Optical spectra of Zn1-xBexTe mixed crystals determined by IR–VIS–UV ellipsometry and photoluminescence measurements

Abstract

Spectroscopic ellipsometry in the photon energy range from 0.04eV to 6.50eV is used for investigation of the optical response of Zn1-xBexTe crystals grown by a high-pressure Bridgman method in the composition range x≤0.12. Infrared spectra display absorption bands centred between 411cm−1 and 420cm−1 associated with BeTe-type optical phonon modes. The positions of the transverse-optical and longitudinal-optical phonon modes have been found by modelling the line shape of the complex dielectric functions, ε˜ and Im(−ε˜−1), using a classical damped Lorentzian oscillator approach. Ellipsometric measurements in the VIS–UV range allow determination of the fundamental energy-gap (E0) and the higher threshold energies (E1, E1+Δ1, E2) originating from the band edge and spin-orbit splitting critical points. We have found that the Be content x=0.12 causes an increase of the fundamental energy gap about 0.15eV at room temperature when compared to the E0=2.23eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30K to room temperature. Luminescence at temperature T>200K is very weak. The peak positions of the exciton emission lines agree well with the E0 band-gaps derived from ellipsometric data if corrected for their temperature dependence.