J O U R N A L O F C H E M I S T R Y Materials Communication Preparation of SrBi2Ta2O9 thin films with a single alkoxide sol–gel precursor Yongtae Kim,a Hee K. Chae,*a Kyu S. Leeb and Wan I. Lee*b aDepartment of Chemistry, Hankuk University of Foreign Studies, Yongin 449-791, Korea bDepartment of Chemistry and Center for Chemical Dynamics, Inha University, Inchon 402-751, Korea Received 10th August 1998, Accepted 15th September 1998 For the first time, a single alkoxide sol–gel precursor The sol–gel precursor solution was prepared as follows. solution for the ferroelectric strontium bismuth tantalate Bismuth 2-methoxyethoxide, Bi(OCH2CH2OCH3)3, was (SrBi2Ta2O9, SBT) was synthesized and utilized for the obtained by modifying the literature method from the reaction fabrication of its thin films.The precursor was prepared between BiCl3 and Na(OCH2CH2OCH3) in tetrahydrofuran. from a 2-methoxyethanol solution of Sr(OCH2CH2- The bismuth complex, Bi(OCH2CH2OCH3)3, was recrys- OCH3)2, Bi(OCH2CH2OCH3)3, and Ta(OCH2CH2OCH3)5. tallized in benzene–hexane. Yield was 78% based on Bi. The 1H and 13C NMR spectra of the precursor in benzene show 1H and 13C NMR spectra of the complex in benzene-d6 show only one set of alkoxy groups, indicating the same chemical only one set of peaks for 2-methoxyethoxide.10 Tantalum 2- environment in solution.This observation suggests that it methoxyethoxide, Ta(OCH2CH2OCH3)5, was synthesized is a single sol–gel precursor, which is ideal for the sol–gel quantitatively by the reaction of alcohol exchange from processing of SBT thin films.The SBT films derived from Ta(OCH2CH3)5 in HOCH2CH2OCH3, and characterized by this precursor present outstanding ferroelectric properties NMR spectroscopy.11 Yellow powders of strontium 2-methoxy- and surface morphology. ethoxide, Sr(OCH2CH2OCH3)2, were also obtained quantitatively from the direct reaction of Sr chips with The sol–gel process is a versatile method for producing cer- HOCH2CH2OCH3.12 We observed that a homogeneous 2- amics and glasses with a variety of applications which include methoxyethanol solution containing Sr(OCH2CH2OCH3)2, electronic, magnetic, optic and optoelectronic materials, as Bi(OCH2CH2OCH3)3, and Ta(OCH2CH2OCH3)5 in a 15252 well as hard and protective coatings.1,2 In recent years this ratio could be prepared by refluxing the mixtures for 2 hours.technique has been extended to the fabrication of thin films The solvent was removed in vacuo to give a colorless syrup- of SrBi2Ta2O9 (SBT), which has been attracting profound like complex. The complex [SrBi2Ta2(OCH2CH2OCH3)18] was interest as a fatigue-free ferroelectric material.3 A variety of characterized by 1H and 13C NMR spectroscopy which shows fabrication methods, such as magnetron sputtering,4 metalthree peaks at 4.65, 3.52 and 3.38 ppm and 76.00, 68.54 and organic chemical vapor deposition (MOCVD),5 pulsed laser 58.84 ppm, respectively, indicating that only one set of 2- deposition (PLD),6 sol–gel process,7 etc., have been applied methoxyethoxide exists in solution (see Fig. 1).13 The peaks for the Bi-based ferroelectric thin films including SBT.Among are quite diVerent from those of the starting materials. This is them, the sol–gel process is considered to be the most successful evidence for the formation of a single alkoxide precursor. The method in terms of composition control. However, the control of stoichiometry in the film has still been a knotty problem because of the relatively high volatility of bismuth components.Conventionally, mixed metal esters including 2-ethylhexanoate have been used for this sol–gel process,3,7 but they are not regarded as suitable precursors to obtain high quality SBT films. Recently, some papers have reported sol–gel processed SBT thin films derived from mixed alkoxide solutions prepared by mixing the individual metal alkoxides, but none of them have prepared any single alkoxide precursors.8,9 In the fabrication of multicomponent metal oxide films, precise control of stoichiometry, crystallographic phase and grain structure in the film is crucial and these properties could be tuned from the molecular level homogeneity, which is likely achieved from a single metal-organic precursor that evolves directly to a mixed-metal oxide, reproducibly. Prior to this study, however, little information was available regarding the preparation and characterization of single precursors for the Sr(OR)2–Bi(OR)3–Ta(OR)5 system.Here, we demonstrate a synthesized single alkoxide sol–gel precursor for the fabrication of SBT thin films. It has been found that only the stoichiometric amount of the Bi precursor, diVerently from other sol–gel solution systems, is necessary for the formation of a ferroelectric SBT phase.SBT thin films with this precursor present outstanding ferroelectric properties and the surface morphology of the film is considerably improved compared with that of films from conventional 2- Fig. 1 NMR spectra of complex isolated from SBT precursor solution in benzene-d6.(a) 1H NMR; (b) 13C NMR. ethylhexanoate sol–gel solution. J. Mater. Chem., 1998, 8(11), 2317–2319 2317complex contains strontium, bismuth and tantalum in a 15252 Fig. 4 shows the hysteresis loop of the Pt/SBT/Pt capacitor. Excellent ferroelectric properties were obtained for a 250 nm ratio as determined by elemental analysis and the complex does not undergo dissociation in alcohol solution.Mass spec- thick SBT film. Remanent polarization (Pr) is 9 mC cm-2 and the coercive field (Ec) is 33 kVcm-1 (or 0.8 V). In addition, trometry data obtained so far indicate that the complex does not exist in the monomeric form, but as a dimer. Hence, the its leakage current density (10-6–10-7 A cm-2) is very low. It has been reported that a pyrochlore phase can be incorpor- synthesized solution could be an ideal sol–gel precursor for the fabrication of SBT thin films.ated under Bi-deficient conditions and 20–30% more of the Bi component is necessary for the sol–gel solution in order to The precursor solutions used for the SBT films contain the stoichiometric amount of Bi (that is, the molar ratio of compensate for the loss of Bi during the heat-treatment.Moreover, several reports indicate that Bi-rich conditions Sr5Bi5Ta is 1.052.052.0) and the concentration was adjusted to 0.10 M. The solution is stable for more than one month and induce better crystallinity in the SBT film and a higher value of remanent polarization as a result.14,15 In this work, however, the aging eVect is negligible.Spin-coated films at 2500 rpm were baked at 120 °C and subsequently at 320 °C to remove we have found a diVerent result, that is, extra Bi is not necessary for the formation of a pure ferroelectric phase, organic solvents. The spin coating and baking cycles were repeated three times to obtain a film of final thickness about which is directly ascribed to the intrinsic role of the single 2- methoxyethoxide precursor.It is deduced that Bi loss is 250 nm. The baked samples were then heat-treated at 800 °C for 1 h in oxygen atmosphere to produce a ferroelectric phase. minimized during the heat-treatment, since the three metals are chemically bonded together. The thermal decomposition The substrates used for the SBT deposition were Pt/Ti/SiO2/Si. On the Si(100) substrate with 300 nm of SiO2 deposited, a behaviour of the prepared single alkoxide sol–gel precursor was analyzed by TGA and DSC.The precursor was dried at 20 nm layer of Ti and a 240 nm layer of Pt were sputterdeposited, respectively. 150 °C and then slowly heated in air with a ramp of 5 °Cmin-1. The weight change and heat exchange as a function of tempera- The glancing angle mode XRD patterns in Fig. 2 indicate that the SBT films consist of a pure ferroelectric phase. The ture were monitored, and are shown in Fig. 5. The dried single field emission SEM image of a fabricated SBT thin film shown in Fig. 3 indicates that the surfaces of prepared thin films are considerably homogeneous, grains are very dense and there seemed to be no secondary structures between the grains.Fig. 4 Hysteresis loop obtained for SBT thin films derived from single alkoxide solution. Fig. 2 XRD patterns of the SBT thin films (glancing angle: 3°). Fig. 5 TGA and DSC curves for the dried single alkoxide sol–gel Fig. 3 Field emission SEM image of SBT thin films (film thickness: ca. 250 nm). precursor. 2318 J. Mater. Chem., 1998, 8(11), 2317–23197 K.Amanuma, T. Hase and Y. Miyasaka, Appl. Phys. Lett., 1995, alkoxide precursor was decomposed slowly and monotonously 66, 221. with no abrupt heat release. This is another important factor 8 I. Koiwa, T. Kanehara, J. Mita, T. Iwabuchi, T. Osaka, S. Ono in retarding the loss of Bi component and in improving grain and M. Maeda, Jpn. J. Appl. Phys., 1996, 35, 4946. structure. 9 T.Hayashi, T. Hara and H. Takahashi, Jpn. J. Appl. Phys., 1997, 36, 5900. 10 IR (Nujol mull, cm-1): 1235 w, 1197 w, 1126 w, 1061 w, 1019 w, 964 w, 894 w, 834 w, 559 w. 1H NMR (C6D6): d 4.98 (t, 6 H, CH2, The financial support for this work from Korean Science and JH–H=4.5 Hz), 3.53 (t, 6 H, CH2, JH–H=4.6 Hz), 3.25 (s, 9 H, Engineering Foundation (KOSEF 97-05-01-03-01-3) is grate- CH3). 13C NMR (C6D6): d 77.75 (s, CH2), 62.45 (s, CH2), 58.17 fully acknowledged. (s, CH3). 11 1H NMR (C6D6): d 4.78 (br, 10 H, CH2), 3.61 (br, 10 H, CH2), 3.27 (s, 15 H, CH3). 13C NMR (C6D6): d 75.61 (s, CH2), 71.18 (s, CH2), 58.47 (s, CH3). 12 1H NMR (C6D6): d 4.27 (br, 4 H, CH2), 3.63 (br, 4 H, CH2), 3.39 Notes and references (s, 6 H, CH3). 13C NMR (C6D6): d 79.19 (s, CH2), 63.08 (s, CH2), 1 C.D. Chandler, C. Roger and M. J. Hampden-Smith, Chem. Rev., 58.66 (s, CH3). 1993, 93, 1205. 13 Anal calc. for C54H126O36SrBi2Ta2: C, 29.23; H, 5.72; Sr, 3.95; Bi, 2 R. C. Mehrotra, A. Singh and S. Sogiani, Chem. Rev., 1994, 94, 18.84; Ta, 16.30. Found: C, 28.95; H, 5.75; Sr, 3.71; Bi, 17.93; Ta, 15.45%. 1H NMR (C6D6): d 4.65 (t, 36 H, CH2), 3.52 (t, 36 H, 1643. CH2), 3.38 (s, 54 H, CH3). 13C NMR (C6D6): d 76.00 (s, CH2), 3 C. A. Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott and 68.54 (s, CH2), 58.84 (s, CH3). J. F. Scott, Nature, 1995, 374, 627. 14 T.-C. Chen, T. Li, X, Zhang and S. B. Desu, J. Mater. Res., 1997, 4 H.-M. Tsai, P. Lin and T.-Y. Tseng, Appl. Phys. Lett., 1998, 12, 1569. 72, 1787. 15 I. Koiwa, Y. Okada, J. Mita, A. Hashimoto and Y. Sawada, Jpn. 5 T. Li, Y. Zhu, S. B. Desu, C.-H. Peng and M. Nagata, Appl. Phys. J. Appl. Phys., 1997, 36, 5904. Lett., 1996, 68, 616. 6 H. Tabata, H. Tanaka and T. Kawai, Jpn. J. Appl. Phys., 1995, 34, 5146. Communication 8/06275G J. Mater. Chem., 1998, 8(11), 2317–2319 2319