Evolution of the “metal–insulator transition” in oxygen-deficient SrTiO3−δ thin films

Oxygen deficient strontium titanate (SrTiO3−δ; STO) shows a range of electronic properties, including metallic conductivity, semiconducting behavior, and superconductivity, in contrast to stoichiometric SrTiO3, which is a band insulator. Several phenomena, including a resistivity transition (referred to in the literature as a “metal–insulator transition”), multiferroic, magnetic behavior, and enhanced superconducting transition temperatures have been observed in oxygen deficient STO thin films. Non-equilibrium conditions during film growth, lattice-mismatch strain, defects, and interface effects are expected to influence the electronic properties of thin films. Understanding the role of these factors is necessary to achieve reproducible modulation of film properties. Here, we report a controlled investigation of STO films on [100] LaAlO3 (STO–LAO) and [100] SrTiO3 (STO–STO), revealing striking differences between the properties of the films on the two substrates. Our results include the first observation of thickness dependent evolution of the resistive transition in partially strain-relaxed STO–LAO films. We show that the extrapolated zero temperature conductivity of these films is finite, indicating that the low temperature phase is not a true insulator. Instead, the temperature dependence indicates a low temperature quantum diffusive metallic state, which may be attributed to disorder-induced weak-localization and enhanced electron–electron interactions. Thus, the resistive transition may be more appropriately termed a “quantum diffusive transition” rather than a “metal–insulator transition.” STO–STO films, in stark contrast, exhibit metallic conductivity for the whole thickness and temperature range. Films on both substrates show magnetoresistance at low temperatures with characteristic differences. We present structural and microstructural characterization supporting our results.