Raman scattering by phonons of Ga1−xAlxSb mixed crystals

Abstract

We present calculations of the Raman scattering spectra by the long-wavelength vibrations of Ga1−xAlxSb mixed crystals for three different cation concentrations. Each mixed crystal is approached using a primitive cell 64 times larger than the primitive cell of the bulk constituents GaSb and AlSb. The phonon modes are calculated on the basis of an 11 parameter Rigid Ion Model and the Raman spectra are calculated using the Bond Polarizability Model (BPM), away from resonance conditions. The parameters of this model (BPM) are not arbitrarily approximated but we have obtained them on the basis of certain relations, involving directly measurable quantities, such as dielectric and elastooptic constants of the bulk crystal. It is shown that for small concentrations the Al ions are not randomly distributed over the whole crystal but almost all tend to concentrate in neighboring lattice planes. Further, we have reproduced the Raman spectra close to resonance conditions, assuming that the value of the first order polarizability of AlSb is increased by an amount of 50% close to resonance conditions. Finally it is shown that disorder produces asymmetric Raman lines spectra with the intensities of the two strongest peaks in the optic frequency ranges of the bulk constituents being concentration dependent.