Analysis of electronic subgap states in amorphous semiconductor oxides based on the example of Zn-Sn-O systems

Crystalline and amorphous zinc-tin-oxide phases (c- and a-ZTO) are analyzed using density functional theory with a focus on deep subgap states, which were recently observed experimentally. Our study reveals that the broader defect band above the valence band in stoichiometric a-ZTO is due to undercoordinated single oxygen atoms whereas the narrower one below the conduction band is caused by strongly miscoordinated tin-oxygen atom complexes. Our conclusion that the deep levels above the valence band clearly originate from undercoordinated oxygen atoms is further supported by the result that hydrogen doping suppresses these levels by creating O-H bonds. This observation has also been made in experiments.