Langmuir 2001, 17, 4267-4273
4267
Stable Surface Coating of Gallium Arsenide with Octadecylthiol Monolayers Klaus Adlkofer and Motomu Tanaka* Lehrstuhl fu¨ r Biophysik E22, Technische Universita¨ t Mu¨ nchen, James-Franck-Strasse, D-85748 Garching, Germany Received October 26, 2000. In Final Form: April 9, 2001 In this paper, we describe the deposition of octadecylthiol (ODT) monolayers on highly doped n-GaAs electrode surfaces, which showed high stability both in air and in aqueous electrolytes. In the first part of this study, four different wet chemical etching procedures were investigated to optimize surface treatment before ODT deposition. The chemical composition of the surface was evaluated by X-ray photoelectron spectroscopy (XPS), demonstrating that the photochemical etching procedure (called “etch P” in this study) can generate a surface enriched with arsenides, which can serve as the binding sites for sulfides. In the second part of this study, the surface prepared by etch P was coated with an ODT monolayer. The monolayer showed high stability in air, as indicated by the constant ellipsometric thickness. In electrolytes, the stability of the monolayer-coated surface was verified by impedance spectroscopy under zero-current potential (Uj)0 ) -360 mV) for more than 10 h; then the stability of the interface was monitored under different bias potentials. Electrochemical passivation of the GaAs surface has been demonstrated for the first time under physiological conditions (in aqueous electrolyte, near neutral pH), which allows for the application of GaAs electrodes to biological systems.
Introduction The design of functional interfaces between semiconductor materials and biological materials attracts both scientific and practical interest.1 The functionalization of electrode surfaces with organic self-assembled monolayers (SAMs) is a very suitable strategy not only for rendering the surface hydrophobic in preparation for further modification but also for forming very thin insulating layers that reduce leak currents and surface decomposition in aqueous electrolytes.2 In this field, the most widely and intensively studied systems are SAMs of organic sulfides and mercapto compounds on gold electrodes;3-8 however, there have been only a few reports on the electrical properties of SAMs on semiconductors.9,10 Despite the rapid development of functionalized electrode devices, realistic applications of semiconductor surfaces in the field of life science have been impeded upon the undesired nature of the materials. For example, GaAs has been claimed to have a high sensitivity because of its high electron mobility in nanostructures such as * Corresponding author: Motomu Tanaka, Lehrstuhl fu¨r Biophysik E22, Technische Universita¨t Mu¨nchen, James-FranckStrasse, D-85748 Garching, Germany. Tel.: ++49-89-28912539. Fax: ++49-89-28912469. E-mail:
[email protected]. (1) Sackmann, E.; Tanaka, M. Trends Biotechnol. 2000, 18, 58-64. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (3) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 465-495. (4) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B. Langmuir 1986, 2, 239-244. (5) Finklea, H. O.; Snider, D. A.; Fedyk, J. Langmuir 1993, 9, 36603667. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (7) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981. (8) Janek, R. P.; Fawcett, W. R.; Ulman, A. Langmuir 1998, 14, 30113018. (9) Fontaine, P.; Goguenheim, D.; Deresmes, D.; Vuillaume, D.; Garet, M.; Rondelez, F. Appl. Phys. Lett. 1993, 62, 2256-2258. (10) Boulas, C.; Davidovits, J. V.; Rondelez, F.; Vuillaume, D. Phys. Rev. Lett. 1996, 76, 4797-4800.
two-dimensional electron gases (2DEGs)11 and quantum wells (QWs).12 However, the application of GaAs to living systems is still difficult because of the complex electrochemical processes that occur at the GaAs/electrolyte interface. Indeed, most of the electrochemical studies of GaAs have been performed in acidic or basic solutions.13-15 There have been several reports on the functionalization of GaAs surfaces with various types of sulfides and mercaptos in contact with air or with metals, and the passivation effects were mostly discussed in terms of photoluminescence (PL) from bulk GaAs16,17 or electrical properties such as the Schottky barrier height.14,18,19 Nevertheless, systematic studies on the functionalization of GaAs surfaces under physiological conditions (in aqueous electrolytes, near neutral pH conditions) are still lacking. Recently, we reported preliminary results on the electrochemical passivation of n-doped GaAs surfaces through the deposition of an octadecylthiol (ODT) monolayer.20 The electrochemical properties of the monolayers were monitored by cyclic voltammetry to determine the zero-current potential, Uj)0 ) -360 mV. In the first part of the present study, the surface preparation methods were systematically compared in their abilities to remove (11) Baumgartner, P.; Engel, C.; Abstreiter, G.; Bo¨hm, G.; Weimann, G. Appl. Phys. Lett. 1995, 66, 751-753. (12) Baumgartner, P.; Engel, C.; Abstreiter, G.; Bo¨hm, G.; Weimann, G. Appl. Phys. Lett. 1994, 64, 592-594. (13) Uhlendorf, I.; Reinecke-Koch, R.; Memming, R. J. Phys. Chem. 1996, 100, 4930-4936. (14) Miller, E. A.; Richmond, G. L. J. Phys. Chem. B 1997, 101, 26692677. (15) Hens, Z.; Gomes, W. P. J. Phys. Chem. B 1999, 103, 130-138. (16) Lunt, S. R.; Ryba, G. N.; Santangelo, P. G.; Lewis, N. S. J. Appl. Phys. 1991, 70, 7449-7467. (17) Asai, K.; Miyashita, T.; Ishigure, K.; Fukatsu, S. Surf. Sci. 1993, 306, 37-41. (18) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Ma¨rtensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 30, 3759-3762. (19) Sheen, C. W.; Shi, J. X.; Ma˘ rtensson, J.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514-1515. (20) Adlkofer, K.; Tanaka, M.; Hillebrandt, H.; Wiegand, G.; Sackmann, E.; Bolom, T.; Deutschmann, R.; Abstreiter, G. Appl. Phys. Lett. 2000, 76, 3313-3315.
10.1021/la001507q CCC: $20.00 © 2001 American Chemical Society Published on Web 06/12/2001
4268
Langmuir, Vol. 17, No. 14, 2001
the native oxide completely and to prepare “arsenide-rich” surfaces, because arsenides on the surfaces serve as the binding sites for sulfides.14,16 Four different chemical etching methods were attempted (etches A, B, C, and P, explained in the following section), and the detailed chemical composition of the surface was characterized by X-ray photoelectron spectroscopy (XPS). XPS spectra suggested that the photochemical etching procedure (etch P) resulted in a surface enriched with elemental arsenide, whose thickness was determined by ellipsometry. In the second part of the study, an ODT monolayer was deposited onto this photoetched surface. After deposition of the monolayer, sulfide-arsenide binding could be observed in the detailed XPS scans. In contrast to the freshly etched surface, the thickness of the monolayer derived from the ellipsometric angles remained constant in air. The electrochemical properties of the surface after SAM deposition were measured by impedance spectroscopy under a zerocurrent potential (Uj)0 ) -360 mV). Furthermore, the electrochemical stability was examined under different bias potentials. Experimental Section Materials. Single-crystal Te-doped n-type GaAs [100] wafers with a doping ratio of 1-4 × 1018 cm-3 were purchased from Freiberger Compound Material GmbH (Freiberg, Germany). For the electrochemical studies, an Ohmic contact was established from the backside of the wafer by electron beam vapor deposition of Ni (100 Å), Ge (200 Å), and Au (2500 Å). Octadecylthiol [ODT, CH3(CH2)17SH] was purchased from Aldrich GmbH (Steinheim, Germany) and recrystalized twice from acetone. All of the other chemicals were also purchased from Aldrich GmbH, and no further purification was performed. Freshly distilled ultrapure water (Millipore, Molsheim, France) was used throughout this study. Sample Preparation. Prior to the wet chemical etching of the native oxide, the samples were briefly sonicated in acetone (ca. 3 min) and rinsed with ethanol and water. The bulk water on the surface was removed by a nitrogen (N2) flow. To optimize the surface composition before functionalization, four wet chemical etching methods were attempted at room temperature. Etch A was a conventional etching method in which the cleaned sample was dipped into concentrated HCl (37%) for 1 min.18,19,21 In another wet chemical etching method, etch B, the cleaned sample was immersed in HCl/EtOH (1/10 by volume) for 1 min.22 In etch C, the sample was exposed to a mixture of H2O2 (30%), H2SO4, and H2O (1/1/100 by volume) for 30 s.16 The other method, etch P, is a so-called “photochemical” etching method.23 The cleaned sample was immersed into HCl/H2O (1/1 by volume) and illuminated from a distance of about 5 cm by a 500-W Xe-Hg arc lamp without any filters for about 30 min. The etching solution was continuously stirred so that the surface was always exposed to fresh solution. Self-assembled monolayers of ODT were deposited by immersing freshly prepared substrates into 1 mM ODT solution in dry ethanol (water content of
