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

Beryllium chalcogenides are a new class of II-VI materials and promising ca ndidates for UV/VIS applications. In this paper we analyse the optical prop erties of the quaternary BeMgZnSe system lattice matched to GaAs. We invest igate the compositional dependence of the fundamental and higher energy gap s (E-0, E-1, E-2). The investigations are performed (a) experimentally by p hotoreflectance and by ellipsometry up to 9.5 eV and (b) theoretically by f irst principles band structure calculations within the virtual crystal appr oximation (VCA). The fundamental energy gap is found to vary from 2.7eV for ZnSe to 3.7eV for a (Be,Mg)-content of 70%. The VCA calculations predict t he correct overall gap dependencies. Especially, a negative bowing of the E -0 gap is predicted by the VCA model. This unusual behaviour may be explain ed by the absence of bond length redistribution dynamics in this lattice ma tched system. (C) 2000 Elsevier Science B.V. All rights reserved.