Transition metal oxide thin films integration on SrTiO₃

Guo, Wei, Ph. D.
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Transition metals (TMs) have an immense range of intriguing physical properties and phenomena. Transition metals and transition metal oxides (TMOs) play a very important role in modern day scientific research and industry. TMs and TMOs exhibit an even broader range of structural, electrical, magnetic, and optical properties when fabricated as thin films on other materials compared to their the bulk forms. SrTiO₃ (STO) is a widely used cubic perovskite oxide material known for its excellent electronic properties. When TMs and TMOs are integrated on STO, effects based on their interaction with STO have lead researchers to explore the physics behind such effects and their possible industrial device applications. In this thesis, we will mainly focus on the integration of TMs and TMOs with STO and with epitaxial STO grown on Si. The interactions of the transition metal Pt and rare earth metal Eu when deposited on STO by molecular beam epitaxy (MBE) were investigated. For Pt growth on STO, I investigated the properties of ultrathin Pt as a function of coverage on TiO₂-terminated SrTiO₃ substrate at different temperatures. I used in situ x-ray photoelectron spectroscopy (XPS), ex situ scanning electron microscopy (SEM) and atomic force microscopy (AFM) to observe the evolution of the electronic structure and surface morphology of Pt. I compared the electronic structure of Pt and the different growth patterns at low and high temperatures. I also performed high temperature annealing of low temperature-grown samples and found a “bubble-up” behavior of the continuous film. We also performed ultra-high vacuum deposition of Eu metal on STO (001) and achieved EuO epitaxy on STO via oxygen scavenging. I explored the oxygen scavenging behavior of Eu using STO films on Si by varying the STO thickness and Eu deposition temperature. In situ XPS was used to investigate the electronic structure of the nominal Eu/STO/Si stack. Our XPS results on the Eu/EuO stack revealed an unusual downward band bending at the interface. This is supported by density functional theory calculations by Gao. This work has been published in J. Appl. Phys. 121, 105302 (2017) and J. Appl. Phys. 124, 235301 (2018). Theoretical calculations performed in our group predicted the existence of a 2-dimensional electron gas (2DEG) at the EuO/STO interface and demonstrated that the 2DEG location can be controlled if an additional layer of BaTiO₃ is included. To explore this effect on the 2DEG, I performed soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) with our collaborators at the Swiss Light Source. The results are currently being summarized for publication. For possible applications in Si photonics, I performed a detailed study of dry oxidation of a Si substrate below a thin epitaxial SrTiO₃. Annealing time and temperature are the key factors to optimize the SiO₂ thickness. I developed a theoretical model based on a modification of the Deal-Grove-Massoud formalism that predicted the thickness of SiO₂ formed underneath STO as a function of time and temperature. The model fits the experimental data well. This work has been published in J. Appl. Phys. 127, 055302 (2020). In addition, I performed preliminary studies on free-standing STO membranes. I developed a fabrication process and performed Raman measurements. We also proposed a quantum well structure design of BaSnO₃/SrTiO₃/Al₂O₃ to make use of the conduction offset as large as ~3.5eV between BaSnO₃ (BSO) and Al₂O₃. The whole deposition process is done by MBE and characterized by reflection high energy electron diffraction (RHEED) and XPS to confirm the BSO film quality. We are still working on improving the quantum well quality to be able to make multiple quantum well structures.