Infrared reflectance and transmission spectra in II-VI alloys and superlattices

Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk Cd _xZn_1-xTe (0 ≤ x ≤ 1) and CdSe_xTe _1-x (0 < x ≤ 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical (ω_TO, ω_LO) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.