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UBC Theses and Dissertations

Soft X-ray reflectometry and X-ray absorption spectroscopy of transition metal oxide thin-films on various substrates: cobaltate (CoO) and lanthanum cobaltate (LaCoO₃) Radi, Abdullah


The newly developed Soft X-ray Reflectometry (SXR) has been used to study the electronic structure of Transition Metal Oxide (TMO) thin-films. The technique is non-destructive, element specific and depth sensitive. The experiments were carried out in the newly installed Resonant Soft X-ray Scattering (RSXS) endstation of the 10ID-2 beamline, the Canadian Light Source (CLS). Simulating and fitting the data required a special home-written software “ReMagX” . X-ray Absorption Spectroscopy (XAS) was measured in Total Fluorescence Yield (TFY) and Total Electron Yield (TEY), followed by on- and off-Resonant Soft X-ray Reflectometry (RSXR) at constant energy and at fixed momentum transfer vector in the z-direction (fixed Qz). The needed samples for the current research were readily available through research collaborators. CoO thin-films were grown with Molecular Beam Epitaxy (MBE) on MgO substrate as an example of compressive strain. The LaCoO₃ thin-films were grown with Pulsed Laser Deposition (PLD), with or without a LaAlO₃ cap, on LaAlO₃, NdGaO₃ or SrTiO₃ substrates as examples of compressive and tensile strain. TEY analysis of the CoO on MgO sample indicates a reduction of symmetry from cubic octahedral to distorted tetragonal with the crystal compressed in the xy plane. The surface contamination layer appears as a distinctive feature in the measured reflectivities. The uncapped LaCoO₃ thin-films show distinctive reconstructed surfaces with more pronounced densities of Co²+ that are energy-feasible ways of compensating the polar surface. The change in the LaCoO₃ on SrTiO₃+ sample results in vertical stripes which are believed to have Co ions in mixed valencies and spin-states and were discussed in two models. The samples have been modeled in ReMagX and the measured TEY signals have been used to generate the needed refractive indices and atomic scattering factors.

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