We demonstrate that surface phonons of clean semiconductor surfaces can be studied by Raman spectroscopy. In this work the vibrational properties of clean InP(110) surfaces are investigated. Two surface phonons of A 0 symmetry at 254 and 270 cm 21 in the band gap between acoustical and optical bulk branches are observed in agreement with theoretical calculations. Because of their limited resolution previous high-resolution electron-energy-loss spectroscopy experiments found only one surface phonon mode near this energy range. Additionally, three other surface modes at 69, 146, and 347 cm 21 are identified by Raman spectroscopy.[S0031-9007 (97)03816-7] PACS numbers: 68.35.Ja, 78.30.FsUp to now surface phonons mostly have been studied by high-resolution electron-energy-loss spectroscopy (HREELS) and by helium atom scattering (HAS). Both techniques are well established experimental methods for mapping the surface dispersion band structure [1][2][3][4]. The main drawback of HAS is its limitation to low energy vibrations (below 250 cm 21 ), whereas HREELS is used to monitor surface phonons with energies up to several 1000 cm 21 . Because of the larger penetration depth of photons compared to that of low energy electrons and atoms, optical methods seem to be less sensitive for surface properties than HREELS and HAS.In recent years, however, resonant Raman spectroscopy (RRS) has been used successfully in surface physics. For resonant Raman conditions the incident optical quantum energy has to approach to an electronic surface state energy, leading to a strong enhancement of the Raman scattering cross section [5]. Because of different electronic band structures of bulk and surface, the corresponding vibrations show different energy dependences of the resonance behavior. Thus it was possible to determine surface vibrations of monolayer terminated semiconductor surfaces like Sb on InP(110) [6,7] and Sb on GaAs (110) [7] and As on Si(111) [7], respectively. Weak signals of adsorbate terminated semiconductors under nonresonant conditions were found for S on InP(001) [8] and H terminated vicinal Si (111) surfaces [9]. Raman signals from surface phonons of clean surfaces, however, have not been reported so far. Raman spectroscopy (RS) as an optical technique is sensitive to vibrations with near zero momentum; however, in general its energy resolution of a few cm 21 is distinctly higher than that of HREELS (approximately 30 cm 21 [1]). Another advantage of RS compared with particle scattering techniques is the possibility of exciting surface phonon modes polarized parallel to the surface plane.It seemed of much interest to apply RS to clean InP(110) surfaces, because this surface has been investigated recently by several other methods. The studies of the vibrational properties of clean InP (110) surfaces include HREELS [1] on the one hand and various calculations on the other hand using the density-functional perturbation theory (DFPT) [10], the phenomenological bond-charge model (BCM) [11], and the pseudopotential frozen-phono...
The electronic and vibrational properties of InP(110), GaAs(110), GaP(110), and InAs(110) surfaces terminated with ordered Sb monolayers are studied by Reflectance Anisotropy Spectroscopy (RAS) and Surface Resonant Raman Spectroscopy (SRRS). In the RAS spectra of the monolayer covered surfaces optical anisotropies attributed to transitions of the electronic surface band structure of the monolayer can be identified. On all four substrates, prominent surface transitions are found in the photon energy range from 1.8 to 2.8 eV. The SRRS, on the other hand, is utilized to monitor surface vibrational modes of the monolayer terminated surfaces. The scattering cross section reveals a clear correlation to the surface transitions detected by RAS; for resonance condition to the surface electronic transitions maximum scattering efficiency occurs. This can be understood in terms of deformation potential scattering. Since the Sb monolayer vibrational modes modulate mainly the bonds of the first few atomic layers, the corresponding modulation of the polarizability is dominated by the surface electronic band structure.
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