2, Northwestern University, Evanston, Illinois, United States
Oxide thin film growth on SrTiO3 by molecular beam epitaxy (MBE) offers a promising pathway for nanoscale control of electronic and structural properties of interfaces. Previous studies have indicated that deposited oxide layers rearrange dynamically during growth , impacting the structure of the resulting interface. However, the impact of a TiO2-rich termination of the substrate , as well as the kinetics and mechanism of the layer-rearrangement process remain unknown. Using X-ray reflectivity (XRR) measurements to determine structure and then first principles calculations to determine surface energetics can offer some insight, but a tighter coupling between theory and experiment can be even more impactful. Integrating constraints from density-functional theory into the refinement of structural models against experimental X-ray data  provides a concurrent description of surface structure and energetics during layer-by-layer growth. We use DFT-constrained structure refinement against resonant and non-resonant XRR measurements performed during shuttered deposition to confirm a double layer TiO2 termination of the bare substrate and demonstrate how this TiO2-rich surface impacts the dynamic layer rearrangement at each step. We combine measurements of the layer-flipping timescale during growth with DFT calculations of the affinity of the surfaces for deposition of a new monolayer to propose mechanisms for growth. We also leverage DFT alongside diffuse scattering data to explain why some growth protocols lead to formation of islands and more disordered fims while others lead to smoother, higher quality films, impacting best practices in oxide MBE.
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