Metal-insulator transition in n-type bulk crystals and films of strongly compensated SrTiO3

We start by analyzing experimental data of Spinelli et al. [Phys. Rev. B 81, 155110 (2010)] for the conductivity of n-type bulk crystals of SrTiO3 (STO) with broad electron concentration n range of 4 x 10(15)-4 x 10(20) cm(-3), at low temperatures. We obtain a good fit of the conductivity data, s (n), by the Drude formula for n >= n(c) similar or equal to 3 x 10(16) cm(-3) assuming that used for doping insulating STO bulk crystals are strongly compensated and the total concentration of background charged impurities is N = 1019 cm(-3). At n < n(c), the conductivity collapses with decreasing n and the Drude theory fit fails. We argue that this is the metal-insulator transition (MIT) in spite of the very large Bohr radius of hydrogenlike donor state aB similar or equal to 700 nm with which the Mott criterion of MIT for a weakly compensated semiconductor, na(B)(3) similar or equal to 0.02, predicts 10(5) times smaller nc. We try to explain this discrepancy in the framework of the theory of the percolation MIT in a strongly compensated semiconductor with the same N = 1019 cm(-3). In the second part of this paper, we develop the percolation MIT theory for films of strongly compensated semiconductors. We apply this theory to doped STO films with thickness d <= 130 nm and calculate the critical MIT concentration nc (d). We find that, for doped STO films on insulating STO bulk crystals, nc (d) grows with decreasing d. Remarkably, STO films in a low dielectric constant environment have the same nc (d). This happens due to the Rytova-Keldysh modification of a charge impurity potential which allows a larger number of the film charged impurities to contribute to the random potential.

Automated summary

This study analyzed the conductivity of n-type bulk crystals of SrTiO3 and films under different electron concentrations, revealing a metal-insulator transition for strongly compensated semiconductors within a certain range of electron concentrations. Furthermore, in low dielectric constant environments, the critical transition concentration for films does not vary with thickness.