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

Spectroscopic ellipsometry in the photon energy range from 0.04 eV to 6.50 eV 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 411 cm−1 and 420 cm−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 0.15 eV at room temperature when compared to the E0 = 2.23 eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30 K to room temperature. Luminescence at temperature T > 200 K 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.