Growth of epitaxial films of sodium potassium tantalate and niobate on single-crystal lanthanum aluminate [100] substrat
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Eliot D. Specht Materials Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
John Z. Larese Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996; and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Ziling B. Xue Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
David B. Beacha) Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 12 March 2008; accepted 28 July 2008)
Epitaxial films of sodium potassium tantalate (Na0.5K0.5TaO3, NKT) and sodium potassium niobate (Na0.5K0.5NbO3, NKN) were grown on single-crystal lanthanum aluminate (LAO) (100) (indexed as a pseudo-cubic unit cell) substrates via an all-alkoxide solution (methoxyethoxide complexes in 2-methoxyethanol) deposition route for the first time. X-ray diffraction studies indicated that the onset of crystallization in powders formed from hydrolyzed gel samples was 550 °C. 13 C nuclear magnetic resonance studies of solutions of methoxyethoxide complexes indicated that mixed-metal species were formed, consistent with the low crystallization temperatures observed. Thermal gravimetric analysis with simultaneous mass spectrometry showed the facile loss of the ligand (methoxyethoxide) at temperatures below 400 °C. Crystalline films were obtained at temperatures as low as 650 °C when annealed in air. -2 x-ray diffraction patterns revealed that the films possessed c-axis alignment in that only (h00) reflections were observed. Pole-figures about the NKT or NKN (220) reflection indicated a single in-plane, cube-on-cube epitaxy. The quality of the films was estimated via (out-of-plane) and (in-plane) scans and full-widths at half-maximum (FWHMs) were found to be reasonably narrow (∼1°), considering the lattice mismatch between the films and the substrate.
I. INTRODUCTION
Sodium potassium niobate (Na0.5K0.5NbO3, NKN) is a lead-free perovskite ferroelectric material with a number of technologically useful properties. Dielectric constants as high as 580 at a thickness of 650 nm have been reported in thin films,1 and a remnant polarization of 23.5 C/cm2 has been reported.2 Blomqvist et al.3 have demonstrated optical waveguides in films of NKN on single-crystal aluminum oxide. Good-quality epitaxial thin films minimize scattering losses, resulting in an increase in light transmission. [Wave-guiding efficiency a)
Address all correspondence to this author. e-mail: [email protected]; [email protected] DOI: 10.1557/JMR.2008.0393 J. Mater. Res., Vol. 23, No. 12, Dec 2008
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decreases due to the scattering losses at sites of imperfection within the wave guiding film such as grain boundaries, film top surfaces, crystalline defects, and the film–substrate interface. For efficient and compact thinfilm waveguides, it is also necessary that (i) the refractive index of the substrate to be lower than that of the electrooptic film, (ii) the film is appropriately
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