STRUCTURAL ANALYSIS OF THE SEMICONDUCTOR-SEMIMETAL ALLOY CD1-XHGXTE BY INFRARED LATTICE-VIBRATION SPECTROSCOPY

Far-infrared reflectivity measurements have been made on traveling-heater-method-grown ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Hg}}_{x}\mathrm{Te}$ bulk single crystals. The spectra at 300, 77, and 25 K display two bands with fine structure, which cannot be explained by the usual two-oscillator model. These lattice spectra are shown to originate from four Cd-Te and Hg-Te vibrational modes in five basic $\mathrm{Cd}(4\ensuremath{-}n)\mathrm{Hg}(n)\mathrm{Te}$ $(n=0,1,2,3,4)$ cells consisting of cations tetrahedrally distributed around a shared Te anion. The mode frequencies are determined by the cation configuration in each cell. They are assumed to be independent of alloy composition, which determines only the mode strengths. The temperature dependence of the Cd-Te and Hg-Te vibrations revealed differences. The Cd-Te frequencies decrease with temperature, as is typical for semiconductor compounds. For the Hg-Te vibrations the temperature dependence is more complicated. Such temperature behavior was explained by influence of the anharmonicity corrections, which change sign with variation of frequency. Dispersion analysis of the dielectric function ${\ensuremath{\varepsilon}}^{\ensuremath{''}}(w)$ restored from the reflection spectra of the alloys at 25 K by Kramers-Kronig analysis revealed a peculiarity at $137{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ which appears with nearly constant strength over the wide compositional range $x=0.1--0.8.$ This mode has the same temperature dependence as the Cd-Te vibrational modes. The most probable explanation of this mode is associated with the alloy disorder or with multiphonon processes.