Formation of Alkanethiolate Self-Assembled Monolayers at Halide

Langmuir 2009, 25, 2013-2025

2013

Formation of Alkanethiolate Self-Assembled Monolayers at Halide-Terminated Ge Surfaces Pendar Ardalan, Charles B. Musgrave,† and Stacey F. Bent* Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305 ReceiVed October 17, 2008. ReVised Manuscript ReceiVed December 10, 2008 We have studied Ge halide passivation and formation of 1-octadecanethiolate self-assembled monolayers (SAMs) at Cl- and Br-terminated Ge(100) and Ge(111) surfaces. The results of water contact angle measurements, ellipsometry, transmission infrared spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy show that good quality 1-alkanethiolate SAMs can be achieved at both Cl- and Br-terminated surfaces via direct Ge-S bonds. The quality of the SAMs depends on the concentration and the solvent of the 1-alkanethiol solution. Moreover, SAMs formed at Ge(100) surfaces have higher water contact angles, thicknesses, and ambient stability than those formed at Ge(111) surfaces. Surface passivation and light are found to play an important role in the packing and stability of the SAMs. Furthermore, well-packed SAMs can be retrieved by repassivation after degradation due to ambient exposure. This work presents novel routes for Ge surface passivation.

1. Introduction Silicon has been the semiconductor of choice in the microelectronics industry for decades, mainly due to the stability and interfacial quality of its oxide. Germanium is of interest for next generation electronic devices because of its high mobility and low dopant activation temperatures.1,2 One of the major drawbacks to the use of Ge is the difficulty in growing an insulating oxide comparable to SiO2 in Si technology. Formation and stability of the germanium oxide on both Ge(100) and Ge(111) has been the subject of a number of experimental and theoretical studies.3-10 The studies suggest that GeO2 is not a good dielectric;7 that GeO, while more stable than GeO2,8 is unstable; and that GeO2 is water soluble.9 Also, upon annealing, GeO2 transforms to GeO, which then desorbs from the surface at ∼420 °C.10 Consequently, in order to achieve high performance Ge-based devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs), alternative solutions must be developed for passivation of the Ge surface prior to deposition of the dielectric materials. Surface passivation of Ge has a direct effect on the quality and characteristics of the interfacial layer between the high-κ dielectric material and the Ge substrate.2,11-14 Interestingly, removal of the germanium oxide alone is not sufficient for * To whom correspondence should be addressed. E-mail: sbent@ stanford.edu. † Current address: Department of Chemical & Biological Engineering, University of Colorado at Boulder, Boulder, CO 80309.

(1) Loscutoff, P. W.; Bent, S. F. Annu. ReV. Phys. Chem. 2006, 57, 467–495. (2) Misra, D.; Garg, R.; Srinivasan, P.; Rahim, N.; Chowdhury, N. A. Mater. Sci. Semi. Proc. 2006, 9, 741–748. (3) Tabet, N. A.; Salim, M. A.; A.L., A.-O. J. Elect. Spectrosc. 1999, 103-103, 233–238. (4) Mayne, A. J.; Rose, F.; Dujardin, G. Surf. Sci. 2003, 523, 157–167. (5) Soon, J. M.; Lim, C. W.; Loh, K. P.; Ma, N. L.; Wu, P. Phys. ReV. B 2005, 72, 115343. (6) Mui, C.; Senosiain, J. P.; Musgrave, C. B. Langmuir 2004, 20, 7604–7609. (7) Rivillon, S.; Chabal, Y. J.; Amy, F.; Kahn, A. Appl. Phys. Lett. 2005, 87, 253101. (8) Onsia, B.; Conard, T.; De Gendt, S.; Heyns, M.; Hoflijk, I.; Mertens, P.; Meuris, M.; Raskin, G.; Sioncke, S.; Teerlinck, I.; Theuwis, A.; Van Steenbergen, J.; Vinckier, C. Solid State Phenom. 2005, 103-104, 27–30. (9) Prabhakaran, K.; Ogino, T. Surf. Sci. 1995, 325, 263. (10) Prabhakaran, K.; Maeda, F.; Watanabe, Y.; Ogino, T. Appl. Phys. Lett. 2000, 76, 2244–2246. (11) Chui, C. O.; Kim, H.; McIntyre, P. C.; Sarasawat, C. IEEE Electron DeVice Lett. 2004, 25, 274–276. (12) Shang, H.; Frank, M. M.; Gusev, E. P.; Chu, J. O.; Bedell, S. W.; Guarini, K. W.; Leong, M. IBM J. Res. DeV. 2006, 4/5, 377–386.

good electrical performance.13 Although Ge surface cleaning in ultrahigh vacuum (UHV) conditions resulted in an oxide-free surface, subsequent deposition of high-κ dielectric materials was shown to result in leaky devices, and consequently, use of an appropriate passivation layer is necessary.14 Several different passivation methods have been explored for germanium, and a number of fundamental studies investigating the passivation and organic functionalization of Ge substrates under gas phase or UHV conditions have been reported.1,8,15-24 Furthermore, various solution-based methods have been studied to achieve chemical passivation of the germanium surface prior to high-κ deposition including hydrohalogenic acids (HF, HCl, HBr, and HI),7,8,13,25-38 hydrogen peroxide (H2O2),7,8,10,31,33,36 deionized (DI) water,7,9,31,33 and ammonium hydroxide (13) Caymax, M.; Elshocht, S. V.; Houssa, M.; Delabie, A.; Conard, T.; Meuris, M.; Heyns, M. M.; Dimoulas, A.; Spiga, S.; Fanciulli, M.; Seo, J. W.; Goncharova, L. V. Mater. Sci. Eng., B 2006, 135, 256–260. (14) Soe, J. W.; Dieker, C.; Locquet, J.-P.; Mavrou, G.; Dimoulas, A. Appl. Phys. Lett. 2005, 87, 221906. (15) Schnell, R. D.; Himpsel, F. J.; Bogen, A.; Rieger, D.; Steinmann, W. Phys. ReV. B 1985, 32, 8052–8056. (16) Roche, J.; Ryan, P.; Hughes, G. J. Appl. Surf. Sci. 2001, 174, 271–274. (17) Weser, T.; Bogen, A.; Konrad, B.; Schnell, R. D.; Schug, C. A.; Steinmann, W. Phys. ReV. B 1986, 35, 8184–8188. (18) Gothelid, M.; LeLay, G.; Wigren, C.; Bjorkqvist, M.; Karlsson, U. O. Surf. Sci. 1997, 371, 264. (19) Ardalan, P.; Davani, N.; Musgrave, C. B. J. Phys. Chem. C 2007, 111, 3692–3699. (20) Filler, M. A.; VanDeventer, J. A.; Keung, A. J.; Bent, S. F. J. Am. Chem. Soc. 2006, 128, 770–779. (21) Cullen, G. W.; Amick, J. A.; Gerlich, D. J. Electrochem. Soc. 1962, 109, 124. (22) Fouchier, M.; McEllistrem, M. T.; Boland, J. J. Surf. Sci. 1997, 385, L905–L910. (23) Bachelet, G. B.; Schlu¨ter, M. Phys. ReV. B 1983, 28, 2302. (24) Cao, S.; Tang, J. C.; Shen, S. L. J. Phys. Condens. Matter 2003, 15, 5261–5268. (25) Sun, S.; Liu, Z.; Lee, D.-I.; Peterson, S.; Pianetta, P. Appl. Phys. Lett. 2006, 88, 021903. (26) Sun, S.; Sun, Y.; Liu, Z.; Lee, D.-I.; Pianetta, P. Appl. Phys. Lett. 2006, 89, 231925. (27) Deegan, T.; Hughes, G. Appl. Surf. Sci. 1998, 123/124, 66–70. (28) Lu, Z. H. Appl. Phys. Lett. 1996, 68, 520–522. (29) Kim, J.; McVittie, J.; Saraswat, K.; Nishi, Y. ECS Trans. 2006, 3, 1191– 1196. (30) Kim, J.; McVittie, J.; Saraswat, K.; Nishi, Y. In 8th International Symposium on Ultra Clean Processing of Silicon Surfaces; Antwerp, Belgium, 2006. (31) Kim, J.; Saraswat, K.; Nishi, Y. ECS Trans. 2005, 1, 214–219. (32) Lu, Y. F.; Mai, Z. H.; Song, W. D.; Chim, W. K. Appl. Phys. A: Mater. Sci. Process. 2000, 70, 403–406.

10.1021/la803468e CCC: $40.75  2009 American Chemical Society Published on Web 01/16/2009

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(NH4OH).31,33,36,37 Deposition of oxynitride,2,11-14 oxysulfide,39 and silicon40 layers has also been utilized for Ge passivation. Acid treatment is one of the most promising approaches to surface passivation. HF treatment leads to hydride termination of Ge substrates,25 analogous to Si substrates41 where the mechanism of the etching is mainly governed by kinetics rather than the thermodynamics of the reactions.41 However, aqueous HF solutions do not fully remove the suboxide layer from the Ge surface, and the resulting surfaces are not resistant toward reoxidation after exposure to ambient, water vapor, or a mixture of O2 and water vapor.8,13,25,26,31 For example, after treating Ge with 20% HF, the H-terminated Ge surface reoxidizes after only 10 min.30 The H-terminated Ge substrates are much less stable in air than H-terminated Si surfaces.42 Passivation using HCl and HBr acids may be more effective. HBr has a higher Ge etch rate than HCl (0.02 nm/min vs 0.0004 nm/min), although both etch significantly slower than H2O2 (40 nm/min).8 Unlike HF etching, high concentrations of aqueous HCl and HBr solutions yield oxide-free Ge substrates, and ambient stability studies reveal that the resulting Cl- or Br-terminated surfaces are stable against reoxidation for 10 min and 6 h after being exposed to ambient atmosphere, respectively.27,29,30,32,33 Therefore, in this work we focus on halide termination of germanium using HCl and HBr. Unfortunately, since even halide termination produces surfaces with limited stability, other passivation methods must be explored. One possibility is to attach an organic group to the surface. Organic functionalization of Ge has been previously reported. For example, hydrogermylation of alkenes and alkynes on H-terminated Ge(100) surfaces has been studied by Choi et al.43 By utilizing ultraviolet photoinduced, thermally initiated, or Lewis acid meditated hydrogermylation chemistry, well-packed alkyl or alkenyl monolayers can be formed at the Ge(100) surface from alkenes and alkynes, respectively. He et al. have also demonstrated the self-assembly of alkyl monolayers on Ge(111) by exposure of Cl-terminated Ge to alkyl Grignard reagents.44 Another class of organic termination is self-assembled alkanethiolate monolayers. This passivation system has the possible advantage of incorporating a Ge-S bond at the interface. A number of studies investigated the use of sulfur as an approach for passivation of Ge. Studies of molecular sulfur adsorption on Ge(100)-2 × 1 in an ultrahigh vacuum have shown that S induces a (1 × 1) reconstruction,16,17 although more recent soft X-ray photoemission studies by Roche et al.16 and STM studies by (33) Kim, J.; Mcvittie, J.; Sarasawat, K.; Nishi, Y. In Study of Germanium Surface for Wet Cleaning Applications; SRC/SEMATECH Engineering Research Center for Environmentally Benign Semiconductor Manufacturing; teleconference, 16 May 2006. (34) Bodlaki, D.; Yamamoto, H.; Waldeck, D. H.; Borguet, E. Surf. Sci. 2003, 543, 63–74. (35) Cao, S.; Tang, J. C.; Wang, L.; Zhu, P.; Shen, S. L. Surf. Sci. 2002, 505, 289. (36) Okumura, H.; Akane, T.; Matsumoto, S. Appl. Surf. Sci. 1998, 125, 125– 128. (37) Hovis, J. S.; Hamers, R. J.; Greenlief, C. M. Surf. Sci. 1999, 440, L815– L819. (38) Ardalan, P.; Pickett, E. R., Jr.; Marshall, A. F.; Bent, S. F. Appl. Phys. Lett. 2008, 92, 252902. (39) Frank, M. M.; Koester, S. J.; Copel, M.; Ott, J. O.; Paruchuri, V. K.; Shang, H.; Loesing, R. Appl. Phys. Lett. 2006, 89, 112905. (40) Wu, N.; Zhang, Q.; Zhu, C.; Chan, D. S. H.; Li, M. F.; Balasubramanian, N.; Chin, A.; Kwong, D.-L. Appl. Phys. Lett. 2004, 85, 4127–4129. (41) Trucks, G. W.; Raghavachari, K.; Higashi, G. S.; Chabal, Y. J. Phys. ReV. Lett. 1990, 65, 504. (42) Rivillion, S.; Chabal, Y. J.; Amy, F.; Kahn, A.; Krugg, C.; Kirsch, P. In Wet Chemical Cleaning of Germanium Surfaces for Growth of High-k Dielectrics; MRS Spring Meeting; San Francisco, CA, United States, 2006; pp 8-17. (43) Choi, K.; Buriak, J. M. Langmuir 2000, 16, 7737–7741. (44) He, J.; Lu, Z. H.; Mitchell, S. A.; Wayner, D. D. M. J. Am. Chem. Soc. 1998, 120, 2660–2661.

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Go¨thelid et al.45 indicate the presence of a nonideal Ge-S phase with a thickness of one to two monolayers. Nevertheless, the observed (1 × 1) structure is attributed to a bulk Ge structure with a disordered Ge-S overlayer.16 Wet sulfidization methods, including hot aqueous (NH4)2S, have also been used46 and in some cases form a monolayer of bridge-bonded Ge-S atoms at the surface that contributes to a smooth (1 × 1) structure. In fact, S-terminated Ge surfaces show ambient stability over a time scale of a few days.34 Moreover, HfO2/GeOS/Ge gate stacks prepared by sulfidization and atomic layer deposition (ALD) have been shown to be a possible alternative for the HfO2/GeOxNy/ Ge architecture for Ge-based MOSFETs, mainly due to their observed lower fixed charge and interface state density.39 Given the promising characteristics of sulfur-germanium bonding, 1-alkanethiols are interesting candidates for Ge surface passivation, as well as for other applications such as area selective atomic layer deposition (ASALD)47-52 and molecular electronics devices that rely on metal-molecule-semiconductor architecture. The use of alkanethiols to form self-assembled monolayers has been most extensively studied on gold surfaces,53-61 where they have been formed from alkanethiol solutions using various techniques such as immersion in liquid,55,62 microcontact printing (µCP),63-65 edge transfer lithography (ETL),66 microtransfer molding (µTM),65 and microdisplacement printing (µDP).62 Relatively few studies of alkanethiolate SAMs on semiconductor surfaces have been reported. For example, McGuiness et al. have carried out extensive preparation and characterization studies on the n-alkanethiolate-GaAs(001) system and concluded that the SAMs exhibit good chemical passivation of GaAs(001) surfaces.67-69 However, these SAMs form an incommensurate adlayer due to the mismatch between the surface and SAM (45) Go¨thelid, M.; LeLay, G.; Wigren, C.; Bjo¨rkqvist, M.; Rad, M.; Karlsson, U. O. Appl. Surf. Sci. 1997, 115, 87–95. (46) Maeda, T.; Takagi, S.; Ohnishi, T.; Lippmaa, M. Mater. Sci. Semi. Proc. 2006, 9, 706–710. (47) Chen, R.; Bent, S. F. AdV. Mater. 2006, 18, 1086–1090. (48) Chen, R.; Bent, S. F. Chem. Mater. 2006, 18, 3733–3741. (49) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F. Chem. Mater. 2005, 17, 536–544. (50) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F. Appl. Phys. Lett. 2004, 84, 4017–4019. (51) Chen, R.; Kim, H.; McIntyre, P. C.; Porter, D. W.; Bent, S. F. Appl. Phys. Lett. 2005, 86, 191910. (52) Chen, R.; Porter, D. W.; Kim, H.; McIntyre, P. C.; Bent, S. F. In Area SelectiVe Atomic Layer Deposition by Soft Lithography; MRS spring meeting; San Fransisco, CA, 2006. (53) Vericat, C.; Vela, M. E.; Benitez, G. A.; Martin Gago, J. A.; Torrelles, X.; Salvarezza, R. C. J. Phys. Condes. Matter 2006, 18, R867–R900. (54) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569. (55) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. ReV. B 1999, 59, R10449. (56) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1986, 109, 3559–3568. (57) Franzen, S. Chem. Phys. Lett. 2003, 381, 315–321. (58) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (59) Bhushan, B.; Liu, H. Phys. ReV. B 2001, 63, 245412. (60) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1170. (61) Cometto, F. P.; Paredes-Olivera, P.; Macagno, V. A.; Patrito, E. M. J. Phys. Chem. B 2005, 109, 21737–21748. (62) Mullen, T. J.; Dameron, A. A.; Weiss, P. S. J. Phys. Chem. B 2006, 110, 14410–14417. (63) Bass, R. B.; Lichtenberger, A. W. Appl. Surf. Sci. 2004, 226. (64) Wilbur, J. L.; Kumar, A.; Biebuyck, H. A.; Kim, E.; Whitesides, G. M. Nanotech. 1996, 7, 452–457. (65) Xia, Y.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153–184. (66) Sharpe, R. B. A.; Titulaer, B. J. F.; Peeters, E.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelesma, B. Nano Lett. 2006, 6, 1235–1239. (67) McGuiness, C. L.; Shaporenko, A.; Mars, C. K.; Uppili, S.; Zhanikov, M.; Allara, D. L. J. Am. Chem. Soc. 2006, 128, 5231–5243. (68) McGuiness, C. L.; Blasini, D.; Masejewski, J. P.; Uppili, S.; Cabarcos, O. M.; Smilgies, D.; Allara, D. L. ACS Nano 2007, 1, 30–49. (69) McGuiness, C. L.; Shaporenko, A.; Zhanikov, M.; Walker, A. V.; Allara, D. L. J. Phys. Chem. C 2007, 111, 4226–4234.

Formation of Alkanethiolate SAMs

intermolecular spacings that leads to poor electrical passivation.70 In addition, alkanethiolate SAMs have been formed on germanium. Maboudian and co-workers demonstrated that well-packed 1-octadecanethiolate SAMs covalently bound to the surface through Ge-S bonds can be formed at the H-terminated Ge(111) surface at room temperature, are thermally stable up to 450 K, and are air-stable up to 12 h.71 The adsorption kinetics of 1-alkanethiolate SAMs at H-terminated Ge(111) were also studied and indicate that the adsorption mechanism involves a two-step process that depends on the concentration and chain length of the 1-alkanethiols.72 Another study showed that 1-alkanethiolate SAMs could block HfO2 ALD on Ge(111), with an efficiency that increased with chain length.73 A disadvantage of the above-mentioned approach using HFetched Ge to form alkanethiolate SAMs is that the starting H-terminated surface is only weakly stable against oxidation, putting severe constraints on the processing time. Moreover, the halide-terminated Ge surfaces exhibit better stability against oxidation than the hydrogen-terminated surface, and acid treatment with HCl, HBr, and HI more completely removes suboxides from Ge than does the HF treatment.27,29-33,74 Consequently, in this work, we explore the formation of 1-alkanethiolate SAMs on germanium starting from the halideterminated surface. Here, the initial surface treatment (HCl or HBr) should not only remove the oxide and contamination but also result in better stability, making the passivated surface suitable for wet functionalization. Although carried out on planar substrates, the study also has implications for Ge nanowire (NW) passivation. Like Ge substrates, passivation of Ge NWs is a key step in fabricating field effect transistors based on these NWs. Previous studies have shown that Ge NWs are even more susceptible to oxidation than flat Ge substrates.75 Various Ge NW passivation techniques have been investigated including hydrohalogenic acids,75-77 sulfidation via aqueous (NH4)2S after a mild HF treatment,75 and organic monolayer passivation via hydrogermylation,75 Grignard reactions,78 and thiolation.75,78 Ultimately, hydrogermylation and thiolation result in the most robust and well-characterized monolayers on the Ge NWs.75,78 However, like for planar surfaces, bromination and chlorination of Ge NWs provide higher stability toward oxidation than does HF treatment.75 Thus, our halogenation/thiolation method may be useful for Ge NWs. In this article, we report the self-assembly of long-chain 1-alkanethiolates on halide-terminated (Cl- and Br-terminated) Ge substrates by immersion in 1-alkanethiol solution. We show that the method forms stable SAMs from halide-terminated Ge. Moreover, we carry out a fundamental study in which the effects of crystallographic orientation of the Ge substrate (i.e., (100) and (111)) and concentration of the 1-alkanethiol solution on 1-alkanethiolate SAM formation and their resultant ambient stability are investigated. We will show that the passivated (100) surface is a better template for SAM formation compared to the (70) Bent, S. F. ACS Nano 2007, 1, 10–12. (71) Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2001, 123, 2422–2425. (72) Kosuri, M. R.; Cone, R.; Li, Q.; Han, S. M.; Bunker, B. C.; Mayer, T. M. Langmuir 2004, 20, 835–840. (73) Chen, R. Surface Modification for Area SelectiVe Atomic Layer Deposition on Silicon and Germanium. Stanford University, 2006. (74) Sun, S. Germanium Surface Cleaning, PassiVation, and Initial Oxidation. Stanford University, 2007. (75) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466–15472. (76) Adhikari, H.; McIntyre, P. C.; Shiyu, S.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 2005, 87, 263109. (77) Jagannathan, H.; Kim, J.; Deal, M.; Kelly, M.; Nishi, Y. ECS Trans. 2006, 3, 1175–1180. (78) Wang, D.; Chang, Y. L.; Liu, Z.; Dai, H. J. Am. Chem. Soc. 2005, 127, 11871–11875.

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Figure 1. Schematic illustration of the formation of 1-alkanethiolate self-assembled monolayers on a Ge substrate by a two-step process, using halide passivation followed by thiolation. Halide passivation is generated by cyclic oxidation/etching or by direct hydrohalogenic acid etching.

(111) orientation. Furthermore, we will show how exposure to light and the choice of passivation method can affect packing and stability of the 1-alkanethiolate SAMs. Finally, repassivation strategies are presented that are capable of retrieving well-packed SAMs after degradation caused by ambient exposure.

2. Experimental 2.1. Materials. Ge samples were cleaved from 2 in. Czochralski (CZ) grown n-type Ge(100) and Ge(111) wafers (Umicore, Belgium) with resistivity ranging from 1.0 to 3.0 Ω-cm. All reagents were used as purchased, including hydrobromic acid (Aldrich, reagent grade, 48 wt %), pyridine (Aldrich, ACS grade, g99.0% purity), hydrochloric acid (EMD Chemicals Inc., ACS grade, 37 wt %), methanol (EMD, ACS grade), chloroform (EMD, ACS grade), hydrogen peroxide (EMD, ACS grade, 30 wt % in H2O), 2-propanol (Fisher, ACS grade), and acetone (Fisher, ACS grade). 1-octadecanethiol (96%, Acros Organics) was dried in a desiccator and used without further purification. 2.2. Substrate Cleaning and Preparation of Cl- and BrTerminated Ge. General cleaning, halide passivation, and SAM formation steps are shown schematically in Figure 1. Particles and adventitious organics at the as-received Ge surfaces were first removed by 10 min sonication in neat acetone followed by chloroform. The samples were subsequently blown dry with N2. Two different treatments were used for the halide passivation step: cyclic oxidation/etching or direct acid dipping. Okumura et al.36 have reported that H2O2/HCl treatment employing consecutive oxidation/etching cycles decreases the Ge surface roughness, with a higher number of cycles (up to 3) leading to smoother surfaces. Hence, in the current work, we explored the use of a cyclic methodology where the surfaces were first oxidized by H2O2, and after a DI water rinse, HCl or HBr treatments were employed. For these studies, Ge samples were oxidized in H2O2 (30 wt % in H2O) for 5 min and then rinsed in deionized (DI) water. The DI water rinse ensured removal of GeO2 from the surface.1,7,9,31 Next, Ge samples were etched in aqueous HCl solution (10 wt %) or HBr solution (10 wt %) for 10 min. The hydrohalogenic etching step also served to remove the Ge oxide species.27,29,30,32,33 These oxidation-rinsingetching cycles were repeated three times to remove traces of the oxide and achieve halide termination of the Ge surfaces. It has been shown that a DI water rinse or exposure to water vapor can remove the halide passivation from the Ge surfaces.26,30 Consequently, the halide-terminated samples were directly blown dry with N2 as the final step. For comparison, we carried out direct acid dipping on some samples. For these sets of experiments, the as-received Ge(100) and Ge(111) samples were directly dipped in various aqueous HBr solutions (10, 20, and 48 wt %) and were subsequently blown dry with N2. For ex situ characterization of the halide-terminated Ge surfaces, the samples were transferred under sealed Ar-purged containers, which were wrapped with Al foil to avoid exposure to light.26 2.3. SAM Formation. To form the SAMs, halide passivated samples were dipped immediately after passivation into 10-4-10-1 M solutions of 1-octadecanethiol (ODT) in 2-propanol (isopropanol, IPA). All of the ODT solutions were prepared freshly before the experiments. We have also studied other solvents; the results of that study will be published separately.79 An Accumet X11 pH meter (79) Ardalan, P.; Bent, S. F. Unpublished results.

2016 Langmuir, Vol. 25, No. 4, 2009 with glass electrode was employed to determine the pH values of the ODT solutions. To eliminate solvent evaporation and exposure of the reaction media to ambient, the SAM containers were sealed by Parafilm and stored in an air-purged glovebox. We have investigated the reaction time required for formation of well-packed ODT SAMs by surveying times between 0 and 96 h. We also investigated the effect of concentration on the SAM formation and found that increasing the ODT solution concentration to values above 0.1 M resulted in formation of a suspension of precipitates after less than 1 h. This precipitation consequently decreased the water contact angles and ODT film thickness values of the resulting SAMs. Such an observation has also been reported by Kousri et al.72 Hence, 0.1 M was chosen as an upper ODT concentration limit throughout this work. It is stated in the literature that bases such as amines facilitate the conversion of thiols to thiolates.80 Moreover, it has been shown that pyridine forms strong hydrogen bonds with thiols, with the majority of the interaction resulting from S-H · · · N bond formation.81 Also, the use of pyridine has been shown to boost the SAM formation rate in the alcohol and Cl-terminated Si systems.82 Hence, we investigated the possible catalytic activity of pyridine on ODT SAM formation on halide-terminated Ge surfaces. However, that effect was found to be negligible on both Ge(100) and Ge(111) surfaces. After the ODT SAM formation, the samples were removed from the glovebox and sonicated in neat 2-propanol for 1 min to remove possible physisorbed molecules from the Ge surface. Next, samples were blown dry with N2 before characterization. We have found that washing the samples with other organic solvents results in a significant decrease in the SAM water contact angles. For ambient stability studies, the samples were either directly exposed to ambient under fluorescent laboratory light or stored in containers that were protected against light (i.e., under dark conditions) by Al foil before characterization. For repassivation studies, air-exposed samples were first dipped in 10% or 48% HBr solutions for 10 min and then dipped in fresh 0.1 M ODT solutions for 72 h. 2.4. Film Characterization. Contact Angle Measurements. An FTA 2000 dynamic contact angle analyzer was used to measure static water contact angles (WCAs). For these measurements, a sessile drop of Millipore-Q water with resistivity of 18.2 MΩ-cm with constant volume of about 10 µL was applied to the surfaces. At least five different points on each sample were measured, and an average WCA value corresponding to measurements on several samples is reported. The accuracy of these measurements was ( 2°. The WCAs were independent of the droplet volume up to 20 µL, which is in agreement with previous studies.71,72 Ellipsometry. Film thicknesses (FTs) were measured using a Gaertner L116C single-wavelength ellipsometer. Real and imaginary parts of the refractive index (ns and ks) that corresponded to the freshly halogenated Ge substrates were recorded for each experiment. To measure the ODT film thicknesses, these refractive indices were used as the optical properties of the substrates. A refractive index of nf ) 1.4645 was used for the ODT films.83 At least six different points were measured on each sample to check the film uniformity, and an average value corresponding to several samples is reported. The accuracy and repeatability of the measurements were (0.3 and (0.1 nm, respectively. ODT surface coverage was approximated by taking the ratio of the film thickness and the expected film thickness for ODT SAMs (∼25 Å).71 X-Ray Photoelectron Spectroscopy (XPS). An SSI S-Probe monochromatized spectrometer system was used in this work. The XP spectra were taken with an Al (KR) X-ray source (1486.6 eV) in an UHV system with base pressure in the 10-9 torr range. To increase the surface/bulk signal ratio, the emitted electrons were (80) Oae, S. Organic Sulfur Chemistry: Structure and Mechanism; CRC Press: Boca Raton, Fl, 1991. (81) The chemistry of the thiol group; John Wiley & Sons, Ltd.: Bristol, 1974; Vol. 1. (82) Zhu, X.-Y.; Boiadjiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Langmuir 2000, 16, 6766–6772. (83) CRC Handbook of Chemistry and Physics, 81st ed.; CRC press LLC: Boca Raton, Fl., U.S.A., 2000-2001.

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Figure 2. XP spectra of (a) as-received Ge(100), (b) Cl-terminated Ge(100), and (c) ODT SAM at the Ge(100) surface. All the spectra are shown on the same scale.

collected at a takeoff angle (i.e., angle between the detector and surface normal) of 55°. The survey scans were collected using a hemispherical electron energy analyzer at a pass energy (PE) of 156.5 eV with 1 eV resolution. Furthermore, the Ge(3d), S(2p), O(1s), C(1s), Cl(2p), Cl(2s), and Br(3d) high-resolution scans were collected at a 55.1 PE with 0.1 eV resolution. All the spectra shown in this paper have a detection sensitivity of ∼0.01 monolayer or ∼0.1 atomic %. The XPS data were processed using a Shirley background correction84 followed by fitting to Voigt profiles. All peaks were adjusted using the bulk C(1s) peak at 284.6 eV to correct the binding energies for the charge shift.85 The Cl or Br coverage at the halogenated Ge surfaces (θ) was determined using the continuum model (eq 1) developed by McFeely et al.86

[ ]

θ ) l - d ⊥ ln

It It - Is

(1)

In this equation It is the total intensity of the Ge(3d) bulk plus the surface emission, and Is represents the intensity of the chemically shifted core levels (surface emission). Also, an electron mean free path (l) of 28.4 Å87 and average layer spacings perpendicular to surface normal (d⊥) of 1.42 Å15 and 1.63 Å15 were used for Ge(100) and Ge(111), respectively. The ODT monolayer thickness was calculated based on the simplified version of the exponential absorption model suggested by Ranke et al.88 as shown below.

( )

d ) -λ cos θ ln

C1s Ge3d

(2)

Equation 2 essentially calculates the attenuation of the Ge(3d) signal after ODT self-assembly (assuming full monolayer coverage). In this equation, θ is the takeoff angle (55°) and λ (36 Å) is the photoionization mean free path (attenuation length), which was determined using an empirical expression for n-alkanethiolate SAMs developed by Whitesides and co-workers.89 Auger Electron Spectroscopy (AES). A PHI 700 Scanning Auger Nanoprobe was used in these studies. This instrument has an elemental detection limit of 0.1 atomic %. A 10 keV 10 nA electron beam was employed to achieve 18-20 nm spatial resolution. At least five different points were measured at the surface of each sample to (84) Shirley, D. A. Phys. ReV. B 1972, 5, 4709–4714. (85) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp. Physical Electronics Division: 1992. (86) McFeely, F. R.; Morar, J. F.; Shinn, N. D.; Landgren, G.; Himpsel, F. J. Phys. ReV. B 1984, 30, 764–770. (87) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Int. Anal. 1991, 17, 911. (88) Ranke, W.; Jacobi, K. Surf. Sci. 1977, 63, 33–44. (89) Laibinis, P. E.; Colin, D. B.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017–7021.

Formation of Alkanethiolate SAMs

Figure 3. High-resolution XP spectra of (a) Ge(3d) and (b) C(1s) for as-received, Cl-terminated, and thiolated Ge(100). For each figure, all three spectra are on the same scale.

confirm the uniformity of the films. The survey scans were collected at 1 eV/step resolution, and each spectrum was averaged over 30 scans to obtain satisfactory signal-to-noise ratios. Elemental analysis was carried out using the Multipak software, and the sensitivity of the Auger signals was calibrated relative to the Cu(LMM) signal at 10 keV. Fourier Transform Infrared (FTIR) Spectroscopy. Transmission IR spectra of the ODT SAMs were collected using a Nicolet 6700 FTIR spectrometer with a nitrogen-cooled indium antimonide (InSb) detector. Spectra were collected 15 min after the introduction of the samples to the holder. This resulted in significant reduction in the H2O and CO2 IR peaks from the ambient air that was introduced upon mounting the samples. Each spectrum was recorded and averaged over 5000 scans with 2 cm-1 resolution in transmission mode. To obtain absorption spectra of ODT-covered Ge, a background transmission IR spectrum was acquired after halide termination. Baseline correction was carried out using a linear function.

3. Results 3.1. Thiols on Cl-Passivated Ge Surfaces. 3.1.1. XPS. X-ray photoelectron spectroscopy was carried out to investigate the elemental composition and oxidation states of the species at the Ge(100) and Ge(111) surfaces before and after each treatment. Results for the Ge(100) studies are included here. Because the trends were very similar, data for the Ge(111) surface can be found in the Supporting Information. Figure 2 shows XPS survey scans, while Figure 3 shows Ge(3d) and C(1s) fine scan XP spectra of the as-received, Cl-terminated and 1-alkanethiol-terminated Ge(100). Respective spectra for the Ge(111) surface are compiled in the Supporting Information. As-received Ge(100) surfaces are characterized by Ge, C, and O XPS peaks (Figure 2(a)). Figure 2(a) shows that as-received Ge(100) samples possess a native oxide that appears as a clear shoulder on the Ge(3d) peak. The Ge(3d) fine scan reveals that this peak is made up of several underlying peaks. Specifically, five peaks are used to fit the data in Figure 3(a). Apart from the Ge(3d) bulk peak (Ge0+), four other components with chemical shifts of +0.8, +1.8, +2.6, and +3.4 eV are also observed that correspond well to shifts reported in the literature for Ge bonded

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to O with oxidation states of +1 to +4, respectively.26,76 The largest contribution comes from the +4 state (GeO2). In addition to oxygen, carbon is present, presumably in the form of adventitious hydrocarbon adsorbed at the native-oxide covered Ge surface. The spectra are very similar for Ge(111), suggesting the presence of oxidized germanium and residual carbon. After chlorination by cyclic oxidation/etching treatments, the XP spectrum of the Cl-terminated Ge(100) surface (Figure 2(b)) shows clear evidence for the presence of Cl. The peak at 269.8 eV is attributed to Cl(2s), while the peak at 198.7 eV is a convolution of the Cl(2p) doublet (2p3/2 and 2p1/2) and a plasmon loss from the Ge(3s).28,85 The binding energy (BE) values further suggest that Cl atoms at the Cl-terminated Ge surface have been shifted by at least 1.2 eV to lower BE compared to elemental Cl.85 Such shifts in BE are expected based on the electronegativity differences between Ge and Cl (the values are 2.01 and 3.16, respectively).83 Hence, the more electronegative Cl shifts to a lower BE upon bonding to Ge. The survey scan also shows that chlorination removes oxygen, as evident by the loss of the Ge(3d) shoulder.26,76 Furthermore, the C signal decreases after chlorination compared to the as-received spectrum, although it is not gone completely (Figure 3(b)). This residual carbon signal may be due to the presence of adventitious C appearing as a result of the ex situ analysis, or possibly from new Ge-C bonding, as was reported for H-terminated Ge(100). Additional studies would be required to further probe the nature of this carbon.7,90 However, the data show that the cyclic chlorination treatment used here removes more C from the Ge(100) surface than does the previously reported HF process.7 The Ge(3d) fine scan peak for the chlorinated surface reveals both the removal of oxygen and the addition of chlorine. The Ge(3d) peak can be fit to three major contributions: the Ge0+ peak and two peaks with chemical shifts of +0.6 and +1.1 eV. These shifted peaks are attributed to the Ge monochloride and Ge dichloride, in agreement with previous studies.25,74 A calculation based on eq 1 indicates that the surface coverage of the monochloride and dichloride species is 0.33 ( 0.05 and 0.68 ( 0.03 monolayer, respectively. No peaks associated with germanium oxides are observed. XP spectra obtained after chlorination of the Ge(111) surface are very similar to those for Ge(100) with one main exception. The fine scan Ge(3d) peak is best fitted to only two peaks instead of the three observed for Ge(100). One is the bulk Ge peak, and the other is chemically shifted by +0.6 eV, which is assigned to Ge monochloride in agreement with previous reports.25,26,28 The estimated surface coverage of chlorine is 1.12 ( 0.01 monolayer. Recent AFM studies suggest similar surface roughness for chlorinated Ge(100) and Ge(111).74 Hence, the presence of different species at the Ge(100) and Ge(111) surfaces appears to be independent of surface roughness. Instead, the absence of dichloride species at Ge(111) surface is probably due to the availability of only one dangling bond per Ge(111) surface atom. After thiolation, the XP spectra again change. Important features are the large increase in the C(1s) peak intensity and the loss of the Cl(2s) peak. Moreover, S(2s) and S(2p) peaks at 226.1 and 162.6 eV are observed from the spectrum in Figure 2(c), in agreement with the presence of surface thiolates.91 It is important to notice that S orbitals have low photoionization crosssections.85 In addition, the S atomic % is expected to be low compared to C and Ge, and the S is buried under the alkyl chains. These three characteristics lead to the observation of low intensity (90) Tabet, N.; Faiz, M.; Hamden, N. M.; Hussain, Z. Surf. Sci. 2003, 523, 68–72. (91) Jun, Y.; Zhu, X.-Y.; Hsu, J. W. P. Langmuir 2006, 22, 3627–3632.

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peaks for S in the final XP spectrum. However, separate AES results clearly confirm the presence of S at the Ge surface (vide infra). Further deconvolution of the C(1s) peak (Figure 3(b)) for the thiolated Ge surface reveals that this peak can be fitted to two components. One is the bulk C peak attributed to the alkyl chain, and the other one has a +1.0 eV chemical shift.92 The shift coincides with the value expected for carbon bonded to sulfur.85 The relative intensity of these two peaks is 15/1, which is close to the 17/1 value expected for C18H37S-Ge species. Careful study of the Ge(3d) peaks for the thiolated surface (Figure 3(a)) reveals the absence of Ge oxide peaks at the surface. Moreover, the flat-topped shape of the Ge(3d) fine scan peak suggests a complex convolution of peaks, which due to limitation in sensitivity and resolution of conventional XPS instruments cannot be completely resolved with the current system. Hence, we will leave detailed study of these shifts to future synchrotron work.93 The carbon-to-germanium peak ratios from the XPS spectra can be used to estimate the thickness of the ODT monolayer, according to eq 2. This calculation yields an estimated film thickness for the thiolate layer made with 0.1 M ODT of approximately 19 Å. XPS scans of the thiolated Ge(111):Cl surface are similar to those for Ge(100):Cl. The results show a lack of Ge oxides and the presence of S. However, contrary to Ge(100), some Cl is still present at the Ge(111) surface after thiolation. Moreover, the thickness of the thiolate films formed from 0.1 M ODT concentration at Ge(111) is estimated to be ∼12 Å using eq 2, a value smaller than that found on Ge(100). The significance of these film thicknesses will be discussed in the next section. On the whole, the above results indicate that 1-octadecanethiol has reacted at both Cl-terminated Ge(100) and Ge(111) surfaces. Hence, we expect that 1-alkanethiols have attached to the surface via Ge-S bonds and have either completely (at Ge(100)) or partially (at Ge(111)) removed the Cl. 3.1.2. Water Contact Angles and Film Thicknesses. Water contact angle measurements and ellipsometry were employed to probe the packing and thickness of the 1-alkanethiolate SAMs. Our results suggest that although 48-72 h is adequate to produce well-packed SAMs at the Ge:Cl surface, increasing the immersion time to 96 h slightly increases the WCAs and FTs. The maximum achievable WCA and FT for the Ge(100):Cl system after 96 h dipping at 0.1 M ODT in IPA are 104.2 ( 1.4° and 20.2 ( 0.6 Å, respectively. In comparison, the WCA and FT for films formed at Ge(111):Cl after employing the same dipping time and ODT concentration are 102.3 ( 0.9° and 9.8 ( 1.7 Å, respectively. The thickness value on the Ge(111) surface is significantly lower than that on Ge(100). The thickness values determined from ellipsometry are in good agreement with those calculated from the XPS data for both (100) and (111) surfaces (19 and 12 Å, respectively). Although there is a significant difference between the (100) and (111) surfaces, for both systems the film thicknesses are lower than that expected for upright ODT molecules (∼25 Å).71 The lower values measured for the ODT SAMs on Ge likely reflect either submonolayer coverages or a significant tilt to the molecules in the SAM. Here we consider each possibility in turn. If the SAMs do not form a complete monolayer, the calculated thickness (∼20 Å) on Ge(100) corresponds to a surface coverage of ∼0.8 (92) A shift to higher binding energies is also expected for C-O-type bonding in alcohols and ethers. However, this peak is expected to exhibit a larger shift (+1.5 eV) than that observed in the spectrum (+1.0 V), indicating that the 2-propanol (IPA) solvent is not adsorbed at the surface at significant concentration. (93) Ardalan, P.; Sun, Y.; Pianetta, P.; Bent, S. F. to be submitted.

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monolayer.71 We note that the XPS calculation from which the thickness was derived assumes a coverage of 1.0 monolayer. The submonolayer coverage may arise if the bulky ODT molecules do not terminate all of the surface Ge atoms due to steric hindrance. Similarly, the FTs calculated for ODT SAMs on Ge(111) would correspond to a surface coverage of ∼0.4 monolayer. It is clear from this analysis that the quality of the SAM is poorer on Ge(111) compared to Ge(100). If we assume instead that there is full monolayer ODT coverage, the thickness value would suggest that the SAM’s tilt angle with respect to surface normal is ∼40° on Ge(100). On Ge(111), we would assign a 1-octadecanethiolate tilt angle of ∼60°. We note that this 60° tilt angle is comparable to reported 1-alkanethiolate films of the same chain length that form at GaAs(001) surfaces.91 However, these values are much higher than that observed on Au(111), where the thickness and tilt angle of 1-octadecanethiolates are ∼22 Å and ∼30°, respectively. We are unable to distinguish between the possibilities of high tilt angle or submonolayer coverage (or a combination of both) with just XPS and ellipsometry data. Our later discussion based on IR data will better clarify this point (see section 3.2.2.3). Different parameters used in the thiolate SAM formation were varied to examine their effect on SAM quality. It has been proposed that increasing the concentration of the thiol solution results in an increase in the surface coverage of the 1-alkanethiolate SAMs formed on the surface of Au60 and on the H-terminated Ge(111) surface.72 We have therefore monitored the WCAs and FTs of the ODT films formed at the Cl-terminated Ge surfaces as a function of the ODT solution concentration. Figures 4(a) and 4(b) show the results of WCA and FT measurements for Ge(100) and Ge(111) as a function of concentration, respectively. From Figure 4, we note that the WCAs and FTs are directly correlated with the concentration. An increase in the WCAs, which signifies an increase in the hydrophobicity of the SAMcoated surface, together with the increase in film thickness suggest that higher SAM surface coverages are achieved with more concentrated solutions.94 Interestingly, Figure 4(b) shows that although the FT values increase with concentration they are still quite low even at high concentrations on Ge(111). A phenomenon known as the “short chain effect”91 has been previously proposed to explain low FTs measured for shorter chain molecules and is attributed to the fact that those alkanethiolates are lying down at the surface instead of forming well-packed monolayers. Here, the high WCAs and very low FTs observed at high concentrations of a long 1-alkanethiol (ODT) may indicate that these molecules are lying down at the (111) surface. 3.1.3. Ambient Stability. The stability of the SAM-coated surface when exposed to ambient was also studied by WCA measurements and ellipsometry, as shown in Figure 5. We have observed that films with higher WCAs and thicknesses formed from concentrated solutions are generally more air-stable, so for these studies ODT SAMs were formed using 0.1 M ODT with 48 h dipping time. It is found that the WCAs decrease as exposure time increases, whereas the FTs increase. This behavior can be explained as follows: oxygen and perhaps water vapor can attack the SAM-coated surface, oxidizing the underlying Ge substrate and leading to a higher measured thickness by ellipsometry. We do not believe that oxidation of the thiolates themselves is likely since previous studies have shown that oxidation of the thiolates (94) In fact, our XPS results show that increasing the concentration of ODT dissolved in IPA from 0.001 to 0.1 M resulted in an increase in the C/Ge atomic ratio from ∼0.73 to ∼1.42 for the Ge(111)-Cl terminated + ODT system. Also, an increase from ∼1.28 to ∼1.83 was observed at the (100) crystallographic orientation upon ODT concentration increase from 0.001 to 0.1 M.

Formation of Alkanethiolate SAMs

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Figure 6. O/Ge atomic ratio as a function of ambient exposure time calculated from XP spectra. Weighted linear fittings are shown. The thiolated Ge(100) surface is more resistant toward oxidation compared to the chlorinated Ge(100) surface.

Figure 4. Static water contact angles (WCAs) and film thicknesses (FTs) of the ODT SAMs formed at (a) Cl-terminated Ge(100) and (b) Cl-terminated Ge(111) surfaces as a function of 1-octadecanethiol concentration using IPA solvent. SAMs were formed by dipping in the ODT solution for 48 h.

Figure 5. Static water contact angles (WCAs) and film thicknesses (FTs) of the 1-octadecanethiolate SAMs at the Cl-terminated Ge(100) and Ge(111) surfaces as a function of time of exposure to ambient. SAMs were formed for 48 h prior to ambient exposure from 0.1 M ODT in IPA solution. Weighted linear fittings are shown as solid or dashed lines.

to sulfonates requires UV radiation and is slow for long chain thiols.60 In support of the surface oxidation mechanism, XPS studies on treated Ge(100) samples show an increase in the O/Ge atomic ratio from 0.08 immediately after SAM preparation to 0.15 and 0.36 after 2 and 8 days of ambient exposure, respectively (Figure 6). At the same time, the C/Ge atomic ratio decreases

from 1.83 to 1.05 and 0.85 after 4 and 8 days exposure, respectively. Thus, the increase in the FTs is attributed mainly to an increase in the oxide layer formed at the Ge interface. The presence of the oxide may disrupt the thiolate packing, leading to a decrease in the intermolecular forces between the alkyl chains and resulting in a decrease in the WCAs. Moreover, as the hydrophobicity of the surface decreases, more water vapor can penetrate to the surface, further promoting the oxidation. In addition, the decrease in the C/Ge ratio suggests that loss of the SAM, which may be dominated by S-C bond cleavage, may also occur.60 Despite the SAM degradation and the oxidation of the interface with ambient exposure, these SAM-terminated surfaces are found to be more stable against oxidation than the Cl-terminated Ge(100) surface. Figure 6 shows that the O/Ge ratio increases more quickly with ambient exposure time for Cl-terminated Ge than for thiol-terminated Ge. 3.2. Thiols on Br-Passivated Surfaces. 3.2.1. Bromine PassiVation Process. To better understand the Br passivation process, various bromination methods were examined. Figures 7(a) to 7(c) show XPS fine scans of the C(1s), O(1s), and Br(3d) peaks after four different bromination methods. These methods include cyclic oxidation and etching with 10% HBr and direct cleaning with different HBr concentrations, namely, 10%, 20%, and 48%. Figure 7 indicates that for the direct acid etching a higher concentration of HBr removes more oxygen and carbon from the surface. Concentrated acids (g20%) remove almost all of the O and leave behind only a small amount of C, although this residual C signal may be from physisorbed hydrocarbon resulting from the ex situ measurement. Another possible source of C is contamination from the solutions used in wet treatments. Examination of the Br(3d) peaks (Figure 7(c)) reveals that the integrated peak area (shown in parentheses in the figure) increases with increasing concentration of the HBr solutions, signifying an increase in the surface coverage of the Br species at the Ge(100) surface. Moreover, cyclic and direct cleaning with 10% HBr show differences in O and C removal. Cyclic treatment removes more C but leaves behind more O than the direct etching treatment. This can be attributed to oxidization of the material in the cyclic treatment, which uses H2O2. However, cyclic treatment with 10% HBr leads to Br surface coverage similar to direct etching with 10% HBr. In the following sections, we will show that the Br surface coverage has a significant effect on the packing and quality of the ODT SAMs formed at Br-passivated surfaces prepared by the different techniques mentioned in this section.

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Figure 9. XP spectra of (a) Ge(3d) and (b) C(1s) for the as-received, Br-terminated, and thiolated Ge(100). For each figure, all three spectra are on the same scale.

Figure 7. XP spectra of (a) C(1s), (b) O(1s), and (c) Br(3d) after asreceived Ge(100) samples were treated by different bromination methods. The numbers in parentheses in (c) show the total integrated area of the Br(3d) peaks after each treatment. For each figure, all four spectra are on the same scale. Figure 10. Auger electron spectrum of a 1-octadecanethiolate SAM formed on the Ge(100):Br surface.

Figure 8. XP spectra of the (a) as-received Ge(100), (b) Br-terminated Ge(100), and (c) ODT SAM at the Ge(100) surface. The Br-terminated spectrum was acquired after as-received samples were brominated by cyclic oxidation and etching with 10% HBr. All the spectra are shown on the same scale.

3.2.2. Thiols. 3.2.2.1. XPS. Figure 8 shows the XP spectra taken before and after as-received Ge(100) samples were brominated by cyclic treatment employing 10% HBr and then thiolated by 72 h dipping in 0.1 M ODT in IPA solution. Figures

9(a) and 9(b) represent the fine scans of the Ge(3d) and C(1s) peaks corresponding to these experiments. Important features of the brominated spectra are the presence of Br(3d), Br(3p), and Br(3s) peaks along with elimination of the oxide shoulder from the Ge(3d) peak (Figure 8(a)). The Br(3d) peak at 68.8 eV is shifted -1.2 eV relative to atomic Br, which can be attributed to attachment of the more electronegative Br (Pauling electronegativity: 2.96) to the more electropositive Ge (Pauling electronegativity: 2.01). Analysis of the Ge(3d) peak of the brominated surface (Figure 9(a)) suggests that it consists of three peaks, namely, bulk Ge(3d) and two other peaks shifted to higher BE with values of +0.5 and +1.0 eV. Although the shoulder is small, a parallel synchrotron study of the same HBr/Ge system shows convincingly that these two small features arise from monobromide and dibromide species.74 Calculation based on eq 1 indicates that the surface coverage of the Ge monobromide and Ge dibromide is 0.31 ( 0.02 and 0.63 ( 0.03 monolayer, respectively. It is interesting to note that, as seen in Figure 9(a), the bulk Ge(3d) peak corresponding to the brominated surface after oxide removal is shifted by ∼0.6 eV to higher BE. However, after

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thiolation this shift is reduced from ∼+0.6 to ∼+0.2 eV with respect to the as-received Ge(3d) spectrum. Hovis et al.37 performed STM and XPS studies on p-type Ge substrates and observed that the bulk Ge(3d) peak shifted to lower BE (-0.5 eV) after oxide removal by annealing. On the other hand, studies by Tabet et al.90 on n-type Ge substrates revealed that upon oxide removal the Ge(3d) bulk peak shifted toward higher BE (+0.6 eV). Such phenomena, observed also on Ge NWs,95 is attributed to Fermi level pinning and band bending caused by interfacial Ge oxide states before treatment which are unpinned upon removal of the oxide. Hence, because n-type Ge substrates were used in the current work, such a shift in the bulk Ge peaks to higher BE is expected upon removal of the oxide. Moreover, the change in the bulk Ge(3d) peak shift after thiolation (from +0.6 to +0.2 eV) may indicate that the thiolated surface is pinned. Upon thiolation, the Br(3d) peak is eliminated; a S(2p) peak appears; and the C(1s) signal increases significantly (Figure 8).96 The fine scan of the C(1s) peak after thiolation (Figure 9(b)) shows that in addition to the bulk C(1s) there is also a smaller peak at higher BE (∼1.0 eV shift). This new feature is ascribed to carbon atoms bonded to the sulfur of the thiol group. The ratio of these two peaks is 1/25, lower than the expected ratio for a C18 chain, indicating that part of the bulk C signal is plausibly from adventitious materials. Analyzing the XPS results by the models referenced earlier, we determine that the maximum thickness of the ODT SAMs is ∼18 Å. Assuming a full monolayer coverage, the thickness value would translate to ∼44° tilt angle with respect to the surface normal. The Ge(3d) peak after thiolation (Figure 9(a)) exhibits a nonLorentzian line shape, suggesting that multiple peaks contribute to the feature. Further understanding of the Ge(3d) peak requires higher-resolution studies, e.g., performed with synchrotron radiation, which we leave for future investigations.93 However, the Ge(3d) peak of the thiolated surface clearly exhibits an absence of Ge oxides, indicated by the lack of a shoulder at high BE. 3.2.2.2. AES. Auger electron spectroscopy (AES) was performed to further study the presence of S at the Ge surface as well as to assess the uniformity of the thiolated surface. Figure 10 shows the AES spectrum of the Ge(100) surface taken after bromination/thiolation steps are completed. The data clearly show the presence of a major S(LMM) transition peak at 155 eV. Other major peaks include Ge(LMM), C(KLL), and O(KLL) at 1150, 275, and 517 eV, respectively.97 No Br was detected at the surface, which suggests complete reaction of the Br-terminated surface with ODT, a result consistent with the XPS data. Calculation of the elemental composition from the AES spectrum shows that ∼2% O is present at the surface, which agrees with the very small O peak observed in XPS (not shown). Moreover, AES scans acquired from different points of the surface indicate that uniform films have been formed. 3.2.2.3. IR. To determine the degree of order in the SAMs, transmission infrared (IR) spectroscopy experiments were performed. Investigation of the C-H stretching vibrational region provides a detailed understanding of the conformational order of the 1-alkanethiolate chains. Specifically, the peak position and peak width of the methylene (CH2) stretching modes are sensitive indicators of packing structures of self-assembled (95) Wang, D.; Chang, Y. L.; Wang, Q.; Cao, J.; Farmer, D. B.; Gordon, R. G.; Dai, H. J. Am. Chem. Soc. 2004, 126, 11602–11611. (96) We have found that both higher ODT concentration and longer dipping time result in removal of more Br from the surface. XPS data exhibit higher growth in the C(1s) peak, which can be attributed to higher thiolate coverage as well as higher conversion in the reaction between thiol molecules and Br-terminated surface. (97) Handbook of Auger Electron Spectroscopy, 3rd ed.; Physical Electronics Inc.: Eden Prairie, MN, 1995.

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Figure 11. Transmission infrared spectrum showing the C-H stretching region for (a) Br-terminated Ge(100) and (b) a 1-octadecanethiolate SAM formed at the Br-terminated Ge(100) surface. The designated peak positions of the thiolated Ge surface coincide with those expected for crystalline SAMs.

monolayer films.98,99 In a liquid-like alkane, the νs(CH2) and νas(CH2) modes appear at 2856 and 2928 cm-1, but for crystalline hydrocarbons, those peaks are shifted to lower wavenumber, appearing near 2850 and 2917 cm-1, respectively. Consequently, the shifts of the methylene stretching modes to lower wavenumber can be used as one indication of increased conformational order of the adsorbed alkyl chains.98,99 Figure 11 shows the C-H stretching region of the IR absorbance spectra of Br-terminated and thiolated Ge(100) surfaces, respectively, for surfaces prepared by cyclic H2O2 and 10% HBr treatments (Figure 11(a)) followed by dipping in 0.1 M ODT for 72 h (Figure 11(b)). The brominated spectrum, ratioed to a background of atmosphere, shows no significant peaks in this wavenumber range. However, the IR spectrum of the SAMcoated surface ratioed to the Br-terminated surface (Figure 11(b)) indicates significant growth in the C-H stretching peaks reported previously for ordered SAMs.54,56,100 The peaks observed at 2918 and 2848 cm-1 coincide well with the CH2 antisymmetric and symmetric stretching vibrational modes, respectively. Moreover, the peak at 2959 cm-1 is characteristic of the CH3 antisymmetric stretching vibrational mode. These three principal peaks are in agreement with the 2958, 2919, and 2850 cm-1 values reported by Kosuri et al. for hexadecanethiolate SAMs formed at the Ge(111)-H terminated surface.72 Importantly, the frequencies measured here are consistent with crystalline-type packing in the hydrocarbon chains.98,99,101-103 This observation (98) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145–5150. (99) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136–6144. (100) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H.; Kruus, E. Langmuir 1997, 13, 5335–5340. (101) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; Van der Maas, J. H.; De Jeu, W. H.; Zuihof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759–1768. (102) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155.

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Ardalan et al.

Table 1. WCAs and FTs Corresponding to ODT SAMs Formed at Ge Surfaces after the As-Received Ge Samples Are Treated by Various Bromination Methods Ge(100) bromination method Cyclic H2O2/DI/HBr 10% HBr 10% HBr 20% HBr 48%

water contact angle (°) film thickness (Å) 103.8 ( 1.5 102.3 ( 1.5 103.2 ( 0.9 99.4 ( 0.9

13.9 ( 0.9 15.3 ( 1.0 11.2 ( 1.6 5.3 ( 1.6

Ge(111) bromination method cyclic H2O2/DI/HBr 10% HBr 10% HBr 20% HBr 48%

water contact angle (°) film thickness (Å) 97.1 ( 2.5 99.9 ( 1.4 99.9 ( 2.2 87.2 ( 3.0

9.7 ( 3.1 8.3 ( 2.3 7.4 ( 1.4 3.6 ( 1.9

indicates that the ODT SAMs formed at the Ge(100)-Br terminated surface are highly ordered, similar to the long-chain alkanethiolate SAMs formed on metal surfaces.54,56,58,104 On the basis of this high degree of order derived from the IR measurements, we suggest that the low thicknesses measured by both XPS and ellipsometry for these SAMs are primarily from a tilt and not from a low coverage since IR studies of ODTS SAMs have shown that the IR frequencies do not reach crystallinelike values until high coverages are reached.105 Using the assumption of full coverage, the tilt value extracted from the XPS measurements (44°) would suggest that the ODT SAMs formed on Ge(100) have a ∼14° larger tilt angle compared to ODT monolayers at the Au(111) surface (i.e., typical tilt angle on Au(111) is ∼30°).54,56,58,104 3.2.2.4. WCAs and FTs. Table 1 presents WCAs and FTs of the ODT SAMs formed at Br-terminated Ge(100) and Ge(111). Various bromination methods were employed to generate the thiolated surfaces. On the basis of the trends discussed in section 3.1.2, 0.1 M ODT dissolved in IPA was employed in these studies to reach the maximum SAM coverage. Table 1 shows that the maximum WCA and FT formed at Ge(100) surfaces after 72 h of dipping are 103.8 ( 1.5° and 15.3 ( 1.0 Å, respectively. These WCA values are significantly higher (more hydrophobic) that that for the as-received Ge (WCA∼