Electronic band structure of quaternary Be-chalcogenides, studied by ultraviolet ellipsometry and photoreflectance spectroscopy

摘要

Beryllium chalcogenides are a new class of II-VI materials and promising candidates for UV/VIS applications. In this paper we analyse the optical properties of the quaternary BeMgZnSe system lattice matched to GaAs. We investigate the compositional dependence of the fundamental and higher energy gaps (E_0, E_1, E_2). Thc investigations are performed (a) experimentally by photoreflectance and by ellipsometry up to 9.5eV and (b) theoretically by first principles band structure calculations within the virtual crystal approximation (VCA). The fundamental energy gap is found to vary from 2.7 eV for ZnSe to 3.7 eV for a (Be,Mg)-content of 70/100. The VCA calculations predict the correct overall gap dependencies. Especially, a negative bowing of the E_0 gap is predicted by the VCA model. This unusual behaviour may be explained by the absence of bond length redistribution dynamics in this lattice matched system.