Tracing the formation of oxygen vacancies at the conductive LaAlO/SrTiO interface via photoemission

The two-dimensional electron gas (2DEG) generated at the LaAlO/SrTiO interface has been in the focus of oxides research since its first discovery. Although oxygen vacancies play an important role in the generation of the insulator-to-metal transition of the SrTiO bare surface, their contribution at the LaAlO/SrTiO interface remains unclear. In this work, we investigated a LaAlO/SrTiO heterostructure with regional distribution of defect-based localized polar sites at the interface. Using static and time-resolved threshold photoemission electron microscopy, we prove that oxygen vacancies are induced near those polar sites, resulting in the increase of carrier density of the 2DEG states. In addition, oxygen-related surface states were uncovered, which we attributed to the release of lattice oxygen during the formation of oxygen vacancies. Such effects are mainly found spatially located around the defect sites at the buried interface, while other regions remain unaffected. Our results confirm that the itinerant electrons induced by oxygen vacancies can coexist with the charge transfer mechanism in the LaAlO/SrTiO heterostructure, together leading to the formation of the metallic interface. These observations provide fundamental insights into the nature of LaAlO/SrTiO interface based 2DEG and unique perspectives for potential applications.