Electrodynamic Properties of Single-Crystal and Thin-Film Strontium Titanate, and Thin-Film Barium Strontium Titanate
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ABSTRACT We have used a coplanar waveguide structure to study broadband electrodynamic properties of single-crystal and thin-film strontium titanate (STO), and thin-film barium strontium titanate (BSTO). We have implemented low-frequency capacitance (100 Hz - 1 MHz), swept-frequency transmittance (45 MHz - 4 GHz), and time-domain transmittance (dc - several GHz) measurements to determine effective refractive index (or, dielectric constant), and dissipation factor (or, loss tangent) as a function of dc bias (up to 4x10 6 V/m) and temperature (20 - 300 K). The STO samples used superconducting electrodes and were designed to operate at cryogenic temperatures, whereas BSTO samples used normal conducting electrodes and exhibited optimal performance around room temperature. By using nearly identical electrode geometries for all devices, we were able to conduct a direct comparative study among them, and investigate not only single-crystal vs thin-film, but also cryogenic vs room-temperature applications. INTRODUCTION Strontium titanate, SrTiO 3 (STO), and barium strontium titanate, Ba•Srl_•TiO 3 (BSTO), with x close to 0.5, are some of the most widely studied materials in condensed matter physics [1,2]. STO and SBTO are also important from a technological point of view: their large dielectric constant and large dielectric breakdown field make them a potential candidate for storage capacitor cells in next generation dynamic random access memories [3], and the large dielectric nonlinearity they exhibit (at cryogenic temperatures for STO, and around room temperature for BSTO) is a desirable property for various applications such as tunable filters and phased array antennas [4,5].
Recently, there has been an increased interest in the microwave applications of nonlinear dielectrics such as STO and BSTO [6]. To better assess their high-frequency application potential, we have conducted a comprehensive study of their broadband (dc - 1010 Hz) electrodynamic characteristics using prototype coplanar waveguide (CPW) devices. In addition, by implementing nearly identical electrode structure for each device, we have been able to do a direct comparative study between single-crystal and thin-film STO [7,8], and also between cryogenic (using STO with superconducting electrodes) and room-temperature (using BSTO with normal conducting electrodes) devices. EXPERIMENT Coplanar Waveguide Device Structure In this study, we have used prototype devices with 8-cm-long meandering CPW electrodes on 1 cm x 1cm chips. The CPW electrodes have approximately 20-jim-wide centerlines, and 10to 20-jim-wide gaps. A schematic top view and generic cross-section with signal and bias configuration of the devices is shown in Fig. 1. Cryogenic devices use 0.4-jim-thick superconducting YBa 2Cu 3OT.5 (YBCO) electrodes with 0.7-jim-thick STO thin films on 0.5-mmthick (100) LaAlO 3 (LAO) substrates, or 1-mm-thick single-crystal (100) STO substrates, whereas room-temperature devices use 0.5-jm-thick Au electrodes with 0.7-jim-thick BSTO thin-films on 1-mm-thick (100)
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