Dielectric properties of Ln(Mg 1/2 Ti 1/2 )O 3 as substrates for high-T c superconductor thin films

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Dielectric properties of Ln(Mg1/2 Ti1/2 )O3 as substrates for high-Tc superconductor thin films Seo-Yong Cho, Chang-Hun Kim, Dong-Wan Kim, and Kug Sun Honga) School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea

Jong-Hee Kim Materials Research Lab., Samsung Electro-Mechanics Co., Ltd. (Received 25 August 1998; accepted 26 January 1999)

Ln(Mg1/2 Ti1/2 )O3 (Ln ­ Dy, La, Nd, Pr, Sm, Y) compositions have been prepared, and their pertinent properties for use as thin film substrates for YBa2 Cu3 Ox (YBCO) were measured. X-ray diffraction shows that Ln(Mg1/2 Ti1/2 )O3 compositions have noncubic symmetry and the GdFeO3 -type structure. Dielectric constant measurements revealed values between 22 and 27, which are larger than those of the LnAlO3 family. Quality factor (­ 1y tan d) of the ceramic specimens measured at room temperature was larger than 3000 at 10 GHz. Among the compounds, La(Mg1/2 Ti1/2 )O3 exhibited the highest dielectric constant and the lowest dielectric loss. Chemical reaction was observed between Ln(Mg1/2 Ti1/2 )O3 (Ln ­ Dy, Sm, Y) and YBCO after annealing a 1 : 1 mixture at 950 ±C. Considering dielectric and physical properties, La(Mg1/2 Ti1/2 )O3 and Sm(Mg1/2 Ti1/2 )O3 were determined to be suitable substrates for YBCO thin film used in microwave applications.

I. INTRODUCTION

The search for substrates that can support the growth of high quality epitaxial YBa2 Cu3 Ox (YBCO) films has centered on materials having the perovskite structure.1–3 These include SrTiO3 , Sr(Al1/2 Ta1/2 )O3 , LaAlO3 , and other derivatives (Table I). These materials should have two kinds of properties: (i) Physically, the substrates should have a lattice constant very similar to YBCO and should be chemically stable in contact with YBCO at growth temperatures. (ii) The substrates should have the appropriate dielectric properties for the particular application. For the design of microwave components, substrate materials having a range of dielectric constant ser d are required. Low dielectric constants less than 10 are ideal values for integrated circuits, while materials having a high dielectric constant are attractive for the reduction of circuit size. In addition, substrate materials for HTS should exhibit low dielectric loss since the conduction loss of a superconductor is negligible. From this viewpoint SrTiO3 has the problem of high dielectric loss; LaAlO3 is currently used for most microwave applications.2 Materials for dielectric resonator applications should have low dielectric loss, high dielectric constant, and temperature stability. If these requirements are satisfied, dielectrics for resonator applications can be used as YBCO thin film substrates, assuming they exhibit small lattice mismatch and chemical compatibility. In other words, substrate materials for HTS films can a)

Address all correspondence to this author. e-mail: [email protected]

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http://journals.cambridge.org

J. Mater. Res., Vol. 14, No. 6, Jun 1999

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