Tunable dielectric response, resistive switching, and unconventional transport in SrTiO3
The first section of this thesis discusses integration of SrTiO3 grown by molecular beam epitaxy (MBE) in vertical device structures. One target application is as a tunable dielectric. Parallel plate capacitors based on epitaxial Pt(001) bottom electrodes and (Ba,Sr)TiO3 dielectric layers grown by MBE are demonstrated. Optimization of structural quality of the vertical stack is shown to produce very low dielectric loss combined with very high tunability of the dielectric constant by DC bias. This results in considerable improvement of common figures of merit for varactor performance in comparison to previous reports.
Another target application for transition metals oxides is in resistive switching memories, which are based on the hysteretic current-voltage response observed in many oxide-based Schottky junctions and capacitors. A study on the role of metal/oxide interface quality is presented. In particular, the use of epitaxial Pt(001) as Schottky contacts to Nb:SrTiO3 is shown to suppress resistive switching hysteresis by eliminating unintentional contributions to interface capacitance. Such uncontrolled factors are discussed as a probable root cause for poor reproducibility in resistive switching memories, currently a ubiquitous challenge in the field. Potential routes towards stabilizing reproducible switching through intentional control of defect densities in high-quality structures are discussed, including a proof of concept demonstration using Schottky junctions incorporating intentionally non-stoichiometric SrTiO3 interlayers grown by MBE.
The second section of this thesis is concerned with unconventional electronic transport in SrTiO3. A systematic description of scattering mechanisms will be presented for three related material systems: uniformly-doped SrTiO$_3$, two-dimensional electron liquids (2DEL) at SrTiO3/RTiO3 interfaces (R = Gd, Sm) and confined 2DELs in RTiO3/SrTiO3/RTiO3 quantum wells. In particular, the prevalence of a well-defined T^2 scattering rate in doped SrTiO3 will be discussed as being incompatible with its traditional assignment as electron-electron scattering in a Fermi liquid.
In the case of ultrathin SrTiO3 quantum wells bound by RTiO3, evidence will be presented for the existence of a quantum critical point. This refers to a quantum phase transition at zero temperature towards an ordered phase in SrTiO3. This transition is driven by increasing confinement of the 2DEL, with a critical point located at the 5 SrO layer thickness of SrTiO3. It is manifested in anomalous temperature exponents of the power law resistivity. Additionally, a well-defined trend for the separation of the Hall and longitudinal scattering rates will be presented, analogously to a similar effect observed in the normal state of high-Tc superconductors. In particular, a unique pattern of residual scattering separation was documented, consistent with a quantum critical correction to the Hall lifetime that is divergent at the quantum critical point.