Subgap states, doping and defect formation energies in amorphous oxide semiconductor a-InGaZnO4 studied by density functional theory

Amorphous In-Ga-Zn-O (a-IGZO) is expected for channel layers in thin-film transistors (TFTs). It is known that a-IGZO is sensitive to an O/H-containing atmosphere; therefore, it is important to clarify the roles of oxygen and hydrogen in a-IGZO. This paper provides atomic and electronic structures, formation energies of defects and bond energies in a-IGZO calculated by first-principles density functional theory (DFT). It was confirmed that oxygen deficiencies having small formation energies (2-3.6 eV) form either deep fully-occupied localized states near the valence band maximum or donor states, which depend on their local structures. All the hydrogen doping form -OH bond and work as a donor. The stable -OH bonds have small formation energy of similar to 0.45 eV and consist of three metal cations coordinated to the O ion. The bond energy of Ga-O is calculated to be similar to 2.0 eV, which is the largest among the chemical bonds in a-IGZO (1.7 eV for In-O and 1.5 eV for ZnO). This result supports the idea that the incorporation of Ga stabilizes a-IGZO TFTs. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim