Observations of p-type semiconductivity in titanium dioxide at room temperature

Titanium dioxide (TiO2) is widely considered to be a strongly n-type semiconductor due to its tendency for oxygen deficiency. In this work, however, room temperature p-type semiconductivity has been observed in rutile TiO2, as determined using surface photovoltage spectroscopy and cyclic voltammetry subsequent to controlled processing at elevated temperatures. In particular, room temperature p-type semiconductivity has been imposed via strong oxidation [p(O2)=75 kPa] at elevated temperatures (1273 K), followed by rapid cooling in the same gas phase. It is reasoned that under such conditions, the observed p-type behaviour is due to the formation of titanium vacancies (acceptor-type ionic defects) at the TiO2 surface. It is also concluded that the extremely slow diffusion kinetics of these defects towards the bulk is what limits the p-type behaviour to the surface and near surface regions. Nevertheless, this could be overcome by applying appropriately lengthy annealing times. The reported observation of p-type semiconductivity in TiO2 is expected to have far reaching consequences for this intensely researched material.