Origin of Oxygen-Induced Abnormal Hump in Bottom-Gated Polycrystalline Zinc Oxide Thin Film Transistors

Abstract

In this paper, we investigate the origin of abnormal hump after O2 annealing in bottom-gated polycrystalline ZnO thin film transistors (TFTs). The channel width dependency and X-ray diffraction (XRD) analysis show that the hump is not related with the parasitic edge or highly conductive upper layer by the superior crystal quality to channel/gate oxide interface. It is generally known that there is the grain boundary potential barrier (GBPB) caused by the intrinsic point defects such as Znix, VOx and VZnx localized in the grain boundary of ZnO film. X-ray photoelectron spectroscopy (XPS) and The room temperature photoluminescence (RTPL) analysis reveals that the thermal annealing in O2 gas reduces GBPB near ZnO surface forming the additional neutral zinc oxide lattice by the chemical interaction of defects. That is, the oxygen-induced hump is well explained by the reduced GBPB near ZnO surface, which is also supported by the clean transfer curve of ZnO TFTs with floating Schottky contact on the back side of the channel.