Probing Charge Dynamics in Amorphous Oxide Semiconductors by Time-of-flight Microwave Impedance Microscopy

TL;DR

This study uses microwave impedance microscopy to spatially and temporally analyze charge transport in amorphous IGZO, revealing complex mechanisms involving trap states and hopping conduction with implications for flexible electronics.

Contribution

It introduces a novel application of time-of-flight microwave impedance microscopy to study charge dynamics in amorphous oxide semiconductors with spatial resolution.

Findings

Drift mobility of 2-3 cm²/Vs consistent with device measurements

Charge transport involves two mechanisms with millisecond timescales

Charge dynamics are governed by trap-and-release and hopping conduction processes

Abstract

The unique electronic properties of amorphous indium gallium zinc oxide (a-IGZO) thin films are closely associated with the complex charge dynamics of the materials. Conventional studies of charge transport in a-IGZO usually involve steady-state or transient measurements on field-effect transistors. Here, we employed microwave impedance microscopy to carry out position-dependent time-of-flight (TOF) experiments on a-IGZO devices, which offer spatial and temporal information of the underlying transport dynamics. The drift mobility calculated from the delay time between carrier injection and onset of TOF response is 2 - 3 cm2/Vs, consistent with the field-effect mobility from device measurements. The spatiotemporal conductivity data can be nicely fitted to a two-step function, corresponding to two coexisting mechanisms with a typical timescale of milliseconds. The competition between multiple-trap-and-release conduction through band-tail states and hopping conduction through deep trap states is evident from the fitting parameters. The underlying length scale and time scale of charge dynamics in a-IGZO are of fundamental importance for transparent and flexible nanoelectronics and optoelectronics, as well as emerging back-end-of-line applications.