The Impact of a Long-Ranged Random Potential on the Transport Properties of Amorphous Semiconductors

Abstract

ABSTRACTWe discuss electrostatic random potentials in doped and in compensated amorphous semiconductors. These potentials are caused by the residual inhomogeneity of a random distribution of charged dopants and their compensating charges. Random potentials are also present in undoped material with negative-U defects. A high density of positive-U defects can also give rise to a random potential in undoped material.We demonstrate with the help of detailed model calculations the effect of such random electrostatic potentials on the transport properties. For transport in extended states the random potential does not give rise to a mere shift of the mobility edge. Instead several new features are observed: the activation energy of the resulting Ohmie dc conductivity is virtually unaffected by the random potential in contrast to the activation energy of the thermoelectric power and that of the Hall effect, respectively. The Ohmie dc current changes at high fields into a superlinear current. The random potential contributes to the dispersion of the transients in time-of-flight experiments but leaves the field dependence of the TOF mobility unaltered. Comparing our results with experimental data we discuss under which circumstances the effect of random potentials can be identified.