Quasi-two-dimensional electron gas at the interface of γ-Al₂O₃/SrTiO₃ heterostructures grown by atomic layer deposition

We report the formation of a quasi-two-dimensional electron gas (2-DEG) at the interface of γ-Al₂O₃/TiO₂-terminated SrTiO₃ (STO) grown by atomic layer deposition (ALD). The ALD growth of Al₂O₃ on STO(001) single crystal substrates was performed at temperatures in the range of 200-345 °C. Trimethylaluminum and water were used as co-reactants. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and ex situ cross-sectional transmission electron microscopy were used to determine the crystallinity of the Al₂O₃ films. As-deposited Al₂O₃ films grown above 300 °C were crystalline with the γ-Al₂O₃ phase. In situ x-ray photoelectron spectroscopy was used to characterize the Al₂O₃/STO interface, indicating that a Ti³⁺ feature in the Ti 2p spectrum of STO was formed after 2-3 ALD cycles of Al₂O₃ at 345 °C and even after the exposure to trimethylaluminum alone at 300 and 345 °C. The interface quasi-2-DEG is metallic and exhibits mobility values of ∼4 and 3000 cm² V^-1 s^-1 at room temperature and 15 K, respectively. The interfacial conductivity depended on the thickness of the Al₂O₃ layer. The Ti³⁺ signal originated from the near-interfacial region and vanished after annealing in an oxygen environment.