Effect of hydrogen incorporation on sub-gap density of states in amorphous InGaZnO thin-film transistors

Abstract

Amorphous semiconducting transparent oxides like InGaZnO4 (a-IGZO) have a broad distribution of metal and oxygen vacancy defects that determine thin film transistor (TFT) characteristics and impact device reliability metrics such as hysteresis. Here, we demonstrate how hydrogen modifies the density of states (DoS) through a novel on-chip method that spectrally resolves trap concentration in a-IGZO spanning the bandgap. Requiring laser energies continuously tunable from 0:26 to 3:1 eV, this method also employs difference frequency generation to access shallow states near the conduction band. We characterize the effect of hydrogen incorporation on the sub-gap peaks of the DoS of an a-IGZO TFT. Specifically, our data suggests hydrogen hybridizes with vacancy defects through metal-hydrogen (M-H) bonds that passivate oxygen vacancy sites and O-H bonds that passivate metal vacancy sites. These interactions result in a suppression of oxygen vacancy and metal vacancy- related trap states in the sub-gap and an enhancement of a metal-hydrogen bonding peak near the VBM. Temperature dependent, photon energy-dependent hysteresis, and transient defect lifetime measurements further reveal the strong impact of hydrogen concentration on a-IGZO TFT performance germane to current optical display technology.