Growth and Properties of Epitaxial YBa 2 Cu 3 O 7 thin Films on{1102}Al 2 O 3
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GROWTH AND PROPERTIES OF EPITAXIAL YBa 2Cu 30 7 THIN FILMS ON {1102 1A12 0 3 D. K. Fork*, T. H. Geballe*, K. Char"*, S. S. Ladernan*** and R. Taber***, F. Bridges****, F. Ponce*****, J. B. Boyce*****, and G. A. N. Connell***** * Dept. of Applied Physics, Stanford University, Stanford, CA 94305 ** Conductus, Inc., Sunnyvale, CA 94086 "**Circuit Technology R&D, Hewlett-Packard Company, Palo Alto, CA 94304 **** Dept. of Physics, University of California, Santa Cruz, CA ***** Xerox Palo Alto Research Center, Palo Alto, CA 94304
ABSTRACT Epitaxial YBa 2Cu 30 7 (YBCO) films were grown on r-plane A120 3 11102) by laser ablation. X-ray diffraction shows that films are epitaxial with the c-axis perpendicular to the substrate and the a or b axes parallel to (2_201), although the full width at half maximum of the rocking curve is larger compared to those of epitaxial films on SrTiO 3 . The critical temperatures (zero resistance) are between 85 K and 88 K with transition widths between 0.5 K and 3 K. The 300 K resistivity of 250 pi.Qcm extrapolates to zero at zero temperature and the critical current is as high as 5 x 106 A/cm 2 at 4.2 K according to magnetization hysteresis measurements. Surface resistance data shows that 2000 A thick epitaxial films on 11102) have about 1 mK at 13 GHz at 4.2 K. INTRODUCTION The physical properties of sapphire make it a highly attractive material for thin films of the high temperature superconductors, particularly yttrium barium copper oxide (YBCO). Its low loss tangent makes sapphire an ideal choice as a dielectric for microwave applications such as transmission lines, delay lines and high-Q resonators. Sapphire has high internal optical transmittance from 150nm (vacuum ultraviolet) to 6000 nm (mid IR). Its high mechanical strength makes it possible to produce wafers that are less than 25 gtm thick. For application to infrared bolometers, this property is essential in reducing the heat capacity, since the relaxation time 'r = C/G[l]. The high Debye temperature, hence high thermal conductivity G, of A1203 is also clearly advantageous. Since it may be produced as large wafers, A12 0 3 is comparatively less expensive than the perovskite materials which are proven to promote good epitaxial growth of the high temperature superconductors. Efforts to grow high quality epitaxial YBCO films on A1203 with properties comparable to films grown on SrTiO 3 , MgO, yttria-stabilized-zirconia (YSZ) and LaAlO 3 have been rendered difficult due to the reaction of YBCO with Al. Attempts to overcome this problem using buffer layers have been reported[2]. Substrate reaction is a more severe problem on silicon, where buffer layers may be unavoidable[3]. In this paper we report the successful growth of epitaxial YBCO films directly on r-plane sapphire 111021 along with structural and transport properties. Mat. Res. Soc. Symp. Proc. Vol. 169. 01990 Materials Research Society
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DEPOSITION TECHNIQUE Reference 4 details the laser ablation technique. Briefly, pellets of the bulk YBCO material are ablated by focuss
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