Optical and vibrational properties of Be-Zn chalcogenide alloys and superlattices

The results of a comprehensive theoretical study for the optical and vibrational behavior of Be-Zn chalcogenide alloys and superlattices (SLs) are presented using a realistic lattice dynamical approach. Raman-scattering data for the zone-center optical modes and the results from first-principles calculations for the elastic, lattice constants, and critical-point phonons are employed in constructing an optimized secondneighbor rigid-ion model for BeS, BeSe, and BeTe. A simple explanation is provided for the unusual characteristics of the optical phonon dispersions found in Be chalcogenides, alloys and (BeSe)(m)/(ZnSe)(n) SLs compared to those of many other Zn-based II-VI compound semiconductors. An elastic continuum model has offered a strong corroboration to the observed doublets by Raman spectroscopy in the BeTe/ZnSe superlattices near 25-30 cm(-1) as the folded LA modes due to mini-zone folding in the z(xx)(z) over bar scattering geometry. By using a standard methodology of multilayer optics with appropriate alloy dielectric functions, we have analyzed the IR reflectance spectra of BeZnSe/GaAs thin epilayers at an oblique incidence with a three-oscillator model. Comparison of our results with the reflectivity data has provided a strong validation to the fact that in the similar to 400-575 cm(-1) frequency region there exist at least two types of Be-Se bonds with optical modes involving two kinds of local atomic arrangements.