Impact ionization phenomenon in single-crystalline rutile TiO2

Titanium oxide (TiO2) is known as a material suitable for photocatalysis, where photo-excited high-energy holes in TiO2 play an important role. We applied high electric fields for TiO2 to generate high-energy holes using an impact ionization phenomenon. In current–voltage (I–V) measurements, currents rose suddenly at a threshold voltage, indicating that an impact ionization phenomenon occurred in TiO2. The threshold voltage was controllable from 10 to 170 V by varying the metal electrode thickness. This means that an electric field enhancement effect was significant at the edge of the thin film electrode. At higher voltages, there was observed a relation I / V2, indicating that space-charge-limited currents dominated the currents flowing in the specimen.