Microstructure of Epitaxial SrTiO 3 /Pt/Ti/ Sapphire Heterostructures
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The microstructure and orientation relationships of epitaxial (111)-oriented SrTiO3 thin films grown by radio frequency magnetron sputtering on epitaxial (111)-oriented Pt/Ti electrodes on sapphire were investigated using x-ray diffraction, conventional and scanning transmission electron microscopy. We show that the epitaxial growth of (111)-oriented SrTiO3 films was promoted by thin Ti adhesion layers underneath the Pt electrode. The SrTiO3 films nucleated with two twin-related orientation variants, rotated by 180° about the 〈111〉 surface normal. The twin boundaries were oriented approximately normal to the film plane, but no strong preference for a specific boundary plane was observed. Growth mechanisms and the relationships to the dielectric properties are discussed.
SrTiO3 is a prototype, incipient ferroelectric with the cubic perovskite structure above 105 K. Thin films of SrTiO3 are investigated for their high dielectric constant, which is attractive for applications in integrated capacitors.1 At low temperatures, SrTiO3 shows a nonlinear, electric field tunable dielectric constant, which is of interest for tunable microwave devices operating at cryogenic temperatures.2,3 Most parallel plate capacitor structures use elemental metal electrodes such as Pt, because of their chemical stability and the high electrical conductivity needed for electrodes for microwave applications.4 The lattice mismatch between Pt and SrTiO3 is small (0.4% at room temperature). Many different film orientations have been reported for SrTiO3 and other ferroelectric films on Pt electrodes.5–7 Epitaxial (001)- and (110)-oriented SrTiO3 thin films on metal electrodes have been obtained on epitaxial Pt/MgO substrates.8 Film orientation and extended defect density determine the film dielectric properties. For example, we have recently predicted that (111) oriented SrTiO3 films that are under a biaxial tensile strain due to the thermal mismatch with the substrate have higher dielectric constants and tunability than films with other orientations.9 In this paper, we show that epitaxial, (111)-oriented SrTiO3 films can be grown on epitaxial (111) Pt electrodes with Ti a)
Present address: Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0282
adhesion layers. We also characterize extended defects in these films. Pt electrodes with a film thickness of about 200 nm were deposited by electron-beam evaporation with no intentional heating of the substrate on basal plane sapphire substrates that were covered with ∼3 nm Ti adhesion layers. The Ti adhesion layers were also deposited at room temperature in the same system. SrTiO3 films were grown by radio-frequency magnetron sputtering from a stoichiometric SrTiO3 target. The base pressure of the sputter chamber was less than 5 × 10−9 Torr. During sputtering, substrates were heated to ∼700 °C. The sputter power was 150 W, the total
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