Electronic transport and localization in low mobility solids and liquids

Abstract

This article is concerned with the electronic transport properties of the large group of solids (and also some liquids) which possess carrier mobilities from 10 cm²V^-1sec^-1 to many orders below that value. Its main aim is to trace the close relation between transport and localization. To this end the physical basis of three important localization models is discussed and the predicted transport mechanisms are reviewed in the light of the experimental information. After a brief summary of the experimental methods, the first subject is the localization arising from strong electron-phonon interaction. Here we consider in some detail the formation of a small polaron in a molecular crystal and its transport by phonon-assisted intermolecular hopping. The second part of the paper deals with the localization of an electron in a so-called electronic bubble, which occurs for example in simple liquids such as Ne or He. The stability and transport of this entity is discussed. The third section is devoted to non-crystalline solids. In these localization is associated with the lack of long-range order and does not depend on phonon interaction. The transport through the various parts of the density of states spectrum is summarized and the suggested models are reviewed on the basis of recent experimental work on amorphous silicon. Finally, the possibility of polaron formation and transport in some chalcogenide alloy glasses is briefly considered. The paper leads to the conclusion that available experimental evidence supports the general validity of the above three models, although it must be emphasized that this is based on an as yet limited range of experimental information.