Ultrabroadband Density of States of Amorphous In-Ga-Zn-O

TL;DR

This study investigates the sub-gap density of states in amorphous In-Ga-Zn-O using ultrabroadband photoconduction, revealing donor and acceptor vacancy states that influence device behavior and threshold voltage shifts.

Contribution

It provides the first detailed characterization of sub-gap states in a-IGZO using UBPC and DFT simulations, linking vacancy states to electrical properties.

Findings

Donor-like oxygen vacancy states peak at 10^{17}-10^{18} cm^{-3}eV^{-1}.

Deep acceptor-like metal vacancy states peak at 10^{18} cm^{-3}eV^{-1}.

Metal vacancy acceptors cause threshold voltage shifts and hysteresis in TFTs.

Abstract

The sub-gap density of states of amorphous indium gallium zinc oxide ($a$-IGZO) is obtained using the ultrabroadband photoconduction (UBPC) response of thin-film transistors (TFTs). Density functional theory simulations classify the origin of the measured sub-gap density of states peaks as a series of donor-like oxygen vacancy states and acceptor-like Zn vacancy states. Donor peaks are found both near the conduction band and deep in the sub-gap, with peak densities of $10^{17}-10^{18}$ cm$^{-3}$eV$^{-1}$. Two deep acceptor-like metal vacancy peaks with peak densities in the range of $10^{18}$ cm$^{-3}$eV$^{-1}$ and lie adjacent to the valance band Urbach tail region at 2.0 to 2.5 eV below the conduction band edge. By applying detailed charge balance, we show increasing the density of metal vacancy deep-acceptors strongly shifts the $a$-IGZO TFT threshold voltage to more positive values. Photoionization (h$\nu$ > 2.0 eV) of metal vacancy acceptors is one cause of transfer curve hysteresis in $a$-IGZO TFTs owing to longer recombination lifetimes as they get captured into acceptor-like vacancies.