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 ∼0.45 eV and consist of three metal cations coordinated to the O ion. The bond energy of GaO is calculated to be ∼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.