Revisiting multiple trapping and release electronic transport in amorphous semiconductors exemplified by a-Si:H

Multiple trapping and release (MTR) is a typical transport mechanism of electron carriers in amorphous and other disordered semiconductors where localized states are significant. Quantitative description of MTR, however, has been based on an "abrupt" mobility edge model, which relies on two underpinning simplifications: (i) states above the conduction band mobility edge are extended and any of them is omnipresent in space, whereas states below the mobility edge are localized and they exist in space as pointlike sites; (ii) all states are evenly distributed in space, and the local density of states (DOS) distribution is spatially invariant. The prequel to this paper [Y. Luo and A. J. Flewitt, Phys. Rev. B 109, 104203 (2024)] demonstrates that neither of these simplifications is valid. Hence, this paper reinvestigates MTR transport and focuses on two similar scenarios: steady DC conduction and non-dispersive time-of-flight conduction. We have managed to reveal that, first, the experimentally measured mobility edge is an effective quantity which is different from the actual critical energy that demarcates extended states and localized states of an amorphous semiconductor. Second, the experimentally derived extended-state mobility is also an effective quantity which turns out to be higher than the actual mobility of free electrons in the material. These two effective quantities are quantified using the hydrogenated amorphous silicon (a-Si:H) discussed in the prequel paper as an example.