Revealing the Partially Coherent Nature of Transport in IGZO

TL;DR

This paper uncovers that charge transport in amorphous oxide semiconductors like IGZO involves partially coherent electrons and is dominated by tunneling between coherent regions, challenging previous assumptions of percolation or hopping.

Contribution

It introduces the FEAFIT model that explains low-temperature transport via partially coherent states and accurately predicts experimental data across various conditions.

Findings

Transport is dominated by electron transfer across insulating gaps.

The FEAFIT model accurately fits experimental data.

Transport parameters correlate with coherence dimensions and disorder.

Abstract

Thin-film transistors based on amorphous oxide semiconductors (AOS) are promising candidates for enabling further DRAM scaling and 3D integration, which are critical for advanced computing. Despite extensive research, the charge transport mechanism in these disordered semiconductors remains poorly understood. In this work, we investigate charge transport in the archetypical AOS material, indium gallium zinc oxide (IGZO), across a range of compositions and temperatures using thin-film transistors and Hall bar structures. Our results show that the electrons involved in transport exhibit partially spatial coherence and non-degenerate conduction. Under these conditions, transport is dominated by electron transfer across insulating gaps between locally coherent regions, rather than by degenerate percolative transport above a mobility edge, or by localized-state hopping, both of which are widely assumed in the literature. While fluctuation-induced tunnelling has previously been invoked to describe low-temperature transport in oxide transistors, we show that such behavior originates from partially coherent electronic states and develop a field-effect-aware fluctuation-induced tunnelling (FEAFIT) framework that explicitly accounts for gate modulation of the tunneling landscape. The FEAFIT model accurately predicts experimental data across all compositions, temperatures, and gate voltages, enabling extraction of fundamental transport parameters. These tunnelling parameters are then correlated with electron coherence dimensions and the degree of energetic disorder obtained from first-principles calculations. Our findings advance the fundamental understanding of charge transport in AOS-based transistors and provide a foundation for further performance improvements