Subgap States near the Conduction-Band Edge Due to Undercoordinated Cations in Amorphous In-Ga-Zn-O and Zn-Sn-O Semiconductors

Abstract

In amorphous oxide semiconductors (AOSs), defects induced by disorder and stoichiometric imbalance significantly affect the electrical conduction and thereby threshold stability of AOS-based devices. While band tails and deep subgap states near the valence-band edge stem from the disordered coordination of anions, the role of disordered cations is poorly understood. Here, based on first-principles calculations, we report the formation of undercoordinated cations in AOSs, such as amorphous In-Ga-Zn-O and Zn-Sn-O semiconductors, which are classified into two groups: undercoordinated cation pairs and undercoordinated single cations. Cation-pair defects belong to the category of O-vacancy defects and exhibit deep subgap states formed by the interaction between cations, whereas single-cation defects act as electron traps, similar to dangling bonds. The undercoordinated single cations have a tendency that the defect level near the conduction-band edge becomes deep as the coordination number decreases. Our results provide not only explanations for the origin of unresolved subgap states near the conduction band but have implications to control defect states and thereby improve the stability of AOS-based devices