Structure and Transport in Epitaxial BaSnO3: Doping, Mobility and the Insulator-Metal Transition

Abstract

The recent discovery of high room temperature electron mobility in wide band gap BaSnO3 (BSO) has generated exceptional interest in this perovskite oxide for electronic devices. Outstanding issues with regards to epitaxial films include understanding transport mechanisms, determining the optimal dopant, and understanding the role of structural defects (like dislocations) in limiting mobility. Here, we discuss detailed temperature and field-dependent electronic transport in both oxygen vacancy and La-doped BSO films grown via high pressure oxygen sputter deposition. High-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) confirm phase-pure, close to stoichiometric, smooth, epitaxial BSO(001). Film thickness, growth rate, deposition temperature, and substrate (i.e., lattice mismatch) have all been systematically varied and related to mobility. Detailed transport accompanied with STEM has been used to understand the structure-electronic property relationships and reveal the correlation between misfit and threading dislocations in BSO thin films. As-grown undoped, insulating films can be made conductive with controllable n-type doping by vacuum reduction, resulting in 300 K Hall mobilities up to 35 cm2V-1s-1 (on LaAlO3(001)) at 5×1019 cm-3. The mobility-electron density relation has been probed in this manner, down to 2×1017 cm-3, the lowest electron density probed in BSO till date. 2% La-doped BSO films, on the other hand, demonstrate 300 K electron mobilities up to 70 cm2V-1s-1 at ~2 ×1020 electrons per cm3. With increasing film thickness a clear insulator-metal transition is observed with both dopants, likely related to defect density near the substrate. The low temperature upturn in resistivity observed in metallic-like BSO has been analyzed using out-of-plane and in-plane magnetoresistance (MR) measurements. Two-dimensional weak localization (WL) has been identified as the underlying mechanism behind this low temperature quantum correction. Overall, the results not only validate the technique of high-pressure oxygen sputtering as a viable approach to produce high quality BSO films, but also provide insight into the mobility-electron density relation, and mobility-limiting factors in these films. The mobility values reported in this thesis are record values for sputtered films and are comparable to that obtained via pulsed laser deposition (PLD) in previous studies.