Interfacial defect complex at the MgO/SrTiO₃ heterojunction and its electronic impact

Atomic defects exist in various heterojunctions and can remarkably affect their properties, yet it remains a challenging task to identify each individual defect. Here, by combining advanced transmission electron microscopy, spectroscopy and first-principles calculations, we obtain a structural and element-selective imaging of defects at an interface between MgO and SrTiO₃. We demonstrate that even for structurally simple MgO and SrTiO₃, their interface involves a defect complex comprising of substitutional Ti and Mg vacancies, which induce a marked shift in electronic states. The Ti atoms diffused into MgO exhibit a valence state of +4 and a certain degree of covalency to the surrounding oxygen. Such an ability to determine defects allows us to precisely alter heterojunctions so as to critically improve the device performances.