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Analysis of hydrogen-terminated Si(1 1 1) surface by infrared multiple-angle incidence resolution spectroscopy
Chemical Physics Letters 415 (2005) 172–175 www.elsevier.com/locate/cplett
Analysis of hydrogen-terminated Si(1 1 1) surface by infrared multiple-angle incidence resolution spectroscopy Hiroyuki Kakuda a, Takeshi Hasegawa a,b,*, Taishi Tanaka c, Kentaro Tanaka b,c, Mitsuhiko Shionoya c a
Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275–8575, Japan b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 15 July 2005; in final form 2 September 2005 Available online 28 September 2005
Abstract The infrared multiple-angle incidence resolution spectroscopy (MAIRS) has been employed to analyze the hydrogen-terminated Si(1 1 1) 1 · 1–H surface. The new technique of infrared MAIRS provides two absorption spectra simultaneously on an identical sample, which separately reveal the surface-parallel and surface-normal vibration spectra. It has been exhibited in the present study that the infrared MAIRS technique is powerful to discuss some infrared key bands in terms of molecular orientation concerned with Si–H vibrational modes on the Si(1 1 1) 1 · 1–H surface with the use of the two spectra. 2005 Elsevier B.V. All rights reserved.
1. Introduction The surface analysis of a semiconductor wafer is an important matter to control and regulate the atomic arrangement in the surface layer [1,2]. The hydrogen termination of silicon wafers has long been attracting much interest, since it energetically stabilizes dangling bonds that are available on a freshly cleaved surface, and efficiently suppresses spontaneous rearrangement of silicon atoms in the surface layer. The surface-termination reaction with hydrogen can be carried out by some established wet etching techniques by use of hydrogen fluoride [3], and the structure of the reacted surfaces is analyzed from the major viewpoints of surface morphology and molecular vibrations. The surface morphology can be analyzed by scanning tunneling microscopy (STM) [4] and its related microscopic techniques, and the molecular vibrations are analyzed mainly by Raman [5] and infrared spectroscopy [6].
*
Corresponding author. E-mail address: [email protected] (T. Hasegawa).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.017
Infrared (IR) spectroscopy is powerful to characterize the hydrogen-terminated silicon surface, and the polarized attenuated total reflection (ATR) technique [7] is often employed. This technique enables us to observe the Si–H stretching vibration modes, and their tilt angle can be discussed. When an s-polarization ATR spectrum is measured, for example, the surface-parallel vibrational modes limitedly appear, while the surface-normal modes appear strongly in a p-polarization spectrum [7,8]. With the two polarized spectra, therefore, the orientation angle can be estimated, if precise optical parameters including dielectric functions and thickness of each layer are available. In other words, the discussion of molecular orientation with ATR spectra is limited in a qualitative manner without the optical parameters. This is because the infrared p-polarized ATR spectra contain information of both surface-parallel and -normal vibrational modes [7]. Therefore, the observation of pure surface-normal modes on a silicon wafer has been a challenging topic. To our best knowledge, the only report thus far has been given by Nagai et al., [9] which is the Ôair-gap ATRÕ technique. When an ATR crystal is very closely placed to the
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
sample wafer by a minute air-gap, the electric field in the surface-normal direction is greatly enhanced in the gap, and nearly pure surface-normal modes in the sample are selectively observed. Nonetheless, the band intensity quantitatively depends on the air-gap distance that is difficult to regulate exactly. For the analysis of surface adsorbates in general, IR transmission and reflection-absorption (RA) [10,11] spectrometries are useful, which reveal molecular vibrations along the surface-parallel and -normal directions, respectively. When a semiconductor wafer is analyzed, however, the RA technique defined by Greenler [11] cannot be employed for the purpose of measuring almost-pure surface-normal vibrations, since it requires a metallic surface to generate a strong surface-normal electric field at the surface. Therefore, to reveal vibrations in the surface-normal direction on a nonmetallic surface, the infrared external-reflection [12] and the ATR techniques have been used instead. Recently, a totally new analytical tool, infrared multiple-angle incidence resolution spectroscopy (IR-MAIRS), has been developed [13–15], which enables us to measure two IR spectra at a time on a transparent material, which correspond to the conventional normal-incidence transmission spectrum on a transparent material and the RA spectrum on a metallic surface. In the present paper, the former and the latter spectra are named surface-parallel ðkÞ and surface-normal (^) spectra, respectively. The principle of this technique is briefly summarized as follows. The collected single-beam spectra form the spectra matrix, S (Eq. (1)). One part of information in this matrix is interrelated to the ÔdescriptorÕ matrix, R, which converts oblique-angle transmission measurements to normal-incidence ones 3 2 3 2 rk1 r?1 s1 6 s 7 6 rk2 r?2 7 7 sk 6 27 6 sk 7 7¼6 S¼6 R þ U þ Ud . ð1Þ d 6 s3 7 6 rk3 r?3 7 s s? 5 ? 4 5 4 .. .. .. . . .
173
Outstanding characteristics of MAIRS are summarized as follows. (1) Both surface-parallel and -normal spectra are simultaneously yielded by oblique-incidence transmission measurements from an identical sample. (2) The surface-normal spectrum agrees the conventional p-polarized RA spectrum on a metallic surface, although it is measured on a nonmetallic surface. (3) Polarized spectra are obtained in spite of using no polarizer [13]. Some MAIRS-related papers have already been published [16,17] to exhibit that IR bands having surface-parallel orientation in a thin film dominantly appear in the surface-parallel spectrum of MAIRS, which proves the MAIRS technique working theoretically. Nevertheless, there has been no report for Ôpurely surface-normalÕ modes, since there are few examples to have the surface-normally oriented chemical groups in thin organized films. In the present study, the IR-MAIRS technique has first been employed for the analysis of the Si–H bonds formed on the Si wafer surface, in which the Si–H vibration mode is essentially expected to have the surface-normal orientation. 2. Experimental An n-type silicon double-faced-mirror wafer (50 · 20 · 0.5 mm, 1–40 X cm) was purchased from Sumitomo Mitsubishi Silicon Co. (Tokyo, Japan), which has (1 1 1) plane on the surfaces. The Si(1 1 1) topmost layer was hydrogen-terminated by an etching technique as described elsewhere [18]. The infrared MAIRS spectra were measured on a Thermo-Electron Nicolet (Madison, WI) Magna 550 FT-IR equipped with an MCT detector with a resolution of 4 cm1 . The aperture was fully opened [14]. 3. Results and discussion Fig. 1 presents the Si–H stretching vibration region of the IR-MAIRS spectra observed on the H-terminated Si (Si(1 1 1) 1 · 1–H) surface. Note that the surface-parallel spectrum is 20-times magnified in intensity. Although the
Here, the subscripts (1, 2, 3, . . .) represent spectra indices for different angles of incidence. The Ôdescribed partÕ can be extracted from S to be a solution as stated by Eq. (2). The solutions, si and s^, correspond to the single-beam spectra virtually measured by normal-incidence transmission spectrometries. The residual part in S is stored in the undescribed (Ud) matrix in Eq. (1) sk ð2Þ ¼ ðRT RÞ1 RT S. s? The superscripts, T and 1, indicate transpose and reverse matrix, respectively. The matrix, R, is given by Eq. (3) [13], where h is the angle of incidence 2 3 2 2 2 2 2 1 þ cos h1 þ sin h1 tan h1 tan h1 7 4 6 6 1 þ cos2 h2 þ sin2 h2 tan2 h2 tan2 h2 7. ð3Þ R¼ 5 p 4 .. .. . .
Fig. 1. FT-IR MAIRS spectra of Si(1 1 1) 1 · 1–H surface. The surfacenormal and surface-parallel spectra correspond to the conventional RA spectrum and normal-incidence transmission one, respectively. The surface-parallel spectrum is 20· magnified in intensity.
174
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
spectra are derived from a single monolayer of Si–H, clear spectra with good quality have been obtained. When transmission measurements are performed for wafers, strong interference fringes often appear on the spectra due to multiple-reflections of the light in the wafer. In the IR-MAIRS spectra, however, such fringes are effectively removed, since the optical fringe is a typical ÔundescribedÕ factor in the regression equation (Eq. (1)). This fringe-removal effect is another benefit of IR-MAIRS. It is known that the Si surface prepared by the etching technique retains 1 · 1 structure, since the atomic rearrangement by oxidization is suppressed by the hydrogen coverage. Regardless, even the hydrogen-terminated Si surface is known to be oxidized with time in air, which gradually makes the topmost surface be amorphous-glass. Therefore, it is expected that an aged hydrogen-terminated Si wafer would have major two different Si–H modes derived from the terrace of a fresh-surface layer and of an amorphous-glass one [19–21]. The Si–H stretching vibration band of the uncoupled monohydride (m(„Si–H); A1 [22]), which is specifically found on a flat Si(1 1 1) terrace of a fresh (high-coverage) sample, appears strongly at 2083 cm1 in the surface-normal spectrum, while it is very weak in the surface-parallel spectrum (Fig. 1). In this manner, the surface-normal spectrum corresponds to the conventional RA spectrum, as if it were measured on a metallic surface. This MAIRS dichroism strongly suggests that the Si–H stretching vibration mode is oriented nearly perpendicular to the surface (Fig. 2a). When the surface is subjected to infrared ATR spectrometry, according to the previous reports [8,18,22], this band appears only in the p-polarization spectrum, while it is not available in the s-polarization one. Therefore, it has been believed that the Si–H bond is oriented perpendicularly to the surface, which is basically consistent with the present MAIRS analysis. It has been difficult thus far, however, to perform the precise quantitative molecular orientation analysis with the polarized ATR spectra. Since the MAIRS surface-parallel and surface-normal spectra have a common absorbance scale, it is easy to evaluate the molecular orientation angle (/) with the use of the intensity ratio between the two spectra. The equation for the evaluation of molecular orientation is given as follows [13]:
/ ¼ tan1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2I parallel =I normal .
ð4Þ
Here, Iparallel and Inormal are band intensities of an identical mode in the surface-parallel and -normal spectra, respectively. As a result, the Si–H bond that corresponds to the band at 2083 cm1 has been revealed to have an orientation angle of 5 from the surface normal. This angle is reasonable when the Si–H groups are packed on the surface with some molecular space, although the groups schematically have the perpendicular orientation (Fig. 2a). Peaks associated with the dihydride (@SiH2, 2090– 2120 cm1) and the trihydride (–SiH3, 2120–2150 cm1), which are available on steps [8,22,23], are all silent in the MAIRS spectra. This result suggests that the Si(1 1 1) 1 · 1–H wafer has an atomically flat surface. On the other hand, a sharp band appears at 2077 cm1 in the surface-parallel spectrum on a tail of the band at 2083 cm1. This band has scarcely been recognized thus far, since most of the experiments have been performed in an ultrahigh vacuum (UHV) condition for a fresh sample. In the present analysis, the sample was prepared by the wet technique, and it was measured in air that contains oxygen. A fact that the band appears only in the surfacenormal spectrum suggests that the mode is similar to that for the 2083 cm1 band. Therefore, it is considered that the band at 2077 cm1 is attributed to the Si–H bond in amorphous-glass layers (Fig. 2b). Of another interest is the band at 2071 cm1 found in the MAIRS surface-parallel spectrum. This mode is theoretically reported to have the B1 mode, and its orientation is mostly parallel to the surface [22]. In the report, the orientation was ambiguous and it was available both in s- and p-polarized ATR spectra, partly because the surface of the wafer had Ômisorientation.Õ In the present analysis, this mode appears only in the surface-parallel spectrum, which strongly supports the theoretical expectation of orientation, and the Si wafer used for the present study is, therefore, suggested to have no misorientation. In fact, the misorientation angle has been estimated by AFM measurements (data not shown) to be ca. 0.14, which is negligibly small relative to experimental error. In addition, it is interesting that the band intensity of this mode is much smaller than the other bands. This is understandable when we consider that this mode is oriented surface-parallel. In the parallel orientation, a number of oscillators of this mode are distributed randomly in the surface layer, which diminishes the total transition moment. In other words, the very weak intensity reversely supports the surface-parallel orientation of the mode. In this manner, the IR-MAIRS technique has been found useful to characterize semiconductor surfaces. Acknowledgements
Fig. 2. Schematic image of Si–H bonds on: (a) a fresh Si(1 1 1) 1 · 1–H surface and (b) a slightly oxidized surface with a thin amorphous-glass layer.
This work was financially supported by Grant-in-Aid for Scientific Research (B) (No. 16350048) from the Ministry of Education, Science, Sports, Culture, and
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
Technology, Japan, and also by The Futaba Electronics Memorial Foundation, Japan. References [1] [2] [3] [4] [5] [6] [7]
[8] [9]
[10]
S. Ye, M. Osawa, K. Uosaki, Shinku 476 (2004) 449. D.D.M. Wayner, R. Wolkow, J. Chem. Soc., Perkin Trans. 2 1 (2002) 23. H. Ubara, T. Imura, A. Hiraki, Solid State Commun. 50 (1984) 673. H. Yu, L.J. Webb, R.S. Ries, S.D. Solares, W.A. Goddard III, J.R. Heath, N.S. Lewis, J. Phys. Chem. B 109 (2005) 671. M.A. Hines, Y.J. Chabal, T.D. Harris, A.L. Harris, J. Chem. Phys. 101 (1994) 8055. W.W. Chen, X.H. Sun, S.D. Wang, S.T. Lee, B.K. Teo, J. Phys. Chem. B 109 (2005) 10871. V.P. Tolstoy, I.V. Chernyshova, V.A. Skryshevsky, Handbook of Infrared Spectroscopy of Ultrathin Films, Wiley-Interscience, New York, 2003. T. Hattori, J. Vac. Sci. Technol. B 11 (1993) 1528. N. Nagai, Y. Izumi, H. Ishida, Y. Suzuki, A. Hatta, in: J.A. de Haseth (Ed.), Proceedings of 11th International Conference of Fourier Transform Spectroscopy, Mater. Sci., 28, American Institute of Physics, 1998, p. 581. J. Umemura, Reflection-absorption spectroscopy of thin films on metallic substrates, in: John M. Chalmers, Peter R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, vol. 2, Wiley, Chichester, 2002, p. 982.
175
[11] R.G. Greenler, J. Chem. Phys. 44 (1966) 310, The RA technique was originally defined as a technique for grazing-angle incidence measurements on a metallic surface. In recent years, however, the term ÔRAÕ has been used for a wide range of reflection measurements including the ER measurements on dielectric materials. [12] T. Hasegawa, S. Takeda, A. Kawaguchi, J. Umemura, Langmuir 11 (1995) 1236. [13] T. Hasegawa, J. Phys. Chem. B 106 (2002) 4112, The equation for molecular orientation analysis (Eq. 5) is misprinted in the original paper, and Eq. (4) in the present Letter is the corrected one. [14] T. Hasegawa, L. Matsumoto, S. Kitamura, S. Amino, S. Katada, J. Nishijo, Anal. Chem. 74 (2002) 6049. [15] T. Hasegawa, Anal. Bioanal. Chem. 375 (2003) 18. [16] T. Hasegawa, J. Umemura, C. Li, R.M. Leblanc, J. Phys. Chem. B 107 (2003) 11996. [17] T. Hasegawa, H. Kakuda, N. Yamada, J. Phys. Chem. B 109 (10) (2005) 4783. [18] G.S. Higashi, R.S. Becker, Y.J. Chabal, A.J. Becker, Appl. Phys. Lett. 58 (1991) 1656. [19] Y.J. Chabal, Phys. Rev. Lett. 50 (1983) 1850. [20] Y.J. Chabal, G.S. Higashi, S.B. Christman, Phys. Rev. B 28 (1983) 4472. [21] Danial D.M. Wayner, Robert A. Wolkow, J. Chem. Soc., Perkin Trans. 2 (2002) 23. [22] P. Jacob, Y.J. Chabal, J. Chem. Phys. 95 (1991) 2897. [23] V.A. Burrows, Y.J. Chabal, S.G. Higashi, K. Raghavachari, S.B. Christman, Appl. Phys. Lett. 53 (1988) 998.
Analysis of hydrogen-terminated Si(1 1 1) surface by infrared multiple-angle incidence resolution spectroscopy Hiroyuki Kakuda a, Takeshi Hasegawa a,b,*, Taishi Tanaka c, Kentaro Tanaka b,c, Mitsuhiko Shionoya c a
Applied Molecular Chemistry, College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275–8575, Japan b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan c Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 15 July 2005; in final form 2 September 2005 Available online 28 September 2005
Abstract The infrared multiple-angle incidence resolution spectroscopy (MAIRS) has been employed to analyze the hydrogen-terminated Si(1 1 1) 1 · 1–H surface. The new technique of infrared MAIRS provides two absorption spectra simultaneously on an identical sample, which separately reveal the surface-parallel and surface-normal vibration spectra. It has been exhibited in the present study that the infrared MAIRS technique is powerful to discuss some infrared key bands in terms of molecular orientation concerned with Si–H vibrational modes on the Si(1 1 1) 1 · 1–H surface with the use of the two spectra. 2005 Elsevier B.V. All rights reserved.
1. Introduction The surface analysis of a semiconductor wafer is an important matter to control and regulate the atomic arrangement in the surface layer [1,2]. The hydrogen termination of silicon wafers has long been attracting much interest, since it energetically stabilizes dangling bonds that are available on a freshly cleaved surface, and efficiently suppresses spontaneous rearrangement of silicon atoms in the surface layer. The surface-termination reaction with hydrogen can be carried out by some established wet etching techniques by use of hydrogen fluoride [3], and the structure of the reacted surfaces is analyzed from the major viewpoints of surface morphology and molecular vibrations. The surface morphology can be analyzed by scanning tunneling microscopy (STM) [4] and its related microscopic techniques, and the molecular vibrations are analyzed mainly by Raman [5] and infrared spectroscopy [6].
*
Corresponding author. E-mail address: [email protected] (T. Hasegawa).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.017
Infrared (IR) spectroscopy is powerful to characterize the hydrogen-terminated silicon surface, and the polarized attenuated total reflection (ATR) technique [7] is often employed. This technique enables us to observe the Si–H stretching vibration modes, and their tilt angle can be discussed. When an s-polarization ATR spectrum is measured, for example, the surface-parallel vibrational modes limitedly appear, while the surface-normal modes appear strongly in a p-polarization spectrum [7,8]. With the two polarized spectra, therefore, the orientation angle can be estimated, if precise optical parameters including dielectric functions and thickness of each layer are available. In other words, the discussion of molecular orientation with ATR spectra is limited in a qualitative manner without the optical parameters. This is because the infrared p-polarized ATR spectra contain information of both surface-parallel and -normal vibrational modes [7]. Therefore, the observation of pure surface-normal modes on a silicon wafer has been a challenging topic. To our best knowledge, the only report thus far has been given by Nagai et al., [9] which is the Ôair-gap ATRÕ technique. When an ATR crystal is very closely placed to the
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
sample wafer by a minute air-gap, the electric field in the surface-normal direction is greatly enhanced in the gap, and nearly pure surface-normal modes in the sample are selectively observed. Nonetheless, the band intensity quantitatively depends on the air-gap distance that is difficult to regulate exactly. For the analysis of surface adsorbates in general, IR transmission and reflection-absorption (RA) [10,11] spectrometries are useful, which reveal molecular vibrations along the surface-parallel and -normal directions, respectively. When a semiconductor wafer is analyzed, however, the RA technique defined by Greenler [11] cannot be employed for the purpose of measuring almost-pure surface-normal vibrations, since it requires a metallic surface to generate a strong surface-normal electric field at the surface. Therefore, to reveal vibrations in the surface-normal direction on a nonmetallic surface, the infrared external-reflection [12] and the ATR techniques have been used instead. Recently, a totally new analytical tool, infrared multiple-angle incidence resolution spectroscopy (IR-MAIRS), has been developed [13–15], which enables us to measure two IR spectra at a time on a transparent material, which correspond to the conventional normal-incidence transmission spectrum on a transparent material and the RA spectrum on a metallic surface. In the present paper, the former and the latter spectra are named surface-parallel ðkÞ and surface-normal (^) spectra, respectively. The principle of this technique is briefly summarized as follows. The collected single-beam spectra form the spectra matrix, S (Eq. (1)). One part of information in this matrix is interrelated to the ÔdescriptorÕ matrix, R, which converts oblique-angle transmission measurements to normal-incidence ones 3 2 3 2 rk1 r?1 s1 6 s 7 6 rk2 r?2 7 7 sk 6 27 6 sk 7 7¼6 S¼6 R þ U þ Ud . ð1Þ d 6 s3 7 6 rk3 r?3 7 s s? 5 ? 4 5 4 .. .. .. . . .
173
Outstanding characteristics of MAIRS are summarized as follows. (1) Both surface-parallel and -normal spectra are simultaneously yielded by oblique-incidence transmission measurements from an identical sample. (2) The surface-normal spectrum agrees the conventional p-polarized RA spectrum on a metallic surface, although it is measured on a nonmetallic surface. (3) Polarized spectra are obtained in spite of using no polarizer [13]. Some MAIRS-related papers have already been published [16,17] to exhibit that IR bands having surface-parallel orientation in a thin film dominantly appear in the surface-parallel spectrum of MAIRS, which proves the MAIRS technique working theoretically. Nevertheless, there has been no report for Ôpurely surface-normalÕ modes, since there are few examples to have the surface-normally oriented chemical groups in thin organized films. In the present study, the IR-MAIRS technique has first been employed for the analysis of the Si–H bonds formed on the Si wafer surface, in which the Si–H vibration mode is essentially expected to have the surface-normal orientation. 2. Experimental An n-type silicon double-faced-mirror wafer (50 · 20 · 0.5 mm, 1–40 X cm) was purchased from Sumitomo Mitsubishi Silicon Co. (Tokyo, Japan), which has (1 1 1) plane on the surfaces. The Si(1 1 1) topmost layer was hydrogen-terminated by an etching technique as described elsewhere [18]. The infrared MAIRS spectra were measured on a Thermo-Electron Nicolet (Madison, WI) Magna 550 FT-IR equipped with an MCT detector with a resolution of 4 cm1 . The aperture was fully opened [14]. 3. Results and discussion Fig. 1 presents the Si–H stretching vibration region of the IR-MAIRS spectra observed on the H-terminated Si (Si(1 1 1) 1 · 1–H) surface. Note that the surface-parallel spectrum is 20-times magnified in intensity. Although the
Here, the subscripts (1, 2, 3, . . .) represent spectra indices for different angles of incidence. The Ôdescribed partÕ can be extracted from S to be a solution as stated by Eq. (2). The solutions, si and s^, correspond to the single-beam spectra virtually measured by normal-incidence transmission spectrometries. The residual part in S is stored in the undescribed (Ud) matrix in Eq. (1) sk ð2Þ ¼ ðRT RÞ1 RT S. s? The superscripts, T and 1, indicate transpose and reverse matrix, respectively. The matrix, R, is given by Eq. (3) [13], where h is the angle of incidence 2 3 2 2 2 2 2 1 þ cos h1 þ sin h1 tan h1 tan h1 7 4 6 6 1 þ cos2 h2 þ sin2 h2 tan2 h2 tan2 h2 7. ð3Þ R¼ 5 p 4 .. .. . .
Fig. 1. FT-IR MAIRS spectra of Si(1 1 1) 1 · 1–H surface. The surfacenormal and surface-parallel spectra correspond to the conventional RA spectrum and normal-incidence transmission one, respectively. The surface-parallel spectrum is 20· magnified in intensity.
174
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
spectra are derived from a single monolayer of Si–H, clear spectra with good quality have been obtained. When transmission measurements are performed for wafers, strong interference fringes often appear on the spectra due to multiple-reflections of the light in the wafer. In the IR-MAIRS spectra, however, such fringes are effectively removed, since the optical fringe is a typical ÔundescribedÕ factor in the regression equation (Eq. (1)). This fringe-removal effect is another benefit of IR-MAIRS. It is known that the Si surface prepared by the etching technique retains 1 · 1 structure, since the atomic rearrangement by oxidization is suppressed by the hydrogen coverage. Regardless, even the hydrogen-terminated Si surface is known to be oxidized with time in air, which gradually makes the topmost surface be amorphous-glass. Therefore, it is expected that an aged hydrogen-terminated Si wafer would have major two different Si–H modes derived from the terrace of a fresh-surface layer and of an amorphous-glass one [19–21]. The Si–H stretching vibration band of the uncoupled monohydride (m(„Si–H); A1 [22]), which is specifically found on a flat Si(1 1 1) terrace of a fresh (high-coverage) sample, appears strongly at 2083 cm1 in the surface-normal spectrum, while it is very weak in the surface-parallel spectrum (Fig. 1). In this manner, the surface-normal spectrum corresponds to the conventional RA spectrum, as if it were measured on a metallic surface. This MAIRS dichroism strongly suggests that the Si–H stretching vibration mode is oriented nearly perpendicular to the surface (Fig. 2a). When the surface is subjected to infrared ATR spectrometry, according to the previous reports [8,18,22], this band appears only in the p-polarization spectrum, while it is not available in the s-polarization one. Therefore, it has been believed that the Si–H bond is oriented perpendicularly to the surface, which is basically consistent with the present MAIRS analysis. It has been difficult thus far, however, to perform the precise quantitative molecular orientation analysis with the polarized ATR spectra. Since the MAIRS surface-parallel and surface-normal spectra have a common absorbance scale, it is easy to evaluate the molecular orientation angle (/) with the use of the intensity ratio between the two spectra. The equation for the evaluation of molecular orientation is given as follows [13]:
/ ¼ tan1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2I parallel =I normal .
ð4Þ
Here, Iparallel and Inormal are band intensities of an identical mode in the surface-parallel and -normal spectra, respectively. As a result, the Si–H bond that corresponds to the band at 2083 cm1 has been revealed to have an orientation angle of 5 from the surface normal. This angle is reasonable when the Si–H groups are packed on the surface with some molecular space, although the groups schematically have the perpendicular orientation (Fig. 2a). Peaks associated with the dihydride (@SiH2, 2090– 2120 cm1) and the trihydride (–SiH3, 2120–2150 cm1), which are available on steps [8,22,23], are all silent in the MAIRS spectra. This result suggests that the Si(1 1 1) 1 · 1–H wafer has an atomically flat surface. On the other hand, a sharp band appears at 2077 cm1 in the surface-parallel spectrum on a tail of the band at 2083 cm1. This band has scarcely been recognized thus far, since most of the experiments have been performed in an ultrahigh vacuum (UHV) condition for a fresh sample. In the present analysis, the sample was prepared by the wet technique, and it was measured in air that contains oxygen. A fact that the band appears only in the surfacenormal spectrum suggests that the mode is similar to that for the 2083 cm1 band. Therefore, it is considered that the band at 2077 cm1 is attributed to the Si–H bond in amorphous-glass layers (Fig. 2b). Of another interest is the band at 2071 cm1 found in the MAIRS surface-parallel spectrum. This mode is theoretically reported to have the B1 mode, and its orientation is mostly parallel to the surface [22]. In the report, the orientation was ambiguous and it was available both in s- and p-polarized ATR spectra, partly because the surface of the wafer had Ômisorientation.Õ In the present analysis, this mode appears only in the surface-parallel spectrum, which strongly supports the theoretical expectation of orientation, and the Si wafer used for the present study is, therefore, suggested to have no misorientation. In fact, the misorientation angle has been estimated by AFM measurements (data not shown) to be ca. 0.14, which is negligibly small relative to experimental error. In addition, it is interesting that the band intensity of this mode is much smaller than the other bands. This is understandable when we consider that this mode is oriented surface-parallel. In the parallel orientation, a number of oscillators of this mode are distributed randomly in the surface layer, which diminishes the total transition moment. In other words, the very weak intensity reversely supports the surface-parallel orientation of the mode. In this manner, the IR-MAIRS technique has been found useful to characterize semiconductor surfaces. Acknowledgements
Fig. 2. Schematic image of Si–H bonds on: (a) a fresh Si(1 1 1) 1 · 1–H surface and (b) a slightly oxidized surface with a thin amorphous-glass layer.
This work was financially supported by Grant-in-Aid for Scientific Research (B) (No. 16350048) from the Ministry of Education, Science, Sports, Culture, and
H. Kakuda et al. / Chemical Physics Letters 415 (2005) 172–175
Technology, Japan, and also by The Futaba Electronics Memorial Foundation, Japan. References [1] [2] [3] [4] [5] [6] [7]
[8] [9]
[10]
S. Ye, M. Osawa, K. Uosaki, Shinku 476 (2004) 449. D.D.M. Wayner, R. Wolkow, J. Chem. Soc., Perkin Trans. 2 1 (2002) 23. H. Ubara, T. Imura, A. Hiraki, Solid State Commun. 50 (1984) 673. H. Yu, L.J. Webb, R.S. Ries, S.D. Solares, W.A. Goddard III, J.R. Heath, N.S. Lewis, J. Phys. Chem. B 109 (2005) 671. M.A. Hines, Y.J. Chabal, T.D. Harris, A.L. Harris, J. Chem. Phys. 101 (1994) 8055. W.W. Chen, X.H. Sun, S.D. Wang, S.T. Lee, B.K. Teo, J. Phys. Chem. B 109 (2005) 10871. V.P. Tolstoy, I.V. Chernyshova, V.A. Skryshevsky, Handbook of Infrared Spectroscopy of Ultrathin Films, Wiley-Interscience, New York, 2003. T. Hattori, J. Vac. Sci. Technol. B 11 (1993) 1528. N. Nagai, Y. Izumi, H. Ishida, Y. Suzuki, A. Hatta, in: J.A. de Haseth (Ed.), Proceedings of 11th International Conference of Fourier Transform Spectroscopy, Mater. Sci., 28, American Institute of Physics, 1998, p. 581. J. Umemura, Reflection-absorption spectroscopy of thin films on metallic substrates, in: John M. Chalmers, Peter R. Griffiths (Eds.), Handbook of Vibrational Spectroscopy, vol. 2, Wiley, Chichester, 2002, p. 982.
175
[11] R.G. Greenler, J. Chem. Phys. 44 (1966) 310, The RA technique was originally defined as a technique for grazing-angle incidence measurements on a metallic surface. In recent years, however, the term ÔRAÕ has been used for a wide range of reflection measurements including the ER measurements on dielectric materials. [12] T. Hasegawa, S. Takeda, A. Kawaguchi, J. Umemura, Langmuir 11 (1995) 1236. [13] T. Hasegawa, J. Phys. Chem. B 106 (2002) 4112, The equation for molecular orientation analysis (Eq. 5) is misprinted in the original paper, and Eq. (4) in the present Letter is the corrected one. [14] T. Hasegawa, L. Matsumoto, S. Kitamura, S. Amino, S. Katada, J. Nishijo, Anal. Chem. 74 (2002) 6049. [15] T. Hasegawa, Anal. Bioanal. Chem. 375 (2003) 18. [16] T. Hasegawa, J. Umemura, C. Li, R.M. Leblanc, J. Phys. Chem. B 107 (2003) 11996. [17] T. Hasegawa, H. Kakuda, N. Yamada, J. Phys. Chem. B 109 (10) (2005) 4783. [18] G.S. Higashi, R.S. Becker, Y.J. Chabal, A.J. Becker, Appl. Phys. Lett. 58 (1991) 1656. [19] Y.J. Chabal, Phys. Rev. Lett. 50 (1983) 1850. [20] Y.J. Chabal, G.S. Higashi, S.B. Christman, Phys. Rev. B 28 (1983) 4472. [21] Danial D.M. Wayner, Robert A. Wolkow, J. Chem. Soc., Perkin Trans. 2 (2002) 23. [22] P. Jacob, Y.J. Chabal, J. Chem. Phys. 95 (1991) 2897. [23] V.A. Burrows, Y.J. Chabal, S.G. Higashi, K. Raghavachari, S.B. Christman, Appl. Phys. Lett. 53 (1988) 998.