Optical Properties of Zinc Telluride

The optical absorption of ZnTe single crystals grown by the Bridgeman method has been studied from visible wavelengths to 50μ, at temperatures from 1.4 to 300°K. Lattice combination bands are observed from which assignments of zone-boundary phonon energies are made: LO=TO=22, LA=16, TA=7×${10}^{-3\mathrm{}}$ eV. The temperature dependence of the absorption is discussed. Absorption due to inter-valence-band transitions has been observed, which indicates the presence of two bands degenerate at $k=0\mathrm{}$ and a split-off band lying 1 eV below. Infrared absorption is observed for As impurity in the range 0.05 to 0.1 eV and for a residual impurity in undoped material, probably a zinc vacancy, in the range 0.1 to 0.9 eV. For each impurity, absorption due to transitions to the split-off valence band is also observed near 1 eV. Sharp excitation lines are seen in the spectrum of the residual impurity. The lines are repeated with emission of optical phonons of energy 0.026 eV, from which the electron-phonon coupling coefficient is estimated. Some structure is observed also in the spectrum of As impurity. The intrinsic absorption edge has been measured at 1.7, 80, and 300°K. The energy gap is determined to be 2.385, 2.37, and 2.25 eV, respectively. A tail is observed at the absorption edge which shows the effect of various impurity levels in the undoped material and in the samples doped with As, In, or Fe. In two samples grown from the vapor phase, some sharp absorption lines are present in the range 2.2 to 2.35 eV. These lines are apparently due to exciton-impurity complexes.