Dynamics of the metal-insulator transition of donor-doped SrTiO3

The electrical properties of donor-doped SrTiO3 (n-STO) are profoundly affected by an oxidation-induced metal-insulator transition (MIT). Here we employ dynamical numerical simulations to examine the high-temperature MIT of n-STO over a large range of time and length scales. The simulations are based on the Nernst-Planck equations, the continuity equations, and the Poisson equation, in combination with surface lattice disorder equilibria serving as time-dependent boundary conditions. The simulations reveal that n-STO, upon oxidation, develops a kinetic space charge region (SCR) in the near-surface region. The surface concentrations of the variously mobile defects (electrons, Sr vacancies, and O vacancies) are found to vary over time and to differ considerably from the values of the new equilibrium. The formation of the SCR in which electrons are strongly depleted occurs within nanoseconds, i.e., it yields a fast MIT in the near-surface region during the oxidation process. As a result of charge (over-)compensation by Sr vacancies incorporated at the surface of n-STO, this SCR is much more pronounced than conventionally expected. In addition, we find an anomalous increase of O vacancy concentration at the surface upon oxidation caused by the SCR. Our simulations show that the SCR fades in the long term as a result of the slow in-diffusion of Sr vacancies. We discuss implications for the electrical conductivity of n-STO crystals used as substrates for epitaxial oxide thin films, of n-STO thin films and interfaces, and of polycrystalline n-STO with various functionalities.