Tiffany C. Kaspar

Experimental determination of valence band maxima for SrTiO3,TiO2, and SrO and the associated valence band offsets with Si(001)

We address the issue of accurate determination of the valence band maximum (VBM) for SrTiO3(001) single crystals and epitaxial films, as well as TiO2(001) anatase and SrO epitaxial films. These measurements are of critical importance in determining valence band offsets in heterojunctions of these oxides with Si. Three different methods are analyzed: (1) fitting a Gaussian broadened theoretical density of states to the x-ray photoelectron valence band spectrum; (2) finding the intersection of a regression line that spans the linear portion of the x-ray photoelectron valence band leading edge with the background between the valence band maximum and the Fermi level; and (3) determining the energy at which high-resolution ultraviolet photoemission intensity at the leading edge goes to zero. We find that method 1 yields physically unreasonable results when used in conjunction with density functional theory because the latter does not predict the detailed shape of the valence bands in these oxides with sufficie...

Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films

Epitaxial thin films of cobalt-doped ZnO (Co : ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2 O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10−4–105 Ω cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co : ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co : ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.

Advanced spectroscopic synchrotron techniques to unravel the intrinsic properties of dilute magnetic oxides: the case of Co:ZnO

The use of synchrotron-based spectroscopy has revolutionized the way we look at matter. X-ray absorption spectroscopy (XAS) using linear and circular polarized light offers a powerful toolbox of element-specific structural, electronic and magnetic probes that is especially well suited for complex materials containing several elements. We use the specific example of Zn1−xCoxO (Co:ZnO) to demonstrate the usefulness of combining these XAS techniques to unravel its intrinsic properties. We demonstrate that as long as phase separation or excessive defect formation is absent, Co:ZnO is paramagnetic. We can establish quantitative thresholds based on four reliable quality indicators using XAS; samples that show ferromagnet-like behaviour fail to meet these quality indicators, and complementary experimental techniques indeed prove phase separation. Careful analysis of XAS spectra is shown to provide quantitative information on the presence and type of dilute secondary phases in a highly sensitive, non-destructive manner.

Band offsets for the epitaxial TiO2/SrTiO3/Si(001) system

We have used x-ray photoelectron spectroscopy with high energy resolution to determine band discontinuities at the two buried interfaces of the epitaxial TiO2 (anatase)/SrTiO3/Si(001) system. The valence band offsets are −2.1±0.1 eV and +0.2±0.1 eV at the SrTiO3/Si and TiO2/SrTiO3 heterojunctions, respectively. Assuming bulk band gaps for the SrTiO3 and TiO2 epitaxial films, the associated conduction band offsets are +0.1±0.1 eV and +0.1±0.1 eV. Si at the interface is in a flatband state, indicating a very low density of electronic states. These results suggest that spin-polarized electron injection from ferromagnetic Co-doped TiO2 anatase into Si should be facile.

Band alignment at epitaxial BaSnO3/SrTiO3(001) and BaSnO3/LaAlO3(001) heterojunctions

We have spectroscopically determined the optical bandgaps and band offsets at epitaxial interfaces of BaSnO3 with SrTiO3(001) and LaAlO3(001). 28 u.c. BaSnO3 epitaxial films exhibit direct and indirect bandgaps of 3.56 ± 0.05 eV and 2.93 ± 0.05 eV, respectively. The lack of a significant Burstein-Moss shift corroborates the highly insulating, defect-free nature of the BaSnO3 films. The conduction band minimum is lower in electron energy in 5 u.c. films of BaSnO3 than in SrTiO3 and LaAlO3 by 0.4 ± 0.2 eV and 3.7 ± 0.2 eV, respectively. This result bodes well for the realization of oxide-based, high-mobility, two-dimensional electron systems that can operate at ambient temperature, since electrons generated in the SrTiO3 by modulation doping, or at the BaSnO3/LaAlO3 interface by polarization doping, can be transferred to and at least partially confined in the BaSnO3 film.

Room-temperature ferromagnetism in ion-implanted Co-doped TiO2(110) rutile

Ferromagnetic Co-doped rutile TiO2 single crystals were synthesized by high-temperature ion implantation and characterized by a variety of techniques. Co is uniformly distributed to a depth of ∼300 nm with an average concentration of ∼2 at. %, except in the near-surface region, where the concentration is ∼3 at. %. Ferromagnetic behavior is exhibited at room temperature with an effective saturation magnetization of ∼0.6 μB/Co atom. The Co is in a formal oxidation state of +2 throughout the implanted region, and no Co(O) is detected.

Nonstoichiometric material transfer in the pulsed laser deposition of LaAlO3

Inequivalent angular distributions have been found for La and Al in the ablation plume from LaAlO3 single crystal targets using a KrF laser during pulsed laser deposition. Angular distributions and stoichiometries in the condensate were measured and reveal decidedly nonstoichiometric transfer from target to substrate over most of the angular range. Composition varied dramatically for plume angles parallel to the long axis of the laser spot with the on-axis position exhibiting a peak in the La/Al atom ratio at ∼1.5. The distributions were more diffuse in the perpendicular direction. Stoichiometric LaAlO3 was found in the condensate only at an extreme off-axis position.

Optical absorption and spectral photoconductivity in α-(Fe1−xCrx)2O3 solid-solution thin films

Hematite, α-Fe2O3, is an attractive narrow gap oxide for consideration as an efficient visible light photocatalyst, with significant potential for band gap engineering via doping. We examine optical absorption in α-(Fe1-xCrx)2O3 epitaxial films and explain the observed excitations, and the nature of the band gap dependence on x, through first-principles calculations. The calculated and measured optical band gap becomes smaller than that of bulk α-Fe2O3 and reaches a minimum as the Cr cation fraction increases to 50%. The lowest energy transitions in the mixed-metal alloys involve electron excitation from occupied Cr 3d orbitals to unoccupied Fe 3d orbitals, and they result in a measurable photocurrent. The onset of α-Fe2O3 photoconductivity can be reduced by nearly 0.5 eV (to 1.60 eV) through addition of Cr.

Co-doped anatase TiO2 heteroepitaxy on Si(001)

Pure anatase TiO2 and CoxTi1−xO2(0.01<x<0.04) epitaxial thin films were deposited by oxygen-plasma-assisted molecular beam epitaxy on Si(001) for evaluation as a potential dilute magnetic semiconductor material suitable for Si-based spintronic devices. Epitaxial growth on Si(001) was facilitated by the deposition of 1∕4 or 1∕2ML Sr metal on the clean Si(001) surface to form an oxidation resistant silicide layer, followed by deposition of a thin SrTiO3 buffer layer. Using 1∕2ML Sr metal to form the silicide allowed the deposition of 10 ML SrTiO3 without oxidation of the Si interface. Epitaxial anatase could be grown on this heterostructure, although use of the oxygen plasma during deposition resulted in significant SiO2 formation. Pure anatase films consisted of epitaxial anatase surface particles on a continuous anatase film. For Co-doped films, Co segregation to surface particles of epitaxial anatase was observed by Auger electron spectroscopy and transmission electron microscopy (TEM); faceting of the p...

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O K-edge and Mn L{sub 3,2}-edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn L{sub 3} resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment (800 C ambient pressure of O{sub 2}) alone results in La deficiency (severest near the surface with Sr doping >0.55) and a stronger Mn{sup 4+} contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O K-edge X-ray absorption measurements support Sr/La enrichment nearer the surface, along with the formation of mixed Sr{sub x}Mn{sub y}O{sub z} and/or passive MnO{sub x} and SrO species.