Oxygen Vacancies and Complex Oxide Heterostructures

Oxygen vacancies are known to be critical to the behavior of complex oxide heterostructures. Here we describe two studies exploiting in situ synchrotron X-ray techniques. The first regards SrTiO₃, 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. Using a suite of in situ techniques to monitor both the atomic and electronic structure of the SrTiO₃ (001) surface prior to and during growth, we observe the disappearance and re-appearance of a 2D electron gas (2DEG) after the completion of each SrO and TiO₂ monolayer. The study resolves a number of longstanding issues associated with complex oxide interfaces and highlight the importance of defect engineering in oxide electronics.

The other study involves SrCoO_3-δ, a material known to behave as an oxygen “sponge”. As oxygen exits from the surface, δ approaches 0.5, and SrCoO_3-δ forms the brownmillerite phase, creating a superlattice peak from oxygen vacancy ordering. As oxygen is re-incorporated into SrCoO_3-δ via a surface redox reaction, the superlattice peak disappears as the perovskite phase is re-formed. Utilizing in situ coherent X-ray scattering at the Advanced Photon Source, we monitored speckle from epitaxial SrCoO_3-δ thin films, gaining insight into the dynamics of oxygen-induced phase evolution in the complex oxides. We will discuss the kinetics and dynamics of the vacancy-ordering phase transition and the methods used to distinguish the different atomic mechanisms taking place.