Improvement in the Photo-Bias Stability of Zinc Tin Oxide Thin-Film Transistors by Introducing a Thermal Oxidized ${\rm TiO}_{2}$ Film as a Hole Carrier Blocking Layer

Abstract

This paper examined the morphological, structural, and electrical properties of thermal titanium oxide (TiOx) films as a function of the physical thickness. All the thermal TiOx films were assigned to a TiO2 chemical state irrespective of the film thickness. The thinner TiO2 films (≤ 5 nm) showed an amorphous phase, whereas the thicker TiO2 film (≥ 7 nm) had a nanocrystalline structure. This intriguing thickness-dependent crystallization behavior can be explained by the dimensional effect. The mobility of the resulting zinc tin oxide (ZTO) thin-film transistors (TFTs) with a gate-stack of silicon nitride (SiNx) and TiO2/SiNx was monotonously reduced with increasing TiO2 film thickness, which can be attributed to the enhanced Columbic scattering effect of TiO2 films. On the other hand, the negative bias illumination stress instability of the ZTO TFTs can be suppressed significantly to -2.4 V by the adoption of a 5-nm-thick TiO2 film compared with that (-14.4 V) of the ZTO device without a TiO2 film, which is discussed based on the valence band-off structure and the amorphous nature of thermal TiO2 films.