Non-localized states and high hole mobility in amorphous germanium

TL;DR

This paper demonstrates that amorphous germanium can exhibit high hole mobility and non-localized states, challenging the traditional view of localized states in amorphous semiconductors and suggesting new device applications.

Contribution

It provides direct electrical evidence of non-localized valence band states in amorphous germanium, with high mobility comparable to crystalline materials, using a novel preparation method.

Findings

High hole mobility (~100 cm^2/(Vs)) in amorphous Ge

Presence of non-localized valence band states

Temperature-dependent resistivity shows intrinsic and extrinsic regions

Abstract

Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to be by the hopping mechanism through localized states. The carrier mobility of these materials is usually very low, in the order of ~10^-3 - 10^-2 cm^2/(Vs) at room temperature. In this study, we present the Hall effect characterization of a-Ge prepared by self-ion implantation of Ge ions. The a-Ge prepared by this method is highly homogenous and has a mass density within 98.5% of the crystalline Ge. The material exhibits an exceptionally high electrical conductivity and carrier mobility (~100 cm^2/(Vs)) for an amorphous semiconductor. The temperature-dependent resistivity of the material is very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results are direct evidence for a largely-preserved band structure and non-localized states of the valence band in a-Ge, as proposed by Tauc et al. from optical characterization alone. This finding is not only significant for the understanding of electrical conductivity in covalent disordered semiconductors, but the exceptionally high mobility we have observed in amorphous Ge opens up device applications not previously considered for amorphous semiconductors.