Functionalization of GaAs Surfaces with Aromatic Self-Assembled

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4992

Langmuir 2003, 19, 4992-4998

Functionalization of GaAs Surfaces with Aromatic Self-Assembled Monolayers: A Synchrotron-Based Spectroscopic Study A. Shaporenko,† K. Adlkofer,‡ L. S. O. Johansson,§ M. Tanaka,*,‡ and M. Zharnikov*,† Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany, Lehrstuhl fu¨ r Biophysik E22, Technische Universita¨ t Mu¨ nchen, James-Franck-Strasse, D-85748, Garching b. Mu¨ nchen, Germany, and Department of Physics, Karlstad University, Universitetsgatan 1, S-65188 Karlstad, Sweden Received November 12, 2002. In Final Form: March 12, 2003 Stoichiometric GaAs(100) surfaces were functionalized with monolayers of an aromatic compound, 1,1′biphenyl-4-thiol (BPT), and the engineered surfaces were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy. BPT molecules were found to form a well-ordered and densely packed self-assembled monolayer on these substrates. The attachment to the substrate occurs over the thiolate headgroup while the intact biphenyl moieties have an upright orientation with an average tilt angle of 31.5° ( 5°. The functionalization of GaAs by BPT was found to prevent an oxidation and contamination of the substrate, keeping the GaAs surface in a pristine state. The surface engineering of GaAs with functionalized aromatic monolayers can provide a crucial link for combining GaAs-based semiconductor nanostructures with bio-organic molecular assemblies.

1. Introduction The design of functional interfaces and links between semiconductor materials and biological objects is of great scientific and practical interest.1 One of the most promising semiconductors in view of flexible band-gap engineering is GaAs, which exhibits high electron mobility in nanostructures and low-dimensional arrangements.2,3 Regrettably, the above-mentioned applications are difficult to realize due to chemical instability of GaAs surface, especially in physiological electrolytes, and lack of a strategy for reliable surface passivation with functional interlayers. However, these problems could be partly avoided through the functionalization of GaAs surfaces with self-assembled monolayers (SAMs), which, by present, were the SAMs of alkylthiolates (AT).4-11 In particular, Dorsten et al. reported that the AT passivated GaAs(100) * To whom correspondence may be addressed: M. Zharnikov ([email protected]) and M. Tanaka ([email protected]). † Universita ¨ t Heidelberg. ‡ Technische Universita ¨ t Mu¨nchen. § Karlstad University. (1) Sackmann, E.; Tanaka, M. Trends Biotechnol. 2000, 18, 58. (2) Baumgartner, P.; Engel, C.; Abstreiter, G.; Bo¨hm, G.; Weimann, G. Appl. Phys. Lett. 1994, 64, 592. (3) Baumgartner, P.; Engel, C.; Abstreiter, G.; Bo¨hm, G.; Weimann, G. Appl. Phys. Lett. 1995, 66, 751. (4) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (5) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663. (6) Dorsten, J. F.; Maslar, J. E.; Bohn, P. W. Appl. Phys. Lett. 1995, 66, 1755. (7) Onho, H.; Motomatsu, M.; Mizutani, W.; Tokumoto, H. Jpn. J. Appl. Phys. 1995, 34, 1381. (8) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol., A 1996, 14, 1844. (9) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. (10) Adlkofer, K.; Tanaka, M.; Hillebrandt, H.; Wiegand, G.; Sackmann, E.; Bolom, T.; Deutschmann, R.; Abstreiter, G. Appl. Phys. Lett. 2000, 76, 3313. (11) Adlkofer, K.; Tanaka, M. Langmuir 2001, 17, 4267.

surfaces were more stable to environmental degradation, over time and under temperature stress, than the surfaces treated with inorganic sulfide.6 Later, Adlkofer et al. communicated that the GaAs/electrolyte interface was stabilized in a physiological buffer for more than 24 h after the AT functionalization.10,11 Further, this functionalization approach could be successfully applied to GaAs cap layers covering near-surface InAs quantum dots, which resulted in a significant enhancement in photoluminescence yield from these dots.12,13 As a next step, it would be useful to extend the range of thiol-derived compounds used for the GaAs surface functionalization and try molecules with a rigid aromatic backbone. Among other advantages, a functionalization with this type of the molecules can be combined with subsequent nanopatterning of SAMs within a conventional lithographic approach or SAM-based “chemical lithography” framework, which includes a tailored chemical modification of a functional group attached to the aromatic spacer of the SAM constituent.14-16 Recently, first results on functionalization of GaAs(100) surfaces with a nonsubstituted aromatic thiol, 1,1′biphenyl-4-thiol (BPT), have been presented.17 Characterization of the engineered surface with ellipsometry, contact angle measurements, and atomic force microscopy implied that BPT forms a stable monolayer on GaAs.17 The presence of such a monolayer significantly suppressed charge transfer at the GaAs/electrolyte interface at neutral (12) Duijs, E. F.; Findeis, F.; Deutschmann, R. A.; Bichler, M.; Zrenner, A.; Abstreiter, G.; Adlkofer, A.; Tanaka, M.; Sackmann, E. Phys. Status Solidi B 2001, 224, 871. (13) Adlkofer, K.; Duijs, E. F.; Findeis, F.; Bichler, M.; Zrenner, A.; Sackmann, E.; Abstreiter, G.; Tanaka, M. Phys. Chem. Chem. Phys. 2002, 4, 785. (14) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (15) Go¨lzha¨user, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806. (16) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793. (17) Adlkofer, K.; Eck, W.; Grunze, M.; Tanaka, M. J. Phys. Chem. B 2003, 107, 587.

10.1021/la020909q CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003

Functionalization of GaAs Surfaces

pH conditions and stabilized this interface, as shown by cyclic voltammetry and electrochemical impedance spectroscopy.17 In this paper, we perform a spectroscopic analysis of SAMs formed from BPT on stoichiometric GaAs(100) surfaces. We have applied synchrotron-based high-resolution X-ray photoelectron spectroscopy (HRXPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy to characterize these systems. Both techniques are highly sensitive and chemically specific. They provide complementary information on chemical identity of the BPT film and the BPT/GaAs interface as well as give an insight into the BPT SAM structure. In the next section, we describe the experimental procedure and techniques, which is followed by presentation (section 3) and discussion (section 4) of the experimental data. Finally, the results are summarized in section 5. 2. Experimental Section Single crystalline n-type GaAs(100) wafers doped with (2.23.4) × 1018 cm-3 Si were purchased from American Xtal Technology Inc. (Fremond, CA). An undoped GaAs (d ) 1000 Å) layer was grown on the surface of these wafers by molecular beam epitaxy using suitable equipment at the Walter Schottky Institute of the Technical University Munich. Synthesis of the biphenylthiol was reported elsewhere.18 All other chemicals were purchased from Aldrich (Steinheim, Germany) and used without further purification. Prior to the SAM fabrication, the samples were sonicated in acetone for ≈3 min and rinsed intensively with ethanol. After this pretreatment, native oxide of GaAs was removed by etching the sample in concentrated HCl for 1 min (a residual acid was rinsed with water), resulting in a stoichiometric GaAs(100) surface. To prepare BPT SAMs, freshly etched substrates were immersed into 0.1 mM BPT solution in dry ethanol at 50 °C for 20 h.17 Both pretreatment and immersion were carried out under nitrogen (N2) atmosphere to avoid surface oxidation. After the SAM formation, the samples were taken out from the reactor, rinsed with ethanol, and dried by a N2 flow. The HRXPS measurements were performed at the synchrotron storage ring MAX II at MAX-Lab in Lund, Sweden, using the D1011 and I311 beamlines. Both beamlines are equipped with a Zeiss SX-700 plane-grating monochromator and a two-chamber ultrahigh vacuum experimental station with a SCIENTA analyzer. Excitation energies in the range of 130-650 eV were used. The Ga 3d, As 3d, C 1s, O 1s, and S 2p narrow scan spectra were acquired for both BPT-coated GaAs and two reference samples of as-prepared (after the pretreatment) and freshly etched GaAs(100). The choice of photon energy for every individual spectrum was based on the optimization of the photoionization cross section for the corresponding core level19-21 as well as on adjustment of either surface or bulk sensitivity. The energy resolution was better than 100 meV, which allowed us to distinguish between different chemical species and to decompose the spectra into the respective components. In addition, due to the high-energy resolution, emissions associated with individual chemical species reflect the intrinsic energy spread of the respective core-level photoemission process. Energy calibration was performed individually for every spectrum to avoid effects related to the instability of the monochromators. The energy scale was referenced to the pronounced Au 4f7/2 “bulk” peak (83.93 eV) of a reference C12/Au sample,22,23 which was (18) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (19) Band, I. M.; Kharitonov, Yu. I.; Trzhaskovskaya, M. B. At. Data Nucl. Data Tables 1979, 23, 443. (20) Goldberg, S. M.; Fadley, C. S.; Kono, S. J. Electron Spectrosc. Relat. Phenom. 1981, 21, 285. (21) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1. (22) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (23) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8.

Langmuir, Vol. 19, No. 12, 2003 4993 attached to the same sample holder as the probed one. The value of 83.93 eV was derived from an independent calibration to the Fermi edge of a clean Pt foil. The spectra were fitted using Voigt peak profiles and a Shirley background. To fit the doublet emissions (Ga 3d, As 3d, and S 2p), we used two peaks with the same full width at half-maximum (fwhm), reasonable spin-orbit splitting, and branching ratios of 2:1 (2p3/2/2p1/2) and 3:2 (3d5/2/ 3d3/2). The resulting accuracy of the binding energies (BEs) and fwhm’s reported here is 0.04-0.05 eV. These values are noticeably higher than the ultimate accuracy of the experimental setup (see, e.g., ref 24); they mostly reflect the distribution of the resulting fit parameters over the spectra of different samples in the same spectral region. The NEXAFS spectroscopy measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The spectra acquisition was carried out at the C 1s absorption edge in the partial electron yield mode with a retarding voltage of -150 V. Linear polarized synchrotron light with a polarization factor of ≈92% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90° (E-vector in surface plane) to 20° (E-vector nearsurface normal) in steps of 10°-20° to monitor the orientational order of the BPT molecules within the films. This approach is based on so-called linear dichroism in X-ray absorption, i.e., the strong dependence of the cross section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.25 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. Before the normalization, a spectrum of freshly etched GaAs was subtracted from the raw spectrum of BPT-coated GaAs.26,27 The energy scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite at 285.35 eV.28 Both the HRXPS and NEXAFS experiments were performed at room temperature and a base pressure lower than 1.5 × 10-9 Torr. The time for either the NEXAFS or XPS characterization was selected as a compromise between the spectra quality and the damage induced by X-rays.16,23,29,30 The samples of freshly etched GaAs were exposed to ambience for 2-3 min, which were spent to take the samples out of the HCl solution, rinse them with water, dry them, attach them to the sample holder, put it in a load-lock chamber, and start pumping.

3. Results 3.1. HRXPS Characterization. The Ga 3d and As 3d HRXPS spectra of as-prepared (after the pretreatment), freshly etched, and BPT-coated GaAs samples are presented in Figures 1, 2, and 3, respectively. The corresponding C 1s, O 1s, and S 2p spectra are displayed in Figure 4. The Ga 3d and As 3d spectra were acquired at photon energies of 130 eV (upper panels) and 600 eV (lower panels). Considering that the electron mean free path reaches its minimum value at kinetic energies of 50-100 eV,32,33 the spectra in the upper panels of Figures 1-3 are mainly representative for the sample surface, whereas the spectra in the bottom panels are dominated by (24) Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888. (25) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992. (26) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M., Tamada, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Isr. J. Chem. 2000, 40, 81. (27) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697. (28) Batson, P. E. Phys. Rev. B 1993, 48, 2608. (29) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 245. (30) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch. Z. Phys. Chem. 1997, 202, 263. (31) Chang, S.; Vitomirov, I. M.; Brillson, L. J.; Rioux, D. F.; Kirchner, P. D.; Pettit, G. D.; Woodall, J. M. J. Vac. Sci. Technol., B 1991, 9, 2129. (32) Lindau, I.; Spicer, W. E. J. Electron Spectrosc. 1974, 3, 409. (33) Powell, C. J. Surf. Sci. 1974, 44, 29.

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Figure 1. Ga 3d and As 3d HRXPS spectra of an as-prepared GaAs (after the pretreatment, see section 2) sample: (Ga 3d spectra) light gray, GaAs; (As 3d spectra) light gray, GaAs; gray, elementary As. The spectra acquired at photon energies of 130 eV (upper panels) and 600 eV (bottom panels) are dominated by contributions from the sample surface and bulk, respectively. A decomposition of distinct spectral features by doublets related to individual chemical species is shown. The broad peaks at the high binding energy side of the shadowed doublets correspond to Ga and As oxides.

Figure 2. Ga 3d and As 3d HRXPS spectra of freshly etched GaAs: (Ga 3d spectra) light gray, GaAs; gray, a Ga oxide or the surface Ga 3d component;31 (As 3d spectra) light gray, GaAs; gray, elementary As. The spectra acquired at photon energies of 130 eV (upper panels) and 600 eV (bottom panels) are dominated by contributions from the sample surface and bulk, respectively. A decomposition of distinct spectral features by doublets related to individual chemical species is shown. The shoulders at the high binding energy side of the shadowed doublets correspond to Ga and As oxides.

contributions from the bulksthe effective sampling depth (≈3λ) is about 18 Å (130 eV) and 33 Å (600 eV),32,33 with

Shaporenko et al.

Figure 3. Ga3d and As3d HRXPS spectra of GaAs functionalized with BPT: (Ga 3d spectra) light gray, GaAs; gray, a Ga oxide or the surface Ga 3d component;31 (As 3d spectra) light gray, GaAs; gray, elementary As; black, S-As. The spectra acquired at photon energies of 130 eV (upper panels) and 600 eV (bottom panels) are dominated by contributions from the sample surface and bulk, respectively. A decomposition of distinct spectral features by doublets related to individual chemical species is shown. The shoulders at the high binding energy side of the shadowed doublets correspond to Ga and As oxides.

Figure 4. C 1s, O 1s, and S 2p HRXPS spectra for as-prepared, freshly etched, and BPT-coated GaAs. Different photon energies were used. A decomposition of some spectra by singlets (C 1s) or doublets (S 2p) related to individual chemical species or excitations is shown. A low binding energy shoulder in the S 2p spectrum of BPT-coated GaAs is related to either atomic sulfur or Ga 3s emission. The presented fit corresponds to the case of atomic sulfur (doublet), but the spectrum can be well fitted with a Ga 3s singlet as well.

a larger weight for the contributions from the topmost layers. Due to the ultimate energy resolution of the HRXPS and the availability of the spectra dominated by a single doublet, we were able to derive directly and/or verify the parameters (fwhm, spin-orbit splitting, and branching ratio) of the Ga 3d and As 3d doublets, which were used as elementary fitting units. This allowed us to decompose the spectra in Figures 1-3 into individual components with the parameters presented in Table 1. The decom-

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Table 1. Parameters of the Individual Emissions in the Ga 3d and As 3d Spectra in Figures 1-3a

Ga 3d5/2 As 3d5/2

binding energy (eV)

assignment

fwhm (eV)

spin-orbit splitting (eV)

branching ratio

19.21 ( 0.04 19.8 ( 0.1 >19.8 41.10 ( 0.03 41.79 ( 0.05 42.37 ( 0.05 >43.0

GaAs Ga2O3 or the surface Ga 3d component Ga oxides GaAs elementary As (As0) As-S As oxides

0.54 ( 0.04

0.43

3/2

0.65 ( 0.05

0.69

3/2

a The parameters were derived from the self-consistent fitting procedure (see text for details). The error bars reflect the scattering of the fitting parameters between the spectra for different samples and photon energies. The assignments were performed in accordance with refs 11, 31, and 34-39.

position was carried out self-consistently over the entire set of the spectra. The assignments of the individual emissions were performed in accordance with refs 11, 31, and 34-39. Within the fitting procedure, only the spectral features with visible individual components (doublets) were decomposed, whereas the broad, structureless maxima, related to Ga and As oxides, were not considered. Though technically possible, any fitting of broad, structureless features by a variety of narrow peaks or doublets is rather arbitrary. The surface-sensitive Ga 3d and As 3d spectra of the as-prepared sample in Figure 1 (upper panels) are dominated by the emissions associated with Ga and As oxides: ≈90% of Ga atoms and ≈75% of As atoms in the surface region are oxidized. The emissions assigned to stoichiometric GaAs (light gray) and elemental arsenide (gray) are hardly perceptible in these spectra. The respective doublets become quite pronounced in the bulksensitive spectra (bottom panels in Figure 1). However, the features related to the oxides are still intense in the bulk-representative spectra in bottom panels of Figure 1, which shows that the entire near-surface region is heavily oxidized. Accordingly, there are pronounced C 1s and O 1s emissions in the corresponding spectra in Figure 4. The extensive oxidation is probably responsible for the noticeable deviation of the Ga:As ratio in the surface region from the stoichiometric 1:1 valuesthe content of Ga is larger by ≈40% than that of As. Both surface-sensitive and bulk-sensitive Ga 3d and As 3d spectra of the freshly etched GaAs sample in Figure 2 are dominated by the emissions related to stoichiometric GaAs (light gray) while the total surface stoichiometry corresponds approximately to theoretical Ga:As ratio of 1:1sthere is only a small excess of As. In addition, there is a distinct As 3d component (41.79 eV) related to elemental arsenide (gray), which has a comparable intensity with respect to the “GaAs” doublet in the surfacesensitive As 3d spectra. Despite the etching, there is still some contamination and oxides on the sample surface as implied by the shoulders at the high binding energies in the Ga 3d and As 3d spectra in Figure 2 as well as by the corresponding C 1s and O 1s spectra in Figure 4. As evident (34) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, J. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1979. (35) Mao, D.; Kahn, A.; Le Lay, G.; Marsi, M.; Hwu, Y.; Margaritondo, G.; Santos, M.; Shayegan, M.; Florez, L. T.; Harbison, J. P. J. Vac. Sci. Technol., B 1991, 9, 2083. (36) Lunt, S. R.; Ryba, G. N.; Santangelo, P. G.; Lewis, N. S. J. Appl. Phys. 1991, 70, 7449. (37) Shin, J.; Geib, K. M.; Wilmsen, C. W. J. Vac. Sci. Technol., B 1991, 9, 2337. (38) Lunt, S. R.; Santangelo, P. G.; Lewis, N. S. J. Vac. Sci. Technol., B 1991, 9, 2333. (39) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corp.: Eden Prairie, MN, 1992.

from Figure 2, the intensity of the oxide-related shoulders in the Ga 3d and As 3d spectra is noticeably higher at a photon energy of 130 eV, which suggests that only the topmost part of the GaAs sample is affected by the oxidation; ≈20% of Ga and As atoms in this region are oxidized. Upon a prolonged exposure of the sample to ambience, the extent of oxidation and contamination progressively increases, so that the entire presurface region becomes oxidized and the spectra change toward those for the as-prepared sample. In contrast to the case of a strongly oxidized and contaminated GaAs surface, the spectra of the BPTfunctionalized GaAs in Figures 3 and 4 look completely different even though this sample was kept in ambience for a long time: The extent of oxidation is rather small (≈18% at the surface) and even the surface-sensitive spectra at a photon energy of 130 eV are dominated by the emissions related to the stoichiometric GaAs (light gray). Along with these emissions, doublets assigned to elemental As (gray) and As bonded to the sulfur headgroup of BPT (black) are also observed in the As 3d spectra. The attachment of BPT to GaAs is also supported by the observation of the S 2p doublet (Figure 4) at a BE of 162.6 eV (S 2p3/2), which is characteristic for thiolate species22,40-42 (there is also a small contribution at a BE of 161.7 eV (S2p3/2) related either to atomic sulfur22,43-45 or to Ga 3s emission39). A comparison of the C 1s and O 1s spectra for the as-prepared, freshly etched, and BPTcoated GaAs samples in Figure 4 supports the conclusions derived from the Ga 3d and As 3d data. The functionalization of GaAs by BPT prevents an oxidation and contamination of the surface: The extent of the oxidation for the BPT-coated GaAs is comparable with that for the freshly etched sample and the total stoichiometry of the GaAs surface corresponds approximately to theoretical Ga:As ratio of 1:1 (a small excess of As). Note that the C 1s spectrum for BPT-coated GaAs is mostly representative for a well-ordered BPT SAM, as follows from the observation of the characteristic shake-up shoulder at higher binding energies with respect to the main emission.22,24,42 3.2. NEXAFS Measurements. Raw C 1s NEXAFS spectra of freshly etched and BPT-functionalized GaAs acquired at so-called magic angle of X-ray incidence (55°) are presented in the upper panel of Figure 5, along with the respective spectrum for a freshly sputtered Au sample. (40) 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. (41) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 139. (42) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter. 2001, 13, 11333. (43) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (44) Takiguchi, H.; Sato, K.; Ishida, T.; Abe, K.; Yase, K.; Tamada, K. Langmuir 2000, 16, 1703. (45) Yang, Y. W.; Fan, L. J. Langmuir 2002, 18, 1157.

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Figure 5. (Upper panel): Raw C 1s NEXAFS spectra of freshly etched and BPT-coated GaAs acquired at a angle of X-ray incidence of 55°, along with the respective spectrum for clean Au. (Lower panel): The same spectra divided by the spectrum for clean Au.

Note that the spectra acquired at the magic angle exclusively reflect the electronic structure of the unoccupied molecular orbitals of the investigated films and are not affected by the angular dependence of the absorption cross sections.25 As compared to the absorptionfree spectrum for the clean Au, the spectrum of the freshly etched GaAs reveals an absorption structure, which becomes clearly visible in the normalized spectrum in the lower panel of Figure 5. Presumably, this structure is related to carbon contamination in the near-surface region. The raw and normalized spectra of BPT-coated GaAs in Figure 5 exhibit additional absorption maxima, which are characteristic for intact aromatic rings. In particular, a distinct π1* resonance at a photon energy of 285.2 eV is clearly perceptible even in the raw spectrum. As soon as the contribution of the substrate is excluded within the subtraction/division normalization procedure (see section 2), this resonance becomes a dominant spectral feature while the entire NEXAFS spectra exhibit a shape characteristic for SAMs of aromatic compounds.42,46-50 The respective normalized spectra for different incidence angles of X-rays are depicted in Figure 6. Along with the π1* resonance, they reveal other characteristic absorption maxima,42,46-50 such as R*/C-S* resonances at photon energies of 287.25 and 287.75 eV,51-53 a π2* resonance at (46) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (47) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (48) Frey, S.; Stadler, V.; Heister, K.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (49) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Wo¨ll, Ch. Langmuir 2001, 17, 3689. (50) Zharnikov, M.; Ku¨ller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Langmuir, in press. (51) Horsley, J. A.; Sto¨hr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099. (52) Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891. (53) Weiss, K.; Gebert, S.; Wu¨hn, M.; Wadepohl, H.; Wo¨ll, Ch. J. Vac. Sci. Technol., A 1998, 16, 1017.

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Figure 6. Normalized C 1s NEXAFS spectra of a BPT-coated GaAs sample acquired at angles of X-ray incidence of 70°, 55°, 30°, and 20° (see section 2 for details of the normalization procedure).

a photon energy of 288.9 eV (there is also an alternative σ*(CH) assignment for this resonance),54,55 and σ* resonances at photon energies of ≈293.0 and ≈300.0 eV. The absorption edge is presumably located at a photon energy of ≈287 eV.46-48 As seen in Figure 6, the intensities of the π* resonances vary strongly when the angle of incidence of the X-rays is changed. The pronounced linear dichroism suggests a high orientational order for the BPT monolayers on GaAs. In addition, the increase in the π* resonance intensity with increasing X-ray incidence angle implies an upright orientation of the biphenyl moieties because the transition dipole moment of the π* resonances is perpendicular to the ring plane. Except for the qualitative conclusions on the orientational order in the BPT monolayer on GaAs, a value of the average tilt angle of the aromatic chains in these systems can be obtained by a quantitative analysis of the angular dependence of the NEXAFS resonance intensities.25 For this analysis, the π1* resonance was selected as the most intense and distinct resonance in the absorption spectra of BPT-coated GaAs. For the BPT monolayer, the intensity I of the π1* resonance is related to the average tilt angle R of the π1* orbital with respect to the surface normal and the X-ray incidence angle θ by25

1 I(R) ∝ 1 + (3 cos2 θ - 1)(3 cos2 R - 1) 2

(1)

which is a standard25 expression for a vector-type orbital. The term cos2 R in eq 1 can be expressed through the twist angle ϑ of the aromatic rings with respect to the plane spanned by the surface normal and the molecular axis and through the average tilt angle φ of the molecular axis with respect to the surface normal by56

cos R ) cos ϑ sin φ

(2)

Here, a planar conformation of the biphenyl moieties (the (54) Yokoyama, T.; Seki, K.; Morisada, I.; Edamatsu, K.; Ohta, T. Phys. Scr. 1990, 41, 189.

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dihedral angle is equal to zero) within the densely packed SAMs is assumed in accordance with literature data for crystalline biphenyl57 and thioaromatic SAMs on Au.46 Consequently, the average tilt angle φ of the molecular axis for a known twist angle ϑ is defined by

1 I(ϑ,φ) ∝ 1 + (3 cos2 θ - 1)(3 cos2 ϑ sin2 φ - 1) 2

(3)

Assuming a herringbone arrangement of BPT molecules such as in the bulk materials and the aromatic SAMs on Au,46,58,59 we get two different spatial orientations of the biphenyl moieties with reverse twist angles ϑ1 ) -ϑ2 and the same tilt angles φ1 ) φ2. In this particular case, the contributions of each spatial orientation to the resonance intensity (3) are the same and eq 3 can be used for the data evaluation without any modification. To avoid normalization problems, we did not analyze the absolute intensities but the intensity ratios I(θ)/I(20°), where I(θ) is the intensity of the π1* resonance at an X-ray incidence angle θ.52,25 In addition, we assumed the same twist angle of 32° for the biphenyl moieties in BPT monolayers on GaAs as found for thioaromatic bulk materials.60-63 This assumption is based on theoretical estimates for the molecular arrangements in biphenyl and naphthalene mercaptan films on Au58 and on the experimental data for a series of oligo(phenylethynyl)benzenethiols.59 Of course, there is the difference between the thiolate-noble metal and thiolate-As linkages, which can affect the film structure to some extent. However, this influence cannot be that large to distort significantly the herringbone structure of the aromatic moieties, which is the only way to optimize the intermolecular interaction in the densely packed 2D layers. The results of the NEXAFS data analysis are presented in Figure 7, where the angular dependence of the π1* resonance intensity ratio I(θ)/I(20°) for BPT-coated GaAs is depicted, along with theoretical dependences (solid and dotted lines) for several selected average tilt angles of the biphenyl moieties in the BPT film. As is seen in Figure 7, the experimental points could be best-fitted with the theoretical curve corresponding to the angle of 31.5° ( 5° (the large error bars are mostly related to the uncertainty in the value of the twist angles). This angle is slightly larger than the respective values for BPT SAMs on noble metal substrates (23° for Au and 18° for Ag)48 but quite close to the corresponding angle for SAMs formed from 4-hydroxy-1,1′-biphenyl (HBP) on hydrogenated Si(111) substrates (28.7°).50 Note that the same assumptions concerning the 2D arrangements of the biphenyl moieties have been made for BPT on Au,48 BPT on Ag,48 and HBP on Si,50 which allows the direct comparison of the derived tilt angles. 4. Discussion The presented experimental data imply that the functionalization of GaAs with BPT monolayers prevents an (55) A° gren, H.; Vahtras, O.; Carravetta, V. Chem. Phys. 1995, 196, 47. (56) Rong, H. T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582. (57) Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1989, 111, 8576. (58) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792. (59) Dhirani, A.-A.; Zehner, W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. J. Am. Chem. Soc. 1996, 118, 3319. (60) Cruickshank, D. W. J. Acta Crystallogr. 1956, 9, 915. (61) Kitaigorodskii, I. A. Organic Chemical Crystallography; Consultants Bureau: New York, 1961. (62) Trotter, J. Acta Crystallogr. 1961, 14, 1135. (63) Charbonneau, G.-P.; Delugeard, Y. Acta Crystallogr. 1976, B32, 1420.

Figure 7. The angular dependencies of the π1* resonance intensity ratio I(θ)/I(20°) for a BPT-coated GaAs sample. For comparison, the theoretical dependencies for two selected average tilt angles of the π1* orbital are added as dashed lines. The best fit is marked by the solid line.

oxidation and contamination of the surface, keeping the BPT/GaAs interface in a similar state as the surface of freshly etched GaAs. It is clearly demonstrated that the BPT molecules form well-ordered, densely packed SAMs on GaAs(100) substrates. The covalent attachment to the substrate occurs over the thiolate headgroup as indicated by the appearance of the respective emissions in the S 2p and As 3d HRXPS spectra. The presence of the entire variety of the characteristic absorption resonances of phenyl rings in the C 1s NEXAFS spectra suggests the intact character of the biphenyl backbone in the BPT films on GaAs. The quantitative analysis of these spectra implies an upright orientation of the biphenyl moieties in the BPT film with an average tilt angle of 31.5° ( 5°. Taking this angle and a length of the BPT molecule of 12 Å,18,48 one gets a film thickness of 10.2 Å, which is in an excellent agreement with the BPT layer thickness measured by ellipsometry, 10 ( 2 Å.17 Whereas we have clear experimental evidence that the anchoring to the substrate occurs over the S-As bond, we cannot conclude what fraction of the As atoms in the topmost layer of the substrate is involved in the bonding. Among other factors, it depends on the exact surface geometry and the adsorption site, which are not known. As mentioned above, the derived average angle of the biphenyl moieties in the BPT films on GaAs is slightly larger than the values found for the same thioaromatic SAMs on noble metal substrates48 but comparable to that for the HBP SAMs on Si(111) surfaces.50 As compared to noble metals, the larger inclination of the aromatic moieties in the case of GaAs substrate can be related to both the different contribution of the thiolate-substrate interaction to the balance of structure-building forces in the monomolecular films42 and a slightly lesser degree of orientational order. In addition, a larger lattice spacing between As atoms on the GaAs(100) surface (5.6 Å as compared to ≈5.0 Å for (111) Au and Ag)64,65 can be of importance, because a larger separation of the anchor sites generally results in a less dense packing with a larger inclination of the SAM constituents. As to the orientational order argument, the assumed partial disordering can be presumably related to the surface contamination before the SAM deposition. In fact, both HRXPS and NEXAFS (64) Hellwege, K.-H. Landolt-Bo¨ rnstein Numerical Data and Functional Relationships in Science and Technology; Springer-Verlag: Berlin, 1983. (65) Ulman, A. Chem. Rev. 1996, 96, 1533.

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spectroscopy data suggested the presence of oxides and carbon contamination to some extent, which might disturb the anchoring and subsequently the ordering of the BPT molecules. The latter hypothesis is indirectly supported by the similarity in the average tilt angles of biphenyl moieties in BPT on GaAs and HBP on Si, which is not contamination free as well.50 Improvement in the wet chemical etching or the development of a more sophisticated cleaning procedure (see, e.g., ref 66) might be helpful to remove the residual oxide and other contaminants before the BPT coating. Note, however, that the etching procedure applied in this study is the result of a long-time optimization work,11 including the analysis and repetition of most suitable approaches from the available literature.4,67-71 A straightforward and unambiguous analysis of the photoemission data was only possible due to the ultimate energy resolution of HRXPS. In addition, the use of the synchrotron radiation with the tunable photon energy allowed us to probe different regions within the samples, which also helped to distinguish between the individual emissions. In general, the synchrotron-based HRXPS made it possible to avoid a “standard” problem of arbitrary decomposition of broad emission features into individual spectral components, which is especially crucial for GaAsbased systems due to a doublet nature of the basic emissions and a small binding energy difference between different chemical states of As and Ga. Complementary information was provided by NEXAFS spectroscopy. In general, a combination of the synchrotron-based HRXPS and NEXAFS spectroscopy applied in this study seems to be well suited for quantitative characterization of complex systems, containing a large variety of different chemical species. 5. Conclusions Synchrotron-based HRXPS and NEXAFS spectroscopy were applied to characterize SAMs formed from BPT on stoichiometric GaAs(100) surfaces. These SAMs were found to be well ordered and densely packed. The attachment to the substrate occurs over the thiolate (66) Razek, N.; Otte, K.; Chasse, T.; Hirsch, D.; Schindler, A. Frost, F.; Rauschenbach, B. J. Vac. Sci. Technol., A 2002, 20, 1492. (67) Vasquez, R. P.; Lewis, B. F.; Grunthar, F. J. J. Vac. Sci. Technol. 1983, B1, 791-794. (68) Woodall, J. M.; Oelhafen, P.; Jackson, T. N.; Freeouf, J. L.; Pettit, G. D. J. Vac. Technol. 1983, B1, 795. (69) Sze, S. M. Semiconductor Devices: Physics and Technology; John Wiley & Sons: New York, 1985. (70) Lunt, S. R.; Ryba, G. N.; Santangelo, P. G.; Lewis, N. S. J. Appl. Phys. 1991, 70, 7449. (71) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Ma¨rtensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 30, 3759.

Shaporenko et al.

headgroup, while the intact aromatic backbones have an upright orientation with an average tilt angle of 31.5° ( 5°. The obtained value seems to be slightly larger than the corresponding values for the aromatic SAMs on noble metal substrates. This can be explained by several reasons or their combination: (i) the larger As-As spacing between the anchor sites (As atoms) on (100) GaAs as compared to (111) Au and Ag, (ii) a different character of the thiolateAs bond as compared to the case of Au and Ag, and (iii) a lesser degree of the orientational order in the films on GaAs, which is presumably related to residual contaminants and oxides on the surface of freshly etched GaAs substrate. The functionalization of GaAs with BPT was found to prevent oxidation and contamination of the surface, keeping the BPT/GaAs interface in a similar state as the freshly etched GaAs surface. Densely packed, chemically stable, and hydrophobic biphenyl monolayers on GaAs can be used for the protection of GaAs in air and in water, as well as for the further functionalization with polymer films and model biomembranes.1,72-74 In addition, applications within the SAM-based chemical lithography framework14,16 enable the fabrication of multifunctional surfaces. The functionalization protocol can also be directly transferred onto various semiconductor heterostructures in the proximity of the GaAs surface, such as near-surface quantum dots,12,13 which can be effectively used for sensitive detection of specific recognition and selective transport in biological membranes. Acknowledgment. The authors are very grateful to M. Grunze and E. Sackmann for the support and A. Ulman for useful suggestions. We thank the BESSY II and MAXlab staff for technical help and Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment for the NEXAFS measurements. We are also thankful to M. Bichler, D. Schuh, and G. Abstreiter (Walter Schottky Institute, TUM) for providing MBE-grown GaAs wafers. This work has been supported by the German BMBF (GRE1HD), the Deutsche Forschungsgemeinschaft (Ja 883/4-2 and Ta259/1), and the Fonds der Chemischen Industrie. M.T. is a recipient of Emmy Noether fellowship of DFG. LA020909Q (72) Hillebrandt, H.; Wiegand, G.; Tanaka, M.; Sackmann, E. Langmuir 1999, 15, 8451. (73) Tanaka, M.; Kaufmann, S.; Nissen, J.; Hochrein, M. Phys. Chem. Chem. Phys. 2001, 3, 4091. (74) Hillebrandt, H.; Tanaka, M.; Sackmann, E. J. Phys. Chem. B 2002, 106, 477.