Fig. 1. Total reflection fluorescence XAFS spectrum near Ba K-edge of (Ba,Sr)TiO 3 thin film. 44 XAFS STUDIES ON ( Ba , Sr ) TiO 3 THIN FILMS Strontium titanate (SrTiO 3 ) and a related compound, barium strontium titanate ((Ba,Sr)TiO 3 ) a r e a t t r a c t i v e c a n d i d a t e s f o r t h e d i e l e c t r i c capacitors of large-scale integrated circuits devices because of their high dielectric constants [1]. The precise dielectric properties of SrTiO 3 and (B a , S r) T i O 3 d e p e n d , h o w e v e r , u p o n t h e i r deposition method and atomic composition. The composition dependence of the dielectric properties of SrTiO 3 thin films has been reported previously [2]. Stoichiometric (Ba,Sr)TiO 3 ((Ba+Sr)/Ti=1) thin films possess superior dielectric properties compared to SrTiO 3 . Furthermore, the dielectric properties are inferior when the (Ba+Sr)/Ti ratio is moved away from unity. Characterization by X-ray diffraction revealed no clear relation between d i e l e c t r i c p r o p e r t i e s a n d c r y s t a l s t r u c t u r e . Therefore, to clarify the relationship between dielectric properties and structure, we studied local structure in (Ba,Sr)TiO 3 thin films. Three kinds of (Ba,Sr) TiO 3 thin films were prepared: stoichiometric ((Ba+Sr)/Ti = 1), Ti-rich ((Ba+Sr)/Ti < 1) and (Ba+Sr )-rich ((Ba+Sr)/Ti > 1). 3 5 n m - t h i c k ( B a , S r) T i O 3 t h i n f i l m s w e r e deposited on metal-coated Si substrates using an electron cyclotron resonance plasma chemical vapor deposition method at low temperature [3]. After deposition, a rapid thermal annealing was performed in order to crystallize the film. Ba L 3 -edge XAFS was studied in order to obtain the local structure around Ba atoms. However, it was difficult to analyze the Ba L 3 -edge XAFS spectra of (Ba,Sr)TiO 3 due to interference with Ti K -edge XAFS. Therefore, we planned to measure the Ba K -edge XAFS at SPring-8, because high- energy X-rays of sufficient flux are available for use. XAFS measurements were carried out using a Si(311) double crystal monochromator at beamline BL01B1 . Fluorescence measurements of total reflection were carried out on the Ba K -edge at room temperature using a Ge-SSD detector in order to obtain the XAFS signal of the surface layer selectively. A typical absorption spectrum near the Ba K - edge of the (Ba,Sr)TiO 3 thin film is shown in Fig.1. Figure 2 shows the Fourier transforms of EXAFS oscillation around Ba atoms. The peak positions and heights appear to be different from one other, indicating that differences in the local structure around Ba atoms correspond to variations in dielectric properties. Thus, the aggravation of the dielectric properties in (Ba,Sr)TiO 3 thin films may be due to the imperfections in local structure and/or the impurity phases. Photon Energy (keV) I / I 0 37.0 37.2 37.4 37.6 37.8 38.0 38.2 0.01 0.02 0.03 0.04 0.05 0.06 0.00 Fig. 2. Fourier transforms of EXAFS oscillation around Ba atoms of the (Ba,Sr)TiO 3 thin films. (Ba, Sr)/Ti ∼ 1 (Ba, Sr)/Ti < 1 (Ba, Sr)/Ti > 1 R (Å) 0 2 4 6 FT(R) 0 2 4 O Ti+Sr+Ba 45 Hidekazu Kimura Fundamental Research Laboratories, NEC Corporation E-mail: kimurah@sci.cl.nec.co.jp References [1] K. Koyama et al. , IEDM Technical Digest (1991) 823. [2] H. Yabuta et al. , Mat. Res. Soc. Symp. Proc. 361 (1995) 325. [3] M. Yoshida et al. , Electrochem. Soc. 142 (1995) 244.
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