SrTiO 3 films on platinized (0001) Al 2 O 3 : Characterization of texture and nonstoichiometry accommodation
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Transmission electron microscopy and x-ray diffraction were used to study SrTiO3 films grown on platinized (0001) Al2O3 substrates. The Pt films were epitaxial with an orientation relationship described by (111)Pt㛳(0001)Al2O3 and [110]Pt㛳[101¯0]Al2O3. SrTiO3 films with two different Sr to Ti ratios, 1.02 and 1.4, were deposited by radio-frequency magnetron sputtering. In the film with a large amount of Sr excess, the grain sizes were smaller and a high density of planar defects was observed. The films were predominantly (111) textured, but a weaker (110) texture component was also found, independent of stoichiometry. While the (111) texture could be explained with the excellent lattice match with (111) Pt, the (110) textured grains had a large mismatch with the Pt electrode. We propose that the presence of the (110) oriented grains is due to nucleation at Pt surface defects. Planar defects in the films with a large amount of Sr excess served to accommodate the nonstoichiometry. Comparison with homoepitaxial SrTiO3 films showed that the density of planar defects in the SrTiO3 films on (111)Pt/Al2O3 is insufficient to accommodate all the excess Sr. The influence of the film microstructure on the dielectric properties is also discussed.
I. INTRODUCTION
Film nonstoichiometry, interfaces with the electrode, and film stress due to lattice and thermal mismatch with the substrate are known to strongly affect the dielectric permittivity and dielectric losses (tan ␦) of perovskite ferroelectric thin films.1–4 Several recent studies have investigated the accommodation of nonstoichiometry and defects in epitaxial perovskite thin films,5–8 and phenomenological models have established the relationship between stress and permittivity in epitaxial films.1,2 However, most practical planar capacitor structures use metal electrodes such as Pt. Perovskite films on metal electrodes typically are polycrystalline or textured. Their microstructures and the relationship to the dielectric properties are less well understood than those of epitaxial structures. SrTiO3 is a prototype, incipient ferroelectric with the perovskite structure. Thin films of SrTiO3 are investigated for their high-dielectric constant, which is attractive for applications in integrated capacitors. At low temperatures, SrTiO3 shows a nonlinear, electric field tunable dielectric constant,9–11 which is of interest for tunable microwave devices operating at cryogenic temperatures. Pt is often used as an electrode in ferroelectric a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Agile Materials Inc., Goleta, CA 93117 DOI: 10.1557/JMR.2004.0197 J. Mater. Res., Vol. 19, No. 5, May 2004
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devices and integrated capacitors, due to its excellent chemical stability and good conductivity. Pt films on SiO2/Si substrates are often highly (111) textured.12,13 Despite the small lattice mismatch (0.4% for SrTiO3 at room temperature) that should promote epitaxial growth, many diffe
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