Dzumedzei R. Scattering of charge carriers in polycrystalline thin films and pressed materials based on lead and tin tellurides.

The technological features of the deposition of thin films or the formation of bulk materials by powder pressing contribute to the directed formation of numerous grain boundaries such as a surface, which contribute to the effective charge carriers and phonons scattering, which in turn causes the high values of thermoelectric parameters. Based on a set of theoretical calculations and experimental studies, an analysis of the transport of charge carriers in binary bulk materials (PbTe, SnTe) obtained by powder pressing and thin films deposited by vacuum methods, as well as upon doping of these compounds (PbTe:Bi(Sb)) or the formation of solid solutions based on them (PbSnTe, PbTeSe)is done. Based on the use of the variational principle, the contributions of individual charge carriers scattering to kinetic phenomena are determined, and the surface effects and scattering at grain boundaries are separately identified. The role of the effects associated with surface in the analysis of the charge carriers scattering mechanisms and, consequently, their influence on the thermoelectric properties of thin films are determined. The dominant influence of carriers scattering on intergranular boundaries (the Mayadas and Shatzkes theory) is established, regardless of the grains size. The influence of the surface effects, which is described by Fuchs and Sondheimer theory, becomes significant in decreasing of the thickness of films. In particular, for PbTe:Bi, the surface of the film substantially affects on transport phenomena for thicknesses ~ 100 nm, that is, for thin films whose thickness is of an order of mean free of path. The obtained results allow setting the technological regimes for optimization of the material parameters in order to obtain the maximum values of thermoelectric figure of merit ZT. Technological factors are determined that influence to the formation of grains of various sizes, which are responsible for the correlation between surface scattering and scattering of charge carriers on the grain boundaries. The conditions under which surface scattering becomes dominant compared with the classical mechanisms characteristic of bulk materials (scattering by phonons or vacancies) are studied.