Spectrum and phase mapping across the epitaxial γ-Al₂O₃/SrTiO₃ interface

Epitaxial heterostructures of γ - Al 2 O 3 / SrTiO 3, grown by atomic layer deposition (ALD) and molecular beam epitaxy, have been characterized by advanced electron microscopy techniques, including aberration-corrected negative-Cs imaging, electron-energy-loss near-edge fine-structure analysis, and off-axis electron holography. Analysis of two-dimensional spectrum maps from samples that previously showed highly conductive interfacial layers revealed partial reduction of the Ti oxidation state in the SrTiO₃ layer from Ti⁴⁺ to Ti³⁺, which was confined to within ∼1-2 unit cells of the interface. Electron holography of an ALD-grown sample revealed a phase profile within the SrTiO₃ layer that rose sharply over a distance of about 1 nm moving away from the interface. Taken together, these results suggest a strong connection between reduction of oxidation state, which could be caused by oxygen vacancies and the quasi-two-dimensional electron gas present at the γ - Al 2 O 3 / SrTiO 3 interface.