Dispersive (non-Gaussian) transient transport in disordered solids

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Abstract
The experimental manifestations of dispersive (non-Gaussian) transient transport in disordered solids are discussed and compared with the predictions of theoretical treatments. The mathematical equivalence of the two theoretical approaches based on the formalisms of continuous-time random-walk (CTRW) and generalized multiple-trapping is demonstrated. Several transport mechanisms are discussed, viz. extended state motion with multiple trapping, hopping and trap-controlled hopping. Experimental studies on the chalcogenide glasses a-Se and a-As2Se3 are emphasized but results for organic solids and a-SiO2 are included. There is independent evidence that transport occurs by a hopping process for the organic systems, but not such clear evidence exists for the inorganic solids. Nevertheless, on the basis of the temperature behaviour of the transit time dispersion and the values of parameters obtained from numerical analysis, we argue that hopping is also the microscopic transport mechanism in the inorganic solids. For a-As2Se3 and a-SiO2 the hopping time distribution function assumes the algebraic form ω(t) ∼ t −(1+α) where 0 < α < 1 and α ∼ const. For the organic systems and a-Se, more complicated time and temperature dependences of the distribution function are necessary to fit the data at all temperatures. In this context the observation of a transition from dispersive to non-dispersive transport as a function of increasing temperature in a-Se and poly-(N-vinylcarbazole) (PVK) is of particular interest. The subtle role played by local morphology in generating a transit time dispersion is demonstrated by comparing PVK and its brominated derivative 3Br-PVK. A special section will be devoted to time-dependent electrical phenomena of metal semiconductor surfaces. That discussion will include a description of the experimental procedures necessary to identify the nature of contacts and their influence on the interpretation of steady-state conductivity data.