Study of one-mode, two-mode and three-mode phonon behavior in mixed zinc blende alloys

In this paper, we put forth a microscopic interpretation of the characteristics of vibrational spectra of zinc blende semiconductor alloys arising due to either difference in masses or contrast in bond lengths. Previous Raman and infrared experiments have helped in identifying two mode vibrational behaviors in mixed systems of Zn1−xMgxSe and Zn1−xMgxTe in contrast to the one-mode behavior in Zn1−xMgxS. Our lattice dynamics computations have elucidated that in addition to the mass of the anion, bond length anomalies, energy separation between the two sets of optical modes, and the magnitudes of the scattering cross section play an important role in the observance of one mode behavior in the S system, two mode behavior for the whole of the composition range in the Se and Te systems, and an additional Be–Te like vibrational doublet in case of Zn1−xBexTe. Our calculations incorporate the treatment of disorder through a supercell approach. The calculated lattice constants for different concentrations, the bimodal bond length distribution, as well as the phonon frequencies at the Brillouin zone centre are in good agreement with the available experimental data.