Two-dimensional electron systems in perovskite oxide heterostructures: Role of the polarity-induced substitutional defects

The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as $\mathrm{LaAl}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$ has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, the fundamental origins of the 2DES are still not understood, e.g., the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation on the depth profile of 2DES-relevant $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ interface carriers using depth- and element-specific techniques like standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti $3d$ interface carriers, which give rise to the 2DES are located within three unit cells from the n-type interface in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first three unit cells of the opposing $\mathrm{LaAl}{\mathrm{O}}_{3}$ layer (\ensuremath{\sim}10 \AA{}). Our findings provide a microscopic picture of how the localized and mobile Ti $3d$ interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the $\mathrm{LaAl}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ interface have disparate explanations as originating from different types of interface carriers.