Deep subgap feature in amorphous indium gallium zinc oxide: Evidence against reduced indium

Amorphous indium gallium zinc oxide (a-IGZO) is the archetypal transparent amorphous oxide semiconductor. Despite the gains made with a-IGZO over amorphous silicon in the last decade, the presence of deep subgap states in a-IGZO active layers facilitate instabilities in thin film transistor properties under negative bias illumination stress. Several candidates could contribute to the formation of states within the band gap. Here, we present evidence against In⁺ lone pair active electrons as the origin of the deep subgap features. No In⁺ species are observed, only In⁰ nano-crystallites under certain oxygen deficient growth conditions. Our results further support under coordinated oxygen as the source of the deep subgap states. (Graph Presented).