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.

We thank Dr. Intae Bae for assistance with the TEM measurements. L.F.J.P. gratefully acknowledges startup funding from Binghamton University and funding from the Integrated Electronics Engineering Center at the State University of New York at Binghamton. The IEEC is a New York State Center for Advanced Technology and receives funding from the New York State Foundation for Science, Technology, and Innovation (NYSTAR) as well as a consortium of industrial members. A.W. acknowledges support from the Royal Society University Research Fellowship scheme and EPSRC Grant No. EP/J017361/1. Access to the HECToR supercomputer was facilitated through membership of the HPC Materials Chemistry Consortium (EP/F067496). The NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Beamline X24a at the NSLS is supported by the National Institute of Standards and Technology. This material is based upon work supported by the National Science Foundation under CHE-1230246 as well as start-up funds provided by University of Toledo.