Investigation of the disorder of Ga1–xAlxAs mixed crystals. Part I: Small concentrations

Abstract

We present calculations of the Raman intensities scattered by the phonons of the mixed crystals Ga1–xAlxAs for five small concentrations up to x = 0.14, away from resonance conditions. Each mixed crystal is approached by a primitive cell, the volume of which is 64 times the volume of the bulk zincblende primitive cell. The phonons have been calculated using a version of a three‐dimensional Valence Overlap Shell Model (VOSM) and the Raman spectra with the bond‐polarizability model (BPM). The values of the bond‐polarizability (BP) parameters of Ga–As and Al–As bonds necessary for the calculation of the Raman intensities are, for the first time, not estimated arbitrarily, but we have evaluated them using certain relations involving directly measurable quantities. We also show that the frequencies of the AlAs LO(Γ)‐like modes and their Raman profiles and intensities as well, depend, for a given Al concentration, strongly on the way the cations are distributed over the mixed‐crystal cation planes. Concerning the GaAs LO(Γ)‐like modes, differences in the cation distribution affect only a little their frequencies and their intensities, but may result in pronounced left asymmetries of the profiles of the corresponding strong Raman peaks. The comparison between the calculated and the experimental Raman spectrum for the Ga0.86Al0.14As is very encouraging, especially when the mixed crystal is described by a disorder configuration where the Al ions are gathered to neighboring cation planes.