Transport and Magnetism in Bulk and Thin Film Strontium Titanate

Abstract

SrTiO3 is a wide band-gap perovskite oxide semiconductor that is widely investigated in the bulk form, due to its remarkable electronic properties. These properties arise from its quantum paraelectric nature which enables unique features, such as, a high-mobility low-density metallic state, quantum transport in an unusual limit, and the most dilute superconducting state thus reported. Recent advances in deposition of oxide thin films and heterostructures have further led to some remarkable observations, such as, the strain-enhancement of mobility in doped thin films of SrTiO3, and the presence of 2D electron gases at interfaces and in delta-doped layers. The presence of magnetic moments and their possible ordering, and the simultaneous observation of quantum oscillations and superconductivity, have been reported in these 2D electron gases. While magnetism has been observed in heterostructures of SrTiO3, there have been limited reports on magnetism in bulk SrTiO3. The first part of this thesis (Chapter 3) discusses how circularly polarized light can induce an extremely long-lived magnetic moment in slightly oxygen-deficient but otherwise nominally pure SrTiO3-δ bulk crystals. These magnetic signals, which are induced at zero applied magnetic field and at low temperatures below ~ 18 K, can be controlled in both magnitude and sign by means of the circular polarization and wavelength of sub-bandgap illumination (400-500 nm), and point to the existence of optically polarizable "V" _"O" -related complexes in the forbidden gap of SrTiO3-δ, rather than collective or long-range magnetic order. The methods used to detect optically induced magnetization are also discussed (Appendix A). The phenomenal progress reported in thin films and heterostructures of SrTiO3 has been possible only by precise control of stoichiometry and defect density in SrTiO3 using techniques such as oxide/LASER MBE or high-temperature PLD. The next part of the thesis (Chapter 4) demonstrates that high pressure oxygen RF sputtering from a ceramic target is similarly capable of growth of high-quality, precisely stoichiometric thin films of SrTiO3. By employing homoepitaxy on SrTiO3(001) substrates, it is shown that optimization of deposition temperature (above 750 C), oxygen pressure (above 2.5 mBar) and deposition rate (below 1.5 Å/min) leads to films that are indistinguishable from the substrate via grazing incidence and wide-angle X-ray scattering. The importance of pre-annealing of substrates in oxygen above 900 C and polishing the target prior to deposition, to obtain bulk-like lattice parameters and eliminate interfacial scattering contrast, is reiterated. Detailed transport measurements were also performed on reduced films grown on LaAlO3(001) and LSAT(001) substrates. The films were found to be semiconducting with mobilities at least an order lower than bulk. Detection and quantification of trace impurities was carried out using PIXE, and the possible causes of low mobility and semiconducting transport characteristics are discussed. Despite the rapid recent progress in thin film deposition techniques, controlled dopant incorporation and attainment of high mobility in thin films of SrTiO3 remain problematic. The last part of the thesis (Chapter 5) discusses the use of analytical scanning transmission electron microscopy to study the local atomic and electronic structure of Nb-doped SrTiO3 both in ideally substitutionally-doped bulk single crystals, and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric undoped SrTiO3, as discussed in the previous chapter, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what is deduced to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO3 films compared to undoped SrTiO3, but without a clear shift in the Fermi edge, that is seen in bulk single crystal Nb-doped SrTiO3. Analysis of atomic-resolution annular dark-field images leads to the conclusion that the interstitial Nb is in the Nb0 state, confirming that it is electrically inactive. It is argued that this approach will enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO3 thin films.