Oxygen diffusion through dielectrics: A critical parameter in high critical temperature superconductors multilayer techn
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We have studied the relative diffusion rates of oxygen through dielectric/buffer layers used in high critical temperature superconducting multilayer structures. Epitaxial bilayer films of dielectric (CeO2, LaGaO 3 , NdGaO 3 , LaAlO 3 , MgO, SrTiO3, LaLiTi2O6, or LaNaTi2O6) on YBa 2 Cu 3 0 7 -,5 (YBCO) have been deposited onto (001) oriented single-crystal MgO substrates using pulsed laser deposition. These bilayers have been investigated for oxygen diffusion over the temperature range 350 to 650 °C by postdeposition annealing the films for 20 min in 0.5 atm of I8 O enriched molecular oxygen gas. Secondary ion mass spectroscopy was used to depth profile the relative concentration of 18O to 16O in each bilayer. Compared to YBCO, the dielectrics MgO, SrTiO3, LaLiTi2O6, and LaNaTi 2 O 6 are relatively slow diffusers, while CeO 2 , LaGaO 3 , NdGaO 3 , and LaA10 3 are relatively fast diffusers.
I. INTRODUCTION Thin films of high critical temperature superconductors (HTSC) have been utilized for a variety of devices1"7 since the discovery of this class of materials in 1986 by Bednorz and Miiller.8 Because of the chemical composition, crystal structure, and anisotropic superconducting properties of HTSC's, 12 stringent requirements are placed on the substrate used for thin film growth. To achieve the best superconducting properties, the substrate must be chemically compatible and have reasonably good matches in thermal expansion coefficient and lattice parameter with the HTSC. 9 For microwave device applications, the substrate also should be low loss, isotropic with respect to microwave radiation, and have a low dielectric constant. For most device applications, the HTSC is deposited with the c-axis normal to the substrate surface since these fully oxygenated films possess the highest critical transition temperature (Tc), smallest critical transition temperature width (ATC) and largest critical current density (Jc).w For HTSC multilayer structures, the substrate and dielectric or buffer layer must satisfy other stringent requirements. Since superconducting properties are sensitive to oxygen stoichiometry,11 the diffusion rate of oxygen through the substrate and dielectric or buffer a) National
Research Council-EPSD Research Associate; presently, an Electronics Engineer at the Army Research Laboratory. b) Under contract with Geo-Centers, Inc. c) Currently at Motorola MOS 12, 1300 North Alma School Road, Chandler, Arizona 85224.
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J. Mater. Res., Vol. 10, No. 7, Jul 1995 Downloaded: 14 Mar 2015
layer becomes a major issue. For example, growth of a YBa 2 Cu 3 0 7 -5 (YBCO) multilayer device using the pulsed laser deposition (PLD) technique typically requires depositing YBCO onto an appropriate substrate or buffer layer at temperatures of around 750 °C in an oxygen atmosphere of about 200 mTorr.7 The film must subsequently be cooled in an oxygen atmosphere (typically 0.5 atm) so that it gains enough oxygen to change phase from the nonsuperconducting tetragonal structure to the superconduct
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