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 ͑110͒ surface of an Ag crystal was investigated by reflectance anisotropy spectroscopy and angleresolved photoemission spectroscopy. A strong resonance in the optical spectra of the clean surface is assigned to a surface-state transition at the Ȳ point of the surface Brillouin zone. This resonance is absent on the oxygen-covered surface. The accompanying photoemission spectra show the corresponding occupied surface state on the clean surface as well as its disappearance with oxygen coverage. ͓S0163-1829͑98͒52040-1͔Reflectance anisotropy spectroscopy ͑RAS͒ is an optical method which allows the sensitive investigation of surface optical properties of semiconductors 1-3 and metals. 4,5 RAS measures the difference of the complex reflectivity along two perpendicular axes in the surface. In the case of optically isotropic bulk materials, any RAS signal must be related to anisotropies induced by the surface. 6 Several mechanisms may contribute to the surfaceinduced optical anisotropy. ͑i͒ Electronic transitions between localized surface states constitute one of the interesting cases allowing for direct surface state spectroscopy. 7,8 ͑ii͒ Transitions involving near surface bulk states whose symmetry is reduced by the presence of an anisotropically reconstructed surface ͑surface-induced bulk states͒ may be another origin for optical surface ansiotropies. 9,10 They give rise to features in the spectra close to the bulk critical points. Finally, apart from these single electron contributions ͑iii͒ collective freecarrier oscillations at the surface ͑surface plasmons͒ may also affect the optical spectra. 11 The only example for surface-state contributions to the reflectance anisotropy on a metal surface so far is the Cu͑110͒ surface. Here a sharp peak in the spectrum at an energy of 2.1 eV was assigned to electronic transitions involving surface states at the Ȳ point of the surface Brillouin zone. 8,12 However, since in Cu the transition energies of bulk d electrons to the Fermi level are also located in this energy range, the observed feature in the Cu͑110͒ spectra might as well also contain contributions arising from surface modified bulk states. Indeed, the RAS spectra indicate such a contribution from near surface bulk states because part of the anisotropy still remains after exposure of the surface to O or CO. 8 Thus the 2.1 eV structure in the Cu͑110͒ spectra does not constitute a pure surface-state transition.Silver, on the other hand, has a surface electronic structure similar to Cu. However, contributions from the d-band transitions to the optical spectra are expected at much higher energies ͓above 4 eV ͑Ref. 13͔͒ than the surface-state transition energies ͓1.7 eV ͑Ref. 7͔͒. An anisotropic contribution from surface-state transitions to the RAS spectra of the Ag͑110͒ surface thus would be expected in the near infrared region, energetically separated from the d bands.Measurements of the optical anisotropy of Ag͑110͒ under ambient 14 and UHV conditions 15 have been already reported. In these experiments no contribu...
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CdTe epitaxial layers were deposited on clean cleaved InSb(110) substrates by molecular-beam epitaxy at room temperature and elevated temperatures. The formation of interface and layer was investigated using Raman spectroscopy as a growth monitor, i.e., Raman spectra were taken on line without interruption of the deposition process. Fabry–Pérot interference of the incident as well as the scattered light within the heterostructure leads to a characteristic modulation of the substrate phonon scattering intensity. The modulation is calculated and serves as a measure for the layer thickness. For the deposition at elevated temperatures the true surface temperature is determined from the InSb TO phonon frequency shift. While at a substrate temperature of 150 °C the crystalline quality of the CdTe layer was improved compared to room-temperature growth, the deposition of CdTe at 300 °C resulted in the formation of a layer consisting of In2Te3 and liberated Sb. The effect of the laser radiation on the growth process at different temperatures is also discussed.
We report about E P R measurements on Fe-Cu complexes in ZnSe crystals, grown by iodine transport reaction. The crystals were doped with 500 ppm Cu and contaminated with l e s s than 30 ppm Fe (proof limit of the AM). The measurements were carried out with a commercial Varian-E-Line spectrometer .operating in the X-band (9.1 GHz) in the temperature range from 1 . 8 to 6 K. Table 1). The observation of the transitions within the first excited doublet (so-called central doublet) is here re-1 ) Str. d e s 17. Juni 135, 1000 Berlin 12, Berlin (West).
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