Fundamental absorption edge in mixed single crystals of II-VI compounds

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Abstract
The energy and temperature dependence of the absorption coefficient near the fundamental absorption edge has been measured in mixed single crystals of II-VI compounds. In all the crystals the Urbach rule is valid. Prominent differences have been found in the temperature dependence of the σ parameter of these crystals, according to which class the crystal belongs: amalgamated or persistent type. Most of the results are explained by the Dow-Redfield theory of the internal Franz-Keldysh effect. In order to explain the differences in the optical properties of the crystals of the two classes (Raman, infrared reflection, and the optical absorption near the fundamental absorption edge), a model of coupled oscillators was developed. The application of that model, together with Toyozawa’s approach of the self-trapped excitons, helps to explain the above differences. The latter theory was found to be applicable especially in the case where localization of phonons occurs.