Unearthing the emerging properties at buried oxide heterointerfaces: the γ-Al ₂ O ₃ /SrTiO ₃ heterostructure

The symmetry breaking that is formed when oxide layers are combined epitaxially to form heterostructures has led to the emergence of new functionalities beyond those observed in the individual parent materials. SrTiO₃-based heterostructures have played a central role in expanding the range of functional properties arising at the heterointerface and elucidating their mechanistic origin. The heterostructure formed by the epitaxial combination of spinel γ-Al₂O₃ and perovskite SrTiO₃ constitutes a striking example with features distinct from perovskite/perovskite counterparts such as the archetypical LaAlO₃/SrTiO₃ heterostructure. Here, non-isomorphic epitaxial growth of γ-Al₂O₃ on SrTiO₃ can be achieved even at room temperature with the epitaxial union of the two distinct crystal structures resulting in modification of the functional properties by the broken cationic symmetry. The heterostructure features oxygen vacancy-mediated conductivity with dynamically adjustable electron mobilities as high as 140 000 cm² V^-1 s^-1 at 2 K, strain-tunable magnetism and an unsaturated linear magnetoresistance exceeding 80 000% at 15 T and 2 K. Here, we review the structural, electronic and magnetic characteristics of the γ-Al₂O₃/SrTiO₃ heterostructure with a particular emphasis on elucidating the underlying mechanistic origins of the various properties. We further show that γ-Al₂O₃/SrTiO₃ may break new grounds for tuning the electronic and magnetic properties through dynamic defect engineering and polarity modifications, and also for band engineering, symmetry breaking and silicon integration.