Hard x-ray photoemission study of bulk single-crystalline InGaZnO4

We performed hard x-ray photoemission spectroscopy experiments of bulk InGaZnO4 (IGZO-11) single crystals, which were recently synthesized via the optical floating zone method, in order to reveal the bulk intrinsic electronic structure. We find that oxygen vacancies are preferentially located around In atoms in as-grown crystals, which are filled out by post-annealing in an oxygen atmosphere. We also find the presence of intrinsic hydroxy bonds, which persists even after oxygen annealing. The subgap states near the conduction-band minimum were clearly observed only in the as-grown crystals, while the subgap states near the valence-band maximum (VBM) were much weaker than those in the previous study even without oxygen annealing. The absence of the near-VBM subgap states in our single crystals suggests that in addition to oxygen vacancies, the loss of crystallinity is also relevant to the formation of the near-VBM subgap states.