Film Texture, Hole Transport and Field-Effect Mobility in Polycrystalline SnO Thin Films on Glass

Abstract

A p-type oxide semiconductor, tin monoxide (SnO), is expected for high-mobility p-channel oxide thin-film transistors. In this study, we fabricated polycrystalline SnO films by three methods based on pulsed laser deposition, direct deposition at high temperatures, in-situ thermal annealing without exposure to air, and ex-situ thermal annealing after exposure to air. It was found that the crystallite orientation depends largely on the fabrication methods; i.e., the direct deposition enhanced c-axis orientation in particular at higher temperatures, and the in-situ thermal annealing induced homogeneous nucleation of non-oriented crystallites at high temperatures. Further, ex-situ thermal annealing nucleated (101)-oriented crystallites due probably to surface contamination originating from air exposure. In-plane Hall mobility was related to the film texture; i.e., the c-axis oriented films exhibited higher hole mobilities in the a-b plane while the more (101)-oriented films exhibited lower mobilities. This observation was attributed to an intrinsic nature that heavier effective hole mass along the Γ-Z direction ([001] direction) than that of the Γ-M direction ([110] direction), originating from strong anisotropy of the layered crystal structure of SnO. The on-state currents of SnO TFTs fabricated by 300°C deposition or thermal annealing were explained qualitatively by the hole mobilities measured by Hall effect.