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Effect of Ge surface termination on oxidation behavior
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Applied Surface Science 254 (2008) 7544–7548 www.elsevier.com/locate/apsusc
Effect of Ge surface termination on oxidation behavior Younghwan Lee a, Kibyung Park a, Yong Soo Cho b, Sangwoo Lim a,* b
a Department of Chemical Engineering, Yonsei University, Republic of Korea School of Advanced Materials Science and Engineering, Yonsei University, Republic of Korea
Available online 15 January 2008
Abstract Sulfur-termination was formed on the Ge(1 0 0) surface using (NH4)2S solution. Formation of Ge–S and the oxidation of the S-terminated Ge surface were monitored with multiple internal reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. In the 0.5, 5, or 20% (NH4)2S solution, H-termination on the Ge(1 0 0) surface was substituted with S-termination in 1 min. When the S-terminated Ge(1 0 0) surface was exposed in air ambient, the oxidation was retarded for about 3600 min. The preservation time of the oxide layer up to one monolayer of S-terminated Ge(1 0 0) surface was about 120 times longer than for the H-terminated Ge(1 0 0) surface. However, the oxidation of S-terminated Ge(1 0 0) surface drastically increased after the threshold time. There was no significant difference in threshold time between S-terminations formed in 0.5, 5, and 20% (NH4)2S solutions. With the surface oxidation, desorption of S on the Ge surface was observed. The desorption behavior of sulfur on the S-terminated Ge(1 0 0) surface was independent of the concentration of the (NH4)2S solution that forms S-termination. Non-ideal S-termination on Ge surfaces may be related to drastic oxidation of the Ge surface. Finally, with the desulfurization on the S-terminated Ge(1 0 0) surface, oxide growth is accelerated. # 2008 Elsevier B.V. All rights reserved. PACS : 61.72.uf; 66.70.Df; 68.35.bg; 68.37. d Keywords: Germanium; Surface; Termination; Oxidation
1. Introduction Germanium is a promising starting material for advanced high-performance devices with a carrier transport property superior to that of silicon [1,2]. However, for the application of Ge to the fabrication of devices, it is necessary to effectively control its surface condition. As is well known, an H-terminated Ge surface is unstable to oxidation in air ambient, contrary to an H-terminated Si surface [3,4]. Therefore, a robust termination of the Ge surface to retard the formation of native oxide is needed. Moreover, amorphous GeO2, which is one of the oxidized Ge structures, is dissolved in H2O [5], acid, and alkali solution [6]. Therefore, it is crucial to investigate the oxidation characteristics of the Ge surface on the designed termination.
* Corresponding author at: Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Republic of Korea. Tel.: +82 2 2123 5754; fax: +82 2 312 6401. E-mail address: [email protected] (S. Lim). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.022
Up to the present, various terminations and passivations, such as H-termination [3,4,7], halogen-termination [3], alkyltermination [3], Ba-passivation [8], As-passivation [9], Si thin layer-passivation [10], thiol-passivation [11,12], and sulfurtermination [3,13–22] have been proposed. Particularly, it is reported that an S-terminated Ge surface is more stable than an H- or halogen-terminated surface, because of its higher bond energy [23]. Therefore, S-passivation on a Ge surface has been demonstrated in high-k dielectrics deposition [13], the fabrication of an epitaxial heterostructure [14], and metal deposition [15]. S-termination can be formed by gaseous methods with hydrogen sulfide (H2S) [16–18], liquefied methods with ammonium sulfide ((NH4)2S) [3,13–15,19], and solid-state electrochemical methods [20–22]. It is found that S-terminations prepared in those methods exhibits a bridge bond structure between Ge and S [14,17,18,22,24,25]. However, the oxidation mechanism on an S-terminated Ge surface has not been completely established. In the fabrication of Ge-based electronic devices, it is important to effectively control queue time between surface treatment and gate oxidation. Therefore,
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the stability of the S-terminated Ge surface to oxidation needs to be understood. In this paper, using multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR), passivation of Ge(1 0 0) with sulfur in (NH4)2S solution and the oxidation of the S-terminated Ge(1 0 0) surface in air ambient were studied. In addition, stability to the oxidation of S-termination on a Ge surface was compared with that of H-termination. 2. Experimental Undoped double-side polished Ge(1 0 0) wafer (resistivity > 40 V cm) was used. The Ge wafer was cut to 80 mm 10 mm 0.45 mm, and a trapezoid shape at 458 was crafted using 0.3 mm silica lapping film (3 M) to achieve internal reflection of 200 times. To remove particles and organic compounds on the Ge surface, the Ge wafer piece was first cleaned in methanol and then acetone for 5 min, respectively. Then, native oxide was removed in 0.5% hydrofluoric acid (HF) for 5 min. A 35.4 g/m3 ozone was produced by a corona discharge ozone generator and fed into the chamber, where the Ge wafer piece was mounted on the susceptor. At a susceptor temperature of 100 8C, oxidation of the Ge surface was performed for 30 min. Next, the Ge wafer piece was dipped in 10% HF solution for 20 min at 25 8C to etch the oxidized Ge layers and produce H-termination on the Ge(1 0 0) surface. Then, the H-terminated Ge surface was treated in 0.5, 5, or 20% (NH4)2S solution for 40 min to form Stermination on the Ge surface. For the periods of each dipping time, FT-IR spectra were measured. After the formation of Spassivation, the Ge wafer piece was exposed in laboratory air ambient, and the growth of the Ge native oxide was observed using FT-IR. IR vibration spectra were measured with FT-IR (Thermo electron, Nicolet 380) using a horizontal attenuated total reflectance (HATR) accessory (PIKE, 022-1260-45). Measurement was performed at 128 scan with 4 cm 1 resolution. The change of sulfur concentration on the Ge surface for a period of time was observed using XPS (ESCALAB 220i-XL) with a microfocused monochromator and moveable Al anode. 3. Results and discussion Fig. 1(a) shows the change of Ge–H FT-IR vibration mode. First, it is observed that H-termination was successfully formed on the Ge(1 0 0) surface by dipping in 10% HF solution. Here, the FT-IR spectra were taken using the oxidized Ge surface as a reference. The change of FT-IR spectra after treatment in 0.5% (NH4)2S solution is also shown. The FT-IR spectra of a Ge surface treated in 10% HF solution were used as a reference. Ge–H peaks have a negative absorbance after Ge surface treatment in (NH4)2S solution, which suggests that Htermination vanishes and S-termination is formed on the Ge surface instead. It is also noted that regardless of (NH4)2S concentration, the Ge–H vibration mode disappeared in 1 min (data not shown). Hence, it is assured that Ge–S bonds rapidly replace Ge–H bonds on the surface in (NH4)2S solution, which
Fig. 1. FT-IR absorbance spectra of (a) Ge–H vibration mode and (b) Ge–O vibration mode for an H-terminated Ge(1 0 0) surface prepared in 10% HF and an S-terminated Ge(1 0 0) surface prepared in (NH4)2S solution.
may be caused by a higher bond energy for Ge–S (534 kJ/mol) than that of Ge–H (263.2 kJ/mol). The wave number where the Ge–S stretching mode appears is reported at 300–420 cm 1 from trimetric sulfides [26]. However, the Ge–S spectra could not be directly measured because the available transmission range for Ge crystal is 475–5500 cm 1 due to lattice absorption [27]. Fig. 1(b) shows the Ge–O FT-IR spectra measured after treatment in 10% HF solution, referenced by an ozone-oxidized Ge surface, and after treatment in 0.5% (NH4)2S solution for 40 min, referenced by an H-terminated Ge surface. The FT-IR spectrum of Ge–O shows a negative absorbance after HF treatment, which suggests that oxidized Ge layers were removed. However, there was no change in Ge–O absorbance when the H-terminated Ge surface was dipped in (NH4)2S solution. Therefore, it is suggested that there is no effect of oxidation on the Ge surface during the Ge–S passivation process. Ge–O absorbance was not detected, either, when the H-terminated Ge(1 0 0) surface was treated in 5 and 20% (NH4)2S solutions (data not shown). Fig. 2 shows the FT-IR spectra change of the Ge–O vibration mode when the Ge surface was treated in 0.5% (NH4)2S solution for 40 min, and the S-terminated Ge(1 0 0) was exposed in air ambient. Here, the FT-IR spectra of S-terminated Ge(1 0 0) were used as a reference. An increase of Ge–O absorbance with exposure time is observed, which indicates that thickness of oxide layer is increased on the S-passivated Ge
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Fig. 2. Change in the FT-IR absorbance spectra of the Ge–O vibration mode, when the S-terminated Ge(1 0 0) surface prepared in (NH4)2S solution was exposed in air ambient.
surface. Until 3600 min in air ambient, it is found that the Ge–O absorbance is relatively small, and the increase of its peak intensity with time is also negligibly small. A small absorbance peak at 800 cm 1 shown at the beginning of the oxidation process is attributed to GeOx (x < 2) [28]. However, it is noted that Ge–O absorbance increases rapidly after 3600 min of exposure in laboratory air ambient. The Ge–O absorbance peak mainly appears at 820 cm 1 after 3600 min, which is attributed to the GeO2 structure [28]. Therefore, it is concluded that the peak shift to a higher wave number with an increase in exposure time is caused by the change in the oxidation state; GeO2 is formed on the S-terminated Ge(1 0 0) surface after exposure to air for more than 3600 min. The IR peak at 820 cm 1 consists of TO and LO mode vibrations, which are observed at 830 and 940 cm 1, respectively [4]. Fig. 3 shows the change of the intensity of Ge–O absorbance with the variation of exposure time in air ambient for various concentrations of (NH4)2S solution. In the figure, the change in Ge–O absorbance for an H-terminated Ge(1 0 0) surface is also shown [28]. A significant difference in Ge–O absorbance between the H-terminated and S-terminated Ge surface is observed at a given exposure time, which may be caused by a stronger bond energy for Ge–S (534 kJ/mol) than for Ge–H
Fig. 3. Change in FT-IR Ge–O absorbance with exposure time. Change in the Ge–O absorbance of S-terminated Ge surfaces treated in various concentrations of (NH4)2S solution is shown with that of the H-terminated Ge surface [28].
(263.2 kJ/mol), while the bond energy of Ge–O (652.7 kJ/mol) is stronger than those. In the case of the H-terminated Ge surface, the breaking of Ge–Hx and Ge–Ge bonds initiates layer-by-layer oxidation on the surface [28]. However, such oxidation is suppressed on the (2 1) S-passivated Ge(1 0 0) surface. From the results in Fig. 3, it is noted that a thin oxide less than one monolayer (ML) on the S-terminated Ge(1 0 0) surface is preserved for about 3600 min from the exposure in air ambient, and its time for the preservation of the oxide layer up to one ML is about 120 times longer than for the H-terminated Ge(1 0 0) surface. From the oxidation behavior shown in Fig. 3, the oxidation process may be divided into two regions: before and after about 3600 min from exposure in air ambient. In the first region, it is inferred that S-termination on the Ge(1 0 0) surface forms a strong passivation against oxidation. The thickness of the oxide layer was almost identical for the Ge surfaces treated in various concentrations of (NH4)2S solution, which suggests that the concentration of the (NH4)2S solution did not significantly affect the number of S-terminations. In the second region, oxide was grown rapidly, which implies the breaking and/or loss of Stermination. In this region, the change of oxide thickness was almost identical for various (NH4)2S concentrations. The rapid increase in oxidation rate for the S-terminated Ge(1 0 0) surface may result from the change in oxide structure such as film density. The transition oxide layer, including Ge–S bonds, may have a less dense oxide structure after the desorption of sulfur, so that the diffusion of oxidant becomes faster, and the oxidation is accelerated by the desulfurization on the Sterminated Ge surface. Miura et al. reported that the oxidation rate of the H-terminated Si surface was strongly dependent on humidity [29]. Therefore, the rates of oxidation shown in Fig. 3 may be influenced by the humidity in the air. Fig. 4(a) shows the change of the XPS S 2p spectra of the Ge surface. When the H-terminated Ge(1 0 0) surface prepared in 10% HF solution was dipped in 20% (NH4)2S solution for 40 min, the S 2p peak appeared from the as-received sample. This is evidence of the surface modification of H-termination to S-termination, which was not directly measured with MIR FTIR. Sulfur was preserved on the S-terminated Ge(1 0 0) surface until 1000 min of exposure in air ambient. However, after 10,000 min of exposure, desorption of S on the Ge surface is observed. Therefore, it is concluded that, at least for the first 1000 min, S-passivation on the Ge(1 0 0) surface is still effective in air ambient. The S-termination is broken, and sulfur is desorbed, after a certain threshold time. In the change of the FT-IR spectra shown in Fig. 3, a threshold exposure time is given as about 3600 min in this study. Fig. 4(b) shows the change of Ge–S ratio on the S-terminated Ge surface with exposure time to the laboratory air. The Ge–S ratio to the total Ge–S and Ge–O bonds decreased from about 0.8 to below 0.1 in 10,000 min of exposure time. In addition, it is observed that the desorption behavior of sulfur on the Sterminated Ge(1 0 0) surface is independent of the concentration of the (NH4)2S solution that forms S-termination. The XPS Ge 3d spectra shown in Fig. 4(c) are divided into three peaks at 28.8, 29.5, and 30.8 eV. These peaks are
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Fig. 4. (a) XPS spectra of S 2p, (b) change of Ge–S ratio on the S-terminated Ge(1 0 0) surface at given exposure times, and (c) XPS spectra of Ge 3d. The XPS measurements were performed immediately after S-termination and after 1000 and 10,000 min of air exposure.
attributed to bulk Ge [30], Ge–S [14,19,22], and Ge–O [5], respectively. From the result, it is clear that Ge is mainly bonded to sulfur when S-termination was prepared in (NH4)2S solution. However, only a slight increase of the Ge–O peak was observed after 1000 min of exposure in air ambient, which suggests minimal breaking of Ge–S bonds. Finally, Stermination on the Ge(1 0 0) surface significantly decreased, but Ge bonded mainly with oxygen when the surface was exposed to air for 10,000 min. The XPS data in Fig. 4 are consistent with the results of oxide growth behavior shown in Fig. 3. In addition, the shift of Ge–O component to a higher binding energy with exposure time is attributed to the change in oxide structure from GeOx to GeO2. S-passivation on a Ge surface has a stable (2 1) structure [14,17,18,22,24,25], while an H-terminated Ge surface can be easily attacked by oxidants on the reactive Ge–H2 and Ge–H3 sites. Consequently, the S-passivated Ge surface is stable to oxidation. Meanwhile, the bonding of Ge and sulfur can form GeS2 [14,22], or a glassy network of GeSx by penetration of sulfur into Ge lattice [31]. Moreover, Ge–S–S–Ge [16] or GeOS [13] can be formed. The lattice structure of those non-ideal Sterminated Ge surfaces may be related to drastic oxidation of the Ge surface. Bonds that do not form a stable (2 1) Ge–S bridge may be first attacked by oxygen. Otherwise, a residual electron of the O atom on a non-ideal S-terminated Ge surface may bond to a residual electron of the S atom. Then, S atoms are attacked or bonded with O atoms to be desorbed as a form of
SO2, which is similar to the desorption of S on the surface at a higher temperature. Finally, the Ge surface is desulfurized with the breaking of the stable (2 1) network, and oxide growth is accelerated. 4. Conclusion In this study, the formation of S-termination on a Ge(1 0 0) surface and oxidation on the surface were studied using multiple internal reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. H-termination on the Ge(1 0 0) surface was substituted with S-termination in 1 min in (NH4)2S solution. When the S-terminated Ge(1 0 0) surface was exposed in air ambient, the oxidation behavior was divided into two regions. Oxidation on the S-terminated Ge(1 0 0) surface was retarded for about the first 3600 min in air ambient. The preservation time of the oxide layer up to one monolayer of S-terminated Ge(1 0 0) surface was about 120 times longer than for the H-terminated Ge(1 0 0) surface. However, oxidation of the S-terminated Ge(1 0 0) surface drastically increased after the threshold time. No significant threshold time difference was observed between the Sterminations formed in 0.5, 5, and 20% (NH4)2S solutions. Non-ideal S-termination on Ge surfaces may be related to drastic oxidation of the Ge surface. Finally, it is concluded that oxidation is accelerated with desulfurization on the Sterminated Ge(1 0 0) surface.
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Acknowledgements This work was supported through a research project by the Korean Ministry of Commerce, Industry and Energy. Also, this work is outcome of the fostering project of the Specialized Graduate School of Hydrogen & Fuel Cell supported financially by the Ministry of Commerce, Industry and Energy (MOCIE). References [1] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981. [2] C. Kittel, Introduction to Solid State Physics, sixth ed., John Wiley and Sons, New York, 1986. [3] D. Bodlaki, H. Yamamoto, D.H. Waldeck, E. Borguet, Surf. Sci. 543 (2003) 63. [4] S. Rivillon, Y.J. Chabal, F. Amy, A. Kahn, Appl. Phys. Lett. 87 (2005) 253101. [5] X.-J. Xhang, G. Xue, A. Agarwal, R. Tsu, M.-A. Hasan, J.E. Greene, A. Rockett, J. Vac. Sci. Technol. A 11 (1993) 2553. [6] T. Akane, J. Tanaka, H. Okumura, S. Matsumoto, Appl. Surf. Sci. 108 (1997) 303. [7] K. Park, Y. Lee, S. Lim, Appl. Surf. Sci. 254 (2007) 1842. [8] A. Cattoni, R. Bertacco, M. Riva, M. Cantoni, F. Ciccacci, H. Von Ka¨nel, G.J. Norga, Mater. Sci. Semicond. Process. 9 (2006) 701. [9] R.D. Bringans, R.I.G. Uhrberg, R.Z. Bachrach, J.E. Northrup, Phys. Rev. Lett. 55 (1985) 533. [10] F.E. Leys, R. Bonzom, B. Kaczer, T. Janssens, W. Vandervorst, B. De Jaeger, J. Van Steenbergen, K. Martens, D. Hellin, J. Rip, G. Dilliway, A. Delabie, P. Zimmerman, M. Houssa, A. Theuwis, R. Loo, M. Meuris, M. Caymax, M.M. Heyns, Mater. Sci. Semicond. Process. 9 (2006) 679. [11] S.M. Jeon, S.J. Jung, H-.D. Kim, D.K. Lim, H. Lee, S. Kim, J. Phys. Chem. B 110 (2006) 21728.
[12] S.M. Han, W.R. Ashurst, C. Carraro, R. Maboudian, J. Am. Chem. Soc. 123 (2001) 2422. [13] M.M. Frank, S.J. Koester, M. Copel, J.A. Ott, V.K. Paruchuri, H. Shang, R. Loesing, Appl. Phys. Lett. 89 (2006) 112905. [14] T. Maeda, S. Takagi, T. Ohnishi, M. Lippmaa, Mater. Sci. Semicond. Process. 9 (2006) 706. [15] P. Ma, G.W. Anderson, P.R. Norton, Surf. Sci. 420 (1999) 134. [16] M. Houssa, G. Pourtois, B. Kaczer, B. De Jaeger, F.E. Leys, D. Nelis, K. Paredis, A. Vantomme, M. Caymax, M. Meuris, M.M. Heyns, Microelectron. Eng. 84 (2007) 2267. [17] K.T. Leung, L.J. Terminello, Z. Hussain, X.S. Zhan, T. Hayshi, D.A. Shirley, Phys. Rev. B 38 (1988) 8241. [18] K. Newstead, A.W. Robinson, S. d’Addato, A. Patchett, N.P. Prince, R. McGrath, R. Whittle, E. Dudzik, I.T. McGovern, J. Phys.: Condens. Matter 4 (1992) 8441. [19] G.W. Anderson, M.C. Hanf, P.R. Norton, Z.H. Lu, M.J. Graham, Appl. Phys. Lett. 66 (1995) 1123. [20] J. Roche, P. Ryan, G.J. Hughes, Surf. Sci. 465 (2000) 115. [21] J. Roche, P. Ryan, G.J. Hughes, Appl. Surf. Sci. 174 (2001) 271. [22] T. Weser, A. Bogen, B. Konrad, R.D. Schnell, C.A. Schug, W. Steinmann, Phys. Rev. B 25 (1987) 8184. [23] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 86th ed., CRC Press, New York, 2005. [24] P. Kru¨ger, J. Pollmann, Phys. Rev. Lett. 64 (1990) 1808. [25] J. Pollmann, P. Kru¨ger, A. Mazur, Appl. Surf. Sci. 56–58 (1992) 193. [26] F. Gloching, The Chemistry of Germanium, Academic Press, London and New York, 1969. [27] Y.J. Chabal, Surf. Sci. 168 (1986) 594. [28] K.B. Park, Y.H. Lee, J.H. Lee, S.W. Lim, Appl. Surf. Sci., in press. [29] T.-A. Miura, M. Niwano, D. Shoji, N. Miyamoto, J. Appl. Phys. 79 (1996) 4373. [30] T.-C. Chuang, CRC Crit. Rev. Solid State Mater. Sci. 14 (1988) 269. [31] P.F. Lyman, O. Sakata, D.L. Marasco, T.-L. Lee, K.D. Breneman, D.T. Keane, M.J. Bedzyk, Surf. Sci. 462 (2000) L594.
Applied Surface Science 254 (2008) 7544–7548 www.elsevier.com/locate/apsusc
Effect of Ge surface termination on oxidation behavior Younghwan Lee a, Kibyung Park a, Yong Soo Cho b, Sangwoo Lim a,* b
a Department of Chemical Engineering, Yonsei University, Republic of Korea School of Advanced Materials Science and Engineering, Yonsei University, Republic of Korea
Available online 15 January 2008
Abstract Sulfur-termination was formed on the Ge(1 0 0) surface using (NH4)2S solution. Formation of Ge–S and the oxidation of the S-terminated Ge surface were monitored with multiple internal reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. In the 0.5, 5, or 20% (NH4)2S solution, H-termination on the Ge(1 0 0) surface was substituted with S-termination in 1 min. When the S-terminated Ge(1 0 0) surface was exposed in air ambient, the oxidation was retarded for about 3600 min. The preservation time of the oxide layer up to one monolayer of S-terminated Ge(1 0 0) surface was about 120 times longer than for the H-terminated Ge(1 0 0) surface. However, the oxidation of S-terminated Ge(1 0 0) surface drastically increased after the threshold time. There was no significant difference in threshold time between S-terminations formed in 0.5, 5, and 20% (NH4)2S solutions. With the surface oxidation, desorption of S on the Ge surface was observed. The desorption behavior of sulfur on the S-terminated Ge(1 0 0) surface was independent of the concentration of the (NH4)2S solution that forms S-termination. Non-ideal S-termination on Ge surfaces may be related to drastic oxidation of the Ge surface. Finally, with the desulfurization on the S-terminated Ge(1 0 0) surface, oxide growth is accelerated. # 2008 Elsevier B.V. All rights reserved. PACS : 61.72.uf; 66.70.Df; 68.35.bg; 68.37. d Keywords: Germanium; Surface; Termination; Oxidation
1. Introduction Germanium is a promising starting material for advanced high-performance devices with a carrier transport property superior to that of silicon [1,2]. However, for the application of Ge to the fabrication of devices, it is necessary to effectively control its surface condition. As is well known, an H-terminated Ge surface is unstable to oxidation in air ambient, contrary to an H-terminated Si surface [3,4]. Therefore, a robust termination of the Ge surface to retard the formation of native oxide is needed. Moreover, amorphous GeO2, which is one of the oxidized Ge structures, is dissolved in H2O [5], acid, and alkali solution [6]. Therefore, it is crucial to investigate the oxidation characteristics of the Ge surface on the designed termination.
* Corresponding author at: Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Republic of Korea. Tel.: +82 2 2123 5754; fax: +82 2 312 6401. E-mail address: [email protected] (S. Lim). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.022
Up to the present, various terminations and passivations, such as H-termination [3,4,7], halogen-termination [3], alkyltermination [3], Ba-passivation [8], As-passivation [9], Si thin layer-passivation [10], thiol-passivation [11,12], and sulfurtermination [3,13–22] have been proposed. Particularly, it is reported that an S-terminated Ge surface is more stable than an H- or halogen-terminated surface, because of its higher bond energy [23]. Therefore, S-passivation on a Ge surface has been demonstrated in high-k dielectrics deposition [13], the fabrication of an epitaxial heterostructure [14], and metal deposition [15]. S-termination can be formed by gaseous methods with hydrogen sulfide (H2S) [16–18], liquefied methods with ammonium sulfide ((NH4)2S) [3,13–15,19], and solid-state electrochemical methods [20–22]. It is found that S-terminations prepared in those methods exhibits a bridge bond structure between Ge and S [14,17,18,22,24,25]. However, the oxidation mechanism on an S-terminated Ge surface has not been completely established. In the fabrication of Ge-based electronic devices, it is important to effectively control queue time between surface treatment and gate oxidation. Therefore,
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the stability of the S-terminated Ge surface to oxidation needs to be understood. In this paper, using multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR), passivation of Ge(1 0 0) with sulfur in (NH4)2S solution and the oxidation of the S-terminated Ge(1 0 0) surface in air ambient were studied. In addition, stability to the oxidation of S-termination on a Ge surface was compared with that of H-termination. 2. Experimental Undoped double-side polished Ge(1 0 0) wafer (resistivity > 40 V cm) was used. The Ge wafer was cut to 80 mm 10 mm 0.45 mm, and a trapezoid shape at 458 was crafted using 0.3 mm silica lapping film (3 M) to achieve internal reflection of 200 times. To remove particles and organic compounds on the Ge surface, the Ge wafer piece was first cleaned in methanol and then acetone for 5 min, respectively. Then, native oxide was removed in 0.5% hydrofluoric acid (HF) for 5 min. A 35.4 g/m3 ozone was produced by a corona discharge ozone generator and fed into the chamber, where the Ge wafer piece was mounted on the susceptor. At a susceptor temperature of 100 8C, oxidation of the Ge surface was performed for 30 min. Next, the Ge wafer piece was dipped in 10% HF solution for 20 min at 25 8C to etch the oxidized Ge layers and produce H-termination on the Ge(1 0 0) surface. Then, the H-terminated Ge surface was treated in 0.5, 5, or 20% (NH4)2S solution for 40 min to form Stermination on the Ge surface. For the periods of each dipping time, FT-IR spectra were measured. After the formation of Spassivation, the Ge wafer piece was exposed in laboratory air ambient, and the growth of the Ge native oxide was observed using FT-IR. IR vibration spectra were measured with FT-IR (Thermo electron, Nicolet 380) using a horizontal attenuated total reflectance (HATR) accessory (PIKE, 022-1260-45). Measurement was performed at 128 scan with 4 cm 1 resolution. The change of sulfur concentration on the Ge surface for a period of time was observed using XPS (ESCALAB 220i-XL) with a microfocused monochromator and moveable Al anode. 3. Results and discussion Fig. 1(a) shows the change of Ge–H FT-IR vibration mode. First, it is observed that H-termination was successfully formed on the Ge(1 0 0) surface by dipping in 10% HF solution. Here, the FT-IR spectra were taken using the oxidized Ge surface as a reference. The change of FT-IR spectra after treatment in 0.5% (NH4)2S solution is also shown. The FT-IR spectra of a Ge surface treated in 10% HF solution were used as a reference. Ge–H peaks have a negative absorbance after Ge surface treatment in (NH4)2S solution, which suggests that Htermination vanishes and S-termination is formed on the Ge surface instead. It is also noted that regardless of (NH4)2S concentration, the Ge–H vibration mode disappeared in 1 min (data not shown). Hence, it is assured that Ge–S bonds rapidly replace Ge–H bonds on the surface in (NH4)2S solution, which
Fig. 1. FT-IR absorbance spectra of (a) Ge–H vibration mode and (b) Ge–O vibration mode for an H-terminated Ge(1 0 0) surface prepared in 10% HF and an S-terminated Ge(1 0 0) surface prepared in (NH4)2S solution.
may be caused by a higher bond energy for Ge–S (534 kJ/mol) than that of Ge–H (263.2 kJ/mol). The wave number where the Ge–S stretching mode appears is reported at 300–420 cm 1 from trimetric sulfides [26]. However, the Ge–S spectra could not be directly measured because the available transmission range for Ge crystal is 475–5500 cm 1 due to lattice absorption [27]. Fig. 1(b) shows the Ge–O FT-IR spectra measured after treatment in 10% HF solution, referenced by an ozone-oxidized Ge surface, and after treatment in 0.5% (NH4)2S solution for 40 min, referenced by an H-terminated Ge surface. The FT-IR spectrum of Ge–O shows a negative absorbance after HF treatment, which suggests that oxidized Ge layers were removed. However, there was no change in Ge–O absorbance when the H-terminated Ge surface was dipped in (NH4)2S solution. Therefore, it is suggested that there is no effect of oxidation on the Ge surface during the Ge–S passivation process. Ge–O absorbance was not detected, either, when the H-terminated Ge(1 0 0) surface was treated in 5 and 20% (NH4)2S solutions (data not shown). Fig. 2 shows the FT-IR spectra change of the Ge–O vibration mode when the Ge surface was treated in 0.5% (NH4)2S solution for 40 min, and the S-terminated Ge(1 0 0) was exposed in air ambient. Here, the FT-IR spectra of S-terminated Ge(1 0 0) were used as a reference. An increase of Ge–O absorbance with exposure time is observed, which indicates that thickness of oxide layer is increased on the S-passivated Ge
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Fig. 2. Change in the FT-IR absorbance spectra of the Ge–O vibration mode, when the S-terminated Ge(1 0 0) surface prepared in (NH4)2S solution was exposed in air ambient.
surface. Until 3600 min in air ambient, it is found that the Ge–O absorbance is relatively small, and the increase of its peak intensity with time is also negligibly small. A small absorbance peak at 800 cm 1 shown at the beginning of the oxidation process is attributed to GeOx (x < 2) [28]. However, it is noted that Ge–O absorbance increases rapidly after 3600 min of exposure in laboratory air ambient. The Ge–O absorbance peak mainly appears at 820 cm 1 after 3600 min, which is attributed to the GeO2 structure [28]. Therefore, it is concluded that the peak shift to a higher wave number with an increase in exposure time is caused by the change in the oxidation state; GeO2 is formed on the S-terminated Ge(1 0 0) surface after exposure to air for more than 3600 min. The IR peak at 820 cm 1 consists of TO and LO mode vibrations, which are observed at 830 and 940 cm 1, respectively [4]. Fig. 3 shows the change of the intensity of Ge–O absorbance with the variation of exposure time in air ambient for various concentrations of (NH4)2S solution. In the figure, the change in Ge–O absorbance for an H-terminated Ge(1 0 0) surface is also shown [28]. A significant difference in Ge–O absorbance between the H-terminated and S-terminated Ge surface is observed at a given exposure time, which may be caused by a stronger bond energy for Ge–S (534 kJ/mol) than for Ge–H
Fig. 3. Change in FT-IR Ge–O absorbance with exposure time. Change in the Ge–O absorbance of S-terminated Ge surfaces treated in various concentrations of (NH4)2S solution is shown with that of the H-terminated Ge surface [28].
(263.2 kJ/mol), while the bond energy of Ge–O (652.7 kJ/mol) is stronger than those. In the case of the H-terminated Ge surface, the breaking of Ge–Hx and Ge–Ge bonds initiates layer-by-layer oxidation on the surface [28]. However, such oxidation is suppressed on the (2 1) S-passivated Ge(1 0 0) surface. From the results in Fig. 3, it is noted that a thin oxide less than one monolayer (ML) on the S-terminated Ge(1 0 0) surface is preserved for about 3600 min from the exposure in air ambient, and its time for the preservation of the oxide layer up to one ML is about 120 times longer than for the H-terminated Ge(1 0 0) surface. From the oxidation behavior shown in Fig. 3, the oxidation process may be divided into two regions: before and after about 3600 min from exposure in air ambient. In the first region, it is inferred that S-termination on the Ge(1 0 0) surface forms a strong passivation against oxidation. The thickness of the oxide layer was almost identical for the Ge surfaces treated in various concentrations of (NH4)2S solution, which suggests that the concentration of the (NH4)2S solution did not significantly affect the number of S-terminations. In the second region, oxide was grown rapidly, which implies the breaking and/or loss of Stermination. In this region, the change of oxide thickness was almost identical for various (NH4)2S concentrations. The rapid increase in oxidation rate for the S-terminated Ge(1 0 0) surface may result from the change in oxide structure such as film density. The transition oxide layer, including Ge–S bonds, may have a less dense oxide structure after the desorption of sulfur, so that the diffusion of oxidant becomes faster, and the oxidation is accelerated by the desulfurization on the Sterminated Ge surface. Miura et al. reported that the oxidation rate of the H-terminated Si surface was strongly dependent on humidity [29]. Therefore, the rates of oxidation shown in Fig. 3 may be influenced by the humidity in the air. Fig. 4(a) shows the change of the XPS S 2p spectra of the Ge surface. When the H-terminated Ge(1 0 0) surface prepared in 10% HF solution was dipped in 20% (NH4)2S solution for 40 min, the S 2p peak appeared from the as-received sample. This is evidence of the surface modification of H-termination to S-termination, which was not directly measured with MIR FTIR. Sulfur was preserved on the S-terminated Ge(1 0 0) surface until 1000 min of exposure in air ambient. However, after 10,000 min of exposure, desorption of S on the Ge surface is observed. Therefore, it is concluded that, at least for the first 1000 min, S-passivation on the Ge(1 0 0) surface is still effective in air ambient. The S-termination is broken, and sulfur is desorbed, after a certain threshold time. In the change of the FT-IR spectra shown in Fig. 3, a threshold exposure time is given as about 3600 min in this study. Fig. 4(b) shows the change of Ge–S ratio on the S-terminated Ge surface with exposure time to the laboratory air. The Ge–S ratio to the total Ge–S and Ge–O bonds decreased from about 0.8 to below 0.1 in 10,000 min of exposure time. In addition, it is observed that the desorption behavior of sulfur on the Sterminated Ge(1 0 0) surface is independent of the concentration of the (NH4)2S solution that forms S-termination. The XPS Ge 3d spectra shown in Fig. 4(c) are divided into three peaks at 28.8, 29.5, and 30.8 eV. These peaks are
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Fig. 4. (a) XPS spectra of S 2p, (b) change of Ge–S ratio on the S-terminated Ge(1 0 0) surface at given exposure times, and (c) XPS spectra of Ge 3d. The XPS measurements were performed immediately after S-termination and after 1000 and 10,000 min of air exposure.
attributed to bulk Ge [30], Ge–S [14,19,22], and Ge–O [5], respectively. From the result, it is clear that Ge is mainly bonded to sulfur when S-termination was prepared in (NH4)2S solution. However, only a slight increase of the Ge–O peak was observed after 1000 min of exposure in air ambient, which suggests minimal breaking of Ge–S bonds. Finally, Stermination on the Ge(1 0 0) surface significantly decreased, but Ge bonded mainly with oxygen when the surface was exposed to air for 10,000 min. The XPS data in Fig. 4 are consistent with the results of oxide growth behavior shown in Fig. 3. In addition, the shift of Ge–O component to a higher binding energy with exposure time is attributed to the change in oxide structure from GeOx to GeO2. S-passivation on a Ge surface has a stable (2 1) structure [14,17,18,22,24,25], while an H-terminated Ge surface can be easily attacked by oxidants on the reactive Ge–H2 and Ge–H3 sites. Consequently, the S-passivated Ge surface is stable to oxidation. Meanwhile, the bonding of Ge and sulfur can form GeS2 [14,22], or a glassy network of GeSx by penetration of sulfur into Ge lattice [31]. Moreover, Ge–S–S–Ge [16] or GeOS [13] can be formed. The lattice structure of those non-ideal Sterminated Ge surfaces may be related to drastic oxidation of the Ge surface. Bonds that do not form a stable (2 1) Ge–S bridge may be first attacked by oxygen. Otherwise, a residual electron of the O atom on a non-ideal S-terminated Ge surface may bond to a residual electron of the S atom. Then, S atoms are attacked or bonded with O atoms to be desorbed as a form of
SO2, which is similar to the desorption of S on the surface at a higher temperature. Finally, the Ge surface is desulfurized with the breaking of the stable (2 1) network, and oxide growth is accelerated. 4. Conclusion In this study, the formation of S-termination on a Ge(1 0 0) surface and oxidation on the surface were studied using multiple internal reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. H-termination on the Ge(1 0 0) surface was substituted with S-termination in 1 min in (NH4)2S solution. When the S-terminated Ge(1 0 0) surface was exposed in air ambient, the oxidation behavior was divided into two regions. Oxidation on the S-terminated Ge(1 0 0) surface was retarded for about the first 3600 min in air ambient. The preservation time of the oxide layer up to one monolayer of S-terminated Ge(1 0 0) surface was about 120 times longer than for the H-terminated Ge(1 0 0) surface. However, oxidation of the S-terminated Ge(1 0 0) surface drastically increased after the threshold time. No significant threshold time difference was observed between the Sterminations formed in 0.5, 5, and 20% (NH4)2S solutions. Non-ideal S-termination on Ge surfaces may be related to drastic oxidation of the Ge surface. Finally, it is concluded that oxidation is accelerated with desulfurization on the Sterminated Ge(1 0 0) surface.
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