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

TL;DR

This paper critically reevaluates the multiple trapping and release (MTR) transport mechanism in amorphous semiconductors, revealing that traditional models oversimplify the spatial and energetic distribution of states, leading to more accurate effective parameters.

Contribution

It provides a rigorous probabilistic analysis showing that the experimentally defined mobility edge and mobility are effective quantities, differing from their true physical counterparts in amorphous semiconductors.

Findings

The measured mobility edge is an effective quantity, not the true critical energy.

The experimentally derived mobility is higher than the actual free electron mobility.

Revisits MTR transport with a more realistic microscopic model.

Abstract

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. Through a probabilistic analysis of the microscopic charge transport details, this paper rigorously demonstrates 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. The hydrogenated amorphous silicon (a-Si:H) discussed in the prequel, being an intensively studied sample in the past, is used as an example to concretize the analysis.