Effect of oxygen vacancies at the Fe/SrTiO3(001) interface: Schottky barrier and surface electron accumulation layer

We have investigated the interface formation at room temperature between Fe and ${\mathrm{TiO}}_{2}$-terminated ${\mathrm{SrTiO}}_{3}(001)$ surface using x-ray photoelectron spectroscopy. Oxygen vacancies within the ${\mathrm{SrTiO}}_{3}$ lattice in the first planes beneath the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ interface are induced by the Fe deposition. Through a detailed analysis of the Fe $2p$, Sr $3d$, and Ti $2p$ core-level line shapes we propose a quantitative description of the impact of the vacancies on the electronic properties of the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ system. While for an abrupt $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ junction the Schottky barrier height for electrons is expected to be about 1 eV, we find that the presence of oxygen vacancies leads to a much lower barrier height value of 0.05 eV. The deposition of a fraction of Fe monolayer also pushes the surface conduction band edge of the ${\mathrm{SrTiO}}_{3}$ below the Fermi level in favor of the formation of a surface electron accumulation layer. This change in the band bending stems from the incorporation of oxygen vacancies in the near-surface region of ${\mathrm{SrTiO}}_{3}(001)$. We deduce the conduction band profile as well as the carrier density in the accumulation layer as a function of the surface potential by solving the one-dimensional Poisson equation within the modified Thomas-Fermi approximation. Owing to the electric-field dependence of the dielectric permittivity, the ${\mathrm{SrTiO}}_{3}$ with oxygen vacancies at the surface shows original electronic properties. In particular, our simulations reveal that variations of a few percent of the vacancies concentration at the surface can cause changes of several tenths of an eV in the band bending that can lead to important lateral surface inhomogeneities for the potential. We also find through our modeling that the defect states density related to oxygen vacancies at the ${\mathrm{SrTiO}}_{3}$ surface cannot exceed, at room temperature, a critical value of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{13}/\mathrm{c}{\mathrm{m}}^{2}$.