Surface Reactions of 1-Propanethiol on GaAs(100) - Langmuir (ACS

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Surface Reactions of 1-Propanethiol on GaAs(100) Suzy Donev,† Narelle Brack,*,† Nathan J. Paris,‡ Paul J. Pigram,† Nagindar K. Singh,‡ and Brian F. Usher† Centre for Materials and Surface Science, La Trobe University, Melbourne, Victoria 3086, Australia, and School of Chemistry, University of New South Wales, UNSW Sydney, NSW 2052, Australia Received July 19, 2004. In Final Form: November 10, 2004 The adsorption and decomposition pathways of 1-propanethiol on a Ga-rich GaAs(100) surface have been investigated using the techniques of temperature programmed desorption, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). 1-Propanethiol adsorbs dissociatively on a clean GaAs(100) surface to form propanethiolate and hydrogen. Further reactions of these species to form new products compete with the recombinative desorption of molecular propanethiol. The C-S bond scission in the propanethiolate results in the formation of propyl species and elemental sulfur. The generation of propene via β-hydride elimination then follows. In addition, propane and hydrogen form via reductive elimination processes. A recombinative high-temperature propanethiol desorption state is also observed. XPS and TOF-SIMS analyses confirm the presence of sulfur on the GaAs(100) surface following thermal decomposition. This paper discusses the mechanisms by which these products form on the GaAs(100) surface.

1. Introduction The investigation of surface interactions of organothiols and organosulfur molecules on semiconductors such as gallium arsenide is of growing importance as these molecules offer potential applications in areas such as (i) the development of group VI precursors, (ii) in situ dopants for III-V semiconductors, (iii) the formation of passivation layers on semiconductor surfaces, (iv) self-assembled electron beam resists, and (v) the formation of functional interfaces which link semiconductor surfaces to biological materials.1-3 GaAs has been widely used as a substrate for the growth of II-VI semiconductor materials and in the production of optoelectronic devices due to its high electron mobility and wide band gap properties. More recently, the formation of self-assembled monolayers (SAMs) on GaAs has been examined. Thiol-derived selfassembling monolayers such as 1,1-biphenyl-4-thiol (BPT)3 and octadecyl thiol (ODT)4,5 on GaAs have been investigated with particular reference to the design of chemical sensors and microelectronic devices for biological applications. However, the presence of the native oxide layer on GaAs presents a major challenge in device development. A number of sulfur-based processes, including aqueous and gas-phase treatments, have been investigated to enhance the resistance of GaAs to oxidation. Wet chemical treatments including ammonium sulfide ((NH4)2S),6-14 sodium sulfide (Na2S‚9H2O),15-18 thionyl chloride (SO2Cl2),19-21 * Corresponding author: Address: Department of Physics, La Trobe University, Melbourne, Victoria 3086, Australia. Ph: +61 3 9479 3808. Fax: +61 3 9479 1552. E-mail: N.Brack@ latrobe.edu.au. † La Trobe University. ‡ University of New South Wales. (1) Camillone, N.; Khan, K. A.; Osgood, R. M. Surf. Sci. 2000, 453, 83. (2) Singh, N. K.; Doran, D. C. Surf. Sci. 1999, 422, 50. (3) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Tanaka, M.; Zharnikon, M. Langmuir 2003, 19, 4992. (4) Baum, T.; Ye, S.; Uosaki, K. Langmuir 1999, 15, 8577. (5) Adlkofer, K.; Tanaka, M.; Hillebrandt, H.; Wiegand, G.; Bolom, T.; Deutshmann, R.; Abstreiter, G. Appl. Phys. Lett. 2000, 76, 3313. (6) Geib, K. M.; Shin, J.; Wilmsen, C. W. J. Vac. Sci. Technol. B 1990, 8, 838.

thioacetamide (CH3CSNH2),22-24 and organothiols25-27 have all been shown to generate passivating layers on GaAs surfaces with varying degrees of success. Among these compounds, (NH4)2S has been shown to be the most effective; however, even samples treated by this method have shown degradation upon exposure to the atmosphere. Gas-phase treatments involving the use of H2S have been widely reported.28-33 Adsorption studies of small organosulfur molecules, such as methanethiol and (7) Yuan, Z. L.; Ding, X. M.; Hu, H. T.; Li, Z. S.; Yang, J. S.; Miao, X. Y.; Chen, X. Y.; Cao, X. A.; Lu, E. D.; Xu, H. S.; Xu, P. S.; Zhang, X. Y. Appl. Phys. Lett. 1997, 71, 3081. (8) Lebedev, M. V.; Aono, M. J. Appl. Phys. 2000, 87, 289. (9) Dong, Y.; Ding, X. M.; Hou, X. Y.; Li, Y.; Li, X. B. Appl. Phys. Lett. 2000, 77, 3839. (10) Paget, D.; Bonnet, J. E.; Berkovits, V. L.; Chiaradia, P.; Avila, J. Phys. Rev. B 1996, 53, 4604. (11) Kang, M.; Park, H. J. Vac. Sci. Technol. A 1999, 17, 88. (12) Yuan, Z. L.; Ding, X. M.; Hu, H. T.; Li, Z. S.; Yang, J. S.; Miao, X. Y.; Chen, X. Y.; Cao, X. A.; Lu, E. D.; Xu, S. H.; Xu, P. S.; Zhang, X. Y. Appl. Phys. Lett. 1997, 71, 3081. (13) Carpenter, M. S.; Melloch, M. R.; Cowans, B. A.; Dardas, Z.; Delgass, W. N. J. Vac. Sci. Technol. B 1989, 7, 845. (14) Maeyama, S.; Sugiyama, M.; Oshima, M. Surf. Sci. 1996, 357358, 527. (15) Bessolov, V. N.; Konenkove, E. V.; Lebedev, M. V. J. Vac. Sci. Technol. B 1996, 14, 2761. (16) Tiedje, T.; Wong, P. C.; Mitchell, K. A. R.; Eberhardt, W.; Fu, Z.; Sondericker, D. Solid State Commun. 1989, 70, 355. (17) Bessolov, V. N.; Lebedev, M. V.; Binh, N. M.; Friedrich, M.; Zahn, R. T. Semicond. Sci. Technol. 1998, 13, 611. (18) Shin, J.; Geib, K. M.; Wilmsen, C. W. J. Vac. Sci. Technol. B 1991, 9, 2337. (19) Li, Z. S.; Cai, W. Z.; Su, R. Z.; Dong, G. S.; Huang, D. M.; Ding, X. M. Appl. Phys. Lett. 1994, 64, 3425. (20) Wang, X.; Hou, X.; Li, Z.; Chen, X. Surf. Interface Anal. 1996, 24, 564. (21) Ding, X. M.; Yuan, Z. L.; Hu, H. T.; Li, Z. S.; Chen, Y. F.; Chen, X. Y.; Cao, X. A.; Hou, X. Y.; Wang, X.; Lu, E. D.; Xu, S. H.; Xu, S. H.; Xu, P. S.; Zhang, X. Y. Nucl. Instrum. Methods Phys. Res. B 1997, 133, 90. (22) Lu, E. D.; Zhang, F. P.; Xu, S. H.; Yu, X. J.; Xu, P. S.; Han, Z. F.; Xu, F. Q.; Zhang, X. Y. Appl. Phys. Lett. 1996, 69, 2282. (23) Xinyi, Z.; Fapei, Z.; Erdong, L.; Pengshou, X. Vacuum 2000, 57, 145. (24) Xu, F. Q.; Lu, E. D.; Pan, H. B.; Xie, C. K.; Xu, P. S.; Zhang, X. Y. Surf. Rev. Lett. 2001, 8, 19. (25) Lunt, S. R.; Santangelo, P. G.; Lewis, N. S. J. Vac. Sci. Technol. B 1991, 9, 2333. (26) Lunt, S. R.; Santangelo, P. G.; Lewis, N. S. J. Appl. Phys. 1991, 70, 7449. (27) Remashan, K.; Bhat, K. N. Thin Solid Films 1999, 342, 20.

10.1021/la048191x CCC: $30.25 © 2005 American Chemical Society Published on Web 01/25/2005

Surface Reactions of 1-Propanethiol on GaAs(100)

ethanethiol, have been published recently.1,2 Small molecules are attractive as potential precursors for II-VI semiconductor materials and passivating layers for GaAs, as they are reasonably volatile and react at low temperatures with semiconductor substrates.11 The problems of poor reproducibility and contamination with carbon associated with aqueous-phase passivation are avoided by gas-phase treatments. The adsorption and thermal decomposition of H2S on GaAs has attracted considerable attention. Foord et al.30 showed that H2S dissociates on a Ga-rich GaAs(100) surface at 190 K to form SH and H species. Molecular adsorption was observed as the surface coverage increased. Upon heating, the SH and H species undergo recombinative desorption at 320 K and this process competes with dissociation of SH to produce adsorbed S and H species. Further studies by Conrad et al.33 quantitatively established the thermal and photochemical pathways of H2S on GaAs(100). The thermal chemistry of simple model organosulfur compounds such as methanethiol (CH3SH), methyl disulfide (CH3S)2, and dimethyl sulfide (CH3)2S on GaAs(110) has also been reported.1 CH3SH and (CH3)2S exhibit similar behavior in that they both undergo molecular adsorption and desorption, whereas (CH3S)2 decomposes upon adsorption and desorbs predominantly as (CH3)2S at ∼500 K. Singh et al.2 have shown that ethanethiol (C2H5SH) adsorbs dissociatively on GaAs(100) at ambient temperature to form ethanethiolate and hydrogen at the surface. Despite studies investigating the interactions of thiols comprised of one or two carbons on GaAs, the adsorption and decomposition pathways of larger thiols with more than 3 carbon atoms in the aliphatic chain on semiconductor surfaces have received little or no attention. Studies of the thermal behavior of larger thiols have focused only on transition metal surfaces. Temperature programmed desorption studies have been reported for butanethiol on Cu(110),34 Au(100),35,36 and Au(111)37 surfaces. Surface interactions of larger thiols, such as hexanethiol on Au(111),37 octanethiol on Cu(111)38 and Au(111),37 and 2-naphthalenethiol on Ag(111), have also been investigated. In general, it is noted that these thiols decompose on the respective surfaces to form thiolate species. In the current study, the adsorption and decomposition of 1-propanethiol on GaAs(100) are investigated using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). This study focuses on ambient temperature adsorption of 1-propanethiol on GaAs(100) with the aim of elucidating the adsorption and decomposition mechanisms and the potential for sulfur passivation. Propanethiol also provides a suitable ana(28) Massies, J.; Dezaly, F.; Linh, N. T. J. Vac. Sci. Technol. 1980, 17, 1134. (29) Ranke, W.; Finster, J.; Kuhr, H. J. Surf. Sci. 1987, 187, 112. (30) Foord, J. S.; FitzGerald, E. T. Surf. Sci. 1994, 306, 29. (31) Dudzik, E.; Muller, C.; McGovern, I. T.; Lloyd, D. R.; Patchett, A.; Zahn, D. R. T.; Johal, T.; McGrath, R. Surf. Sci. 1995, 344, 1. (32) Tiedje, T.; Colbow, K. M.; Rogers, D.; Fu, Z.; Eberhardt, W. J. Vac. Sci. Technol. B 1989, 7, 837. (33) Conrad, S.; Mullins, D. R.; Xin, Q. S.; Zhu, X. Y. Surf. Sci. 1997, 382, 79. (34) Lai, Y. H.; Yeh, C. T.; Cheng, S. H.; Liao, P.; Hung, W. H. J. Phys. Chem. B 2002, 106, 5438. (35) Bondzie, V.; Dixon-Warren, St. J.; Zhang, Y. Y. Surf. Sci. 1999, 431, 174. (36) Dixon-Warren, S. J.; Bondzie, V.; Burson, N.; Lucchesi, L.; Yu, Y.; Zhang, Z. J. Vac. Sci. Technol. A 1999, 17, 2982. (37) Kodama, C.; Hayashi, T.; Nozoye, H. Appl. Surf. Sci. 2001, 169, 264. (38) Sung, M. M.; Yun, W. J.; Lee, S. S.; Kim, Y. Bull. Korean Chem. Soc. 2003, 24, 610.

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logue for studying fundamental interactions between sulfur and the GaAs surface, which has direct relevance to SAMs based on alkanethiol chemistry. SAMs of propanethiol and its fluorinated derivatives have only been investigated on Au(111) surfaces,39 on zinc,40 on copper,41 and on silver thin films.42 2. Experimental Section GaAs(100) epilayers, the sample surface for all experiments, were prepared by molecular beam epitaxy (Varian) using GaAs substrates. The epilayers were capped in situ using a thick As layer for transfer between deposition and analytical instruments. The sample size was 10 mm × 10 mm. Prior to ambient temperature dosing experiments, the samples were decapped and then argon ion bombarded (1.4 keV Ar+) and annealed to 770 K for three cycles. As reported previously, this method removed adventitious carbon and oxygen and produced a galliumrich (4 × 1) reconstructed surface as determined by XPS and low-energy electron diffraction (LEED) experiments.2 1-Propanethiol (99%, Sigma Aldrich) was used as-received without further purification. After attachment to the gas dosing line, the propanethiol was degassed by several freeze-pumpthaw cycles. This procedure was followed by thorough passivation of the gas dosing lines prior to commencement of TPD and XPS experiments. Sample dosing was performed at room temperature by bringing the sample into close proximity to either a microcapillary array doser (TPD) or a stainless steel dosing capillary tube (XPS). TPD experiments were performed in an ultrahigh vacuum (UHV) system which has been described elsewhere,43 using a linear heating rate of 9 K s-1. The desorption spectra are presented as desorption intensity (arbitrary units) of the ion currents of the selected species as a function of substrate temperature. The propanethiol exposures are reported in L (langmuirs), where 1 L ) 10-6 Torr s, and have been calculated using pressure values measured by the chamber ion gauge and which are uncorrected for ion gauge sensitivity for propanethiol relative to nitrogen (set at a value of 25). By virtue of the dosing arrangement described above, the gas pressure exerted at the surface is much higher than that measured by the ion gauge, and hence the actual exposure values are higher than the values being reported here. However, since the same sample geometry was always maintained during dosing, the dosing enhancement factor at the surface is a constant and therefore the exposure values should be taken as being proportional to the actual values. In addition, no effort was made to convert these exposure values to surface coverages in monolayers (ML). However, the exposure values reported in a series of spectra acquired for any one molecular species or the fragment of that molecular species are indicative of relative surface coverages. The substrate temperatures have an error of (10 K associated with them. Note that the literature mass spectra for the purposes of determining the fragments of products to be monitored were obtained from the NIST Chemistry WebBook,44 and in the case of propanethiol, the spectrum was also acquired by the mass spectrometer utilized in TPD studies. In situ XPS measurements were performed after UHV transfer from the sample heating and dosing chambers using a Kratos Axis Ultra XPS spectrometer. Both monochromated Al KR (1486.6 eV) and nonmonochromated Mg KR (1253.6 eV) sources were operated at 150 W. The Mg KR source was used to determine the presence of sulfur by monitoring the S KLL Auger lines observed (39) Lee, S. Y.; Noh, J.; Hara, M.; Lee, H. Mol. Cryst. Liq. Cryst. 2002, 377, 177. (40) Mekhalif, Z.; Massi, L.; Guittard, F.; Geribaldi, S.; Delhalle, J. Thin Solid Films 2002, 405, 186. (41) Laffineur, F.; Dehalle, J.; Guittard, S.; Geribaldi, S.; Mekhalif, Z. Colloids Surf., A 2002, 198, 817. (42) Pang, Y. S.; Hwang, H. J.; Kim, M. S. J. Phys. Chem. B 1998, 102, 7203. (43) Avery, N. A. Vibration spectroscopy of molecules on surfaces; Yates, J. T., Madey, T. E., Eds.; Plenum Press: New York, 1987; p 223. (44) NIST Mass Spec Data Center, S. E. Stein, director; “Mass Spectra” in NIST Chemistry WebBook; NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; March 2003; National Institute of Standards and Technology: Gaithersburg, MD (http://webbook.nist.gov).

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using the Mg Bremsstrahlung continuum. The monochromated Al KR X-ray source was used for acquiring region scans (20 eV analyzer pass energy). The spectrometer energy scale was calibrated using the Au 4f7/2 photoelectron peak at a binding energy of 83.98 eV. Spectra were charge corrected assuming the As 3d peak associated with GaAs occurred at 40.8 eV.45 The analysis area was 700 µm × 300 µm. Spectra were quantified using Kratos XPS elemental sensitivity data after background subtraction and fitting of Gaussian (70%)-Lorentzian (30%) component peaks. Atomic concentration uncertainties for all fitted spectra are estimated to be (10% of the measured value. TOF-SIMS spectra were obtained using a TOF-SIMS IV instrument (Ion-TOF GmbH, Germany) with a reflectron analyzer, a monoisotopic 69Ga+ ion source (25 keV), and a pulsed electron flood source for charge neutralization. The primary pulsed ion beam current was 2.5 pA, and the primary dose was lower than 1.0 × 1013 ions cm-2 (static SIMS limit). All experiments were performed using a cycle time of 100 µs. The mass resolution was typically greater than 6000 at m/z ) 27. Both positive and negative spectra were acquired from a 100 µm × 100 µm area. Only the negative spectra are presented herein. Samples were exposed to the atmosphere (less than 2 min) during transfer from the XPS system to the TOF-SIMS instrument.

3. Results 3.1. Adsorption of Propanethiol on GaAs(100). Figure 1 shows XPS S 2p, C 1s, Ga 3d, and As 3d region spectra before and after a 300 L exposure of 1-propanethiol on GaAs(100) at room temperature. The clean substrate shows no evidence of the presence of carbon and sulfur species. The single component C 1s peak observed at 285.0 eV for the dosed substrate is consistent with the presence of C-C/C-H (hydrocarbon) species associated with propanethiol. Prior to dosing, the Ga 3s peak consists of a single peak centered at 160 eV. Following dosing, a shoulder is evident on the high binding energy side of the Ga 3s peak and this is assigned to S 2p species. The S 2p3/2 peak occurs at 162.0 eV. The presence of sulfur species was further investigated by angle resolved measurements and measurement of the S KLL Auger line at 2111 eV. It is possible to measure this transition using a nonmonochromated Mg KR source as the Bremsstrahlung continuum provides photons of sufficient energy to produce the S KLL transition. Figure 2 shows the S 2p spectra of a dosed GaAs(100) sample at 90° and 30° and the S KLL spectrum at 90°. The shoulder attributed to S 2p increases in intensity relative to the Ga 3s peak as the electron takeoff angle is reduced. The weak S KLL peak centered at 2111 eV further confirms the presence of sulfur. The Ga 3d and As 3d peaks have been included to demonstrate that there is no clear evidence for either Ga-S or As-S bonding as determined by XPS. The Ga 3d peak consists of a single component centered at 19.1 eV, while the As 3d consists of a doublet centered at 40.8 eV. This result is not surprising since the probing depth of XPS analysis is in the order of 5 nm and hence the sensitivity to the topmost layer is relatively small. Previous studies have suggested that the sulfur should preferentially adsorb at gallium sites rather than arsenic sites, given the presence of a gallium-rich surface and the higher enthalpy of formation of Ga2S3 (Hf(Ga2S3) ) -572 kJ mol-1) compared with that of As2S3 (Hf(As2S3) ) -169 kJ mol-1).2,46 Tables 1 and 2 show the relative percentage atomic concentrations and elemental ratios for etched and annealed GaAs(100) and propanethiol-dosed GaAs(100) prior (45) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992. (46) Aylward, G. H.; Findlay, T. SI Chemical Data, 3rd ed.; Jacaranda Wiley Ltd.: Brisbane, 1994.

Donev et al.

to and following further annealing. The etching and annealing process results in a Ga-rich surface as indicated by the Ga/As ratio of 1.23. This ratio remains constant following subsequent dosing and annealing treatments. After dosing the substrate, the gallium and arsenic concentrations decrease while carbon and sulfur species are detected, consistent with the adsorption of thiol. This is further supported by a C/S ratio of ∼2.7, which is close to the expected ratio of 3 for propanethiol or propanethiolate. XPS could not be used to deduce whether propanethiol adsorbs molecularly or dissociatively to form propanethiolate and hydrogen. The adsorption of propanethiol on GaAs(100) is further confirmed by TOF-SIMS analysis. Figure 3 shows negative TOF-SIMS spectra of GaAs(100) before and after dosing with propanethiol. The assigned fragments are presented in Table 3. The surface of the etched and annealed sample is dominated by O- and OH-, gallium and arsenic oxide fragments, and hydrocarbon fragment ions due to adsorbed surface contamination acquired during brief atmospheric exposure (∼2 min). From the XPS studies, where the dosed GaAs sample was not exposed to the atmosphere, there was no evidence of an O 1s peak and similarly there were no chemically shifted features in the Ga 3d and As 3d spectra due to adsorbed oxygen. Traces of fluorine (m/z ) 19), sulfur (m/z ) 32), and chlorine (m/z ) 35, 37) are present on the surface. Following thiol adsorption, peaks attributable to 1-propanethiol fragments at 32 (S-), 33 (SH-), 46 (CH2S-), and 75 (C3H7S-) are observed, thereby confirming the presence of thiol. 3.2. Thermal Decomposition of Propanethiol on GaAs(100). Temperature programmed desorption studies were undertaken to determine the surface reactions of 1-propanethiol on GaAs(100). A number of desorbing species were detected: propanethiol, propene, propane, and hydrogen. In Figure 4, the desorption profiles for C3H7SH (propanethiol parent ion, m/z ) 76) are presented for increasing exposures. Two peaks are detected, which populate simultaneously. A broad peak exists at 450 K and a sharper peak at 640 K, corresponding to the desorption of propanethiol from two different binding sites. The higher temperature state is more intense and grows more rapidly than the lower temperature state. For the low-temperature state, the temperature maximum (Tmax) appears to decrease with increasing thiol exposure. This behavior is consistent with second-order reaction kinetics and suggests that propanethiol adsorbs dissociatively to form propanethiolate and hydrogen species on the surface. It is postulated the peak observed at low temperature is due to the recombinative desorption of adsorbed propanethiolate and hydrogen. The high-temperature state at 640 K also shows a slight decrease in Tmax with increasing exposure of propanethiol, indicating second-order kinetics for this state. This high-temperature peak arises from recombination of a hydrocarbon species (propyl) and a sulfur-containing species (SH). The two reaction pathways are consistent with a previous study of ethanethiol adsorption on GaAs(100), where the low-temperature peak in the ethanethiol desorption trace was attributed to recombinative desorption of surface ethanethiolate and hydrogen species, and the high-temperature peak was due to recombinative desorption of ethyl and SH species. It was observed that both desorption processes occurred with second-order desorption kinetics.2 To detect desorbing propene, the parent ion (C3H6, m/z ) 42) was monitored and also one of its fragments C3H4 (m/z ) 40). Figure 5 shows the C3H6 spectrum following a 50 L propanethiol exposure and spectra monitoring the


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Figure 1. S 2p, C 1s, Ga 3d, and As 3d photoelectron spectra before (a-d) and after (e-h) a 300 L exposure of GaAs(100) to 1-propanethiol.

C3H4 fragment for four different exposures ranging from 1 to 50 L. While both the C3H6 spectrum and the C3H4 spectra span over a large temperature range, close inspection of the peak shapes shows that the C3H6

spectrum appears to have a shoulder on the hightemperature side. This shoulder is attributed to the mass spectrometer ion source fragmentation contributions from the high-temperature desorbing propanethiol. For this

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Figure 2. (a,b) S 2p and (c) S KLL spectra following 300 L exposure of 1-propanethiol onto GaAs(100). Table 1. Atomic Concentrations (%) of the GaAs(100) Sample (a) after Etching and Annealing, (b) after Dosing with 300 L Propanethiol, and (c) after Annealing to 373, 473, and 673 K, Respectivelya atomic concentration (%) (a) (b) (c)

a

surface

C

S

As

Ga

etched and annealed thiol dosed (300 L) annealed, 373 K annealed, 473 K annealed, 673 K

0.0 8.9 0.0 0.0 0.0

0.0 3.3 3.3 2.4 1.9

44.8 40.4 43.6 44.8 45.4

55.2 47.5 53.2 52.8 52.7

The error is (10% of the measured value.

Table 2. Elemental Ratios of the GaAs(100) Sample (a) after Etching and Annealing, (b) after Dosing with 300 L Propanethiol, and (c) after Annealing to 373, 473, and 673 K, Respectivelya elemental ratios surface (a) etched and annealed (b) thiol dosed (300 L) (c) annealed, 373 K annealed, 473 K annealed, 673 K a

Ga/As C/S C/As S/As C/Ga S/Ga 1.23 1.18 1.22 1.18 1.16

0.00 2.70 0.00 0.00 0.00

0.00 0.22 0.00 0.00 0.00

0.00 0.08 0.07 0.05 0.04

0.00 0.19 0.00 0.00 0.00

0.00 0.07 0.06 0.05 0.04

The error is (20% of the measured value.

reason, the C3H4 fragment of propene was chosen to monitor its desorption characteristics and in particular the desorption kinetics. This fragment does not have contributions from desorbing propanethiol and propane (see below), and hence the Tmax in the C3H4 spectra provides an indication of the true temperature at which propene desorbs from the surface. The C3H4 spectra show a single peak, positioned at 540 K, which increases in intensity as the exposure of 1-propanethiol increases. The Tmax does not remain constant for successive exposures of propanethiol, as expected for a first-order reaction, and in

Figure 3. Negative TOF-SIMS spectra of GaAs(100) (a) etched and annealed and (b) dosed with 300 L 1-propanethiol. Table 3. Peak Assignments for GaAs(100) Following Etching and Annealing and Dosing with 300 L 1-Propanethiol m/z

ion fragment

m/z

ion fragment

m/z

ion fragment

12 13 14 15 16 17 19 24 25

CCHCH2CH3OOHFC2C2H-

26 32 33 35 37 45 46 59

C2H2S or O2SH35Cl37ClCHO2CH2SC2H3O2-

75 85 87 91 101 103 107 123

As-, C3H7S69GaO71GaOAsO69GaO 2 71GaO 2 AsO2AsO3

this case the β-hydride elimination process forms this product. There is a small shift to high temperatures, such that following a 50 L exposure, the peak maximum occurs


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Figure 4. TPD spectra tuned to C3H7SH (m/z ) 76) as a function of increasing 1-propanethiol exposure. Figure 7. TPD spectra of H2 (m/z ) 2) as a function of 1-propanethiol exposure.

Figure 5. TPD spectra of C3H6 (m/z ) 42) after a 50 L exposure of 1-propanethiol and C3H4 (m/z ) 40) as a function of 1-propanethiol exposure.

Figure 6. TPD spectra of C2H5 (m/z ) 29) following a 50 L 1-propanethiol exposure and C3H7 (m/z ) 43) following 10, 20, and 50 L 1-propanethiol exposures.

at 555 K. This small but definite increase of Tmax is attributed to the stabilizing effect of the surface coadsorbed sulfur on the propyl groups from which the propene is formed. Sulfur becomes deposited on the surface during the decomposition of the propanethiolate formed on the surface upon adsorption, and with the increasing number of cycles of propanethiol exposure/temperature ramp during TPD data acquisition, its surface concentration increases. In a previous investigation in our group on ethanethiol reactions on GaAs(100),2 Auger electron spectroscopy was used to confirm the accumulation of surface sulfur during between TPD scans. In the current study, XPS data show that heating the thiol-dosed surfaces to 373, 473, and 673 K increases the S:Ga ratio (Table 2). The next product to be monitored was propane. The detection of the propane parent ion (m/z ) 44) had interference from background carbon dioxide, and hence the C3H7 (m/z ) 43) fragment was used instead. In Figure 6, TPD spectra monitoring this species for exposures spanning from 10 to 50 L are shown. Two features can be discerned, a prominent feature with the peak maximum at 610 K and a broad feature at about 440 K, the latter being attributed to the low-temperature propanethiol desorption. One of the other fragments of propane, the C2H5 fragment, was also monitored. The desorption trace monitoring this fragment following a 50 L propanethiol exposure is shown in Figure 6. One main peak, centered at 610 K, is observed, and no features in the low-

temperature region are noted. According to the literature mass spectra,44 C2H5 is the fragment of propane but not that of propene, while propanethiol fragmentation makes only a small contribution toward the C2H5 ion current. This latter is reconfirmed by the absence of a broad feature in the low-temperature region in this spectrum due to the low-temperature propanethiol desorption. The peak in the C2H5 spectrum confirms that propane desorbs from the surface at 610 K. The Tmax for C3H7 appears to remain invariant with increasing exposures. This result contradicts the anticipated second-order reaction kinetics, expected for propane formation by the reaction of propyl and hydrogen species, which would have shown the Tmax to shift slightly to lower temperatures. There could be two reasons for this anomaly. First, the hydrogen reduction of propyl groups is first order because it is reaction limited. This means that this reaction would rely on a preceding reaction for one of its reacting species, in this case hydrogen from the first-order β-hydride elimination process, and hence would show an apparent first-order kinetics. However, since there is an abundance of hydrogen on the surface, derived from both propanethiol, when it first adsorbs onto the surface as propanethiolate and hydrogen species, and during propene formation from the propyl groups, this is unlikely. In addition, the desorption temperature is about 80 K higher than the propene desorption, which is considerably higher than expected if propane formation were to be reaction limited. It is suggested that the expected Tmax decrease for second-order propane formation is actually offset by the increased stability, and hence higher desorption temperature, of the propyl groups or any products that form from the propyl groups, as the concentration of surface sulfur increases with increasing number of TPD scans taken sequentially, as described above to account for the increase in propene desorption temperature. In Figure 7, the spectra monitoring H2 ion current are presented and show that H2 desorption occurs at a temperature of about 625 K at low exposures, which increases to 640 K at high propanethiol exposures. As for propane, the Tmax does not decrease with increasing exposure, which was expected as recombinative desorption of hydrogen gas from surface hydrogen atoms should occur via a second-order process. The slight increase in the Tmax is again attributed to a consequence of the stabilizing effect of the sulfur on the propyl groups, which somewhat offsets the expected decrease in Tmax. As noted above, one of the two sources of surface hydrogen is the β-hydride elimination of the propyl group. If the propyl groups become more strongly bound onto the surface than the hydrogen, elimination would occur at a higher temperature and hence the desorption temperature of the hydrogen recombination will increase. The stabilizing effect also influences the

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Figure 8. Negative TOF-SIMS ion fragments m/z 75, 101, and 133 for GaAs(100) (a) etched and annealed, (b) 1-propanethiol dosed (300 L), and (c) 1-propanethiol dosed (300 L) and heated to 473 K.

surface hydrogens derived during propanethiol adsorption and hence already present on the surface prior to hydrogen being eliminated from the propyl groups. TPD spectra tuned to sulfur-containing species such as propyl sulfide (C6H14S), propyl disulfide (C6H14S2), and hydrogen sulfide (H2S) were also monitored although they were not detected. A comparison of the peak intensities of all the desorbing species detected indicates that propene is the most intense, suggesting that propene generation is the preferred desorption pathway for the adsorption of 1-propanethiol on GaAs(100). Using peak areas from the TPD data, it was found that the ratio of yields of propanethiol, propene, propane, and hydrogen was approximately 0.04:1.0:0.20: 0.60. In determining this ratio, the relative intensities (either peak areas in TPD scans or peak heights in the literature spectra)44,47 and the sensitivity of each fragment (estimated from its ionization efficiency) were incorporated into the calculation as done previously.47 The thermal decomposition process of propanethiol was also monitored with XPS and TOF-SIMS. XPS analysis of the dosed sample after annealing suggests that sulfur remains on the surface, whereas the presence of carbon decreases to zero (Table 1). The loss of carbon is reflected in the significant decrease in the C/As and C/Ga ratios between the dosed and then annealed samples (Table 2). As the annealing temperature is increased, loss of sulfur is indicated, for example, by a 50% reduction in the S/As and S/Ga ratios following annealing at 673 K. The loss of intact thiol molecules from the surface is illustrated via TOF-SIMS in Figure 8 with the loss of the parent ion peak (m/z ) 75) following heating to 473 K. In addition, the formation of Ga-S interactions, following thiol exposure, is indicated by the presence of GaS- and GaS2fragments as shown in Figure 9. These species are also observed following annealing, further supporting the deposition of sulfur on the surface. The ratio of GaS- to GaO2- species is ∼1:2 and ∼1:2.8 after dosing and annealing, respectively, indicating the loss of sulfur from the surface. 3.3. Sulfur Poisoning of GaAs(100). Experiments were undertaken to investigate the role of preadsorbed (47) Tjandra, S.; Zaera, F. Surf. Sci. 1995, 322, 140.

Donev et al.

Figure 9. C 1s and S 2p region spectra for GaAs(100) after (i) two complete 1-propanethiol dose and heat cycles followed by a third thiol dose (a,c) and (ii) three complete 1-propanethiol dose and heat cycles (b,d). Each dose was 300 L. Table 4. Atomic Concentrations (%) for GaAs(100) after (i) One Thiol Dose, (ii) the Completion of Two Dose and Heat Cycles Plus a Further Dose, and (iii) the Completion of Three Dose and Heat Cycles atomic concentration (%) surface

C

S

Ga

As

i ii iii

8.9 6.3 0.0

3.3 3.1 2.2

47.5 48.8 52.8

40.4 41.8 45.0

sulfur on the surface chemistry of propanethiol. Table 4 shows the relative atomic concentrations of elements present on the GaAs surface after (i) a 300 L dose of thiol, (ii) the completion of two dose and heat cycles plus a further dose, and (iii) the completion of three dose and heat cycles. During a dose and heat cycle, the GaAs surface is dosed with 300 L propanethiol at room temperature and then heated to 673 K. The C 1s and S 2p XPS spectra are shown in Figure 9 for procedures ii and iii. Following the initial dose, carbon and sulfur are detected and the C/S ratio is ∼3 which is consistent with the presence of propanethiol. Subsequent dosing and annealing results in the deposition of excess sulfur onto the GaAs surface. Upon further dosing of the substrate, a decrease in the C/S ratio is observed, indicative of an accumulation of sulfur surface species. For the annealed samples (Figure 9d), a shift of the S 2p signal by 0.5 eV to lower binding energies is observed, indicating the presence of elemental sulfur species on the surface. The sulfidization of the surface may influence the temperature at which the products would desorb from the surface. This effect was indeed observed and will be discussed further below. 4. Discussion Analysis of a clean GaAs(100) surface exposed to 1-propanethiol is consistent with the successful deposition of thiol species, as confirmed by the presence of carbon and sulfur species (XPS) and the respective fragment ions (TOF-SIMS). XPS analysis shows a C:S ratio of ∼3:1. This ratio could mean the presence of either molecular propanethiol or the propanethiolate species. However, TPD


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The thermal desorption results presented suggest that propanethiol adsorbs dissociatively on a gallium-rich GaAs(100) surface at room temperature to form hydrogen and propanethiolate species (eq 1).

CH3CH2CH2SH (g) f CH3CH2CH2S (ads) + H (ads) (1) Figure 10. Reaction scheme of 1-propanethiol adsorbed on GaAs(100) at room temperature.

shows that the low-temperature propanethiol occurs via a second-order reaction from the propanethiolate and hydrogen species. Hence, propanethiol adsorbs dissociatively at room temperature and the carbon-containing species on the surface at low temperatures detected with XPS is the propanethiolate species. Subsequent annealing of the substrate results in the thermal decomposition of the propanethiolate, and the accompanying decrease in the C 1s intensity indicates the loss of carbon-containing species. The S 2p signal shifts to lower binding energies suggesting that elemental sulfur deposits on the surface during thiolate thermal decomposition. XPS analysis cannot clearly identify whether the sulfur (in the thiolate or elemental sulfur form) bonds to the surface Ga or subsurface As atoms. This is because the S 2p peak overlaps with the Ga 3s peak of the substrate and hence makes the determination of chemical shifts difficult, especially as the photoionization cross section of the S 2p level in comparison with the Ga 3s level is relatively low for photon energies beyond 1000 eV.48 Also it is important to note that in a Ga 3d spectrum, the Ga-S peak shift is only 0.6 eV from the Ga-As component.18,29 A more significant shift of 1.6 eV18 from the Ga-As component would be observed for an As-S contribution in the As 3d spectrum, and this was not observed. In a previous study investigating the adsorption of 1,1-biphenyl-4-thiol on GaAs(100) using synchrotron XPS,3 it was reported that the As bonded to the sulfur headgroup. It was also noted that the Ga:As ratio was 1:1; however, the surface was arsenic enriched. The latter plus the high sensitivity achievable with synchrotron radiation may account for the evidence confirming the presence of As-S shifts. In the current study, the experiments were conducted on a gallium-enriched surface and XPS provides no clear evidence for the presence of Ga-S and/or As-S shifts following propanethiol adsorption. However, the detection of GaS- and GaS2- fragments in the thiol-dosed GaAs TOF-SIMS data before and after annealing supports the presence of Ga-S bonding. This result is consistent with previous LEED and Auger studies,2,33 which suggest that sulfur bonds preferentially to gallium on an annealed gallium-rich GaAs(100) surface. Further, Shin et al.18 have reported that S, when present in high concentrations, bonds with atoms present in excess on the GaAs surface, independent of how the surface composition was obtained or whether the source of the sulfur is from vapor (H2S) or wet treatment (NH4)2S. This scenario is not envisaged in the present study involving monolayer adsorption and hence low surface concentrations of sulfur-containing species. The TPD data, supported by the XPS and TOF-SIMS results, can be used to elucidate the thermal chemistry of propanethiol on GaAs(100). The mechanism of the decomposition of propanethiol on GaAs(100) is summarized in Figure 10. (48) Yeh, J. J.; Lindau, I. Atom. Data Nucl. Data Tables 1985, 32, 1.

This behavior is consistent with other studies of thiol molecules on both transition metal surfaces and GaAs(100). For example, ethanethiol adsorbed onto GaAs(100) at room temperature produces ethanethiolate and hydrogen species.2 Similarly, studies of methanethiol and ethanethiol on Si(100)49 and GaAs(110)1 and butanethiol on Au(100)35 and Cu(110)34 at low temperatures reported the formation of the respective thiolate and hydrogen species. These studies imply that S-H cleavage in these molecules is more favorable than C-S bond scission and hence would occur first. The propanethiolate species can either recombine with hydrogen to evolve propanethiol (eq 2) or decompose to form propyl and sulfur species (eq 3).

CH3CH2CH2S (ads) + H (ads) f CH3CH2CH2SH (g) (2) CH3CH2CH2S (ads) f CH3CH2CH2 (ads) + S (ads) (3) The sulfur produced from reaction 3 remains on the surface to high temperatures as determined from XPS. The propyl species react further via β-hydride elimination (eq 4) and hydrogen reduction (eq 5) to form a product mixture of propene and propane.

CH3CH2CH2 (ads) f C3H6 (g) + H (ads)

(4)

CH3CH2CH2 (ads) + H (ads) f CH3CH2CH3 (g) (5) The β-hydride elimination reaction, being a facile process, represents the major pathway for the elimination of propyl groups from the surface, while the formation of propane species corresponds to the minor pathway. The hydrogen formed during the β-hydride elimination reaction and during S-H scission when propanethiol adsorption occurred, as shown in eq 1, can also undergo selfhydrogenation to desorb as molecular hydrogen (eq 6).

2H (ads) f H2 (g)

(6)

Production of a mixture of alkene, alkane, and hydrogen via β-hydride elimination in conjunction with reductive elimination has been previously observed for adsorbed alkyl species. For example, ethanethiol adsorption on GaAs(100) at room temperature2 and on Si(100) at low temperatures49 showed the formation of ethane, ethane, and hydrogen from ethyl groups formed by the decomposition of ethanethiolate. Similarly, a study of 1- and 2-iodopropanes on Ni(100) surfaces50 showed that at high coverages, significant amounts of propane and propene desorb from the surface through the β-hydride elimination of the propyl group and the recombination of propyl groups with surface hydrogen. At low exposures, no propene or propane desorption occurred; instead the total dehydrogenation of the propyl species occurred to form surface carbon and hydrogen. The latter behavior was also (49) Lai, Y. H.; Yeh, C. T.; Yeh, C. C.; Hung, W. H. J. Phys. Chem. B 2003, 107, 9351. (50) Tjandra, S.; Zaera, F. Langmuir 1994, 10, 2640.

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observed for butanethiol on Cu(110),34 where although β-hydride elimination to form C4H8 from the butyl groups was the dominant pathway, some dehydrogenation of the alkyl moiety was also observed resulting in the deposition of carbon on the surface. On Au(100), the decomposition of the chemisorbed butanethiolate species results primarily in the formation of 1-butene at ∼500 K.35 The formation of higher hydrocarbons from the surface reactions of adsorbed propyl species and/or thiolate species or the desorption of new sulfide-containing species such as propyl sulfide (C6H14S), dipropyl disulfide (C6H14S2), and hydrogen sulfide was not observed in the present study. This result differs from a study of ethanethiol on GaAs(100),2 which reported the desorption of hydrogen sulfide and diethyl sulfide from a sulfided surface. The absence of H2S in the current study suggests the surface was not sufficiently sulfided for the S atoms to act as a catalyst for the production of new sulfur-containing species. However, a high-temperature propanethiol peak is observed, which is believed to be formed due to the presence of some sulfur on the surface. In previous investigations, involving ethanethiol, it was postulated that the high-temperature ethanethiol and H2S desorption was a consequence of reactions of the adsorbed ethanethiolate with surface sulfur and the surface intermediate SH. Since no H2S was a detected in this study, a different mechanism for the formation of the high-temperature propanethiol must apply here. It is postulated that the S deposited becomes hydrogenated from background hydrogen to form the SH species, which reacts with the propyl groups to form propanethiol at temperatures greater than 600 K via a second-order process (eq 7).

CH3CH2CH2 (ads) + SH (ads) f CH3CH2CH2SH (g) (7) While no experimental evidence for the formation of the intermediate SH has been presented, eq 7 is the most plausible reaction to account for the high-temperature propanethiol peak, especially as recent high-resolution TPD work in the same UHV chamber showed that surface CH2 species on GaAs(100) were readily converted to CH3

Donev et al.

using background gas-phase hydrogen radicals.51 Two other sources of surface hydrogen for the hydrogenation of elemental sulfur are the β-hydride elimination process and the S-H bond scission in the propanethiol when it first adsorbs onto the surface, discussed above. Hence, for temperatures greater than 600 K, there are three competing reactions involving the surface hydrogen species: the reduction of propyl groups, the self-reduction of hydrogens, and the reduction of the surface sulfur, the latter consequently effecting the desorption of the hightemperature propanethiol state. 5. Conclusions The adsorption and desorption reactions of propanethiol on GaAs(100) have been investigated using a combination of TPD, XPS, and TOF-SIMS. The results indicate that at room temperature 1-propanethiol adsorbs dissociatively on clean GaAs(100) to form propanethiolate molecules and hydrogen species. Two pathways are then available for the propanethiolate; it can recombine with hydrogen to form molecular propanethiol, which subsequently desorbs, or it can undergo further decomposition to form surface propyl species and chemisorbed sulfur. The propyl species further react via β-hydride elimination resulting in the formation of propene species with the additional desorption of molecular hydrogen. A less favorable pathway for the propyl species is dehydrogenation to form propane, which subsequently desorbs. XPS results show that elemental sulfur remains on the surface following high-temperature annealing. This study demonstrates that propanethiol may be deposited onto GaAs via the vapor phase and it then can be decomposed to leave atomic sulfur on the surface. The surface concentration of sulfur is quite small and probably insufficient for propanethiol to be used for passivating GaAs surfaces for electronic device applications. Acknowledgment. We thank the Australian Research Council and the State Government of Victoria, Science Technology and the Innovation Initiative for the provision of XPS and TOF-SIMS infrastructure. LA048191X (51) Kemp, N. T.; Singh, N. K. In preparation.

Surface Reactions of 1-Propanethiol on GaAs(100) - Langmuir (ACS

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