Vibrational sum-frequency observation of synthetic diamonds

Diamond and Related Materials 10 Ž2001. 1643᎐1646

Vibrational sum-frequency observation of synthetic diamonds Hiroyuki Takabaa , Koji Kusafukaa , Mikka Nishitani-Gamo b, Yoichiro Sato b, Toshihiro Ando b, Jun Kubotaa , Akihide Wadaa,U , Chiaki Hirose a a

b

Chemical Resources Laboratory 1, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan Core Research for E¨ olutional Science and Technology (CREST) of Japan Science and Technology Corporation (JST), c r o National Institute for Research in Inorganic Materials (NIRIM), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Abstract Sum-frequency generation ŽSFG. spectroscopy was applied to the study of plasma hydrogenated diamond CŽ100. surface of a synthetic diamond. Two vibrational resonance peaks with the intensities strongly dependent on the polarization combinations of the visible and infrared beams were observed at 2899 and 2924 cmy1. From the results of SFG measurements for various polarization combinations, the 2899 and 2924 cmy1 peaks were assigned, respectively, to the antisymmetric and symmetric CH stretching modes of HC᎐CH group on HrCŽ100.-2 = 1 surface. In addition, a strong background signal with an oscillatory profile was observed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Spectroscopy; Synthetic diamond; Surface; CVD

1. Introduction Diamond thin film has a number of excellent properties such as high thermal conductivity, extreme hardness, and chemical inertness. The epitaxial growth of single-crystal diamond films has been a long-standing goal of diamond technology. Chemical vapor deposition ŽCVD. is the most well developed technique used to fabricate diamond films. In diamond film growth by CVD method, hydrogen atom is crucial for the enhancement of the growth rate by creating vacant sites through abstraction and for the improvement of the quality of the diamond film by suppressing graphite formation. Thus, hydrogen-covered diamond surfaces have attracted much attention because of the roles played by the hydrogen atoms in the CVD process. Vibrational spectroscopy is one of the most useful methods for the observation of surface species w1x, and hydrogen-covered diamond surfaces have been studied

U 1

Corresponding author. Renamed from Research Laboratory of Resources Utilization.

by such vibrational spectroscopic methods as high resolution electron energy loss spectroscopy ŽHREELS. w2᎐4x, sum-frequency generation ŽSFG. spectroscopy w5᎐7x, and surface enhanced Raman scattering ŽSERS. w8x. For diamond Ž111. surfaces prepared by hydrogen plasma or grown homoepitaxially, the presence of CH w5x and CH 3 w2᎐4,6,8x groups was revealed and much valuable information about their bonding geometry was obtained from the spectroscopic studies using those techniques. On the other hand, there are only a few spectroscopic studies of H-terminated CŽ100. surface. Using HREELS measurements on the boron-doped CŽ100.-2 = 1 surface Aizawa et al. observed an energy loss peak at 2928 cmy1 , which was assigned to the CH stretching band of a monohydride ŽCH. group. Anzai et al. observed the SFG spectra of homoepitaxially-grown undoped diamond CŽ100.-2 = 1 surface placed in a glass cell filled with Ar and found the peaks at 2910 and 2960 cmy1 which were assigned to the CH stretching mode of a monohydride group and antisymmetric CH stretching mode of a dihydride ŽCH 2 . group, respectively. Recent results of an ab initio self-consistent field molecular orbital calculation using a C 9 H 14 clus-

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 4 7 - 2

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H. Takaba et al. r Diamond and Related Materials 10 (2001) 1643᎐1646

ter predicted that the CH stretching vibration of the monohydride group consists of two modes, one at 2920 cmy1 and one at 2903 cmy1 , because the diamond CŽ100.-2 = 1 surface has dimer rows and they should have two stretching vibration modes: in-phase stretching Žsymmetric stretching . and out-of-phase stretching Žantisymmetric stretching . modes w8x. Ushizawa et al. observed a broad peak at approximately 2928 cmy1 with a shoulder tailed toward the lower frequency side on the SERS spectrum of HrCŽ100.-2 = 1 surface w8x. The peak at 2928 cmy1 was assigned to the symmetric CH stretching band of the HC᎐CH group which is the dimer unit of the diamond CŽ100.-2 = 1 surface terminated with hydrogen atoms. The shoulder was interpreted as due to the overlap of the antisymmetric CH stretching band of the HC᎐CH group and the symmetric and antisymmetric CH stretching bands of CH 2 group. Unfortunately, previously reported experimental data are insufficient to verify the result of the ab initio calculation. The SFG spectroscopy which detects the resonance enhancement of the SFG signal by the vibrational modes of the surface species has several unique features w9x. The method has surface-selectivity because the SFG process is a three-wave mixing process which is inactive to homogeneous materials, and exhibits excellent molecule- and environment-specificities since the vibrational bands of surface species are detected at a spectral resolution determined by the infrared light. Information on the orientational order of surface species can be derived from an analysis of the polarization characteristics associated with the three light waves w9᎐12x. In this paper we report on the SFG experiments carried out on a high-pressure high-temperature ŽHP᎐HT. synthetic diamond HrCŽ100.-2 = 1 surface placed in a UHV chamber. Two sharp SFG peaks appeared at 2899 and 2924 cmy1 after annealing at 973 K for 20 min and were assigned to antisymmetric and symmetric CH stretching bands of the HC᎐CH species, respectively.

treatment and surface characterization, the sample was transferred to a UHV chamber Žbase pressure; 1 = 10y1 0 torr. for the SFG measurements and cleaned by annealing for 20 min at 973 K under evacuation. The SFG measurements were carried out under the UHV condition at room temperature. The loss of hydrogen from the surface by the annealing is negligible because there was no effect on the SFG spectra by the several cycles of the cleaning procedure. A schematic layout of the SFG experiment is shown in Fig. 1. The fundamental Ž1064 nm. output pulses from a mode-locked Nd:YAG laser Ž35 ps, 10 Hz. were used for the generation of frequency-tunable IR pulses and the second harmonic Ž532-nm. pulses for the SFG. Optical parametric generation and amplification ŽOPGrOPA. using two LiNbO 3 crystals were used for the generation of the IR pulses. The generated IR pulses had a tuning range of 2550᎐4000 cmy1 with the spectral width of 10 cmy1 at 3000 cmy1 . The visible and IR pulses with the energies of 40 and 120 Jrpulse, respectively, were focused on the sample surface with the incident angles of 40 and 50⬚, respectively. The generated SFG pulses coming out in the direction of reflection were passed through dielectric filters and a monochromator, and detected by a photomultiplier tube. The vibrational SFG spectrum was obtained by monitoring the SFG signal intensity as a function of infrared frequency, as the intensity was resonantly enhanced when the frequency of the infrared beam resonates with a vibrational mode of surface species.

3. Results and discussion Fig. 2 shows the observed SFG spectra of HrCŽ100.2 = 1 surface. The spectra shown by curves Ža., Žb., Žc., and Žd. were obtained under the Žpp., Žsp., Žps., and Žss. polarization combinations, respectively, where the first and second letters in parenthesis denote the polarizations of visible and infrared beams, respectively. For the SFG beam, both p- and s-polarized beams were

2. Experimental HP᎐HT synthetic diamond CŽ100.-2 = 1 surface prepared by M.N.-G., Y.S. and T.A. at NIRIM, was investigated. The sample was polished and cleaned in HCl and HNO3 acids, then rinsed with distilled water. Hydrogenation was performed at 1073 K in hydrogen plasma generated by a 2.45-GHz microwave discharge. The discharge condition was similar to that of CVD diamond growth. The 2 = 1 structure of HrCŽ100. surface was confirmed by the low-energy electron diffraction ŽLEED. pattern in which the characteristic Ž2 = 1. half-order spots were clearly observed. After the plasma

Fig. 1. Schematic layout of vibrational SFG spectroscopy. ␭r4, 1r4 wave plate; ␭r2, 1r2 wave plate; D.L., delay line; F., dielectric filter; MC, monochromator; Pol., polarizer; PMT, photomultiplier tube.

H. Takaba et al. r Diamond and Related Materials 10 (2001) 1643᎐1646

Fig. 2. SFG spectra observed on the HrCŽ100.-2 = 1 surface. Polarization combinations of the visible and infrared beams are Ža.: Žpp., Žb.: Žsp., Žc.: Žps., and Žd.: Žss..

detected. It is seen from the figure that the intensities of three peaks observed at 2924, 2899 and 2830 cmy1 are dependent on the polarization combination. The peak at 2899 cmy1 was observed only when the polarization of the infrared beam was s-polarized while the 2924 cmy1 peak appeared only when the polarization of the infrared beam was p-polarized. The dependence of the peak intensity on infrared polarization indicates that the peak at 2899 cmy1 is the vibrational mode with the transition dipole moment parallel to the surface plane and the peak at 2924 cmy1 the vibrational mode having the transition dipole moment vertical to the surface plane. The polarization characteristics of the peaks are summarized in Table 1 where the appearance or absence of the 2899 and 2924 cmy1 peaks are denoted by circles and crosses, respectively, in the second and third columns. The nature of the broad peak observed at approximately 2830 cmy1 will be discussed later. As for the vibrational bands in the CH stretching region of the HrCŽ100.-2 = 1 surface, Aizawa et al. found a loss peak at 2928 cmy1 on the HREEL spectrum and assigned it to the CH stretching vibration of

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monohydride ŽCH. species w4x. Recently, a broad peak at approximately 2928 cmy1 with a shoulder on the low frequency side was observed on the SERS spectrum and was assigned to the symmetric CH stretching mode of the HC᎐CH species, which has C 2v symmetry, on the basis of ab initio calculations using a C 9 H 14 cluster model w8x. The calculation predicted the frequency of the antisymmetric CH stretching mode of the HC᎐CH species at 2903 cmy1 . The occurrences of the enhancement of SFG signal by vibrational resonance of the a 1 Žsymmetric stretching. and b 1 Žantisymmetric stretching . modes of the C 2v symmetry species are depicted in the fourth and fifth columns of Table 1, respectively. The characteristics were derived on the basis of the procedure described in Domen and co-workers w7,12x with the assumptions that the surface species has C 2v symmetry with the C 2 axis standing normal to the surface and the molecular plane lying perpendicular to the dimer row and that the dimer rows are directed to either the Ž110. or Ž110. directions with equal probability. It is seen from the table that the behavior of the peaks at 2899 and 2924 cmy1 agrees with the selection rules for the b 1 and a 1 modes, respectively. The 2924 cmy1 peak is assigned to the symmetric CH stretching Ža 1 . mode of the HC᎐CH from the polarization characteristics shown in Table 1 and from the closeness of the frequency to those derived by HREELS and SERS observations and by ab initio calculation. Similarly, the 2899 cmy1 peak is assigned to the antisymmetric CH stretching Žb 1 . mode of the same species from the polarization dependence and the closeness of the frequency to that obtained from the ab initio calculation. The fact that the 2899 cmy1 peak was not detected on the Žpp. polarization combination in disagreement with the predicted polarization characteristics is ascribed to the presence of strong background signal which appeared for the Žpp. polarization combination and overwhelmed the vibrational resonance signal. On the SERS spectrum of HrCŽ100.-2 = 1 surface w8x, a weak broad shoulder on the low frequency side of the peak at 2928 cmy1 were observed and attributed to

Table 1 Relationship between polarization conditions and appearance of SFG peaks Polarization conditiona

Žpp. Žsp. Žps. Žss. a

Peak frequency Žcmy1 .b

Vibration modec

2899

2924

a1

b1

= = ` `

` ` = =

` ` = =

` = ` `

The first and second letters in parentheses denote the polarizations of visible and infrared beams, respectively. `, ‘observed’; =, ‘not observed’. c `, ‘allowed’; =, ‘forbidden’. b

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H. Takaba et al. r Diamond and Related Materials 10 (2001) 1643᎐1646

Fig. 3. Background SFG signal under the polarization combinations of Ža.: Žpp. and Žb.: Žsp..

the overlap of the antisymmetric band of the HC᎐CH and the symmetric and antisymmetric CH stretching bands of CH 2 groups on the step edge, ledges, kinks, and disordering of the dimer rows and missing rows. In our SFG measurements, however, the peaks originating from the CH 2 groups were not observed presumably because the SFG signal intensity is proportional to the square of a density of molecules on the surface. The broad peak observed at approximately 2830 cmy1 would be ascribed to the SFG signal generated from the bulk of diamond substrate. As a matter of fact, the SFG spectrum of the HrCŽ100.-2 = 1 surface of HP᎐HT synthetic diamond depends on the penetration depth of the overlap-region of the visible and infrared beams. A strong SFG background signal was observed as shown in Fig. 3 when the overlap-region penetrated into the substrate and the profile showed a periodic oscillation with a period of 100 cmy1 . The background signal was weakened and the vibrational resonance signal appeared as the penetration depth was reduced. The origin and the nature of the background signal is not clear, however.

4. Conclusion The vibrational SFG spectroscopy has been applied to the investigation of the plasma hydrogenated dia-

mond CŽ100.-2 = 1 surface of synthetic diamond. Two vibrational peaks were found at 2899 and 2924 cmy1 and assigned to the antisymmetric and symmetric CH stretching modes, respectively, of the HC᎐CH group from the polarization characteristics and the comparison of the frequencies with the values obtained from HREELS and SERS measurements and the ab initio calculation. A strong SFG background signal which displayed a periodic oscillation with a period of 100 cmy1 was observed in addition to the vibrational resonance peaks, but the origin of the signal is not clear yet.

Acknowledgements The present study was supported by Core Research for Evolutional Science and Technology ŽCREST. of Japan Science and Technology Corporation ŽJST.. References w1x J.T. Yates Jr., T.E. Madey, Vibrational Spectroscopy of Molecules on Surfaces, Plenum, New York, 1987. w2x B.J. Waclawski, D.T. Pierce, N. Swanson, R.J. Celotta, J. Vac. Sci. Technol. 21 Ž1982. 68. w3x S. Tong Lee, G. Apai, Phys. Rev. B 48 Ž1993. 2684. w4x T. Aizawa, T. Ando, M. Kamo, Y. Sato, Phys. Rev. B 48 Ž1993. 18348. w5x R.P. Chin, J.Y. Huang, Y.R. Shen, T.J. Chuang, H. Seki, M. Buck, Phys. Rev. B 45 Ž1992. 1522. w6x H. Seki, T. Yamada, T.J. Chuang, R.P. Chin, J.Y. Huang, Y.R. Shen, Diam. Relat. Mater. 2 Ž1993. 567. w7x T. Anzai, H. Maeoka, A. Wada et al., J. Mol. Struct. 352r353 Ž1995. 455. w8x K. Ushizawa, M.N. ᎐Gamo, Y. Kikuchi, I. Sakaguchi, Y. Sato, T. Ando, Phys. Rev. B 60 Ž1999. R5165. w9x Y.R. Shen, Nature 337 Ž1989. 519. w10x C. Hirose, N. Akamatsu, K. Domen, J. Chem. Phys. 96 Ž1992. 997. w11x C. Hirose, H. Yamamoto, N. Akamatsu, K. Domen, J. Phys. Chem. 97 Ž1993. 10064. w12x C. Hirose, N. Akamatsu, K. Domen, Appl. Spectrosc. 46 Ž1992. 1051.