Band Gaps and Phase Transitions in Cubic ZnS, ZnSe and ZnTe

Abstract

The effect of hydrostatic pressure on the fundamental absorption edge and phase transitions of the zinc chalcogenides is investigated by means of optical absorption and reflection at room temperature. The ${E_O}(\Gamma _{15}^V \to \Gamma _1^C)$ gap exhibit a sublinear increase under pressure which changes to linear when plotted as a function of the relative change of volume. Various trends, e.g. sublinearity, deformation potentials etc are clearly demonstrated along the sequence ZnS → ZnTe. A transition to a NaCl — type structure occurs at 15‡0.3 and 13.5‡0.3 GPa for ZnS and ZnSe, respectively. The NaCl structures are stable at least up to 27.0 and 25.0 GPa for ZnS and ZnSe. For ZnTe an intermediate phase ZnTe-II appears in the pressure range between 9.3 and 12.0 GPa. At 12.0 GPa a second phase transition takes place and the new structure ZnTe-III is stable at least up to 30.0 GPa. The structures of the intermediate and the high pressure range of ZnTe are not known at present, however the NaCl structure is to be excluded. Absorption and reflectivity data demonstrate conclusively that — contrary to earlier reports — the NaCl — type structure are semiconducting in ZnS, ZnSe as well as the intermediate ZnTe-II phase. The highest ZnTe-III phase is indeed a metallic phase. The experimental results are compared to theoretical calculations based on local empirical pseudopodential and ab initio LMTO calculations.