Evolution of the Metal-‘Insulator’ Transition in Oxygen-deficient SrTiO_3-δ thin films

The structural and electronic properties of SrTiO₃ are very sensitive to ionic off-stoichiometry due to the resulting charge doping as well as collective adjustments in the ionic positions. The most common ionic defects are oxygen vacancies that get accommodated into the structure through valency change of the bonded Ti ion. Thin film growth techniques such as Pulsed Laser Deposition, provide extra handles to leverage strain engineering, plasma plume characteristics and its interaction with the substrate material, to fine tune the stoichiometry of thin films. Non-equilibrium conditions during film growth, substrate-dependent lattice-mismatch strain, defects associated with strain-relaxation and interface effects are factors that are expected to influence the electronic properties of thin films. This presentation will be centered on our recent results representing the first observation of a thickness dependent evolution of the apparent ‘metal-insulator transition’ temperature in epitaxial SrTiO_3-δ(STO)thin films grown on (100) oriented LaAlO₃ substrate (LAO). Our thinnest STO-LAO films which are compressively strained show purely insulating behavior down to very low temperatures. Films that are partially strain-relaxed undergo a thickness dependent ‘metal-insulator transition’. Remarkably, the extrapolated zero temperature conductivity of the apparently “insulating” state is finite, representing a disorder-induced quantum diffusive metallic state. In stark contrast, films on (100) SrTiO₃ subsrate grown under the same conditions exhibit metallic conductivity for the whole thickness range. Films on both substrates show magnetoresistance at low temperature, albeit with characteristic differences albeit with characteristic differences that are consistent with quantum diffusive transport.