Dynamics of the metal-insulator transition of donor-dopedSrTiO3

Abstract

The electrical properties of donor-doped SrTiO 3 (n-STO) are profoundly affected by an oxidation-induced metal-insulator transition (MIT). Here we employ dynamical numerical simulations to examine the hightemperature 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.

Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3

“…11 In addition, Mikheev et al attributed the magnitude of the resistive switching properties of a Pt/Nb:SrTiO 3 junction to charges trapped in an unintentional contamination layer. 4 [14][15][16][17] A comparable scenario has been proposed also for electron gases formed at interfaces of SrTiO 3 . [18][19][20] Classical defect chemistry of donor-doped bulk SrTiO 3 is generally accepted for elevated temperatures above 1200 K. 21 At lower temperatures, however, it might not be possible to equilibrate either of the ionic sublattices in the entire sample due to sluggish ionic movement.…”

“…2017) and the reported surface potential obtained from a defect chemical oxidation model at 1500 K. 15 Furthermore, Higuchi et al found the differences in the BE of 0.7 eV for 2 at. % donor-doped and 2 at.…”

“…This would freeze out some of the concentrations, leading to an effective charge separation at the surface. 15,20 The lower temperature limit at which the surface defect chemistry of the oxygen and strontium sublattices needs to be considered is already well investigated for acceptor-doped SrTiO 3 . However, it is much more complex for donor-doped SrTiO 3 , due to long equilibration times, secondary phase formation, 22 and the stronger influence of cations as electronic charge compensating species.…”

“…However, it is much more complex for donor-doped SrTiO 3 , due to long equilibration times, secondary phase formation, 22 and the stronger influence of cations as electronic charge compensating species. 15 Complementing defect chemical considerations, at lower temperatures, the presence of charged adsorbates might influence n-SrTiO 3 surfaces in oxidizing conditions. Setvin et al showed the existence of oxygen molecules forming superoxides at TiO 2 surfaces below 300 K. 23,24 Similarly, charged oxygen adsorbate species are proposed on perovskite surfaces at comparably high temperatures, their concentration diminishing with increasing temperature though.…”

“…25 Far above room temperature, the presence of any of those oxygen species on n-SrTiO 3 surfaces is still unconfirmed and any effect on the space charge formation has not been quantified so far. Thus, up to now, it is not known, whether intrinsic electronic surface states, 11 or unintentional defect layers due to carbonate adsorbates, 4 or possibly chemisorbed charged oxygen molecules such as observed for TiO 2 , 23,24,26 or intrinsic ionic surface defects, namely, V Sr , 15 can cause the observed surface space charge layer effect in n-SrTiO 3 13 and its dependence on the oxygen partial pressure (pO 2 ).…”