Electronic transport in doped SrTiO₃: Conduction mechanisms and potential applications

Resistivity, Hall effect, and magnetoresistance are reported on a large set of semiconducting SrTiO3-δ single crystals doped n -type (by reduction or Nb substitution) over a broad range of carrier density (the 1015 to mid 1020 cm^-3 range). Temperature-independent carrier densities, strongly temperature-dependent mobilities (up to 22000 cm2 V^-1 s^-1 at 4.2 K), and a remarkably low critical carrier density for the metal-insulator transition are observed, and interpreted in terms of the known quantum paraelectricity of the host. We argue that an unusual, high mobility, low density, metallic state is thus established at carrier densities at least as low as 8.5× 1015 cm^-3, in contrast to some prior conclusions. At low temperatures, the temperature dependence of the mobility and resistivity exhibit a nonmonotonic carrier density dependence and an abrupt change in character near 2× 1016 cm^-3, indicating a distinct crossover in conduction mechanism, perhaps associated with a transition from impurity-band to conduction-band transport. The results provide a simple framework for the understanding of the global transport behavior of doped SrTiO3. Finally, it is proposed that the large residual resistivity ratios (>3000), and large, temperature independent, Hall coefficients (>1700 cm3 C^-1), demonstrate considerable potential for high-sensitivity resistive thermometry and Hall sensing applications.