Modulated Piezoreflectance in Semiconductors

Abstract

The direct gaps of Ge, GaAs, GaSb, InP, ZnS, CdTe, CdSe, CdS, and ZnO have been measured using the piezoreflectance technique. Thin single crystals of these materials were mounted on lead-zirconate-lead-titanate piezoelectric transducers and cooled to 77°K. Measurements were performed with the stress applied along the [100] and [111] crystallographic directions of the cubic materials and along the [0001] and [$11\overline{2}0$] directions of the hexagonal materials. The shear deformation potentials $b$ and $d$ of the highest valence-band state of the cubic materials were determined from the ratio of the intensity of the light polarized parallel and perpendicular to the direction of the stress and the known values of the hydrostatic deformation potentials. The results show a continuous increase of the ratio $\frac{d}{b}$ from the covalent materials Ge and Si to the partially ionic III-V and II-VI compounds. A simple point-ion model is proposed to explain the increase in the ratio $\frac{d}{b}$ with increasing ionicity for the cubic materials. For the wurtzite materials, similar measurements yield ratios of shear to hydrostatic deformation potentials. Information about the stress-exchange splittings and the corresponding exchange-interaction constant is also obtained.