Enhancing properties of perovskite ferroelectrics and correlated metals via precise stoichiometry control
Restricted (Penn State Only)
- Author:
- Kuznetsova, Tatiana
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 14, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Venkatraman Gopalan, Major Field Member
Jon-Paul Maria, Chair & Dissertation Advisor
Qi Li, Outside Unit & Field Member
Roman Engel-Herbert, Special Member
Ismaila Dabo, Major Field Member - Keywords:
- Molecular beam epitaxy
Complex oxides
Transparent conductors
Thin films - Abstract:
- Complex transition metal oxides in the perovskite structure exhibit an abundance of functionalities, such as ferroelectricity, superconductivity, multiferroicity, and many more, making the perovskite structure an ideal playground for engineering such properties and even discovering new ones. Advances in computational materials science simplify this exploration and streamline the process of trial and error in research. However, the need for experimental verification of such predictions is ever-pressing. While advances in thin film synthesis enabled the creation of many nearly-perfect materials with atomic precision, the growth of transition metal oxides still often presents a challenge. This work aims to facilitate thin film synthesis of transition metal oxides with low vapor pressures for ferroelectrics and transparent conductor applications. This dissertation strives to expand the variety of available high-quality transparent conducting oxides with improved electrical and optical properties. While CaVO3 was one of the first correlated metals explored for transparent conducting applications, its electrical performance was hampered by a high stacking fault density in the films. Careful control of the cation stoichiometry and microstructure enabled achieving superior electrical properties, such as room temperature resistivity of (3.7 ± 0.6) × 10-5 Ω cm and residual resistivity ratio of over 90, in 38 nm thick CaVO3 films grown via hybrid molecular beam epitaxy. A custom effusion cell for preventing reduction of molybdenum oxide in vacuum was designed, which provided a high and stable flux of MoO3. Experimental verification of the growth of SrMoO3 thin films by suboxide molecular beam epitaxy is demonstrated. A very competitive transparent conductor SrMoO3 with room temperature resistivity as low as 5 × 10-5 Ω cm and high optical transparency in the visible spectrum was grown by suboxide MBE. This work demonstrates the growth of BaTiO3/SrTiO3 superlattices via hybrid molecular beam epitaxy for the first time. The dependence of stabilized domain structure on superlattice periodicity and strain state has been investigated by synchrotron-based X-ray diffraction and optical second harmonic generation. This study deepens the understanding of the energy landscape of ferroelectrics. This dissertation is structured as follows. Chapter 1 introduces the reader to the perovskite structure and properties of transition metal oxides in the perovskite structure. Chapter 2 reviews the fundamentals of oxide thin film synthesis and commonly used deposition techniques. Chapter 3 introduces the characterization techniques used in this dissertation, such as X-ray diffraction, reflection high-energy electron diffraction and atomic force microscopy. Chapter 4 discusses synthesis and properties of correlated metal SrMoO3 grown via suboxide molecular beam epitaxy. Chapter 5 describes the growth and characterization of ultraclean correlated metal CaVO3. Chapter 6 studies the growth and domain structures in BaTiO3/SrTiO3 superlattices with different strain states and periodicity. Finally, Chapter 7 discusses future research directions in the area of ferroelectric superlattices and demonstrates the growth of KTaO3/KNbO3 superlattices by suboxide molecular beam epitaxy. Also, Chapter 7 demonstrates the growth of thin films of a solid solution of two incipient ferroelectrics CaTiO3 and SrTiO3 in attempts to find ferroelectric or highly polarizable phases. Further, preliminary work on integration of a UV transparent conductor SrNbO3 with ferroelectric BaTiO3 thin films for the creation of ferroelectric solar cells is demonstrated.