Fractional-unit-cell-doped spinel/perovskite oxide interfaces with switchable carrier conduction

The two-dimensional hole gas (2DHG) at the polar LaAlO3/SrTiO3 interface remains elusive. Different from isostructural perovskite-type interfaces, the spinel/perovskite heterointerface of γ-Al2O3/SrTiO3 (GAO/STO) enables us to control interfacial states with sub-unit-cell precision. Herein, we present the epitaxial growth of fractionally doped GAO/STO heterointerfaces, where GAO is precisely doped on the scale of 1/4-unit-cell (0.2 nm) by ferromagnetic Fe3O4 and nonmagnetic ZnO atomic layers. Notably, the conduction of the engineered interfaces depends critically on the position of the dopant, where a coexistence of electron and hole conduction is measured at even sublayer-doped GAO/STO interfaces. First-principles density functional theory calculations indicate that electron conductivity is from the interfacial TiO2 layers of the STO substrate, while the hole conductivity is from the Zn-doped GAO film. The presence of hole conduction can be explained from the alternating structural feature of a doped layer without oxygen vacancies. This work sheds additional insight on the emergence of 2DHG at oxide interfaces and provides opportunities for atomically engineered oxide interfaces with non-isostructural layers.