Abstract
Epitaxial heterostructures of γ−Al2O3/SrTiO3, 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−Al2O3/SrTiO3 interface.
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