Shportko K. Phase-change chalcogenide compounds and diphosphides: influence of structure and composition on optical properties in the IR range

The dissertation deals with an important problem of modern solid state physics, such as a study of the optical properties of promising functional materials — phase-change chalcogenide compounds and diphosphides. A comprehensive, systematic and consistent study of the physical regularities and mechanisms of the influence of disorder on the optical properties of phase-change chalcogenide materials and zinc and cadmium diphosphides was carried out. It was found that phase-change chalcogenide materials Ge-Sb-Te in the amorphous state demonstrate compositional dependences of vibrational properties. Composition-dependent influence of structural disorder on the optical properties of phase-change chalcogenide materials (GeTe)x(Sb2Te3)1-x in the amorphous state was determined by IR reflectance spectroscopy in a wide temperature range. An important result is that the obtained compositional dependences make it possible to predict the vibrational properties of other compounds of the GeTe-Sb2Te3 line. It was shown that the crystallization of chalcogenide materials Ge-Sb-Te is accompanied by a change in the chemical bonding from covalent to resonant bonding, which is a special case of covalent bonding. This type of coupling is manifested in large values of the high-frequency permittivity, an increase in the peak value of the imaginary part of the permittivity in the range of electron interband transitions, and also in changes in vibrational properties. The influence of structural disorder on the optical properties of phase-change chalcogenides in the crystalline state has been analyzed. It has been shown that by changing the measure of structural disorder, it is possible to obtain tailored optical properties of these compounds in a wide spectral range. The effect of thermally induced disorder on IR and Raman-active phonons in zinc and cadmium diphosphides has been studied. The results have demonstrated the anisotropy of the effect of thermally induced disorder on the vibrational properties of zinc and cadmium diphosphides. This effect is explained by the deformation of phosphorus chains when the temperature changes by changing the lengths and angles of bonds between phosphorus atoms. The effect of structural disorder on the optical properties of cadmium diphosphide has been studied. The structural disorder in the studied samples was caused by the introduction of an impurity into the cationic sublattice and the presence of vacancies in the cationic and anionic sublattices