Finite-size versus interface-proximity effects in thin-film epitaxialSrTiO3

Abstract

The equilibrium electrical conductivity of epitaxial ${\text{SrTiO}}_{3}$ (STO) thin films was investigated as a function of temperature, $950\ensuremath{\le}$ $T$/K $\ensuremath{\le}1100$, and oxygen partial pressure, ${10}^{\ensuremath{-}23}\ensuremath{\le}$ $p$${\text{O}}_{2}$/bar $\ensuremath{\le}1$. Compared with single-crystal STO, nanoscale thin-film STO exhibited with decreasing film thickness an increasingly enhanced electronic conductivity under highly reducing conditions, with a corresponding decrease in the activation enthalpy of conduction. This implies substantial modification of STO's point-defect thermodynamics for nanoscale film thicknesses. We argue, however, against such a finite-size effect and for an interface-proximity effect. Indeed, assuming trapping of oxygen vacancies at the STO surface and concomitant depletion of oxygen vacancies---and accumulation of electrons---in an equilibrium surface space-charge layer, we are able to predict quantitatively the conductivity as a function of temperature, oxygen partial pressure, and film thickness. Particularly complex behavior is predicted for ultrathin films that are consumed entirely by space charge.