Pressure dependence of the optical and vibrational properties of hexagonal and cubic CdS films grown on GaAs

Abstract

The pressure dependence of the excitonic photoluminescence and Raman scattering in both hexagonal and cubic CdS films grown on GaAs substrates is reported. We found that the pressure coefficients of the cubic CdS films are equal to those of the hexagonal films within experimental uncertainties. The main difference between their optical and vibrational properties seems to lie in their exciton energies, the hexagonal phase having a slightly higher energy than the cubic phase. Strong enhancement in multi-longitudinal optical (LO) phonon scattering has been observed by "pressure-tuning" the exciton energy to resonate with the excitation laser line. As a result we have been able to determine the pressure coefficients of the higher order LO phonons in CdS for modes as high as 5LO. We found that the pressure coefficients of these higher order LO modes when normalized by the phonon frequency exhibit a systematic decrease with the order of the mode.