Surface and Interfacial Studies of Perovskite Oxides Grown by Hybrid Molecular Beam Epitaxy
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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Perovskite oxides are well known for their intriguing properties such as ferroelectricity, ferromagnetism, and superconductivity that can be tuned by different crystalline degrees of freedom. SrTiO3 (STO) is a perovskite oxide with simple cubic structure which is compared to the silicon in the semiconductor field because of its suitable lattice parameter for widespread use as a substrate in thin-film perovskite oxide growth. In an STO-based system, photocatalytic capabilities are discovered in addition to the discovery of a quasi two-dimensional electron gas (2DEG). Previous work has shown that STO surface properties determine its applicability for catalytic and interfacial behavior. SrNbO3 (SNO) is another interesting cubic perovskite oxide with lattice parameter of 4.0 Å is known for its strong electron donor capability and photocatalytic behavior. Recent reports on the strain-induced breaking of cubic symmetry of SNO producing large mobilities and large magnetoresistance and mobility in SNO/STO interface open the opportunity for SNO as a topological and quantum material. However, SNO synthesis is challenging due to the metastable nature of the d1 Nb4+ cation and the limitations in the delivery of refractory niobium. This work focuses on the synthesis of SNO thin films by hybrid molecular beam epitaxy (hMBE) to address the growth challenges and application in high mobility 2DEGs. A thin-film deposition technique, hMBE, in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. High-quality epitaxial STO and SNO thin films were grown on multiple substrates by hMBE approach. To address numerous unanswered questions regarding the chemical mechanisms of the growth process and the surface properties of the resulting films, STO thin films were prepared on annealed STO and Nb-doped STO substrates by hMBE. While synthesizing STO thin film, a titanium tetraisopropoxide (TTIP) precursor used for Ti delivery and an elemental Sr source with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. This work demonstrates the growth of SNO thin films by hMBE for the first time using a tris(diethalamido)(tert-butylimido) niobium precursor for niobium. An elemental Sr source is used for supplying strontium and the synthesis is carried out on GdScO3 and as prepared BaSnO3 (BSO) films on Nb-doped STO substrates. To examine the preservation of the Nb 4d1 metastable charge state in atmospheric conditions, an insulating SrHfO3 capping layer was deposited on top of SNO films. Furthermore, the charge transfer mechanism in the SNO/BSO interface is investigated by first-principles density functional theory and angle-resolved X-ray photoelectron spectroscopy to shed light on the possibility of high mobility 2DEG.