Chemical stability of CVD source materials for high-T c superconducting films
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Masaaki Nakabayashi and Tadashi Shiraishi Department of Communications, Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-12, Japan
Youichi Suemune Nihon Kagaku Sangyo Co. Ltd., 80 Nakane-cho, Souka 340, Japan
Takakazu Yamamoto Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Midori-ku, Nagatsuta, Yokohama 227, Japan (Received 30 November 1991; accepted 28 December 1991)
The stability of chemical vapor deposition (CVD) source materials for high-r c superconducting films was examined by 'H-NMR (nuclear magnetic resonance), 13 C-NMR, IR (infrared) absorption, and TG-DTA (thermogravimetry and differential thermal analysis) measurements. Highly purified Ca(DPM)2 (DPM: dipyvaloylmethane: (CH3)3CCOCHCOC(CH3)3) and Sr(DPM)2, utilizing recrystallization and sublimation, easily decomposed at room temperature and changed their thermal properties when they were handled and stored in air or even in an Ar atmosphere without removing adsorbed species such as water. The stability increased by removing a trace of adsorbed species and storing in an Ar atmosphere at - 3 0 °C. Bi(C 6 H 5 ) 3 and Y(DPM) 3 showed higher chemical stability than Ca(DPM)2 and Sr(DPM) 2 .
I. INTRODUCTION High-r c superconducting film preparation has been widely studied not only for practical application but for exploration of fundamental physical and electrical properties. So far, physical methods such as sputtering1 and molecular beam epitaxy (MBE)2 have mainly been employed as superconducting film preparation techniques. Chemical vapor deposition (CVD), which could be another suitable method for film deposition, has also been studied as a process for the preparation of superconducting films. At present, CVD preparation of high-J c superconducting films has been reported using thermal decomposition,3 plasma excitation,4 and photochemical excitation5 of organometallic compounds and metal (3diketonate complexes. Several problems remain, however, one of which is the inadequate purity and chemical stability of commercially available source materials. In our earlier paper,6 we reported that sublimation and recrystallization were effective for purification of metal organic sources for CVD of high-J c superconducting films and that NMR spectroscopy was effective for checking source purity. We also reported that volatility of the sources drastically increased by purification. However, chemical stability of the sources was not sufficient; sources degraded gradually even at room temperature.7 Development of a storing method to prevent source 1336 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 6, Jun 1992
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decomposition was required, but not yet attained. In this paper, chemical stability of the highly purified sources and cause of degradation are examined using ^ - N M R , 13 C-NMR, IR, and TG-DTA measurements. A method to prevent source degradation is also presented. II. EXPERIMENTAL Source materials examined were Ca(DPM)2 (DPM = (CH 3 ) 3 CCOCHCOC(CH 3 ) 3 ), Sr(DPM)2
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