Reactive Coevaporation of SrTiO 3 Thin Films for Tunable Microwave Devices
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Reactive Coevaporation of SrTiO3 Thin Films for Tunable Microwave Devices Luke S.-J. Peng, Nina F. Heinig, and Brian H. Moeckly Conductus, Inc., 969 W. Maude Ave., Sunnyvale, CA 94085, [email protected] ABSTRACT We have successfully grown SrTiO3 films on LaAlO3 and MgO substrates up to 2” in diameter using the deposition technique of reactive coevaporation. We have explored a wide range of deposition temperatures, oxygen pocket pressures, and compositions in order to determine the optimal growth process window. X-ray diffraction analyses indicate that the process window for growth on MgO substrates is narrower than for LaAlO3 substrates. Dielectric property measurements show that samples near stoichiometry have higher tunability, but their peak loss tangents are also high (~0.02). The slightly Sr-poor samples (46-48 at.%) show a reduced loss tangent (104) and tuning (∆εr/εr > 95%), though their peak loss tangents are also high. By careful control of the growth and proper substrate selection, however, we are able to obtain films with moderate tuning (∆εr/εr ~ 30%) and low loss tangents (tanδ ~ 0.002) at 60 to 70 K. The reactive coevaporation (RCE) deposition technique has proven invaluable in facilitating growth of multiple large-area complex oxide thin films such as YBCO, thereby allowing manufacturability of these materials. We are interested in extending this technique to the growth of STO thin films. The oxygen pocket heater we use is a key part of this deposition process, since it provides for both uniform heating as well as oxidation of the material. Multiple substrates rotate on a platen inside the heater between the deposition zone and an oxidation pocket, which maintains an oxygen pressure of 10 mTorr or higher. This deposition technique was first promoted by the group at the Technical University of Munich [4] and is also used by us. [5] The RCE technique allows us to grow highly epitaxial oxide thin films on multiple substrates whose number and size is limited only by the size of the heater. In this paper, we report our efforts at optimizing the growth of STO films by examining the effect of deposition temperature, oxygen pocket pressure, and composition. We also discuss correlations between deposition conditions and dielectric properties. We report on STO growth on LaAlO3 (LAO) and MgO single crystal substrates up to 2” in diameter. Our immediate interest is reducing the STO loss while preserving its tunability, and then integrating these films with YBCO microwave filters for tuning purposes.
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EXPERIMENT General details of the in-situ RCE deposition technique have been described previously. [5] For this work, the Sr and Ti sources were evaporated by a Luxel Radak II evaporation furnace and a Thermionics Hydra-1 e-gun, respectively. Film composition and deposition rate are monitored and controlled by IST hallow-cathode-lamp (HCL) based atomic absorption (AA) and Inficon quartz crystal monitors (QCM). The oxygen pocket pressure is 10 to 35 mTorr, while the background pressure of the chamber during
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