Pulsed laser-induced oxygen deficiency at TiO 2 surface: Anomalous structure and electrical transport properties

Abstract

We have studied pulsed laser-induced oxygen deficiencies at rutile TiO 2 surfaces. The crystal surface was successfully reduced by excimer laser irradiation, and an oxygen-deficient TiO 2− δ layer with 160 nm thickness was formed by means of ArF laser irradiation at 140 mJ/cm 2 for 2000 pulses. The TiO 2− δ layer fundamentally maintained a rutile structure, though this structure was distorted by many stacking faults caused by the large oxygen deficiency. The electrical resistivity of the obtained TiO 2− δ layer exhibited unconventional metallic behavior with hysteresis. A metal–insulator transition occurred at 42 K, and the electrical resistivity exceeded 10 4 Ω cm below 42 K. This metal–insulator transition could be caused by bipolaronic ordering derived from Ti–Ti pairings that formed along the stacking faults. The constant magnetization behavior observed below 42 K is consistent with the bipolaronic scenario that has been observed previously for Ti 4O 7. These peculiar electrical properties are strongly linked to the oxygen-deficient crystal structure, which contains many stacking faults formed by instantaneous heating during excimer laser irradiation.