Variable range hopping in TiO2 insulating layers for oxide electronic devices

TiO2 thin films are of importance in oxide electronics, e.g., Pt/TiO2/Pt for memristors and Co-TiO2/TiO2/Co-TiO2 for spin tunneling devices. When such structures are deposited at a variety of oxygen pressures, how does TiO2 behave as an insulator? We report the discovery of an anomalous resistivity minimum in a TiO2 film at low pressure (not strongly dependent on deposition temperature). Hall measurements rule out band transport and in most of the pressure range the transport is variable range hopping (VRH) though below 20 K it was difficult to differentiate between Mott and Efros-Shklovskii's (ES) mechanism. Magnetoresistance (MR) of the sample with lowest resistivity was positive at low temperature (for VRH) but negative above 10 K indicating quantum interference effects.