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

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 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 (SiN_x) and TiO₂/SiN_x 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.