Characterization of the Surface to Thiol Bonding in Self-Assembled

monolayer (SAM) to that of sputtered InP provides a thickness for the SAM of 14 ... Dmitri Y. Petrovykh , Jennifer C. Smith , Thomas D. Clark , Ro...
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Langmuir 1999, 15, 8640-8644

Characterization of the Surface to Thiol Bonding in Self-Assembled Monolayer Films of C12H25SH on InP(100) by Angle-Resolved X-ray Photoelectron Spectroscopy Hiromichi Yamamoto, R. A. Butera, Y. Gu, and David H. Waldeck* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received April 20, 1999. In Final Form: July 29, 1999 Angle-resolved X-ray photoelectron spectroscopy (ARXPS) was used to characterize the bonding of alkanethiols to n-InP surfaces and to measure the monolayer thickness. A careful comparison of the angle-dependent spectra for three different sample preparations (oxidized InP(100), HF-etched InP(100), and HF-etched InP(100) with a self-assembled monolayer of C12H25SH) shows that the sulfur binds to In atoms on the surface. Comparison of the angle-dependence of the intensity ratios for the In 3d5/2 core level of InP with the self-assembled monolayer (SAM) to that of sputtered InP provides a thickness for the SAM of 14 ( 4 Å, corresponding to alkane chains at a tilt angle of 44 ( 16° from the surface normal.

Introduction Understanding and manipulating the electronic properties of semiconductor surfaces is of great importance. The surface passivation of semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP), is of great significance to the performance characteristics of optoelectronic and photovoltaic devices, and other devices. The control of surface electronic characteristics and reactivity is also important for fundamental studies of electron transfer at the solid-liquid interface and photocatalysis.1-7 For InP, a few studies have reported on the chemical modification of the surface with thiol compounds, such as hydrogen sulfide H2S,8,9 ammonium sulfide (NH4)2S,10-12 and alkanethiols CnH2n+1SH.13,14 An alternative approach to the modification of oxidized InP surfaces (via an OH functionality) was presented by Sturzenegger and Lewis.15 Although a wide range of research and characterization has been performed for alkanethiol self-assembled mono* To whom correspondence should be addressed. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (2) Ulman, A. Introduction to Thin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (3) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948. (4) (a) Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (b) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Martensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 263, 948. (5) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (6) Sandroff, C. J.; Nottenberg, R. N.; Bischoff, J. C.; Bhat, R. Appl. Phys. Lett. 1997, 51, 33. (7) Yablonovitch, E.; Cox, H. M.; Gmitter, T. J. Appl. Phys. Lett. 1988, 52, 1002. (8) Dudzik, E.; Whittle, R.; Mu¨ller, C.; McGovern, I. T.; Nowak, C.; Ma¨rkl, A.; Hempelmann, A.; Zahn, D. R. T.; Cafolla, A.; Braun, W. Surf. Sci. 1994, 223, 307-309. (9) Shimomura, M.; Sanada, S.; Ichikawa, S.; Fukuda, Y.; Nagishi, M.; Mfller, P. J. J. Appl. Phys. 1998, 83, 3071. (10) Tao, Y.; Yelon, A.; Sacher, E.; Lu, Z. H.; Graham, M. J. Appl. Phys. Lett. 1992, 60, 2669. (11) Han, I. K.; Kim, E. K.; Lee, J. I.; Kim, S. H.; Kang, K. N.; Kim, Y.; Lim, H.; Park, H. L. J. Appl. Phys. 1997, 81, 6986. (12) Chasse´, T.; Peisert, H.; Streubel, P.; Szargan, R. Surf. Sci. 1995, 434, 331-333. (13) Gu, Y.; Lin, Z.; Butera, R. A.; Smentkowski, V. S.; Waldeck, D. H. Langmuir 1995, 11, 1849. (14) Gu, Y.; Kumar, K.; Lin, Z.; Read, I.; Zimmt, M. B.; Waldeck, D. H. J. Photochem. Photobiol. A: Chem. 1997, 105, 189. (15) Sturzenegger, M.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 3045.

layer (SAM) films on Au,16 the formation of a SAM of alkanethiols on InP has not yet been fully characterized.13,14,17 This study further characterizes the formation of alkanethiol monolayers on InP. By using quantitative angle-resolved X-ray photoelectron spectroscopy (ARXPS), three primary insights are obtained. First, the earlier demonstration that surface oxidation of InP(100) is minimized when a monolayer of C12H25SH is deposited is further elucidated. Second, this study demonstrates that the sulfur of the alkanethiol bonds to In atom(s) on the surface, but not to P atom(s). Third, it is possible to draw conclusions about the surface coverage, the film thickness, and hence the tilt angle of the SAM on InP(100), by using the angle-dependent intensity of the In 3d5/2 core level of the InP(100) with the SAM. Experimental Section A wafer of n-InP(100) (Crystacomm, with a dopant density of 1.3 × 1016 cm-3) was used in these studies. Reagent grade C12H25SH was purchased from Aldrich and used without further purification. The InP substrates (18 × 10 mm2) were degreased with trichloroethylene, acetone, and methanol using an ultrasonic washer. After this step the InP is degreased but still oxidized. To remove the oxide the sample was then immersed in a 1:1 mixture of HF and deionized water (an electrical resistance, 18 MΩ) for 6 min. A series of ARXP spectra with time of immersion in the HF solution showed that no change was observed in the oxide intensity for etching times of more than 6 min. A SAM of C12H25SH on InP was prepared by the previously reported procedure.17 After etching the InP was washed with deionized water, and then distilled dry ethanol. The HF-etched InP was transferred into a reaction vessel that contained 10 mL of distilled dry ethanol and 2 mL of C12H25SH. The InP was reacted for 18 h at 52-55 °C under N2 atmosphere. Subsequently, the sample was washed with distilled dry ethanol. The formation of the SAM on the InP was confirmed by a contact angle measurement (112°) of a duplicate specimen prepared in the same batch and an S 2p XPS peak, taken at the end of ARXPS measurement. The ARXPS studies used three different InP samples: a sample that was degreased only (no. 1), a sample that was degreased and HF-etched (no. 2), and a sample that was degreased, etched, and had an SAM deposited on it (no. 3). The XPS apparatus was a Physical Electronics model 550. The energy of photoelectrons was analyzed by a cylindrical, double-pass analyzer. The front (16) A. Ulman, Chem. Rev. 1996, 96, 1533. (17) Gu, Y.; Waldeck, D. H. J. Phys. Chem. 1998, 102, 9015.

10.1021/la990467r CCC: $18.00 © 1999 American Chemical Society Published on Web 09/24/1999

Bonding in Self-Assembled Monolayer Films

Figure 1. Normalized ARXP spectra of In 3d5/2 core level of (A) degreased InP, (B) HF-etched InP, and (C) InP with the SAM of C12H25SH at five angles 0°, 30°, 50°, 60°, and 70° to the surface normal. The solid line represents fitting spectra decomposed by the peak of In 3d5/2 core level of the sputtered InP. of the CMA was apertured in order to restrict the range of acceptance angles to (6°. The energy resolution for the apparatus was determined to be about 1 eV by calibration with the Fermi edge of sputter-cleaned rhodium. The work function of the apparatus was determined to be 4.20 eV from the Fermi edge of rhodium. The pass energy was 25 eV. The X-ray source was Mg KR radiation at 1253.6 eV. The manipulator for angle-resolved measurements was built in house. The manipulator was calibrated using a He-Ne laser and its angular precision had a standard deviation of 1.7°. When the InP samples were transferred to the XPS apparatus, they were exposed to the air for about 5 min. To quantitatively analyze the data, a clean surface of InP(100) was prepared by Ar+-sputtering an HF-etched sample at 3 kV (ion current, 0.12 µA). The sputtering was performed until no further changes in the spectral position or shape of the In 3d5/2 and P 2p peaks were found. The ARXP spectra of the In and P core levels that were obtained from the sputter-cleaned InP at normal emission were used to decompose all of the other spectra. The areas of ARXP spectral peaks on the sputtered InP were also used as intensities for the clean surface to determine a thickness of the SAM, as discussed later. The base pressure in the apparatus was 1-3 × 10-9 Torr during measurements on the oxidized, HF-etched and SAM-coated InP. It was about 6 × 10-10 Torr for the studies on the sputter-cleaned InP.

Nature of the Surface to Molecule Bonding Figure 1 shows normalized ARXP spectra of In 3d5/2 core level for the three different samples: InP degreased (no. 1), HF-etched InP (no. 2), and InP/C12H25SH (no. 3). In these spectra the angle between the sample normal and the detector is varied; i.e., the “tilt” angle θ. The figure shows spectra for each of the three samples at five different tilt angles. Because the escape depth of a photoelectron is constant at a particular energy, the depth from which the photoelectrons can be detected should decrease as the cos(θ).18 Hence as the angle increases the spectra become more surface selective. The first column in Figure 1 shows the angle-dependent spectra for no. 1. The spectra reveal a clear broadening and a shift as the tilt angle increases. Using a line shape obtained from the measurement of the sputter-cleaned InP, it is possible to fit the spectra to a sum of two In core level peaks that are centered at 443.8 and 444.8 eV, (18) Hofmann, S. Depth Profiling in AES and XPS. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, New York, Brisbane, Toronto, Singapore, 1996.

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respectively. These data have been normalized to the same height so that the decomposition of the spectrum into two components is more evident. As the tilt angle in the experiment increases it is not necessary to shift the peak centers, but rather one must change the relative intensity of the peak at 444.8 eV to the peak at 443.8 eV. This observation is consistent with the 444.8 eV peak corresponding to a surface In species and the 443.8 eV peak corresponding to a bulk species. It is well-known that the surface of InP oxidizes in the air, and the presence of oxygen was verified by XPS and Auger.13 Hence the peak at 444.8 eV is assigned to indium oxide on the surface of InP. This assignment is in agreement with the assignment reported previously for the In core level shift between InP and indium oxide.11 Columns two and three of Figure 1 show spectra for samples no. 2 and no. 3, respectively. The ARXP spectra of sample no. 2 (HF-etched InP) show the presence of a peak at 444.8 eV for the higher tilt angles. This peak corresponds to that assigned to indium oxide above. Clearly, the HF-etching greatly reduces the amount of oxide that is observed. These spectra were also studied as a function of the time that the sample was in the HF solution and the oxide could not be reduced below the amount shown here. The residual presence of the oxide could have multiple origins: the etchant might leave an oxide residue on the surface, oxide may grow in the rinse stage, and oxide can grow when the sample was exposed to air during mounting for the XPS studies. The third column shows the ARXP spectra for sample no. 3. In this case the freshly etched InP surface was coated with the dodecanethiol. Any signal in these spectra from an oxide was below the noise level. Figure 2 provides a more detailed comparison of the normalized ARXP spectra of In 3d5/2 peak for sample no. 3 at 0, 30, and 70°. As depicted in Figure 2A, the shape of the ARXP spectrum at 30° is identical to that at 0°. On the other hand, the comparison of the 0 and 70° spectra in Figure 2B reveals some differences. A small shoulder structure is observed between 444 and 445 eV in the ARXP spectrum at 70°. Figure 2C shows the differential spectrum between 0 and 70° on an expanded intensity scale. Although the exact peak position cannot be determined because of the high noise level in this spectrum, the peak appears to be shifted from the 444.8 eV position of the oxide in samples no. 1 and 2. The differential spectrum was fit to the line-shape of the sputter-cleaned InP using different binding energies. The best fit gives a center position of 444.5 ( 0.2 eV. The assignment of this peak at 444.5 eV will be discussed later with the ARXP spectra of the P 2p and S 2p core levels. Figure 3 shows ARXP spectra of P 2p core level for each of the three samples: InP degreased (no. 1), HF-etched InP (no. 2), and InP/C12H25SH (no. 3). Each spectrum is normalized by the peak intensity (area) for the P 2p core level of the sputtered InP at the corresponding tilt angle. The spectra in the first column of Figure 3 correspond to the degreased sample (no. 1). The presence of two peaks, located at 127.5 and 132.4 eV, is clearly evident. The intensity of the peak at 127.5 eV decreases with increasing angle, whereas the intensity of the peak at 132.4 eV does not change very much. Thus, the peak at 132.4 eV is assigned to surface-localized phosphorus atoms. The peak centered at 127.5 eV can be assigned to the emission from the P 2p core levels of the bulk. These peak assignments are in good agreement with previously reported values for the shift in the P 2p core in InP and phosphorus oxide.11 The overall decrease in the total

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Figure 4. This figure shows a comparison of the P 2p core level spectra of InP with the C12H25SH monolayer at 70° with that of the sputter-cleaned InP at normal emission. The crosses (+) represent the spectrum for the alkanethiol-coated InP, and the line represents a fit using the line shape of the P 2p core level spectrum of the sputter-cleaned InP.

Figure 2. Comparison of ARXP spectra of In 3d5/2 core level of InP with the SAM of C12H25SH: (A) the line is 0° and (×) is 30° to the surface normal; (B) the line is 0°, the (+) is 70°, and (b) is the difference between the spectra at 0 and 70°; and (C) (b) is an expanded spectrum of the difference between the spectra at 0 and 70°, and the line is a fit using a line shape from the In 3d5/2 core level of the sputtered InP.

Figure 3. ARXP spectra of the P 2p core level of (A) degreased InP, (B) HF-etched InP, and (C) InP with the SAM of C12H25SH at five angles 0°, 30°, 50°, 60°, and 70° to the surface normal. Each spectrum is normalized by the intensity of P 2p core level of the sputtered InP.

intensity of the phosphorus signal reflects the attenuation of the signal by an oxide overlayer.

The spectra in column two of Figure 3 correspond to the HF-etched InP (no. 2). These spectra also show the clear presence of two peaks, one for phosphorus oxide at 132.4 eV and the second for bulk phosphorus. The relative intensity of the oxide peak, as compared to that for sample no. 1, is reduced which is consistent with removal of oxide by HF-etching. It is also evident that as the tilt angle increases the overall intensity of the sample decreases, but not as significantly as that of sample no. 1. This finding is consistent with the presence of a thinner oxide layer for the HF-etched sample. The ARXP spectra in column three of Figure 3 correspond to the SAM-coated InP (no. 3). These spectra do not display a peak for the phosphorus oxide at 132.4 eV. This conclusion becomes more clear by comparison of this phosphorus spectrum with that from the sputter-cleaned InP. Figure 4 compares the P 2p peak of sample no. 3 at 70° with that of the sputtered InP at 0°. No difference between the spectra is observed although the noise level at 70° is significant. Earlier studies of S-passivated InP (treated with (NH4)2S and H2S9-11) showed no shift in the P 2p core level also. The phosphorus peak’s intensity is not attenuated as much for this sample as it is for the other two. A quantitative comparison of the signal intensities of the spectra is also possible. Because of the normalization used in Figure 1, the decrease in the In intensity with the tilt angle is not evident. The spectra from Figure 1 were also normalized by the peak intensity (area) for the In 3d5/2 core level of the sputter-cleaned InP at the corresponding tilt angle. These data are given in the Supporting Information. This allows the intensity ratio of the P core level to the In core level to be compared at each angle. Such a comparison demonstrates that the oxide is a mixture of phosphorus oxides and indium oxides. Although their mixture is likely to be heterogeneous, no clear evidence for segregation along the surface normal is seen from the data. The phosphorus to indium intensity ratios (scaled by the cross-sections) at 0° are 0.58 for no. 1, 0.60 for no. 2, and 0.82 for no. 3. Figure 5 shows an ARXP spectrum of S 2p core level of InP with the SAM at the normal emission angle. As shown in Figure 5, the peak for the S 2p core level of InP with the SAM is observed around 161 eV, which is comparable to the binding energy that has been reported by others for

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Figure 5. An ARXP spectrum of the S 2p core level for the alkanethiol-coated InP, at normal emission.

InP passivated with (NH4)2Sx and H2S.9-12 On the other hand, the sulfur core level has been reported to lie in the range of 162-163 eV for thioalkanes,19 in the 163164 eV range when bonded to phosphorus, and at binding energies of 163-168 when bonded to oxygen.20 This trend in the sulfur core correlates with the S atom being more electropositive. These comparisons indicate that the sulfur core level is shifted when the alkylthiol binds to the substrate and suggests that the sulfur is bonding to the indium (In-S bonding between InP and the alkanethiol). These results support the conclusion that the alkanethiol of the SAM-coated InP sample attaches through In-S bonding. The ARXP spectra of the In 3d5/2 and P 2p core levels in Figures 1- 4 indicate that a small amount of oxide remains on the surface of HF-etched InP; however, it could have several origins (vide supra). If oxide is present on the SAM-coated InP sample, it is not evident in the In and P core level spectra. The conclusion that the thiol binds to the In is supported by the following four observations. First, a small shoulder is observed on the In 3d5/2 core level (ca. 444.5 eV) for the SAM-coated sample. This binding energy is different from that found for the In-O bonding in samples no. 1 and 2. Second, no difference is seen in the peak position or shape of the P 2p core level between the SAM-coated InP and the clean InP. Third, the In and P core level oxide peaks are not observed, indicating that the amount of oxide has been reduced from that in the HF-etched sample. Fourth, the binding energy of the peak of S 2p core level is comparable to that reported for InP surfaces that were passivated by (NH4)2S and H2S. Together, these observations provide a convincing case to conclude that the sulfur of the SAM bonds to In on the surface, but not to P.

Figure 6. Attenuation curves for the In 3d5/2 core level of InP with the alkanethiol. I(θ) and Ic(θ) represent the intensities of In 3d5/2 core level peaks for the alkanethiol-coated sample and the sputter-cleaned sample at the angle θ, respectively. The (b) are the total In 3d5/2 emission, the (×) are the In 3d5/2 emission for In-P bonding, and (0) are the In 3d5/2 emission for In-S bonding. The line is a fit to eq 1 and gives a slope of -0.54.

where I(θ) is the photoelectron intensity from the substrate covered by the thin film, Ic(θ) is the photoelectron intensity from the clean substrate, θ is the angle of the detector with respect to the surface normal, d is the thickness of the film, and λ is the escape depth of photoelectrons from the substrate. Equation 1 predicts that a plot of ln[I(θ)/ Ic(θ)] versus 1/cos θ should be linear with a slope of -(d/ λ). This particular form of eq 1 is useful, because instrumental factors of the apparatus cancel out. As a result, it is possible to discuss the intensity change with respect to the angle, as shown in Figure 3, in a quantitative manner. Figure 6 shows these attenuation curves for the In 3d5/2 core level of the SAM-coated InP sample. From Figure 6, the slope of the line is found to be -0.54 ( 0.13. The relationship between the kinetic energy of photoelectrons and the attenuation length λ in n-alkanethiols was determined experimentally.21 It follows the equation

λ ) 9.0 + 0.022‚Ek

(2)

(1)

over a kinetic energy range Ek of 500-1500 eV, where λ is in Å and Ek is in eV. Becaue the work function of the apparatus is 4.20 eV, the photon energy of the Mg KR line is 1253.6 eV, and the binding energy of the In 3d5/2 core level is 444 eV, one obtains an electron kinetic energy of 805.4 eV. This value corresponds to an escape depth λ of 27 ( 2 Å. Using this value with the slope of -0.54 gives a thickness for the SAM on InP(100) of 14 ( 4 Å. This thickness can be combined with molecular considerations to obtain the tilt angle of the alkane chain of the alkanethiol with respect to the surface normal. Taking the length of an alkanethiol chain to be 19.5 Å, the tilt angle is determined to be 44 ( 16°. The tilt angle depends on the details of the substrate and the packing; however, this value is in reasonable agreement with the range of values reported on other systems. For example, the tilt angle of alkanethiols was reported to be ∼30° on Au(111) and ∼5° on Au(100).16,22 The tilt angle on Ag(111) is reported to be ∼30° 23 and that on GaAs (100) was reported

(19) (a) Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlmann, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 277. (b) Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.-E.; Lindgen, I.; Lindberg, B. Nova Acta Regiae Soc. Sci. Ups. 1965, 20 IV, 5.

(20) Morgan, W. E.; Stec, W. J.; Albridge, R. G.; van Wazer, J. R. Inorg. Chem. 1971, 10, 926. (21) Laibinis, P. E.; Bain C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (22) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Kondo, T.; Yanagida, M.; Shimazu, K.; Uosaki, K. Langmuir 1998, 14, 5656.

The Monolayer Thickness and Coverage It is possible to quantify the thickness of the SAM using the ARXPS data. The XPS intensity from a smooth substrate that is homogeneously covered by a thin film is given by the following equation:18

( )

ln

I(θ) -d ) λ‚cos θ Ic(θ)

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to be 57°.24 The similarity of the tilt angle for the SAM on InP(100) with that found for GaAs(100) is suggestive but more studies are required before conclusions about tilt angle and the surface properties of the substrate are possible. The coverage of the SAM can roughly be estimated by comparing the intensity of the S core level spectrum to that of either the In core level or the P core level. The measured intensity ratio of In to S, IIn/IS, at 0° is 81 ( 12; i.e., the area of the In peak is 81 times larger than that of the S peak. This value will be compared with that predicted for an idealized model of the surface, in particular, the case in which the SAM forms a smooth overlayer film on a perfect InP(100) surface. Using this model, it is possible to compute the expected ratio of XPS intensities in terms of the number of atoms (In and S), their cross-sections, and their escape depths. For bulk InP, the intensity of the In core will be given by

∫0∞exp(-x/λInInP)dx]exp(-dSAM/λInSAM) )

IIn ) BσIn[FIn InPA

In In BσIn[FIn InPAλInP]exp(-dSAM/λSAM) (3)

where σIn is the cross-section for generation of photoelectrons from In (taken to be 13.23), λInSAM is the characteristic attenuation length of In photoelectrons through the SAM (taken to be 27 ( 2 Å), B contains instrumental factors (such as X-ray flux and detector efficiency), and the term in square brackets represents the effective number of In atoms in the solid. A is the area imaged by the detector, λInInP is the characteristic attenuation length of In atoms in the bulk InP (taken from the universal curve to be 30 ( 10 Å), and FInInP is the number density of In atoms in the bulk. Taking the lattice constant of InP to be 5.869 Å 25 and 4 In atoms in a unit cell, their number density will be 0.0198 Å-3 (or 1.98 × 1022 cm-3). The intensity of the S core level will be given by

IS ) BσS[FSA]exp(-dSAM/λSSAM)

(4)

where σS is the cross-section for generation of photoelectrons from S (taken to be 1.74), λSSAM is the characteristic attenuation length of S photoelectrons through the SAM (taken to be 33 ( 2 Å), and the term in square brackets represents the effective number of S atoms on the surface of the InP solid. The quantity FS is the number of S atoms per unit area of the surface. If one assumes that an S atom is bonded to each available In atom (2 per unit cell of InP(100)), then the surface density of S atoms is 0.0581 Å-2 (or 5.81 × 1014 cm-2). The ratio of these two equations gives In In In IIn σIn [FInPλInP] exp(-dSAM/λSAM) ) ‚ ‚ IS σS FS exp(-dSAM/λSSAM)

(5)

Using the values reported above, eq 5 predicts an intensity (23) (a) Ulman, A. J. Mater. Educ. 1989, 11, 205. (b) Laibinis, P. E.; Whitesides, G. M.; Allara. D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (c) Nemetz, A.; Fischer, T.; Ulman, A.; Knoll, W. J. Chem. Phys. 1993, 98, 5912. (24) Sheen, C. W.; Shi, J. X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514.

ratio of 71.26 Given the large uncertainty in the value of λInInP and the highly idealized nature of this model, the agreement with the experimental value of 81 ( 12 is acceptable. A similar analysis for the phosphorus to sulfur ratio predicts a value of 6.0 that should be compared to an experimental value of 6.3 ( 1. Although suggestive of nearly full coverage of the surface by the alkanethiol, the error intrinsic in this analysis precludes such a conclusion. Conclusions ARXP spectra were used to determine that alkanethiols bind to the In atoms on InP(100). Comparison of the ARXP spectra for In 3d5/2 core level of oxidized InP and HFetched InP reveals that the peak for the In-O bonding is at 444.8 eV, while that for the In-P bonding is at 443.8 eV. On the other hand, ARXP spectra of P 2p core level indicate that the peak for the P-O bonding is at 132.4 eV, while that for the In-P bonding is at 127.5 eV. For the InP coated with an alkanethiol monolayer film, the peaks for the In-O and P-O bonding (which are present for the uncoated sample) could not be observed. A small shoulder in the ARXP spectrum of the In 3d5/2 core level was observed at about 444.5 eV and was tentatively assigned to a shift caused by In-S bonding. No difference in the peaks of P 2p core level were found for the alkanethiolcoated InP and the sputter-cleaned InP. The binding energy of the S 2p core level was comparable to that reported previously on sulfur-passivated InP. The shift of the S 2p core level is consistent with bonding to In but not consistent for bonding to P. The combination of these data lead to the conclusion that the sulfur in the SAM bonds to In on the surface and not to P. A quantitative analysis of the relative intensities of the S, In, and P core level spectra was used to characterize the surface coverage and the alkanethiol film’s thickness. The intensity ratio of S to In core level spectra demonstrates significant coverage of the SAM on InP. The angle dependence of the In core level intensities for sputtercleaned InP surfaces and the SAM-coated surface follow the predictions of eq 1 and give a film thickness of 14 ( 4 Å for the alkanethiol monolayer. This thickness corresponds to a tilt angle of 44 ( 16° for the alkane chains from the surface normal. Acknowledgment. This work was supported by the Department of Energy, Division of Chemical Sciences (DE-FG02-89ER14062). Supporting Information Available: Spectra from Figure 1 normalized by the peak intensity (area) for the In 3d5/2 core level of the sputter-cleaned InP at the corresponding tilt angle. This material is available free of charge via the Internet at http://pubs.acs.org. LA990467R (25) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC: Boca Raton, Ann Arbor, London, Tokyo, 1994. (26) This analysis ignores any instrumental factors that depend on the initial photoelectron’s kinetic energy. The transmission of the PHI model 550 instrument displays a weak energy dependence over this range (see ref 18, section 5.3). Its inclusion does not affect the relative S to P intensities because of their similar binding energies; however it would affect the predicted intensity ratio of In to S and In to P by 10-40%. For example, inclusion of a strict 1/KE (where KE is the kinetic energy) dependence for the transmission in eq 5 predicts an intensity ratio of 96sstill in reasonable agreement with the experimental value.