Origin of deep subgap states in amorphous indium gallium zinc oxide: Chemically disordered coordination of oxygen

S. Sallis, K.T. Butler, N.F. Quackenbush, D.S. Williams, M. Junda, D.A. Fischer, J.C. Woicik, N.J. Podraza, B.E. White Jr., A. Walsh, L.F.J. Piper

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Abstract

The origin of the deep subgap states in amorphous indium gallium zinc oxide (a-IGZO), whether intrinsic to the amorphous structure or not, has serious implications for the development of p-type transparent amorphous oxide semiconductors. We report that the deep subgap feature in a-IGZO originates from local variations in the oxygen coordination and not from oxygen vacancies. This is shown by the positive correlation between oxygen composition and subgap intensity as observed with X-ray photoelectron spectroscopy. We also demonstrate that the subgap feature is not intrinsic to the amorphous phase because the deep subgap feature can be removed by low-temperature annealing in a reducing environment. Atomistic calculations of a-IGZO reveal that the subgap state originates from certain oxygen environments associated with the disorder. Specifically, the subgap states originate from oxygen environments with a lower coordination number and/or a larger metal-oxygen separation.
Original languageEnglish
Article number232108
JournalApplied Physics Letters
Volume104
Issue number23
DOIs
Publication statusPublished - 9 Jun 2014

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