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Growth and Strain Relations in (001)-oriented Ferroelectric and Ferromagnetic Perovskite Oxide Thin Films
Norwegian University of Science and Technology, Faculty of Information Technology, Mathematics and Electrical Engineering, Department of Electronics and Telecommunications.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

The wide range of properties offered by perovskite oxides demands high quality thin films and growth control on several interfaces. In this work, growth of ferroelectric BaTiO3 (BTO) on (001)-oriented SrTiO3 (STO) and metallic ferromagnetic La_{0,7}Sr_{0,3}MnO_{3} (LSMO) by pulsed laser deposition has been elucidated. Characterization has been done by reflection high energy electron diffraction, atomic force microscope and x-ray diffraction (XRD). A substrate preparation method consisting of etching in deionized water followed by a 2 hour annealing in constant oxygen flow at 950^{o}C is found to be optimal for the (001)-oriented STO substrates. Root mean square values the order of 0,055 nm are obtained. The optimal deposition conditions for the BTO and LSMO thin films are found to be with 0,1 mbar and 0,35 mbar oxygen ambient at 700^{o}C. A surface roughening over the 5 first UC growth, prior to the the in-plane lattice relaxation, is observed for the BTO films. The surface subsequently smoothens before roughening again. For LSMO growth thermal and nonthermal growth has been identified and shown to have implications on the subsequent BTO layer. The BTO in-plane relaxation is shown to occur at an earlier stage on nonthermally compared to thermally grown LSMO. Elastic in-plane lattice relaxations are observed up to thicknesses of 4,48 nm, 4,16 nm and 2 nm for BTO grown on STO, thermal LSMO and nonthermal LSMO respectively. Such relaxations are characterized by oscillations in the in-plane lattice. Simultaneous elastic and plastic in-plane relaxations are also observed. XRD measurements show an increased BTO tetragonality close to the interfaces. The out-of-plane parameter are measured to 4,15 Å and 4,112 Å at 15 nm on STO and LSMO respectively, and decreases with increasing film thicknesses. LSMO films are shown to be epitaxially strained up to 98,1 nm, with an out-of-plane parameter of 3,85 Å. A Stranski-Krastanov growth mode is identified for BTO with critical thicknesses of 4,16 nm, 4 nm and 1,6 nm for BTO grown on STO, thermal and nonthermal LSMO. For the LSMO(25 nm)/BTO(10 nm)/LSMO(25 nm) superlattice a self-healing effect and a higher strain is observed in the topmost LSMO layer grown on relaxed BTO.

Place, publisher, year, edition, pages
Institutt for elektronikk og telekommunikasjon , 2013. , 128 p.
URN: urn:nbn:no:ntnu:diva-22567Local ID: ntnudaim:10069OAI: diva2:649812
Available from: 2013-09-19 Created: 2013-09-19 Last updated: 2013-09-19Bibliographically approved

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