Optical Phonons and Chemical Bonding in TlSbS2, TlSbSe2, and Tl3SbS3 Crystals

Abstract

The emission lines of bound and free excitons and their phonon replicas were observed in the luminescence spectra of ZnP2-D48 crystals doped with Mn, Sn, Cd and Sb measured at 10K. The emission lines are described by the model of axial center levels. Models of the bands of the bound excitons with different axial centers (Mn, Sn, Cd and Sb) are presented. It was observed that the indirect transitions in the excitonic bands were nonpolarized and that the direct transitions were polarized. The minimal direct energy gaps in the polarization E||c are due to the allowed Г1→Г1 transitions, and the gaps in the polarization E⊥c are due to the Г2→Г1 transitions. The temperature shift coefficient of the bands gaps differs for different polarizations in the temperature interval from 2 to 10K (ΔE/ΔT=3.5meV/K and 1meV/K for E||c and E⊥c, respectively). The optical constants n, k, ε1, ε2, d2ε1/dE2 and d2ε2/dE2 were calculated for the energy interval 1.5–10eV using the Kramers-Kronig analysis of measured reflection spectra. The features observed in these spectra were interpreted using two types theoretical calculations of band structure as optical transitions.