Improvement in the Photo-Bias Stability of Zinc Tin Oxide Thin-Film Transistors by Introducing a Thermal Oxidized Film as a Hole Carrier Blocking Layer

This paper examined the morphological, structural, and electrical properties of thermal titanium oxide (TiO x ) films as a function of the physical thickness. All the thermal TiO x films were assigned to a TiO 2 chemical state irrespective of the film thickness. The thinner TiO 2 films (≤ 5 nm) showed an amorphous phase, whereas the thicker TiO 2 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 (SiN x ) and TiO 2 /SiN x was monotonously reduced with increasing TiO 2 film thickness, which can be attributed to the enhanced Columbic scattering effect of TiO 2 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 TiO 2 film compared with that (-14.4 V) of the ZTO device without a TiO 2 film, which is discussed based on the valence band-off structure and the amorphous nature of thermal TiO 2 films.