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Si(111 ) surfaces by infrared spectroscopy
Surface Science 502–503 (2002) 91–95 www.elsevier.com/locate/susc
Investigation of the bending vibrations of vicinal H/Si(1 1 1) surfaces by infrared spectroscopy Y. Caudano a,*, P.A. Thiry a, Y.J. Chabal b a
Laboratoire de Spectroscopie Mol eculaire de Surface, Facult es Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium b Agere Systems, Electronic and Photonic Materials Physics Research, 600 Mountain Avenue, Murray Hill, NJ 07974, USA
Abstract We studied the vibrational structure of vicinal hydrogenated Si(1 1 1) surfaces by transmission infrared spectroscopy. In particular, we investigated the bending modes of the silicon hydrides present on the terraces and at the step-edges. We find that step-induced surface stress and/or long-range dipolar interactions split the bending modes of the Si(1 1 1) terrace monohydrides into two components, parallel and perpendicular to the step-edge direction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Hydrogen atom; Infrared absorption spectroscopy; Semiconducting surfaces; Silicon; Stepped single crystal surfaces; Surface structure, morphology, roughness, and topography; Vibrations of adsorbed molecules
1. Introduction The stretching vibrations of the silicon hydride groups terminating H/Si surfaces (2100 cm 1 ) were analyzed extensively using infrared absorption spectroscopy in the multiple-internal-reflection (MIR) configuration [1–3]. However, absorptions provoked below 1300 cm 1 by silicon phonons and multi-phonon processes [4] prevent the study of the Si–H bending modes by MIR as a result of the low bending frequencies (<1000 cm 1 ). High-resolution characterizations of the bending modes of H/ Si surfaces are nevertheless primordial to identify the hydride structures unambiguously when the surface orientation and morphology are not known
*
Corresponding author. Fax: +32-81-724707. E-mail address: [email protected] (Y. Caudano).
beforehand. Substantial improvements in the signal-to-noise ratio of infrared spectrometers have recently allowed the study of the bending modes of H/Si surfaces by single-pass transmission spectroscopy, even though the latter has a much weaker surface sensitivity than MIR. The bending modes of vicinal H/Si(1 1 1) surfaces have first been observed with infrared transmission spectroscopy by Watanabe et al. [5–9], as a function of the incidence angle and beam polarization. The hydrogenation was performed by immersion in hot deionized and deoxidized water [10,11]. In contrast to their work, we passivated the Si surfaces with hydrogen using NH4 F and HF solutions. The narrower vibrational line widths resulting from this preparation procedure testify its production of better quality surfaces. Accordingly, a deeper examination of the bending modes can be carried out, yielding thus a more reliable
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 0 4 - 5
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Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
fingerprinting of the low-frequency Si–H vibrations. The intent of this work is both to provide a reference for mode assignment and to evaluate the sensitivity of the Si–H bending vibration to various surface phenomena, such as step-induced surface stress and long-range dipolar interactions.
2. Experimental The Si(1 1 1) samples (3:75 cm 1:25 cm 0:05 cm) come from 100 mm float-zone wafers, n-doped by P (resistivity of 24–34 X cm), covered initially by a thick, protective thermal oxide. The stepped Si(1 1 1) surfaces are produced by applying a miscut polar angle with respect to the [1 1 1] direction along the [1 1 2] azimuth. Miscut angles were 2°, 5°, 9° and 5°, 9°, 15° respectively. Negative miscut angles define surfaces with stepedges decorated by vertical dihydrides (Si–H2 groups), while positive miscut angles result in stepedges terminated by monohydride zigzag chains (Fig. 1). The preparation of ideal hydrogenated Si surfaces involves modified RCA clean and hydrogenation steps. It is function of the surface orientation [3,12–20]. To obtain atomically smooth H/Si(1 1 1)1 1 surfaces, the Si(1 1 1) samples are immersed successively in H2 O/H2 O2 /NH4 OH and H2 O/H2 O2 /HCl solutions (each 10 min at 80 °C). The surface is then hydrogenated with a NH4 F solution (6 min 30 s in NH4 F 40%, pH 7.8). Follows a second oxidation stage in H2 O/H2 O2 /HCl and a last hydrogenation by NH4 F. To prepare the vicinal Si(1 1 1) surfaces, the process has to be modified due to the quick etching by NH4 F. Therefore, removal of the thick surface thermal
Fig. 1. Step-edge structures of H/Si(1 1 1): vertical Si–H2 groups (left) and monohydride zigzag chain (right).
oxide is performed by plunging the sample into buffered HF (concentrated HF 49% and NH4 F 40% in 1:7 proportions, pH 5.5) instead of NH4 F, while the last hydrogenation step consists firstly of dipping the sample into concentrated HF (a few seconds) to remove the surface chemical oxide, and secondly of immersing it into NH4 F to improve the surface structure. The duration of the last immersion depends on the step density of the sample (3 min 45 s for 2°, 1 min 10 s for 5°, 50 s for 9° and 40 s for 15°). It is determined by the etching rate of the surface by NH4 F, which is evaluated qualitatively by monitoring the quantity of bubbles appearing at the sample surface [17,18, 21]. All preparations are completed by a quick rinse of the sample in deionized water. The infrared absorption spectra are measured using nitrogen-purged Fourier-transform infrared (FTIR) spectrometers (Nicolet 750 and 860) combined with a DTGS detector for studying the H/Si(1 1 1)1 1 surface, or a MCT detector in the case of the stepped surfaces. The measurements are performed at ambient pressure and temperature, in a nitrogen atmosphere. In the case of H/Si(1 1 1)1 1, infrared absorption spectroscopy is carried out in transmission at Brewster’s angle (73.7°), with unpolarized radiation since the surface is isotropic. In contrast, because the vicinal hydrogenated Si(1 1 1) surfaces present a specific direction of terrace edges, transmission at normal incidence is then used, with polarized radiation (parallel or perpendicular to the step-edges, respectively). The spectrometer resolution is set to 0.25 cm 1 for H/Si(1 1 1)1 1 and 0.5 cm 1 for stepped Si(1 1 1). The H/Si spectra are all obtained by referencing to the spectra of the same samples after surface oxidation in H2 O/ H2 O2 /HCl. The occurrence of multiple reflections between the parallel surfaces of the sample provoke interferences that induce strong oscillations of the absorption spectra baseline. They can mask completely the absorption bands at high resolution and are particularly pronounced at normal incidence. Therefore, the parts of the interferogram corresponding to the oscillation frequencies are removed. When the spectra quality is still not acceptable after this filtering, a cutoff frequency
Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
is introduced, resulting in a poorer effective resolution (usually 1–2 cm 1 ).
3. Results and discussion 3.1. H/Si(1 1 1)1 1 surface The simple vibrational spectrum of the atomically smooth H/Si(1 1 1)1 1 surface presents only two narrow bands at 626.7 and 2083.7 cm 1 (Fig. 2) with respective full widths at half maximum equal to 1.3 and 1.0 cm 1 . They correspond to the doubly degenerate bending mode and to the well-characterized stretching vibration of the surface Si–H bonds, respectively. Using a three-layer dielectric model with an effective Si–H dielectric constant 2:0 [17,22] to evaluate the sensitivity of the spectroscopy to each vibration at Brewster’s angle, we find from the measured integrated absorbance of the modes that the oscillator strength of the bending vibration is 5.5 times stronger than the stretching one.
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Si–H bonds on hydrogenated 5°, 9° and 15° vicinal Si(1 1 1) surfaces are shown in Fig. 3 (left panel). The stretching mode spectrum (not shown here) reproduces well the previously established characteristic spectrum of monohydride terraces bounded by step-edges decorated with vertical SiH2 groups [23–30]. The peak observed at 655.5 cm 1 , only in the direction parallel to the step-edge and with increasing intensity vs. step density, corresponds to the SiH2 wagging vibration at the step-edge. The growth of the peak is proportional to the expected step density on the surface. (The actual density and the one derived from the sample miscut angle can differ due to kinetically controlled fluctuations during the etching process.) On H/Si(1 0 0) surfaces, we
3.2. Dihydride steps By using infrared spectroscopy in transmission at normal incidence with polarized radiation, we can distinguish between vibrational modes parallel and perpendicular to the step-edge direction. The bending vibrations of the terrace and step-edge
Fig. 2. Infrared absorption spectrum of the H/Si(1 1 1)1 1 surface (recorded in transmission at Brewster’s angle).
Fig. 3. Transmission infrared spectra of H/Si(1 1 1) surfaces miscut by 5°, 9°, and 15°, presenting step-edges decorated with dihydrides (left panels), and by 2°, 5°, and 9° , with stepedges terminated by zigzag monohydride chains (right panels). Polarization parallel (perpendicular) to step-edge: top (bottom) panels. Vertical dashed lines at 626.7 cm 1 : position of the bending mode on H/Si(1 1 1)1 1.
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measured the frequency of this mode at 656.5 cm 1 . This points out that the strong steric interaction occurring at the step between the SiH2 group and the underneath monohydride row does not produce a strong frequency shift of the vibration. The peaks observed around 627 cm 1 with slightly decreasing intensity for both polarizations are attributed to the terrace monohydride bending modes. As a result of the step formation, the terrace bending mode splits into two components, parallel and perpendicular to the step-edge direction, presenting differing frequencies. This observation will be discussed later, in the view of our results on monohydride steps. The estimate of the surface monohydride coverage deduced from the integrated absorbance of the terrace bending modes disagrees totally with the theoretical predictions inferred from the miscut angle. However, the measured absorbance is consistent with the intensity expected if we assume that the first monohydride row of the lower terrace does not contribute to the terrace absorption around 627 cm 1 . This Si–H row interacts strongly with the step-edge SiH2 and its stretching mode is indeed distinct from the terrace stretch [23–30]. This might indicate that this row also possesses specific bending modes (i.e. differing from those of the neighboring terrace Si–H rows), even though these are not detected. 3.3. Monohydride steps The bending vibrations detected on vicinal H/ Si(1 1 1) surfaces with 2°, 5° and 9° miscut angles are presented in Fig. 3 (right panel). The characteristic asymmetric stretch of the step-edge Si–H zigzag chain is present at 2070.5 cm 1 (not shown here) [17,21,31], confirming the monohydride structure of the steps. In agreement with Refs. [5–8], we attribute the bending mode measured at 614 cm 1 , only in the parallel direction, to a step vibration. The latter is identified from the polarization dependence of the mode absorption and the step-edge symmetry. It corresponds to the parallel in-phase bend of the coupled monohydride chain terminating the stepedge.
For both incident beam polarizations, an absorption band is found at around 627 cm 1 . It is assigned to the bending modes of the terrace monohydrides. For larger miscut angles, a contribution of the perpendicular component of the bending mode of the step-edge Si–H zigzag chains is expected to be present in the foot of the peak (on Si(1 1 0), this mode has been observed at 637 cm 1 [7] while we measured it at 632 cm 1 ). This may be evidenced by the relatively constant integrated absorbance of the band, at contrast with the decreasing intensities of the parallel component and of the terrace modes on surfaces with dihydride step-edges. Similarly to what we observe on surfaces with dihydride steps, the terrace bending mode splits into two components, parallel and perpendicular to the step-edge direction, with decreasing and increasing frequency vs. step density, respectively. This splitting is in agreement with the modification of the surface symmetry that is introduced by the parallel step-edge structures. Two major effects can contribute to the opposite frequency shifts of the original mode components. They are step-induced surface stress and long-range dipolar interactions between the terrace monohydrides. Let us note that preliminary calculations of the dipolar interaction between the Si–H bending modes of rectangular terraces indicate that for a single row of oscillators the frequency of the parallel (perpendicular) component decreases (increases) compared to the Si–H bend isolated frequency. This may be an indication that dipolar coupling indeed plays a role in the observed experimental shifts.
4. Conclusions We analyzed the bending vibrations of vicinal H/Si(1 1 1) surfaces by infrared absorption spectroscopy in transmission. We determined that the atomically straight steps of vicinal surfaces split the degenerate terrace bending mode in two components, parallel and perpendicular to the step-edge direction. This effect may arise from stepinduced surface stress and/or long-range dipolar interaction between the bending modes of the terrace monohydrides. On vicinal surfaces with step-
Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
edges decorated by vertical dihydrides, we also deduced that the first monohydride row adjacent to the steps might possess specific bending modes, distinct from those of the other terrace rows. Acknowledgements Y. Caudano acknowledges the Belgian National Fund for Scientific Research (FNRS) for financial support. References [1] Y.J. Chabal, Surf. Sci. Rep. 8 (1988) 211. [2] Y.J. Chabal, Properties of crystalline silicon, EMIS Data Rev. 20 (1999) 211m. [3] Y.J. Chabal, in: T.S. Moss (Ed.), Handbook on Semiconductors, vol. 2, Elsevier Science, Amsterdam, 1994, p. 188. [4] R.J. Collins, H.Y. Fan, Phys. Rev. 93 (1954) 674. [5] S. Watanabe, Y. Sugita, Chem. Phys. Lett. 244 (1995) 105. [6] S. Watanabe, Y. Sugita, Surf. Sci. 357/358 (1996) 536. [7] S. Watanabe, J. Chem. Phys. 108 (1998) 5965. [8] S. Watanabe, Appl. Surf. Sci. 130–132 (1998) 231. [9] S. Watanabe, Surf. Sci. 415 (1998) 385. [10] S. Watanabe, N. Nakayama, T. Ito, Appl. Phys. Lett. 59 (1991) 1458. [11] S. Watanabe, Y. Sugita, Surf. Sci. 327 (1995) 1. [12] Y.J. Chabal, in: W. Kern (Ed.), Handbook of Semiconductor Wafer Cleaning Technology; Science, Technology and Applications, Noyes Publications, Park Ridge, NJ, USA, 1993 (and references therein).
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[13] Y.J. Chabal, G.S. Higashi, K. Ragavachari, V.A. Burrows, J. Vac. Sci. Technol. A 7 (1989) 2104. [14] Y.J. Chabal, A.L. Harris, K. Raghavachari, J.C. Tully, Int. J. Mod. Phys. B 7 (1993) 1031. [15] G.W. Trucks, K. Ragavachari, G.S. Higashi, Y.J. Chabal, Phys. Rev. Lett. 65 (1990) 504. [16] G.S. Higashi, Y.J. Chabal, G.W. Trucks, K. Ragavachari, Appl. Phys. Lett. 56 (1990) 656. [17] P. Jakob, Y.J. Chabal, J. Chem. Phys. 95 (1991) 2897. [18] P. Jakob, Y.J. Chabal, K. Ragavachari, R.S. Becker, A.J. Becker, Surf. Sci. 275 (1992) 407. [19] Y.J. Chabal, Mat. Res. Soc. Symp. Proc., vol. 259, Mat. Res. Soc., Warrendale, PA, 1992, p. 349. [20] P. Dumas, Y.J. Chabal, P. Jacob, Surf. Sci. 269/270 (1992) 867. [21] P. Jakob, Y.J. Chabal, K. Kuhnke, S.B. Christman, Surf. Sci. 302 (1994) 49. [22] Y.J. Chabal, Physica B 170 (1991) 447. [23] M. Morin, P. Jakob, N.J. Levinos, Y.J. Chabal, A.L. Harris, J. Chem. Phys. 96 (1992) 6203. [24] K. Kuhnke, M. Morin, P. Jakob, N.J. Levinos, Y.J. Chabal, A.L. Harris, J. Chem. Phys. 99 (1993) 6114. [25] K. Kuhnke, A.L. Harris, Y.J. Chabal, P. Jakob, M. Morin, J. Chem. Phys. 100 (1994) 6896. [26] K. Ragavachari, P. Jakob, Y.J. Chabal, Chem. Phys. Lett. 206 (1993) 156. [27] Y.J. Chabal, J. Mol. Struct. 292 (1993) 65. [28] M.A. Hines, Y.J. Chabal, T.D. Harris, A.L. Harris, Phys. Rev. Lett. 71 (1993) 2280. [29] M.A. Hines, T.D. Harris, A.L. Harris, Y.J. Chabal, J. Electron Spectrosc. 64/65 (1993) 183. [30] P. Jakob, Y.J. Chabal, K. Raghavachari, S.B. Christman, Phys. Rev. B 47 (1993) 6839. [31] H.Y. Yang, G. Lucovsky, Phys. Status Solidi A 159 (1997) 17.
Investigation of the bending vibrations of vicinal H/Si(1 1 1) surfaces by infrared spectroscopy Y. Caudano a,*, P.A. Thiry a, Y.J. Chabal b a
Laboratoire de Spectroscopie Mol eculaire de Surface, Facult es Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium b Agere Systems, Electronic and Photonic Materials Physics Research, 600 Mountain Avenue, Murray Hill, NJ 07974, USA
Abstract We studied the vibrational structure of vicinal hydrogenated Si(1 1 1) surfaces by transmission infrared spectroscopy. In particular, we investigated the bending modes of the silicon hydrides present on the terraces and at the step-edges. We find that step-induced surface stress and/or long-range dipolar interactions split the bending modes of the Si(1 1 1) terrace monohydrides into two components, parallel and perpendicular to the step-edge direction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Hydrogen atom; Infrared absorption spectroscopy; Semiconducting surfaces; Silicon; Stepped single crystal surfaces; Surface structure, morphology, roughness, and topography; Vibrations of adsorbed molecules
1. Introduction The stretching vibrations of the silicon hydride groups terminating H/Si surfaces (2100 cm 1 ) were analyzed extensively using infrared absorption spectroscopy in the multiple-internal-reflection (MIR) configuration [1–3]. However, absorptions provoked below 1300 cm 1 by silicon phonons and multi-phonon processes [4] prevent the study of the Si–H bending modes by MIR as a result of the low bending frequencies (<1000 cm 1 ). High-resolution characterizations of the bending modes of H/ Si surfaces are nevertheless primordial to identify the hydride structures unambiguously when the surface orientation and morphology are not known
*
Corresponding author. Fax: +32-81-724707. E-mail address: [email protected] (Y. Caudano).
beforehand. Substantial improvements in the signal-to-noise ratio of infrared spectrometers have recently allowed the study of the bending modes of H/Si surfaces by single-pass transmission spectroscopy, even though the latter has a much weaker surface sensitivity than MIR. The bending modes of vicinal H/Si(1 1 1) surfaces have first been observed with infrared transmission spectroscopy by Watanabe et al. [5–9], as a function of the incidence angle and beam polarization. The hydrogenation was performed by immersion in hot deionized and deoxidized water [10,11]. In contrast to their work, we passivated the Si surfaces with hydrogen using NH4 F and HF solutions. The narrower vibrational line widths resulting from this preparation procedure testify its production of better quality surfaces. Accordingly, a deeper examination of the bending modes can be carried out, yielding thus a more reliable
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 0 4 - 5
92
Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
fingerprinting of the low-frequency Si–H vibrations. The intent of this work is both to provide a reference for mode assignment and to evaluate the sensitivity of the Si–H bending vibration to various surface phenomena, such as step-induced surface stress and long-range dipolar interactions.
2. Experimental The Si(1 1 1) samples (3:75 cm 1:25 cm 0:05 cm) come from 100 mm float-zone wafers, n-doped by P (resistivity of 24–34 X cm), covered initially by a thick, protective thermal oxide. The stepped Si(1 1 1) surfaces are produced by applying a miscut polar angle with respect to the [1 1 1] direction along the [1 1 2] azimuth. Miscut angles were 2°, 5°, 9° and 5°, 9°, 15° respectively. Negative miscut angles define surfaces with stepedges decorated by vertical dihydrides (Si–H2 groups), while positive miscut angles result in stepedges terminated by monohydride zigzag chains (Fig. 1). The preparation of ideal hydrogenated Si surfaces involves modified RCA clean and hydrogenation steps. It is function of the surface orientation [3,12–20]. To obtain atomically smooth H/Si(1 1 1)1 1 surfaces, the Si(1 1 1) samples are immersed successively in H2 O/H2 O2 /NH4 OH and H2 O/H2 O2 /HCl solutions (each 10 min at 80 °C). The surface is then hydrogenated with a NH4 F solution (6 min 30 s in NH4 F 40%, pH 7.8). Follows a second oxidation stage in H2 O/H2 O2 /HCl and a last hydrogenation by NH4 F. To prepare the vicinal Si(1 1 1) surfaces, the process has to be modified due to the quick etching by NH4 F. Therefore, removal of the thick surface thermal
Fig. 1. Step-edge structures of H/Si(1 1 1): vertical Si–H2 groups (left) and monohydride zigzag chain (right).
oxide is performed by plunging the sample into buffered HF (concentrated HF 49% and NH4 F 40% in 1:7 proportions, pH 5.5) instead of NH4 F, while the last hydrogenation step consists firstly of dipping the sample into concentrated HF (a few seconds) to remove the surface chemical oxide, and secondly of immersing it into NH4 F to improve the surface structure. The duration of the last immersion depends on the step density of the sample (3 min 45 s for 2°, 1 min 10 s for 5°, 50 s for 9° and 40 s for 15°). It is determined by the etching rate of the surface by NH4 F, which is evaluated qualitatively by monitoring the quantity of bubbles appearing at the sample surface [17,18, 21]. All preparations are completed by a quick rinse of the sample in deionized water. The infrared absorption spectra are measured using nitrogen-purged Fourier-transform infrared (FTIR) spectrometers (Nicolet 750 and 860) combined with a DTGS detector for studying the H/Si(1 1 1)1 1 surface, or a MCT detector in the case of the stepped surfaces. The measurements are performed at ambient pressure and temperature, in a nitrogen atmosphere. In the case of H/Si(1 1 1)1 1, infrared absorption spectroscopy is carried out in transmission at Brewster’s angle (73.7°), with unpolarized radiation since the surface is isotropic. In contrast, because the vicinal hydrogenated Si(1 1 1) surfaces present a specific direction of terrace edges, transmission at normal incidence is then used, with polarized radiation (parallel or perpendicular to the step-edges, respectively). The spectrometer resolution is set to 0.25 cm 1 for H/Si(1 1 1)1 1 and 0.5 cm 1 for stepped Si(1 1 1). The H/Si spectra are all obtained by referencing to the spectra of the same samples after surface oxidation in H2 O/ H2 O2 /HCl. The occurrence of multiple reflections between the parallel surfaces of the sample provoke interferences that induce strong oscillations of the absorption spectra baseline. They can mask completely the absorption bands at high resolution and are particularly pronounced at normal incidence. Therefore, the parts of the interferogram corresponding to the oscillation frequencies are removed. When the spectra quality is still not acceptable after this filtering, a cutoff frequency
Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
is introduced, resulting in a poorer effective resolution (usually 1–2 cm 1 ).
3. Results and discussion 3.1. H/Si(1 1 1)1 1 surface The simple vibrational spectrum of the atomically smooth H/Si(1 1 1)1 1 surface presents only two narrow bands at 626.7 and 2083.7 cm 1 (Fig. 2) with respective full widths at half maximum equal to 1.3 and 1.0 cm 1 . They correspond to the doubly degenerate bending mode and to the well-characterized stretching vibration of the surface Si–H bonds, respectively. Using a three-layer dielectric model with an effective Si–H dielectric constant 2:0 [17,22] to evaluate the sensitivity of the spectroscopy to each vibration at Brewster’s angle, we find from the measured integrated absorbance of the modes that the oscillator strength of the bending vibration is 5.5 times stronger than the stretching one.
93
Si–H bonds on hydrogenated 5°, 9° and 15° vicinal Si(1 1 1) surfaces are shown in Fig. 3 (left panel). The stretching mode spectrum (not shown here) reproduces well the previously established characteristic spectrum of monohydride terraces bounded by step-edges decorated with vertical SiH2 groups [23–30]. The peak observed at 655.5 cm 1 , only in the direction parallel to the step-edge and with increasing intensity vs. step density, corresponds to the SiH2 wagging vibration at the step-edge. The growth of the peak is proportional to the expected step density on the surface. (The actual density and the one derived from the sample miscut angle can differ due to kinetically controlled fluctuations during the etching process.) On H/Si(1 0 0) surfaces, we
3.2. Dihydride steps By using infrared spectroscopy in transmission at normal incidence with polarized radiation, we can distinguish between vibrational modes parallel and perpendicular to the step-edge direction. The bending vibrations of the terrace and step-edge
Fig. 2. Infrared absorption spectrum of the H/Si(1 1 1)1 1 surface (recorded in transmission at Brewster’s angle).
Fig. 3. Transmission infrared spectra of H/Si(1 1 1) surfaces miscut by 5°, 9°, and 15°, presenting step-edges decorated with dihydrides (left panels), and by 2°, 5°, and 9° , with stepedges terminated by zigzag monohydride chains (right panels). Polarization parallel (perpendicular) to step-edge: top (bottom) panels. Vertical dashed lines at 626.7 cm 1 : position of the bending mode on H/Si(1 1 1)1 1.
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measured the frequency of this mode at 656.5 cm 1 . This points out that the strong steric interaction occurring at the step between the SiH2 group and the underneath monohydride row does not produce a strong frequency shift of the vibration. The peaks observed around 627 cm 1 with slightly decreasing intensity for both polarizations are attributed to the terrace monohydride bending modes. As a result of the step formation, the terrace bending mode splits into two components, parallel and perpendicular to the step-edge direction, presenting differing frequencies. This observation will be discussed later, in the view of our results on monohydride steps. The estimate of the surface monohydride coverage deduced from the integrated absorbance of the terrace bending modes disagrees totally with the theoretical predictions inferred from the miscut angle. However, the measured absorbance is consistent with the intensity expected if we assume that the first monohydride row of the lower terrace does not contribute to the terrace absorption around 627 cm 1 . This Si–H row interacts strongly with the step-edge SiH2 and its stretching mode is indeed distinct from the terrace stretch [23–30]. This might indicate that this row also possesses specific bending modes (i.e. differing from those of the neighboring terrace Si–H rows), even though these are not detected. 3.3. Monohydride steps The bending vibrations detected on vicinal H/ Si(1 1 1) surfaces with 2°, 5° and 9° miscut angles are presented in Fig. 3 (right panel). The characteristic asymmetric stretch of the step-edge Si–H zigzag chain is present at 2070.5 cm 1 (not shown here) [17,21,31], confirming the monohydride structure of the steps. In agreement with Refs. [5–8], we attribute the bending mode measured at 614 cm 1 , only in the parallel direction, to a step vibration. The latter is identified from the polarization dependence of the mode absorption and the step-edge symmetry. It corresponds to the parallel in-phase bend of the coupled monohydride chain terminating the stepedge.
For both incident beam polarizations, an absorption band is found at around 627 cm 1 . It is assigned to the bending modes of the terrace monohydrides. For larger miscut angles, a contribution of the perpendicular component of the bending mode of the step-edge Si–H zigzag chains is expected to be present in the foot of the peak (on Si(1 1 0), this mode has been observed at 637 cm 1 [7] while we measured it at 632 cm 1 ). This may be evidenced by the relatively constant integrated absorbance of the band, at contrast with the decreasing intensities of the parallel component and of the terrace modes on surfaces with dihydride step-edges. Similarly to what we observe on surfaces with dihydride steps, the terrace bending mode splits into two components, parallel and perpendicular to the step-edge direction, with decreasing and increasing frequency vs. step density, respectively. This splitting is in agreement with the modification of the surface symmetry that is introduced by the parallel step-edge structures. Two major effects can contribute to the opposite frequency shifts of the original mode components. They are step-induced surface stress and long-range dipolar interactions between the terrace monohydrides. Let us note that preliminary calculations of the dipolar interaction between the Si–H bending modes of rectangular terraces indicate that for a single row of oscillators the frequency of the parallel (perpendicular) component decreases (increases) compared to the Si–H bend isolated frequency. This may be an indication that dipolar coupling indeed plays a role in the observed experimental shifts.
4. Conclusions We analyzed the bending vibrations of vicinal H/Si(1 1 1) surfaces by infrared absorption spectroscopy in transmission. We determined that the atomically straight steps of vicinal surfaces split the degenerate terrace bending mode in two components, parallel and perpendicular to the step-edge direction. This effect may arise from stepinduced surface stress and/or long-range dipolar interaction between the bending modes of the terrace monohydrides. On vicinal surfaces with step-
Y. Caudano et al. / Surface Science 502–503 (2002) 91–95
edges decorated by vertical dihydrides, we also deduced that the first monohydride row adjacent to the steps might possess specific bending modes, distinct from those of the other terrace rows. Acknowledgements Y. Caudano acknowledges the Belgian National Fund for Scientific Research (FNRS) for financial support. References [1] Y.J. Chabal, Surf. Sci. Rep. 8 (1988) 211. [2] Y.J. Chabal, Properties of crystalline silicon, EMIS Data Rev. 20 (1999) 211m. [3] Y.J. Chabal, in: T.S. Moss (Ed.), Handbook on Semiconductors, vol. 2, Elsevier Science, Amsterdam, 1994, p. 188. [4] R.J. Collins, H.Y. Fan, Phys. Rev. 93 (1954) 674. [5] S. Watanabe, Y. Sugita, Chem. Phys. Lett. 244 (1995) 105. [6] S. Watanabe, Y. Sugita, Surf. Sci. 357/358 (1996) 536. [7] S. Watanabe, J. Chem. Phys. 108 (1998) 5965. [8] S. Watanabe, Appl. Surf. Sci. 130–132 (1998) 231. [9] S. Watanabe, Surf. Sci. 415 (1998) 385. [10] S. Watanabe, N. Nakayama, T. Ito, Appl. Phys. Lett. 59 (1991) 1458. [11] S. Watanabe, Y. Sugita, Surf. Sci. 327 (1995) 1. [12] Y.J. Chabal, in: W. Kern (Ed.), Handbook of Semiconductor Wafer Cleaning Technology; Science, Technology and Applications, Noyes Publications, Park Ridge, NJ, USA, 1993 (and references therein).
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