Oxide thin films and surfaces for electro-optics and superconductivity

Abstract

In this dissertation, I describe work completed on several oxide materials. Firstly, I discuss the effects of doping the cuprate YBa$_2$Cu$3$O${7-\delta}$ with calcium, which introduces an alternate cleavage plane and allows for the inspection of bulk-like superconducting properties via scanning tunnelling microscopy. Secondly, I discuss thin film growth of BaTiO$_3$ using conventional oxide molecular beam epitaxy (MBE), focusing particularly on the effects of stoichiometry and strain. Thirdly, I introduce preliminary results of using hybrid MBE to grow thin film BaTiO$_3$. Finally I briefly describe work growing the binary oxide anatase TiO$_2$ via both conventional and hybrid MBE. To the best of my knowledge, this report is the first of hybrid MBE-grown anatase. \YBCO\ has favorable macroscopic superconducting properties of $T_\mathrm{c}$ up to 93 K and $H_{c2}$ up to 150 T. However, its nanoscale electronic structure remains mysterious because bulk-like electronic properties are not preserved near the surface of cleaved samples for easy access by local or surface-sensitive probes. It has been hypothesized that Ca-doping at the Y site could induce an alternate cleavage plane that mitigates this issue. We use scanning tunneling microscopy to study both Ca-free and 10% Ca-doped YBCO, and find that the Ca-doped samples do indeed cleave on an alternate plane, yielding a spatially-disordered partial (Y,Ca) surface. Our density functional theory calculations support the increased likelihood of this new cleavage plane in Ca-doped \YBCO. On this surface, we image a superconducting gap with average value 24 $\pm$ 3~meV and characteristic length scale 1--2 nm, similar to Bi-based high-$T_\mathrm{c}$ cuprates, and the first map of gap inhomogeneity in the YBa$_2$Cu$3$O${7-\delta}$ family. Thin films of barium titanate show promise as an electro-optic candidate for integrated photonic devices due to their high Pockels coefficient of up to 1300 pm/V --- over 30 times higher than lithium niobate, the most widely used electro-optic material. In this work we show that both excess Ba and excess Ti have a higher solubility in strained thin films than in bulk crystals. Excess Ti causes defects in the crystal structure of the film that seem proportional in quantity to the amount of excess Ti, as expected for an element with a sticking factor of one. Excess Ba, on the other hand, does not incorporate into the film. Instead it seems to rise to the surface of the thin film during growth to form a water-sensitive BaO-based compound that can be rinsed off post-growth to expose a single crystal BaTiO$_3$ film. To explore strain effects on BaTiO$3$, 80 nm films are grown on a variety of scandate substrates with epitaxial compressive strains of up to 1.5%. Films have a low surface roughness, and are single crystal with high crystallinity. We find that the nature of the scandate substrates makes it challenging to find the index of refraction of the films, but see features in the UV-vis spectra that correspond to the expected bandgap values. We construct a confocal measuring setup that can be used to access the electro-optic tensor of interest ($r{42}$), and show preliminary results that indicate a strong electro-optic response. Exploratory work of hybrid MBE growth of BaTiO$_3$ is conducted, mapping out the complex phase space of adsorption controlled growth. Effects of Ba:Ti ratio, growth speed, liquid nitrogen cooling, substrate temperature, and oxygen plasma are each briefly demonstrated and discussed.