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
$({\rm TiO}_{x})$
films as a function of the physical thickness. All the thermal
${\rm TiO}_{x}$
films were assigned to a
${\rm TiO}_{2}$
chemical state irrespective of the film thickness. The thinner
${\rm TiO}_{2}$
films
$({\leq}{\rm 5}~{\rm nm})$
showed an amorphous phase, whereas the thicker
${\rm TiO}_{2}$
film
$({\geq}{\rm 7}~{\rm nm})$
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
$({\rm SiN}_{x})$
and
${\rm TiO}_{2}/{\rm SiN}_{x}$
was monotonously reduced with increasing
${\rm TiO}_{2}$
film thickness, which can be attributed to the enhanced Columbic scattering effect of
${\rm TiO}_{2}$
films. On the other hand, the negative bias illumination stress instability of the ZTO TFTs can be suppressed significantly to
${-}{\rm 2.4}~{\rm V}$
by the adoption of a 5-nm-thick
${\rm TiO}_{2}$
film compared with that
$({-}{\rm 14.4}~{\rm V})$
of the ZTO device without a
${\rm TiO}_{2}$
film, which is discussed based on the valence band-off structure and the amorphous nature of thermal
${\rm TiO}_{2}$
films.