Sol-gel derived (La, Sr)CoO 3 thin lms on silica glass

Transcription

1 Thin Solid Films 341 (1999) 13±17 Sol-gel derived (La, Sr)CoO 3 thin lms on silica glass B.J. Kim, J. Lee*, J.B. Yoo Department of Materials Engineering, Sung Kyun Kwan University, Suwon, , South Korea Abstract (La 0.5,Sr 0.5 )CoO 3 (LSCO) lms were prepared by a sol-gel method on silica grown by various methods such as thermal oxidation, plasma enhanced chemical vapor deposition (PECVD) and atmospheric pressure chemical vapor deposition (APCVD). Polyvinyl alcohol (PVA) was used as a wetting agent to improve the surface wetting behavior of the LSCO precursor solution onto the silica. The surface wetting of LSCO precursor solution on the silica prepared by PECVD or APCVD was progressively improved with adding PVA in the solution. At 5 wt.% of PVA addition, the surface wetting became uniform and the resulting microstructure of the crystallized LSCO lms was homogeneous. Crystallization and densi cation of the LSCO lms were improved by modifying the heating rate during crystallization. Dense and uniform LSCO lms were obtained with a ramp rate of 28C/min to 4008C for pyrolysis of PVA, followed by heating with a ramp rate of 108C/min to crystallization temperatures. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Conducting oxide; Sol-gel; (La,Sr)CoO 3 1. Introduction * Corresponding author. Tel.: ; fax: address: jclee@yruim.skku.ac.kr (J. Lee) (La 12x,Sr x )CoO 3 (LSCO) has been widely used for oxidation catalysts and oxide electrodes for high temperature fuel cells since the physical properties of (La 12x,Sr x )CoO 3 can be modi ed by varying the La/Sr ratio [1±3]. Moreover, (La 12x,Sr x )CoO 3 has been used as an electrode material for ferroelectric memories [4±10]. Ferroelectric thin lm capacitors with conducting oxide electrodes, (La 12x,Sr x )CoO 3, exhibited signi cantly improved reliability characteristic such as fatigue which has been a critical issue in ferroelectric memory technology. Especially, lead based ferroelectric perovskites such as Pb(Zr,Ti)O 3 and (Pb,La)(Zr,Ti)O 3 with the conducting oxide electrodes having a strong fatigue resistance. Recently, ferroelectric thin lms have attractions for micro-electro-mechanical systems (MEMS) applications. Since ferroelectric materials have inherently piezoelectric properties, the ferroelectric thin lms can be used as actuating or sensing materials in micro actuators or sensors [11± 12]. In such devices, the ferroelectric thin lms in a capacitor structure (e.g. metal/ferroelectric/metal) exerts a driving force or undergo mechanical stresses, possibly causing delamination between each layer and further device failure. Therefore, mechanical endurance and adhesion between each layer in the devices will be important for device performance and reliability. Conventionally, metals such as Pt and Au have been used as an electrode material for driving the piezoelectric devices. In general, it would be expected that the capacitor structure consisting of all the oxide layers (e.g. LSCO/PZT/LSCO) in the devices have better mechanical adhesion between the layers than that consisting of oxide and metal layers (e.g. Pt/PZT/Pt). In this study, therefore, we have fabricated the conducting oxide, i.e. LSCO, as an electrode for MEMS devices. Since the application is the piezoelectrically actuated silica waveguide, we have made LSCO thin lms by the sol-gel method on silica glass fabricated on silicon by various methods, i.e. thermal oxidation, plasma enhanced chemical vapor deposition (PECVD) using silane, and atmospheric pressure chemical vapor deposition (APCVD) using ozone and TEOS. In order to improve the mechanical adhesion between silica substrate and LSCO layer, surface wetting of LSCO precursor solution on the silica substrate was considered. We have used polyvinyl alcohol (PVA) to modify the wetting behavior. This paper reports the effect of PVA addition on wetting of the precursor solution on the silica substrate and resulting LSCO microstructure after crystallization. 2. Experiments For the synthesis of (La 0.5 Sr 0.5 )CoO 3 lms on silica, lanthanum nitrate (La(NO 3 ) 3 6H 2 O), strontium nitrate (Sr(NO 3 ) 2 ), and cobalt nitrate (Co(NO 3 ) 3 6H 2 O) have been used. These precursors were dissolved in distilled water at /99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S (98)

2 14 B.J. Kim et al. / Thin Solid Films 341 (1999) 13±17 Fig. 1. Flow diagram of LSCO thin lm synthesis. 908C with the La/Sr/Co ratio of 1/1/2, followed by the addition of polyvinyl alcohol (PVA). The resulting solution was re uxed for 24 h at 110±1208C. During re ux, the solution concentration was modi ed to be 0.5 M by distillation. This precursor solution was spin coated on SiO 2 /Si at 6000 rpm for 20 s, followed by drying at 1208C. After multicoating was carried out to obtain the required thickness of the LSCO lms, the spin coated lms were heat treated for pyrolysis and crystallization. The heat treatments after drying were carried out at 500±8008C for 30 min in either an air or oxygen owing atmosphere. Fig. 1 shows the ow diagram for the synthesis of LSCO lms. The resulting structures of the LSCO thin lms were characterized by an X-ray diffraction method. Surface wetting behavior of the precursor solution on the silica was examined at each step, i.e. after spin coating, drying and crystallization. 3. Results and discussion First LSCO lms were coated on thermally oxidized Si with various amounts of PVA addition. The amounts of PVA wetting agent were varied in the range of 0±5 wt.%. The surface wetting of the LSCO precursor solutions was poor although the surface wetting was improved progressively with PVA addition, resulting in inhomogeneous dried lms. After crystallization, the morphology of the LSCO lms was irregular and most of the LSCO lms were peeled off from the silica. On the other hand, the surface wetting of the LSCO precursor solutions on silica was improved when it was grown by PECVD. Fig. 2 illustrates surface microstructures of crystallized LSCO lms coated on SiO 2 grown on Si by PECVD with various amounts of PVA addition. Similar to the surface wetting behavior on the thermally oxidized silica, surface wetting of the LSCO precursor solution was poor at small amounts of PVA. However, surface wetting of the LSCO precursor solution on SiO 2 grown on Si by PECVD or APCVD was improved progressively with increasing the amounts of PVA addition. The surface wetting became uniform on the silica for the precursor solution with 5 wt.% PVA addition. As a result, the LSCO lms spin coated on the silica had a uniform microstructure after crystallization, as shown in Fig. 2. Similarly, surface wetting of the LSCO precursor solution on silica grown by APCVD became uniform and the resulting microstructure of the crystallized LSCO lms was homogeneous. It is known that SiO 2 lms grown by PECVD or APCVD have larger amounts of hydroxyl groups that those grown by thermal oxidation of Si [13,14]. In general, it is also well known that the reduction of surface energy between a liquid and a solid facilitates all wetting processes such as spreading, adhesion and immersion [15]. If the hydroxyl groups exist at the surface of silica, the surface will become more hydrophilic, leading to the reduction of surface energy between the aqueous precursor solution and the silica. Therefore, the wettability of the silica becomes improved when the silica was grown by PECVD or APCVD. Crystallization behaviors of the LSCO lms using the precursor solution with 5 wt.% PVA were also examined. Fig. 3 shows X-ray diffraction patterns of LSCO lms deposited on thermally oxidized Si. The LSCO lms had the perovskite structure in the crystallization temperatures of 600±8008C. The LSCO lms also showed a randomly oriented polycrystalline nature. The LSCO lms grown on PECVD or APCVD exhibited similar crystallization behavior to the LSCO lms grown on thermally oxidized Si. In other words, the crystallization temperature was in the range of 600±8008C, irrespective of the growth methods of the silica. Fig. 4 illustrates a cross-section of LSCO/SiO 2 /Si. A 250 nm thick LSCO lm was grown on silica by multicoating (10±15 times). Surface morphology of the LSCO lms crystallized at various temperatures were examined. The LSCO lms were crystallized with a heating rate of 108C/min from the precursor solution with 5 wt.% PVA. The LSCO lms crystallized at 6008C were uniform. As the crystallization temperature increased, the grains in the lms became large. Fig. 5 shows the surface morphology of the LSCO

3 B.J. Kim et al. / Thin Solid Films 341 (1999) 13±17 15 Fig. 2. Surface morphology of LSCO lms grown on silica from precursor solutions containing (a) 2 wt.%, (b) 3 wt.%, (c) 4 wt.% and (d) 5 wt.% of PVA. lms with different heating rates during the crystallization. The LSCO lms crystallized with a 108C/min heating rate had a porous microstructure. On the other hand, when the LSCO lms were heated to 4008 with a ramp rate of 28C/min and further heated to 7008C with a heating rate of 108C/min, the LSCO lms become dense and have a ne grained microstructure. In general, PVA is thermally decomposed at 200±3008C [16]. It is noted that PVA in the LSCO lms should be decomposed before the crystallization takes place. Fig. 6 illustrates that the thermally decomposition of PVA with a heating rate of 108/min is not complete at 6008, while PVA is thermally decomposed around 4008 at a heating rate of 28/min. The heating rate of 108C/min for the crystallization is too fast to remove PVA by thermal decomposition. In this case, residual PVA in the LSCO lm will inhibit further crystallization and densi cation (sintering) of the lm, leading to the porous LSCO lms, as shown in Fig. 5a. Therefore, it is important to remove the PVA (organic component) prior to crystallization in the crystallization process. Surface morphology of LSCO lms crystallized in an oxygen owing atmosphere was similar to that of being crystallized in air. However, the electrical resistivity of the LSCO lms was increased by the oxygen owing treatment, which is attributed to the reduction of the oxygen vacancy concentration. This leads to the increase of hole concentration and further increases the electrical conductivity of the LSCO lms [17]. The electrical resistivity of the LSCO lms treated in an oxygen owing atmosphere was, V cm. 4. Conclusion Fig. 3. X-ray diffraction patterns of LSCO/SiO 2 /Si with various crystallization temperatures for LSCO lms. (La 0.5,Sr 0.5 )CoO 3 lms were prepared by a sol-gel method on silica grown by various methods such as thermal oxida-

4 16 B.J. Kim et al. / Thin Solid Films 341 (1999) 13±17 Fig. 4. Cross-section of LSCO/SiO2/Si. tion, PECVD and APCVD. Polyvinyl alcohol (PVA) was used as a wetting agent to improve surface wetting behavior of the LSCO precursor solution onto the silica. The surface wetting was poor on the silica grown by thermal oxidation irrespective of the amounts of PVA addition in the precursor solution. On the other hand, the surface wetting of the precursor solution onto the silica grown by PECVD or APCVD was improved as the amounts of the PVA wetting agent in the precursor solution increased. This wetting behavior was attributed to the hydroxyl groups existing in the silica prepared by PECVD or APCVD. The heating rate during crystallization was varied to obtain dense LSCO lms. It is important to remove the organic PVA for densi cation before crystallization of the LSCO lms occurs. Acknowledgements This work is nancially supported by Ministry of Information and Telecommunication of Korea. Fig. 5. Surface morphology of LSCO lms with (a) a heating rate of 108C/min and (b) a heating rate of 28C/min to 4008C for pyrolysis, followed by rapid heating (108C/min) to a crystallization temperature (7008C). The silica was prepared by PECVD.

5 B.J. Kim et al. / Thin Solid Films 341 (1999) 13±17 17 References Fig. 6. Thermogravimetric analysis of PVA. [1] Q. Nguyen, J. Minh, Am. Ceram. Soc. 76 (1993) 563. [2] H.J. Kweon, S.T. Kuk, H.B. Park, D.G. Park, K. Kim, J. Mater. Sci. Lett. 15 (1996) 428. [3] J.F.M. Cillessen, R.M. Wolf, A.E.M. De Veirman, Appl. Surf. Sci. 69 (1993) 212. [4] J.T. Cheung, P.E. Morgan, D.H. Lowndes, X.Y. Zheung, J. Breen, Appl. Phys. Lett. 62 (1993) [5] R. Ramesh, H. Gilchrist, T. Sands, V.G. Keramidas, R. Haakenaasen, D.K. Fork, Appl. Phys. Lett. 63 (1993) [6] R. Ramesh, J. Lee, T. Sands, V.G. Keramidas, O. Auciello, Appl. Phys. Lett. 64 (1994) [7] R. Dat, D.J. Lichtenwalner, O. Auciello, A.I. Kingon, Appl. Phys. Lett. 64 (1994) [8] F. Wang, S. Leppavuori, J. Appl. Phys. 82 (1997) [9] R. Ramesh, B. Dutta, T.S. Ravi, J. Lee, T. Sands, V.G. Keramidas, Appl. Phys. Lett. 64 (1994) [10] J. Lee, R. Ramesh, V.G. Keramidas, Mater. Res. Soc. Symp. Proc. 361 (1995) 67. [11] D.A. Barrow, T.E. Petroff, R.P. Tandon, M. Sayer, J. Appl. Phys. 81 (1997) 876. [12] I. Kanno, S. Fujii, T. Kamada, R. Takayama, Appl. Phys. Lett. 70 (1997) [13] A.C. Adams, Solid State Technol. 26 (1983) 135. [14] S.S. Chao, G. Lucovsky, S.Y. Lin, et al., J. Vac. Sci. Technol. A 4 (1986) [15] D.J. Shaw, Introduction to Colloid and Surface Chemistry, 3rd ed., Butterworth, London, 1980 p. 13. [16] S. Budavaris (Ed.), The Merck Index 11th Ed., Merck, Rahway, NJ, 1989, p [17] S. Madhukar, S. Aggarwal, A.M. Dhote, R. Ramesh, A. Krishnan, D. Keeble, E. Poindexter, J. Appl. Phys. 81 (1997) 3543.