Origin of the 2D Electron Gas at the SrTiO₃ Surface

Bulk SrTiO₃ is a well-known band insulator and the most common substrate used in the field of complex oxide heterostructures. Its surface and interface with other oxides, however, have demonstrated a variety of remarkable behaviors distinct from those expected. In this work, using a suite of in situ techniques to monitor both the atomic and electronic structures of the SrTiO₃ (001) surface prior to and during growth, the disappearance and re-appearance of a 2D electron gas (2DEG) is observed after the completion of each SrO and TiO₂ monolayer, respectively. The 2DEG is identified with the TiO₂ double layer present at the initial SrTiO₃ surface, which gives rise to a surface potential and mobile electrons due to vacancies within the TiO_2-x adlayer. Much like the electronic reconstruction discovered in other systems, two atomic planes are required, here supplied by the double layer. The combined in situ scattering/spectroscopy findings resolve a number of longstanding issues associated with complex oxide interfaces, facilitating the employment of atomic-scale defect engineering in oxide electronics.