Synthesis and characterization of LiTaO 3 thin films deposited on Si by the sol-gel method
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R. E. Avila and G. J. Piderit Comision Chilena de Energia Nuclear, Cas. 188-D, Santiago, Chile (Received 17 February 1994; accepted 24 July 1995)
Polycrystalline LiTaO3 (LT) thin films have been prepared on (OOl)Si substrates by the sol-gel method. A Li-Ta double alkoxide prepared from lithium methoxide and tantalum ethoxide precursors was spin coated on Si and heated up to 950 °C. The dependence of the film quality upon the process variables, alkoxide concentration before hydrolysis, the water to double alkoxide ratio, and the final double alkoxide concentration, has been established. Preferential alignment of the (104) LT planes was observed parallel to the (100)Si surface. Most films present resistivities of the order of 30 kCl • cm and breakdown field in excess of 200 kV/cm.
I. INTRODUCTION The excellent optoelectronic, ferroelectric, and piezoelectric properties of mixed oxides of the form LiMO 3 (M = Nb or Ta) have motivated much research activity on these materials in recent years. Sol-gel methods, based on the controlled hydrolysis of metal alkoxides, are an attractive route to obtain these compounds in thin film form. In these methods a polymeric gel is obtained as a result of crosslinking of polymeric products generated by polycondensation of partially hydrolyzed alkoxides. This gel is further converted to the corresponding ceramic film by heating below 1000 °C, making the technique compatible with standard integrated circuit processing. The solution may be applied over large, flat areas by spinning, or over uneven surfaces by dipping. However, the drying and shrinkage stresses occurring during the gel to dense oxide conversion frequently result in porous or cracked films. The problem of film cracking may be ameliorated by the careful control of the hydrolysis conditions, which determine the gel structure, and the design of the drying and sintering process. Special care is necessary because of the extreme reactivity to moisture of most of the metal alkoxides, which can induce differential alkoxide hydrolysis or the appearance of undesirable precipitates during the synthesis reactions. The use of molecularly modified alkoxide precursors should be a useful approach to overcome these difficulties. Several modifications of metal alkoxides carried out through partial substitution of alkoxy groups have been reported.1-4 On the other hand, the use of specifically designed modified precursors to prepare ceramic gels, powders, or thin films has been pioneered by Phule et a/.5"8 Thus, LiTaO3 as ultrafine powder,9 thin films,5 or whiskers8 has J. Mater. Res., Vol. 10, No. 11, Nov 1995 http://journals.cambridge.org
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been produced, using precursors such as lithium acetate and a molecularly modified tantalum ethoxide with both ethoxy and acetate groups.6 On the other hand, to obtain smooth and crack-free films, the addition of drying control chemical agents10 (DCCA), among other experimental variations, have been reported. Furthermore, the technique employed for the deposition of the solution
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