Processing Effects on the Microstructure of Sol-Gel Derived SBN Thin Films
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application of a dc electric field. The objective of this study is to more closely examine the microstructures of sol-gel derived SBN thin films and to determine how they are influenced by changes in processing variables. The electrical and optical properties of the material are highly dependent upon its crystalline orientation and structure; therefore SBN thin films of controlled microstructure are necessary for reliable device applications. Our approach in this study is to vary the chemistry of the SBN precursor as well as the processing conditions of the thin film and to monitor their effects on the films' microstructure by transmission electron microscopy (TEM). Effects of precursors, hydrolysis, pH, substrate and heating rate on the microstructure were examined. SOL-GEL THIN FILM DEPOSITION AND CHARACTERIZATION Thin films of Sr 0 .5 Ba 0 . 5Nb 2O 6 (50/50 SBN) were derived from alkoxide precursors using the sol-gel technique similar to that shown in reference [1]. Acetates, methoxyethoxides and chelated alkoxides of strontium and barium were used as precursors to avoid the insolubility and high hydrolysis rates commonly seen in the alkoxides of Group IIA metals [3]. The barium and strontium alkoxide precursors were derived from the reaction of barium and strontium metals with dried alcohol (ethanol or butanol) stabilized with 2 moles of 2-methoxyethanol per mole of metal or directly with 2-methoxyethanol. The mixed Ba,Sr alkoxide was refluxed for 4-6 hours to ensure good mixing. A stoichiometric amount of niobium ethoxide was added to the mixed alkoxide and the resulting solution was refluxed overnight (- 18 hrs.) to form double alkoxides. The solution was then hydrolyzed and diluted to produce 0.2M SBN sols. 297 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
The filtered 0.2M sols were then deposited onto annealed and degreased single crystal (100) MgO, (110) sapphire or Pt coated Si (100) substrates. The films were spin coated at 3000 rpm for 30 seconds and subsequently pyrolyzed at 3250 C for 5 minutes after each deposition to remove residual organics. Three sequential layers were deposited producing films of approximately 6000A in thickness. To prevent substantial film cracking due to drying stresses in the films, the first film layer was crystallized at 750'C before the deposition of additional layers. The microstructure of the SBN thin films were examined by transmission electron microscopy (TEM). Examinations were done at 200keV on a Philips CM20 TEM. Films on MgO substrates were prepared by etching in a phosphoric acid solution followed by ion milling. Films on sapphire were prepared by dimpling and ion milling. All TEM samples were carbon coated to get a conductive surface. The crystalline structure of the SBN films were analyzed by x-ray diffraction analysis and the thermochemical behavior of dried sol powders were measured by a DuPont thermal analysis system. RESULTS
Effect of precursors Four different barium and strontium precursors were investigated: acetates, ethoxides, m
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