Raman Scattering from Lattice Vibrations of Ga Asx PI-x
First-order Raman scattering from the lattice vibrations of the mixed semiconductor GaAsxP1-x has been studied with both He:Ne and YAG:Nd lasers. Polarization studies have allowed separation of the Raman spectra into two parts, which have the scattering symmetries expected of transverse and longitudinal phonon polarizations in the zincblende structure. The spectra show lattice oscillations in two distinct frequency bands, one of which (345–405 cm-1) is identified with GaP, and the other of which (250–295 cm-1) with GaAs. The prominent feature of each band is a pair of peaks, the lower peak having the symmetry of a transverse phonon, the upper having the symmetry of a longitudinal phonon. In the arsenic band of the phosphorus-rich samples there is a weaker resonance whose strength relative to the “main” transverse resonance increases with decreasing arsenic concentration and is in fact stronger than the “main” resonance in an x ≈ 0.07 sample. Two weak spectral peaks occur in the phosphorus band; in the x ≈ 0. 07 sample they correlate with peaks in the phonon density of states of GaP. The nature of this fine structure indicates an inadequacy of the Verleur-Barker mixed crystal model in the interpretation of Raman and infrared spectra. A theory is also developed for the Raman scattering spectrum of an isotropic, multiply-resonant, infrared-active lattice.
KeywordsArsenic Concentration Longitudinal Mode Transverse Mode Mixed Crystal Phonon Dispersion Curve
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