Thin film deposition and interface characterization of YBCO on LiNbO 3 substrates
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H.D. Shih Texas Instruments Inc., Corporate Research and Development, P.O. Box 655936, MS 150, Dallas, Texas 75265
(Received 22 May 1995; accepted 22 August 1995)
High transition temperature superconducting YBa2Cu30i, (YBCO) thin films have been epitaxially grown on YZ-cut LiNb03 (LNO) substrates by the pulsed laser deposition technique. The interface between YBCO and LNO has been systematically investigated by scanning electron microscopy, atomic force microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. Off-stoichiometry LiNbO, phases are found to segregate on the substrate surface because of lithium and oxygen vacancies formed during the high temperature YBCO growth. These submicrometer particles are observed along the Z-axis on the X-2plane of LNO with height of -30 nm above the LNO surface. This rough growth surface results in YBa2Cu30i, thin films grown on the LNO surface that have reduced J , and T, , possibly limiting the use of YBCO/LNO heterostructures for surface acoustic wave (SAW) devices. I. INTRODUCTION
Fabrication of high transition temperature superconducting (HTS) films on piezoelectric LiNb03 is of special interest because devices, such as surface acoustic wave devices (SAW), acousto-optic diffractors, and traveling wave optical modulators, employing normal metal electrodes suffer from large attenuation,' whereas HTS superconducting electrodes promise increased performance. Both 50 MHz and 100 MHz interdigital transducers (IDT's) have been fabricated from HTS thin films of YBa2Cu?O7, (YBCO) deposited on YZ-LiNbOs (LNO), and have dramatically improved the efficiency of delay lines by over -12 dB at the superconducting transition temperature T, of YBC0.2 The IDT's in an ordinary surface acoustic wave (SAW) filter are thin films of normal metal and, therefore, exhibit finite conductivity. The YBCO superconducting interdigital transducer electrode, however, not only increases signal level by eliminating any phase lag that sheet resistance produces between input voltage and response current, but it also decreases the background noise level. As a result, there is a dramatic increase in signal-to-noise ratio. This enhancement has now created an opportunity for future studies in superconducting SAW devices. However, few successful depositions of epitaxial HTS films on LNO have been r e p ~ r t e d . Specifically, ~.~ the surface morphology of YBCO films grown on YZLiNb03 was rough and n o n ~ n i f o r m in ~ , ~comparison to YBCO thin films deposited on SrTi03, LaA103, and MgO substrate^.^-^ There is a report on making good J. Mater. Res., Vol. 10, No. 12, Dec 1995 http://journals.cambridge.org
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epitaxial YBCO on LNO with a Y203-stabilized Zr02 (YSZ) buffer layer"; however, a thick buffer (-100 nm) may not support some acoustic device concepts because of added attenuation. Our previous study of the superconducting SAW devices2 with analysis based on the crossed-field model indicates that a residual insertion loss in IDT's could still remain after all
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