Browsing by Subject "SrTiO3"

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    Hybrid Molecular Beam Epitaxy of Complex Oxide Heterostructures with Ultrahigh Electron Density
    (2017-01) Xu, Peng
    Two-dimensional electron gases (2DEG) at complex oxide interfaces have attracted lots of attention for fundamental physics studies and potential applications in novel oxidebased electronics. While most researches focuses on LaAlO3/SrTiO3 heterostructures, many interesting phenomena were also discovered at titanates heterostructures, such as LaTiO3/SrTIO3 and GdTiO3/SrTiO3. In this study, we chose another material system, NdTiO3/SrTiO3, to investigate the 2DEG at the interfaces. NdTiO3 and SrTiO3 thin films and heterostructures were grown using the hybrid molecular beam epitaxy approach. 3E14 cm-2 (0.5 e-/u.c.) was realized at NdTiO3/SrTiO3 interfaces, and precise control of carrier density and metal-to-insulator transition were achieved by intentionally introducing Nd vacancies. Moreover, ultrahigh carrier density (~1E15 cm-2) was discovered by band engineering NdTiO3/SrTiO3 heterostructures. The charge transfer model was proposed to explain such high carrier density. Both experimental results and computational modeling suggest the broken-gap type band offset drives charge transfer from NdTiO3 to SrTiO3. Finally, we will also discuss environmental oxygen effects on electronic transport properties of NdTiO3/SrTiO3.
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    Hybrid Molecular Beam Epitaxy Of Strain-Engineered Srsno3 Films And Heterostructures
    (2018-05) Wang, Tianqi
    Recently, perovskite oxides have been experiencing a resurgence of research interest because of their richness in functionalities and tunability by external stimuli. In particular, owning to the combination of high mobility and large band gap, alkaline earth stannate SrSnO3 starts to gain popularity among researchers and industries for transparent conducting and high power electronics. On the other hand, the hybrid molecular beam epitaxy opens up new revenues for self-regulating growth of perovskite oxides by incorporating chemical precursors into their growth approaches. Driven by the demands of optimal electronic properties and investigation of scattering mechanism, SrSnO3 thin film with excellent structural quality grown by molecular beam epitaxy becomes especially desirable. Thereby, the marriage between these two conceives great potential of scientific discovers in the very beginning. However, pioneering a novel growth approach is challenging and requires preparation work at many aspects such as selection of Sn-based chemical precursors, understanding of the strain relaxation related defect formation, and the control over duel valence states of Sn. In the attempt to high mobility SrSnO3 using hybrid molecule beam epitaxy, a record high room-temperature mobility of 55 cm2V-1s-1 was achieved through systematic control of cation stoichiometry. In addition, the correlation between electronic transport and defect associated with non-stoichiometry and dislocations were revealed. It turned out that non-stoichiometry could lead to a crossover from weak to strong localization of electronic carriers in La-doped SrSnO3. In contrast, substrate-induced dislocations can have a strong influence on the electron phase coherence length resulting in two-dimensional to three-dimensional weak localization crossover. With the growth conditions of high performance SrSnO3, structure and properties of coherent stoichiometric SrSnO3 was then further tuned via strain engineering. Meanwhile, synchrotron-based reciprocal space mapping and half-order diffraction were employed to characterize structural distortions. We demonstrated that in SrSnO3 thin films three distinct phases of tetragonal I4/mcm, high-temperature orthorhombic Imma, and room temperature orthorhombic Pnma can be stabilized under compressive, tensile, and nearly zero strain, respectively. Remarkably, stabilization of high symmetry tetragonal phase SrSnO3 corresponded to a shift of the 1st order phase transition shift by over 800 C and gave rise to an enhancement of 300 % in electron mobility at room temperature. Moreover, chemical doping (< 1 %) by La in SrSnO3 was also found to influence phase stabilization and electronic transport possibly related to anti-site defects. In summary, through the development of hybrid molecular beam epitaxy of SrSnO3 superior structural and electronic properties were attained. Also, structure distortions induced by strain engineering and chemical doping were found to dramatically tailor and enhance SrSnO3 properties. The study in SrSnO3 followed and showcased the very strength of perovskite oxide in research value.