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/TiO ₂/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.