feature plotted as a function of the substrate annealing temperature.

ratio for the temperature range of 240-720 K is 1.21 ( 0.06, which is consistent with the clean-surface value of 1.25 ( 0.02. Peak fits to the As 3d and Ga 3d photoemission features presented in Figures 1 and 2, respectively, and measurements of the fwhm values were conducted to determine the effect of changing surface chemistry of the adsorbed CF3CH2I species on the substrate atoms. The fwhm values of Ga 3d are plotted as a function of substrate temperature in Figure 7a, and an equivalent plot for the As 3d feature is shown in Figure 7b. In each case, the 210 K spectrum was the lowest-temperature spectrum that allowed peak fits, but the determination of the fwhm values, when multilayers are still present on the surface, has a larger error associated with them as a result of the low signal-to-noise ratio in those spectra and hence may not be meaningful. For temperatures greater than 240 K, the fwhm shows an apparent decrease in each case, which we believe is an artifact of measuring the fwhm of spectra of surfaces with physisorbed layers. An interesting difference between the plots is that in the case of As 3d the decrease is more gradual and for temperatures >370 K the value becomes a constant at 1.73 ( 0.01 eV, consistent with the value measured for the clean-surface spectrum that was determined to be 1.74 ( 0.01 eV. In the case of Ga 3d, the decrease in the fwhm is much sharper, and for temperatures >350 K, the fwhm value increases and starts to decrease only for temperatures greater than 600 K. By 720 K, it is the same value as that measured from the clean-surface spectrum (1.62 ( 0.01 eV). The observed increase in the Ga 3d fwhm for

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temperatures >350 K coincides with the commencement of the β-fluoride elimination process in the adsorbed CF3CH2 species (see thermal desorption data given below) and is due to the formation of a chemically shifted peak due to the reaction of the eliminated fluorines with the substrate atoms to form GaFx species. The presence of a chemically shifted peak, due to GaFx formation, was confirmed by peak fitting, and in Figure 8a, fitted peaks for a select number of Ga 3d spectra are presented. In the case of the clean-surface spectrum and the spectrum taken following annealing to 720 K, a single peak at a binding energy of 19.1 eV27 and a fwhm of 1.62 ( 0.01 eV with and a Gaussianbroadened (16% Gaussian/84% Lorentzian) Lorentzian peak shape was used to fit the experimental data following Shirley background subtraction. In spectra taken following annealing to 470 and 570 K, it was found that the data could not be fitted with a single peak in either case using the clean-surface parameters noted above, and this was confirmed by the nonzero residual (not shown) obtained when the single fitted peak was subtracted from the experimental data. However, the data could be fitted with two peaks (with similar fitting parameters as above for both peaks), as was confirmed by the zero residual when the sum of the fitted peaks was subtracted from the raw data. The second peak exists on the higher-binding-energy side of the main peak, with the peak area being ∼5% of the dominant peak and chemically shifted by ∼1.5 ( 0.2 eV for both temperatures. This chemical shift is consistent with that observed for GaF formation on GaAs(100) and GaAs(110),32 which was reported to be within the range of 0.8-1.6 eV. The absence of a chemically shifted peak at ∼2.5 eV on the higher-binding-energy side of the main Ga 3d peak suggests that GaF3 did not form in this study, although it has been shown to form on GaAs(100) during XeF2 etching reactions.32 In contrast to the Ga 3d spectra, peak fitting conducted for the As 3d data acquired for temperatures greater than 400 K could be fitted with a single peak using the same fitting parameters (binding energy 40.8 eV, fwhm 1.74 eV, 15% Gaussian/85% Lorentzian line shape) as for the clean-surface data; no additional peaks were needed to fit the experimental data. The clean surface and the 470, 570, and 720 K spectra for As 3d are shown in Figure 8b. The observation suggests that no chemically shifted As 3d peaks are detectable, which is different from the observation of McLean et al. for atomic fluorine adsorption on GaAs(100)42 where a chemically shifted As 3d peak was observed as a result of the formation of AsF. 3.2. Low-Energy Electron Diffraction (LEED) Studies of Adsorption and Annealing of CF3CH2I on GaAs(100)-(4 × 1). LEED studies conducted on the surface dosed at 120 K showed that the (4 × 1) reconstruction of the clean surface was lifted and very faint (1 × 1) spots were visible only with a very high background. The spots sharpened after heating to 300 K, when the multilayers become desorbed and the background is reduced in intensity. Heating to higher temperatures did not result in a (32) Simpson, W. C.; Yarmoff, J. A. Annu. ReV. Phys. Chem. 1996, 47, 527. (33) Mass Spectra. In NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD, March 2003 (http://webbook.nist.gov). (34) Li, L. Surf. ReV. Lett. 2000, 7, 625. (35) Wu, H.-J.; Chiang, C.-M. J. Phys. Chem. B 1998, 102, 7075. (36) Huang, W. X.; White, J. M. J. Am. Chem Soc. 2003, 125, 10798. Huang, W. X.; White, J. M. J. Am. Chem Soc. 2004, 126, 14527. (37) Maitlis, P. M. J. Organomet. Chem. 2004, 689, 4366-4374. (38) FitzGerald, E. T.; Foord, J. S. J. Phys.: Condens. Matter 1991, 3, S347. (39) Ditlevson, P. D., Van Hove, M. A.; Samorjai, G. A. Surf. Sci. 1993, 292, 267. (40) Khatri, A.; Ripalda, J. M.; Krzyzewski, T. J.; Jones, T. S. Surf. Sci. 2004, 549, 143. (41) Creighton, J. R. J. Vac. Sci. Technol., A 1990, 8, 3984. (42) McLean, A. B.; Terminello, L. J.; McFreely, F. R. Phys. ReV. B 1989, 40, 11778.

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Figure 8. (a) Numerical fits to the Ga 3d photoemission feature of the clean surface and following annealing at the temperatures indicated. (b) Numerical fits to the As 3d photoemission feature of the clean surface and following annealing at the temperatures indicated.

change in the LEED pattern except that the spots became sharper, and this pattern remained until 720 K when the (1 × 1) spots became dim. These observations are different from our ethyl

iodide studies on gallium-rich GaAs(100)13 where an arsenicrich phase c(4 × 4) was observed at 600 K following GaI desorption and prior to As2/As4 desorption for temperatures greater

Coupling Versus Surface-Etching Reactions

Figure 9. Thermal desorption spectra monitoring from top to bottom: CF3CH2I+ (m/z ) 210), I+ (m/z ) 127), CF3+ (m/z ) 69), CF2CH2+ (m/z ) 64), CF3CH2CHdCF2+ (m/z ) 146), and CF3CH2CH2CF3+ (m/z ) 166) ion currents following 10 L CF3CH2I exposures at 150 K.

than 800 K. However, in both the ethyl iodide and CF3CH2I cases the clean surface (4 × 1) reconstruction did not return until after at least two cycles of argon bombardment of the surface and annealing to 800 K. 3.3. Temperature-Programmed Desorption Studies of CF3CH2I on GaAs(100)-(4 × 1). Figure 9 shows thermal desorption spectra tuned to a number of ion currents following 10 L CF3CH2I exposure at 150 K. Although adsorption was performed at 150 K, the data are displayed in the temperature range of 290-700 K because including the physisorbed peak obscured the details of the high-temperature feature. Thermal desorption spectra tuned to the parent ion CF3CH2I+ (m/z ) 210) show a broad feature spanning 350-520 K with two apparent maxima at ∼425 and ∼500 K. Our previous thermal desorption results in this work showed that a weakly chemisorbed molecular state exists at 290 K, which dissociates below room temperature to form CF3CH2 and I species.11 Hence, the molecular species observed in the 350-520 K region in Figure 9 is formed by the recombination of the adsorbed CF3CH2 and I species. The doublet nature of the desorption trace is most likely due to the fact that the iodines are adsorbed at both arsenic dangling bonds and the gallium dimers that exist in the (4 × 1) reconstruction discussed above. The binding energies of the iodines at these sites are expected to be different, as confirmed by the different desorption temperatures of AsI and GaI (see below). Adsorption at two inequivalent sites may also explain why desorption occurs over a large temperature range (170 K). The I+ (m/z ) 127) desorption trace is similar to the CF3CH2I+ desorption trace and is due to the mass spectrometer ion source fragmentation pattern of the desorbing CF3CH2I.33 The CF3+ (m/z ) 69) monitored under same conditions is similar to the CF3CH2I+ and I+ traces except that this trace extends to higher temperatures, and this is because the CF3+ ion current contains contributions from the mass spectrometer ion source fragmentation contributions from desorbing CF3CH2I as well as coupling products CF2dCHCH2CF3 and CF3CH2CH2CF3 (see below). The latter desorbs at a higher temperature (∼500 K) and gives rise to the high-temperature component in the CF3+ ion current above 500 K. Experiments conducted to observe the further reactions of the surface CF3CH2 and I, as reported previously,11 yielded the following products:11 CF2dCH2, CF3CH2CHdCF2, and CF3(CH2)2CF3. The desorption traces of these products, monitoring the parent ion currents, are also shown in Figure 9. CF2d CH2 is the major reaction product and shows desorption at 430

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K. A series of CF2dCH2+ ion currents monitored for increasing CF3CH2I exposures had shown that the temperature at the peak maximum remains constant at this value, suggesting first-order desorption kinetics. Similar to the β-hydride reaction observed for surface alkyls,5-7 CF2dCH2 is formed by β-fluoride elimination in the surface CF3CH2 species. The desorption of this species occurs over a narrow temperature range compared to that for other products, confirming that the β-fluoride elimination process is facile and rapid. Because the CF2dCH2 desorption temperature is lower than the recombinative CF3CH2I desorption and the desorption temperatures of the two coupling products discussed below, β-fluoride elimination appears to be the most favorable pathway for CF3CH2 reactions. We have shown previously that two coupling products form from reactions of the adsorbed CF3CH2, these being CF3CH2CHdCF2 and CF3(CH2)2CF3. The respective ion currents (CF3CH2CHdCF2+ (m/z ) 146), CF3(CH2)2CF3+ (m/z ) 166)) of these products are shown in Figure 9. CF3CH2CHdCF2 desorption occurs as a doublet with two apparent peak maxima separated by ∼50 K. However, the CF3(CH2)2CF3 desorption occurs as one distinct peak at ∼500 K, and we have shown this peak maximum to shift to lower temperatures with surface coverage, indicative of second-order reaction kinetics and suggesting that CF3(CH2)2CF3 forms by self-coupling of the surface CF3CH2 species. Note that CF3(CH2)2CF3 forms after CF3CH2CHdCF2 and CF2dCH2 formation, which is different from alkyl coupling reactions on transition-metal surfaces,5 where the direct coupling of the alkyl to form the corresponding higher alkane and not the alkene is the preferred carbon-carbon bond-formation pathway. Its product yield is also lower than that of CF3CH2CHdCF2 if we compare the only relative peak areas of the two products for the same exposure, assuming that the ionization cross sections of the two molecules are not very different. The desorption profiles of CF3CH2CHdCF2 and CF3(CH2)2CF3 are quite dissimilar both in terms of the shapes and the temperature range over which the desorption of each species occurs. If the mass spectrometer ion source fragmentation of CF3(CH2)2CF3 makes a contribution to the CF3CH2CHdCF2+ ion current, then this must be negligible. This is evident from a comparison of the high-temperature components of the desorption peaks of both CF3CH2CHdCF2+ and CF3(CH2)2CF3+ ion currents that show that the latter desorption trace extends to 580 K whereas the former trace extends only to 530 K. The etch products formed during CF3CH2I reactions were AsF, AsI, GaF, GaI, and As2, and the respective ion currents are presented in Figure 10 following CF3CH2I exposure. Higher arsenic halides (AsFx and AsIx, (x ) 1-3)) and gallium halides (GaFx and GaIx, (x ) 1-3)), F2, and HF were not detected. The AsF desorption feature is broad, and it can be noticed that its desorption at 450 K commences following CH2dCF2 formation and it occurs at a lower temperature than GaF desorption at 480 K. The fact that both AsF and GaF desorption commences after CH2dCF2 formation suggests that the fluorines for their formation are derived from the β-fluoride elimination reaction. The experiments conducted to monitor arsenic dimers were conducted in a different manner from those for reaction products noted above. Initially, a blank spectrum monitoring As2+ (m/z ) 150) ion current was acquired from a cleaned surface that had been annealed to 800 K and had no prior CF3CH2I exposure, shown as the bottom As2+ trace in Figure 10. The sample when heated to only 700 K shows a rise in the background that commences at 680 K and is attributable to incongruent As2 evaporation from the bulk. The incongruent evaporation occurs when GaAs melts into its components (As2 and Ga) rather than

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Figure 10. Thermal desorption spectra monitoring from top to bottom: AsF+ (m/z ) 94), AsI+ (m/z ) 202), GaF+ (m/z ) 89), and GaI+ (m/z ) 197) ion currents following 10 L CF3CH2I exposures at 150 K except for GaF+ and GaI+, which had 72 L exposures. The bottom As2+ (m/z ) 150) ion current was acquired from a clean sample annealed to 800 K, and the top As2+ ion current was acquired following three successive cycles of annealing to 700 K/100 L CF3CH2I dose. Scheme 1. Reaction Scheme for the Adsorption and Decomposition of CF3CH2I on Gallium-Rich GaAs(100)-(4 × 1)

forming a liquid phase itself. The top As2+ trace is from a surface that had three successive cycles of annealing to 700 K/100 L CF3CH2I dose such that by the fifth cycle the onset occurs at ∼600 K. Hence, if the surface is annealed to 700 K only it becomes arsenic-rich and the As2 species that desorb arise from the surface arsenic dimers.

4. Discussion The reaction scheme for the adsorption and decomposition of CF3CH2I on GaAs(100) is shown in Scheme 1. In our previous report on CF3CH2I reactions on gallium-rich GaAs(100)-(4 × 1),11 we showed that the gallium and arsenic etch products and the fluorine-substituted hydrocarbon products are all formed by the thermal activation of the adsorbed CF3CH2I. The product distribution is similar to those in our studies of ethyl iodide13 and bromochloroethane2 on GaAs(100) but is different from that in our study of CH2I212 where no etch products were formed. We showed that CF3CH2I adsorbs molecularly at liquid-nitrogen temperature on GaAs(100) but undergoes thermal dissociation via C-I scission below room temperature to form a chemisorbed monolayer containing CF3CH2 and I species, consistent with CF3CH2I adsorption on Ag(111)16,17 and Si(100),16 as shown below where (ads) represents chemisorption:

CF3CH2I(ads) f CF3CH2(ads) + I(ads)

(1)

These species most probably adsorb at gallium sites by breaking the Ga dimers in the (4 × 1) structure discussed above. This assumption is based on observations in a number of studies on gallium-rich GaAs(100) (e.g., ref 34) where it has been shown that the gallium atoms are the more favorable adsorption sites. Given bond energies44 of the Ga-Ga dimer (112 kJ mol-1), C-I (237 kJ mol-1), Ga-C (294 kJ mol-1), and Ga-I (339 kJ mol-1) bonds, the energetics for the dissociative adsorption of CF3CH2I at a Ga-Ga dimer is exothermic (-284 kJ mol-1) and hence consistent with the process occurring below room temperature. The reverse of reaction 1, the recombination of CF3CH2I species, occurs at high temperatures (400-500 K) because it is endothermic, and the temperature range is large (∼160 K) because iodine can be derived from arsenic sites. Although recombinative desorption of alkyl halides does not occur on transition-metal surfaces, on GaAs(100) this has been observed previously for ethyl iodide13 reactions, in addition to that reported in the present study. The difference in the reaction behavior can be linked to the extent of β-hydride and β-fluoride elimination processes induced in the adsorbed alkyls by surfaces. The fact that on GaAs recombinative desorption is observed suggests that on this surface the rate constants for β-hydride and β-fluoride processes are smaller than on transition metals and hence not all adsorbed alkyls undergo β-hydride/β-fluoride elimination to form the respective alkenes. Therefore, GaAs allows the recombination of the alkyls, for example, CF3CH2 in this study and CH3CH2 in the case of ethyl iodide reactions, with the adsorbed iodines liberating the respective molecular species. The desorption of CF2dCH2, formed from CF3CH2 by β-fluoride elimination, occurs as a single intense peak spanning ∼100 K for the same heating rate. This reaction commencing at ∼350 K exhibits a rapid fall-off in the desorption peak on its high-temperature side, thus giving rise to an asymmetrical peak and a constant temperature at the peak maximum (430 K) with increasing exposure.11 Both features are consistent with the typical characteristics of a first-order process. β-Fluoride elimination represents the predominant pathway for the removal of surface CF3CH2, consistent with the previous studies of CF3CH2I on Ag(111)16,17 and Si(100).18

CF3CH2(ads) f CF2dCH2(g) + F(ads)

(2)

However, β-fluoride elimination on GaAs(100) is not as facile as it is on Ag(111),17,16,35,36 where CF2dCH2 and CF2dCD2 were formed at 250 and 262 K, respectively, from the respective trifluoroethyl species. There is no evidence in our thermal desorption data that β-fluoride elimination occurs below room temperature. This is supported by XPS data that clearly shows a marked increase in the fwhm of the Ga 3d photoemission feature only for temperatures greater than 400 K when the onset of β-fluoride elimination commences with the subsequent desorption of both AsF and GaF. One of the two coupling products, CF3CH2CHdCF2, desorbs immediately after CF2dCH2 formation, and the second product, CF3CH2CH2CF3, desorbs at a temperature that is ∼70 K higher. We have previously shown that CF3CH2CH2CF3 desorption exhibits second-order kinetics because it is formed by the selfcoupling of the adsorbed CF3CH2 species:11

2CF3CH2(ads) f CF3CH2CH2CF3(g)

(3)

(43) Williston, L. R.; Bello, I.; Lau, W. M. J. Vac. Sci. Technol., A 1992, 10, 1365. (44) CRC Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1986.

Coupling Versus Surface-Etching Reactions

The formation of this recombinative coupling product is different from the behavior of CF3CH2 on Ag(111)17,35,36 where no self-coupling occurred and CF2dCH2 was the only reaction product, an unexpected result because Ag(111) is known to facilitate alkyl coupling reactions exclusively. In that study, it was postulated that fluorine substitution at the β position destabilized the coupling transition state relative to the trifluoroethyl initial state and hence the coupling reaction rate was reduced as a consequence of an increased activation barrier to coupling.17 In another study on the same surface, it has been postulated that the reduced surface mobility of CF3CH2, resulting from increased Ag‚‚‚F interactions due to its tilted geometry, suppressed the migratory coupling reaction, thereby reducing the coupling rates.16 We have no evidence that the adsorbed CF3CH2 has tilted geometry on GaAs, but the fact that self-coupling of the adsorbed CF3CH2 does occur on GaAs suggests that if some mobility is lost as a result of possible fluorine‚‚‚surface interactions then the loss has not been sufficient to hinder the coupling process. CF3CH2CHdCF2 desorption commences immediately upon CF2dCH2 formation, and the similarity of its desorption profile to that of the recombinative parent molecule suggests that CF3CH2CHdCF2 formation is dependent on both CF2dCH2 and CF3CH2 being available on the surface at the same temperature. The lower desorption temperature for this product suggests that CF3CH2CHdCF2 formation is thermodynamically more favorable than the formation of the higher alkane, CF3CH2CH2CF3. This is not altogether uncommon and has similarly been observed in Fischer-Tropsch synthesis of higher hydrocarbons7,23,37 where there is a preference for vinyl (sp2)-alkyl (sp3) coupling to form alkenes rather than alkyl (sp3)-alkyl (sp3) coupling to form alkanes. The exact mechanism of CF3CH2CHdCF2 formation from CF2dCH2 and CF3CH2, however, is not known at this stage because without vibrational spectroscopy data it is difficult to ascertain whether the CF2dCH2 readsorbs onto the surface before reaction with the adsorbed CF3CH2. It is known for ethene to adsorb onto GaAs(100)2,38 and Pt(111)39 via π-bonding and σ-bonding configurations, and on GaAs(100)2 these occur with activation energies of desorption of 107 and 145 kJ mol-1. With an activation energy of desorption of 102 kJ mol-1 for CF2d CH2, calculated using the Redhead equation with a heating rate of 12 K s-1 and a preexponential factor of 1013 s-1, it is conceivable that CF2dCH2 following its formation could be accommodated on the surface via π bonding. In this configuration, greater mobility is possible, and hence migratory insertion into the Ga-C bond of the adsorbed CF3CH2 species with the concurrent elimination of hydrogen becomes feasible. This postulate is supported by the fact that this coupling product forms before the self-coupling product, CF3CH2CH2CF3, where CF3CH2 bonded to surface sites by σ bonding is not expected to be as mobile as the π-bonded CF2dCH2.

CF3CH2(ads) + CF2dCH2(ads) f CF3CH2CHdCF2(g) + H(ads) (4) This mechanism, known as the olefin migratory insertion mechanism, was also responsible for the formation of butene in the case of ethanethiol reactions on gallium-rich GaAs(100)10 where the butene was observed to form from ethyl groups immediately after CH2dCH2 formation. It has been observed previously on transition-metal surfaces6,7 and during the FischerTropsch synthesis of higher hydrocarbons from CO and H2 over cobalt catalysts.23 The fate of the hydrogen eliminated in such a process is not certain. It does not desorb as gaseous H2, HF, or HI but most probably serves as an etchant for the continual

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removal of substrate arsenic subsurface layers as AsHx at low temperatures40 (∼70 °C), which is much lower than recombinative hydrogen desorption in the range of 480-510 K.41 Five etch products were detected (GaF, GaI, AsF, AsI, and As2), and this distribution represents, to date, the largest number of products observed from an alkyl halide reaction on galliumrich GaAs(100). The gallium halides (GaF and GaI) and arsenic halides (AsF and AsI) desorb before the As dimers, and the fluorides desorb before the iodides. The fluorines, liberated from the β-hydride process in the adsorbed CF3CH2, desorb as GaF and AsF. This observation is consistent with the core-level photoemission investigation of atomic F (generated by XeF2 dissociative adsorption) on GaAs(110)42 that demonstrated that F, unlike chlorine, interacts with both arsenic and gallium atoms. XPS data shows that a chemically shifted peak in the Ga 3d photoemission feature is observed whereas no equivalent peak shift is detectable in the As 3d feature. This latter observation is not easy to explain, especially because AsF desorption is detected in thermal desorption studies and it desorbs prior to GaF commencing at ∼400 K. No AsF3 desorption was detectable, but this could be because it is stable only on GaAs at or near room temperature32 when β-fluoride elimination to liberate fluorines for its formation does not occur. In a similar manner, GaF3 does not form, which is confirmed both in XPS and thermal desorption studies, and the GaF that forms desorbs at ∼425 K, which is a much lower temperature than the desorption temperature of 578 K (305 °C) when GaF is formed on GaAs by bombardment with low-energy fluorine ions.43 We have previously proposed that AsF and GaF form from the eliminated fluorines (by breaking two C-F bonds) via the reaction11

GaAs(subs) + 2F(ads) f GaF(g) + AsF(g)

(5)

based on the mechanism proposed by Simpson and Yarmoff in their study of XeF2 interactions with GaAs,32 where it was shown that F can diffuse into the bulk. We observe AsF desorption immediately after the β-fluoride elimination process (eq 2), and hence this mechanism is plausible. Given the bond strengths44 Ga-As (211 kJ mol-1), C-F (480 kJ mol-1), C-C (346 kJ mol-1) CdC (614 kJ mol-1), Ga-C (294 kJ mol-1), Ga-F (576 kJ mol-1), and As-F (413 kJ mol-1), the energetics of the overall process, taking into consideration the number of bonds broken (2 C-F, 2 Ga-C, 2 C-C, and 1 Ga-As) and bonds formed (2 CdC, 1 As-F, and 1 Ga-F) becomes an endothermic one (+234 kJ mol-1) requiring high temperatures and is consistent with the observation that the β-fluoride elimination process occurs at 430 K. For eliminated fluorines to be adsorbed onto the surface requires vacant sites adjacent to the adsorbed CF3CH2 species. The doublet nature of the AsF desorption suggest that desorption occurs from two inequivalent arsenic sites. In the surface layer of the (4 × 1) reconstruction, there are three distinct As sites, and two of these have dangling bonds directed toward the vacuum.15 We believe that these sites are also possible sites at which eliminated fluorines can become adsorbed and subsequently liberate AsF. The liberated AsF could immediately react with the substrate via the reaction

AsF(g) + GaAs(subs) f GaF(g) + 2As(ads)

(6)

We postulate this reaction on the basis of a proposal put forward previously by Bernstein and Grepstad for CF4 plasma etching of GaAs(100).45 The reaction explains two observations in this study. It explains the formation of GaF immediately after AsF formation and ∼100 K lower than for the species previously (45) Bernstein, R. W.; Grepstad, J. K. J. Appl. Phys. 1990, 68, 4811.

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formed by interactions of low-energy fluorine ions with GaAs.43 It also accounts for the concentration of arsenic at the reaction interface that subsequently desorbs as arsenic dimers at high temperatures.

2As(ads) f As2(g)

(7)

The formation of GaF by direct reaction of eliminated fluorines with gallium sites may also be possible, but the gallium dimers may not be available for fluorine adsorption because of site blocking by adsorbed CF3CH2 and iodine species. AsI desorption follows AsF/GaF desorption and commences prior to GaI desorption, and both show single peaks. Both the gallium and arsenic sites available in the (4 × 1) structure must become populated with iodine when C-I scission occurs by thermal activation in the molecularly adsorbed CF3CH2I. On the basis of the observation of iodine mobility between sites in the case of I2 adsorption on GaAs(100),30 we postulate that some diffusion does occur to enable iodine to move between surface sites and between the first few layers. In that study, it was also found that iodine adsorbed at surface sites was in abundance (gallium vs arsenic), and hence in our study where the experiments were conducted on a gallium-rich surface these preferred sites for iodine adsorption are expected to be the gallium sites. Evidence for this comes from our observation, discussed in our previous paper on this work,11 that if the surface is made progressively more gallium-rich the yield of GaI could be increased for the same CF3CH2I exposure. The dissociated CF3CH2 moieties most probably also adsorb at gallium sites, and steric hindrance from this moiety means that not all adjacent sites become populated. Because AsI desorption is observed, we believe that dissociated I species also become adsorbed at arsenic sites in the surface layer of the (4 × 1) reconstruction. I2 has been shown to be nondisruptive on arsenic-rich GaAs(100)-c(2 × 8)30 and antimonyrich InSb(001)-c(4 × 4)46 in studies where the initial surface structures and stoichiometries were regenerated following annealing of the iodine-dosed surface and evolution of I2, and this was attributed to its milder etching behavior compared to that of chlorine and fluorine. The Ga/As ratio was maintained constant at 1.21 ( 0.06 throughout the annealing experiment starting with a CF3CH2Idosed gallium-rich surface at about 210 K. This observation is consistent with I2 reactions on arsenic-rich GaAs(100)-c(2 × 8)30 and for atomic fluorine studies on GaAs(110).42 Our study, however, is different from previous investigations involving realtime mass spectroscopic monitoring during modulated molecular beam scattering experiments of Cl2 on GaAs(100)3 and during surface photoabsorption studies in the case of HCl on GaAs(100),47 which both showed that the surface stoichiometry (Ga/ As ratio) changed over the 300-700 K temperature range in which the reactions were conducted. A difference that exists between the I2 studies on arsenic-rich GaAs(100)-c(2 × 8)30 and this investigation is that the original clean-surface reconstruction is not regenerated after annealing to 720 K but rather a (1 × 1) pattern is observed. The difference in product distribution between the two adsorption systems during annealing may account for this observation. In the case of I2 studies, the adsorbed iodine desorbs recombinatively as I2 from GaAs(100)-c(2 × 8),30 and we observe a plethora of etch products (AsF, GaF, AsI, GaI, and As2) with CF3CH2I. It is important to note that these arsenic- and gallium-containing etch products evolve in an alternating manner (46) Jones, R. G.; Singh, N. K.; McConville, C. F. Surf. Sci. 1989, 208, L34. (47) Fang, H.; Eng, J., Jr.; Su, C.; Vemuri, S.; Herman, I. P.; Bent, B. E. Langmuir 1998, 14, 1375.

with the onset temperature for desorption for each product: AsF (400 K), GaF (450 K), AsI (480 K), GaI (520 K), and As2 (>600 K). A consequence of this desorption behavior is that the change in surface stoichiometry would be expected to be minimal over the annealing temperature range. A similar observation was made for atomic fluorine reaction with GaAs(110)42 where the near constancy of the bulk Ga/As ratio for the entire fluorine exposure range was attributed to the removal of both Ga and As atoms from the surface as the respective trifluorides. It is equally important to note that the etch products in this study form only after β-fluoride elimination in CF3CH2 and the deposition of surface fluorine have occurred and that the ability of the fluorine to disrupt the Ga-As bonds is the precursor to all possible etch products being evolved. This wide range of etch products has not been observed previously with any other alkyl halide reactions on GaAs(100). For example, in our previous investigation of ethyl iodide13 on gallium-rich GaAs(100)-(4 × 1) no AsI desorption was detected, although GaI and As2 were formed. In the case of bromochloroethane2 reactions on the same surface, although AsCl, GaCl, and GaBr were detected, no AsBr was detected. The similar Ga/As ratio of the clean surface and following CF3CH2I dosing and annealing, the evolution of both arsenic and gallium etch products during annealing could be expected to exhibit the same LEED pattern at the end of the experiments. This was not the case in this study where a (1 × 1) pattern was observed rather than the clean-surface (4 × 1) structure. There could be two reasons for this. XPS data show that adsorbed iodine (∼5%) remains on the surface even after annealing to 720 K, and with iodine atoms bonded to surface atoms, the (4 × 1) reconstruction will not be possible. It is also possible that the disruption of the Ga-As bonds by the fluorines causes damage a few layers deep into the bulk and the order does not return until after the damaged layers have been physically removed by cycles of argon ion sputtering and annealing. The disruption of the Ga-As bonds may be spatially inhomogeneous, as was postulated for atomic fluorine reactions with GaAs(110),42 and hence GaGa dimer formation does not occur during annealing. However, this must remain a conjecture until confirmed by scanning tunneling microscopy studies.

5. Conclusions The adsorption of CF3CH2I on gallium-rich GaAs(100)-(4 × 1) at 150 K results in a weakly chemisorbing molecular state (in addition to the physisorbed multilayers) that undergoes dissociation below room temperature to form surface CF3CH2 and I species by C-I scission in the molecular state. In this article, we provide a summary of products that form by the surface reactions of CF3CH2, which are as follows: (i) CF2dCH2 via the β-fluoride elimination reaction, as noted above; (ii) CF3CH2CH2CF3, by a second-order reaction of adsorbed CF3CH2; and (iii) CF2dCHCH2CF3 by migratory CF2dCH2 insertion into the carbon-surface bond of adsorbed CF3CH2. Detailed discussions of these reactions can be found in a previous report on this work.11 The iodine species desorbs as GaI and AsI etch products, suggesting that the iodine species adsorbs at both gallium dimer and arsenic sites available in the (4 × 1) reconstruction of GaAs(100). The fluorines released during β-fluoride elimination react with the surface gallium atoms and also diffuse into the bulk where they react directly with the Ga-As back-bonds to form both GaF and AsF, which subsequently desorb. The concomitant evolution of As atoms during these reactions causes the preferential segregation of arsenic atoms, which desorb im-

Coupling Versus Surface-Etching Reactions

mediately as arsenic dimers (As2) for temperatures greater than 600 K. The coupling products and etch products desorb concurrently. The etch products desorb sequentially in the order of increasing desorption temperature as (i) AsF, (ii) GaF, (iii) AsI, (iv) GaI, and (v) As2. The alternating pattern of arsenic and gallium etch products results in a nearly constant Ga/As ratio over the entire annealing temperature range. However, the cleansurface (4 × 1) reconstruction of GaAs(100) is not regenerated at the end of the annealing experiments, but rather a (1 × 1) surface structure results. The lack of (4 × 1) reconstruction may be due to the presence of small amounts of adsorbed iodine up to 720 K, which does not allow Ga-Ga dimer formation to occur.

Langmuir, Vol. 22, No. 23, 2006 9565

This study has provided valuable insights into mechanisms of a complex molecule, CF3CH2I, that shows a variety of surface reactions encompassing surface-etching and carbon-carbon bond formation via one of the previously postulated Fischer-Tropsch (FT) mechanisms. The release of the toxic arsenic dimers at high temperatures renders GaAs unsuitable as an FT catalyst. However, the surface-etching reactions of the liberated fluorine and iodine atoms and the fact that the adsorbed CF3CH2 is completely removed from the surface as gaseous organic compounds suggest that under appropriate conditions CF3CH2I can be used as an etchant for GaAs in microelectronics processing. LA061207U