Origin of interfacial conductivity at complex oxide heterointerfaces: Possibility of electron transfer from water chemistry at surface oxygen vacancies

Abstract

A variety of conducting heterointerfaces have been made between ${\mathrm{SrTiO}}_{3}$ substrates and thin capping layers of distinctly different oxide materials that can be classified into polar band insulators (e.g., ${\mathrm{LaAlO}}_{3}$), polar Mott insulators (e.g., ${\mathrm{LaTiO}}_{3}$), apparently nonpolar band insulators (e.g., $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$), and amorphous oxides (e.g., amorphous ${\mathrm{SrTiO}}_{3}$). A fundamental question to ask is if there is a common mechanism that governs interfacial conductivity in all these heterointerfaces. Here, we examined the conductivity of different kinds of heterointerfaces by annealing in oxygen and surface treatment with water. It was found that the conductivity of all the heterointerfaces shows a strong dependence on annealing, and can be universally tuned by surface treatment whose effect is determined by the annealing condition. These observations, together with ambient-pressure x-ray photoelectron spectroscopy measurements, suggest that water chemistry at surface oxygen vacancies is a common mechanism that supplies electrons to the interface.