Direct Observation of Photoexcited Localized Energy States of Atomically Deposited ZnO Transistors by Analyzing Transfer Characteristics.

Abstract

Zinc oxide (ZnO) thin-film transistors (TFTs) can be promising for applications in wide-band light absorption. However, they suffer from retarded photoresponse characteristics due to atomic defects and the resulting localized electronic states. To investigate the photoinduced localized states of the ZnO TFTs, here, we combine X-ray photoelectron spectroscopy, atomic force microscopy, and density functional theory (DFT) calculations. Specifically, we derive a relationship between the density of states (DOS) and the thermally activated field-effect mobility. The derived model allows us to extract the DOS of photoexcited ZnO TFTs, which notably increased under light exposure, indicating that the lattice structure of the ZnO film changes. DFT calculations further support this finding, showing that photoinduced oxygen vacancies result in lattice distortions along the c-axis. These results suggest that the sluggish photoresponse of ZnO TFTs originates from light-induced lattice distortion caused by photoinduced oxygen vacancies, which create extended localized states.