Complex oxides comprised of transition metal cations exhibit a host of intriguing properties for new technologies that can be tuned by the choice of ions from the 3d, 4d, and 5d blocks of the periodic table. Perovskite oxides with the chemical formula ABO₃ have some of the richest behavior, where they can exhibit ferroelectricity, ferromagnetism, or superconductivity depending on the choice of B-site metal ion. This combination of properties in a single class of materials offers rich opportunities for engineering of unusual behavior through the design of multi-layer thin films that incorporate epitaxial strain and interfacial electronic band offsets. Using hybrid metal-organic molecular beam epitaxy (MBE), we are able to control these materials down to the atomic level so that interfaces between two different materials can be tuned to produce novel quantum phenomena. In this talk, we will show how novel behavior can be tuned and studied using in situ techniques to understand the film growth process and resulting functional properties. We have employed MBE and in situ X-ray photoelectron spectroscopy (XPS) to explore 4d and 5d oxide films that exhibit strong spin-orbit coupling and interfacial charge transfer. We have demonstrated the growth of hard-to-grow materials including SrNbO₃, SrIrO₃, and SrHfO₃ using metal-organic precursors and examined how interfacial phenomena can be tuned via charge transfer into materials such as BaSnO₃ and SrCoO₃. Ongoing work focuses on the use of these materials to produce novel oxide heterostructures for topological phases and high electron mobility 2D electron gases.

We have probed the relationship between electron trapping at the surface and electron transfer across the interface of MBE-grown SrTiO₃ and unintentionally doped Si(001). The latter, driven by shallow O donors in the near-surface region, gives rise to a 2D hole gas on the semiconductor side of the interface if enough charge transfer occurs to reach inversion. The former results in surface depletion within the top ~2 nm of the film as a consequence of charge trapping at the surface. By varying the composition of the film surface, we have found that charge transfer from Si to STO and thus hole gas formation in Si can be controlled. That is, the initiating step in charge transfer across the interface is charge trapping at the surface and first-principles modeling points to extra oxygen at the STO surface as being the electron trap that initiates the process. Surface compositional changes that prevent the trapping of extra oxygen at the surface quench charge transfer across the interface. As a result, the two electric fields constitute a coupled state that can be manipulated by means of surface composition engineering. Resonant soft x-ray photoemission near the Ti L₃-to-e_g excitation yields valuable information on the electronic properties of gap states associated with trapped electrons at the surface and itinerant electrons in the subsurface region of the films. This measurement nicely complements hard x-ray photoemission that has been used to probe the coupled electric fields.