Microstructure and microwave properties of YBCO thin films grown on MgO and SrTiO 3 by CVD
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T. Yamashita Department of Electronics, Nagaoka University of Technology, Nagaoka, Niigata 940-21, Japan
T. Hirai Institute for Materials Research, Tohoku University, Sendai, Miyagi 980, Japan
H. Suzuki and H. Kurosawa Technological Research Center, Riken Corporation, Kumagaya 810, Saitama 360, Japan
L. Drabeck and G. Griiner Department of Physics, University of California-Los Angeles, Los Angeles, California 90024 (Received 4 April 1991; accepted 24 June 1991)
The microstructure and microwave properties of YBa2Cu307_,5 thin films grown on SrTiO3 (100) and MgO (100) substrates by chemical vapor deposition have been studied. Both 100 GHz cavity measurement of surface resistance and ground plane substitution in a 5 GHz microstrip resonator show that films on SrTiO3 have better microwave properties than those on MgO. Although there are some a-axis grains and secondary phases on the surface, a large fraction of each film on SrTiO3 is epitaxial with its c axis normal to the substrate. The 100 GHz surface resistance of these films is less than copper for temperature ^ 82 K, and approaches the detection limit at 10-20 K. For the films on MgO, c-axis grains of different in-plane rotation are found together with some a-axis needle-like grains. The grain boundaries are detrimental to the microwave properties, and the resulting surface resistance at 100 GHz and 10-20 K is about 20 mil higher than that of films on SrTiO3. I. INTRODUCTION
II. EXPERIMENTAL
Several methods have been used to produce high Tc superconductor (HTSC) thin films with good microwave performance. Examples include laser deposition plus thermal anneal,1"3 coevaporation plus thermal anneal,4 in situ pulsed laser ablation,5'6 and in situ offaxis magnetron sputtering.78 Chemical vapor deposition (CVD) of HTSC materials has some advantages over the growth methods mentioned above. The deposition area, film uniformity, and growth rate can be much improved by using CVD to grow HTSC thin films. By changing the gas composition within a short transient time, multilayer structures by CVD are possible. For HTSC compounds that include a volatile element such as the T l - B a - C a - C u - O system, CVD may have some attractions over other methods.
The details of film synthesis were described before.10'16-17 The CVD apparatus used was a vertical hot-wall type. The source materials were the volatile /3diketonates Y(thd) 3 , Ba(thd)2, and Cu(thd)2 (thd:2,2,6,6tetramethyl-3,5-heptanedionate). These materials were evaporated at 132, 250, and 130 °C, respectively. The polished substrates for this study are 10 x 10 x 0.5 mm SrTiO3 (100) and MgO (100). These substrates were degreased in solvent baths before film deposition. Substrate temperature was held between 850 and 900 °C during film deposition. Total pressure was maintained at 10 Torr, with 150 ml/min flow of Ar over each source compound, and 250 ml/min flow of O 2 . The duration of deposition was 30—60 min and film thickness is about 1 /im. After deposition the films were cooled to room temperature at a rate of 15
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