Structural and electrical properties of SrBi Nb O thin films prepared by chemical aqueous solution at low temperature

Transcription

1 July 1999 Materials Letters Structural and electrical properties of SrBi Nb O thin films 9 prepared by chemical aqueous solution at low temperature S.M. Zanetti a, E.B. Araujo b, E.R. Leite a,), E. Longo a, J.A. Varela c a Departamento de Quımica-UFSCar, P.O. Box 676, Sao Carlos, SP, Brazil b Departamento de Fısica-UFSCar, P.O. Box 676, Sao Carlos, SP, Brazil c Instituto de Quımica-UNESP, P.O. Box 355, Araraquara, SP, Brazil Received 18 December 1998; accepted 13 January 1999 Abstract SrBi Nb O Ž SBN. 9 thin films were prepared by the polymeric precursors method and deposited by dip coating onto PtrTirSiO rsiž 100. substrates. The dip-coated films were specular and crack-free and crystallized during firing at 7008C. Microstructure and morphological evaluation were followed by grazing incident X-ray diffraction Ž GIXRD., scanning electron microscopy Ž SEM. and atomic force microscopy Ž AFM.. The films exhibited somewhat porous grain structure with rounded grains of about 100 nm. For the electrical measurements, gold electrodes of 300 mm in diameter were sputter deposited on the top surface, forming a metal-ferroelectric-metal Ž MFM. configuration. The remanent polarization Ž P. r and coercive field E were 5.6 mcrcm and 100 kvrcm, respectively. q 1999 Elsevier Science B.V. All rights reserved. c Keywords: Ferroelectic; SrBi Ta O ; Bismuth layer; Thin films; PtrTirSiO rsi substrates; Perovskite phase 9 1. Introduction In recent years, several studies have been reported on thin films for memory applications. The most popular ferroelectric material for nonvolatile memories is lead zirconate titanate Ž PZT.. However, the PZT thin films on Pt electrode present serious problems of degradation due to oxygen vacancies created at the interface wx 1. An alternative material is a new class of ferroelectrics based on Bi-layered perovskites. The bismuth layer structure was originally described by Aurivillius wx to have the formula Ž Bi. O q - Ž A B O. y, where A is Bi, Ba, Sr, Ca, Pb, my 1 m 3mq1 ) Corresponding author. Fax: q KorNa;B is Ti, Nb, Ta, Mo, W or Fe. Recently, Bi-layered perovskite thin films such as Bi 4Ti 3O 1, SrBi Ta O Ž SBT. or SrBi Nb O Ž SBN. 9 9 which showed little or no fatigue have been developed. Among them, SBT has received special attention for NvRAM device application w3 7 x. The main disadvantage presented by SBT thin film is the high crystallization temperature required for good electrical properties, normally about 8008C, which is incompatible with Si wafer technology wx 8. Due to this problem, the studies led to the search for low temperature process or material. In a previous work wx 9, the authors reported the synthesis of SBN phase Ž powder and thin film. using the polymeric precursor method developed by Pew10 x. The Pechini process is based on chini the Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S X

2 34 ( ) S.M. Zanetti et al.rmaterials Letters polymeric precursors originated from a polyesterification reaction between the desired metallic citrate and a polyalcohol, usually, ethylene glycol. The polymeric precursor method has been sucessfully used to prepare mixed oxides w11,1x and has the advantage of using common reagents and not requiring special atmosphere, and moreover, no aging effects have been reported so far. It was observed that the SBN phase formation around 6508C represents a reduction in the temperature process. In this work, SBN thin films were prepared by dip coating a chemical solution on PtrTirSiŽ 100. substrate and the crystallinity, surface morphology and electrical properties of the films were investigated.. Experimental Strontium carbonate, SrCO Ž Merck. 3, niobium ammonium oxalate, NH H wnbo Ž C O. x P 3H O Ž CBMM, Araxa, Brazil. and bismuth oxide, Bi O3 Ž Aldrich. were used as reagents to synthesize SrBi Nb O Ž SBN. 9. Niobium hydroxide was formed by dissolution of the niobium ammonium oxalate in water and precipitated as NbŽ OH. 5 by addition of NH 4OH. After filtration, niobium hydroxide was dissolved in an aqueous solution of citric acid to form niobium citrate. The content of Nb was gravimetrically determined as NbO 5. To this niobium citrate solution were added stoichiometric amounts of SrCO3 as salt and Bi O3 dissolved in water with HNO 3. Ethylenediamine was added dropwise into the solution with constant stirring until the ph reached 7 8. After homogenization of the solution, ethylene glycol was added as a second solvent. The molar ratio between strontium, bismuth and niobium was 1::, the citric acidrmetal molar ratio was fixed at 3.95 and the citric acidrethylene glycol ratio was fixed as 60r40 Ž mass ratio.. The viscosity of the deposition solution was adjusted by water evaporation until it reaches 15 mpa s. Films were dip coated from SBN deposition solution onto PtrTirSiOrSi substrate and heat-treated in two steps: first, heating at 3008C for h Žfor complete elimination of organic materials. and after at 7008C for h Žfor crystallization of perovskite phase.. Film thickness were controlled by adjusting the number of repetitions of the coatingrheating cycles. Films were characterized by Grazing Incidence X-ray Diffraction Ž GIXRD; Siemens, D5000., using an incident angle of 8 Ž a. and a LiF100 monochromator. Scanning Electron Microscopy ŽSEM; Zeiss, DSM940A. and Atomic Force Microscopy ŽAFM; Digital, NanoScope 3A. were used for surface morphology characterization. Surface roughness Ž RMS. was examined by AFM, using tapping mode. Fig. 1. GIXRD patterns of SBN films heat-treated at different temperatures prepared on PtrSiO rsi Ž 100. substrates: Ž a. 4008C; Ž b. 5008C; Ž. c 6008C; Ž. d 6508C and Ž. e 7008C.

3 ( ) S.M. Zanetti et al.rmaterials Letters For the electrical measurements, gold electrodes of 300 mm in diameter were sputter deposited on the top surface, forming a metal-ferroelectric-metal Ž MFM. configuration. Capacitance and dissipation factor Ž tan d. values, at 100 Hz 10 MHz frequency range, and capacitance voltage Ž C V. were measured with a HP 4194A impedance analyzer. Ferroelectricity was investigated using a modified Sawyer Tower circuit attached to a Tektronix 3 digital oscilloscope. 3. Results and discussion 3.1. Crystallization and surface morphology of SBN thin films Fig. 1 presents GIXRD patterns of SBN films heat-treated at different temperatures. It can be observed that the perovskite phase is already crystallized at 6508C, but the increase of the temperature to 7008C improved the crystallinity. The films are polycrystalline, showing no c-axis-preferred orientation, but rather random orientation. The lattice parameters calculated from Ž 105., Ž 110. and Ž 15. peaks of the film heat-treated at 7008C are asbs A and cs A. Fig. presents the AFM micrographs of surface topography of SBN films heat-treated at different temperatures. The evolution of grain structure of the films with the increase of the heat temperature is clearly observed. No grain structure is seen once the film is amorphous at 4008C, as showed by XRD pattern. At 5008C, a great number of small spherical grains, which was assumed to be the intermediate fluorite phase from which the perovskite SBN phase origi- Fig.. AFM micrographs of surface topography of SBN films heat-treated at: Ž. a 4008C; Ž. b 5008C; Ž. c 6008C; Ž. d 6508C and Ž. e 7008C Ž Areas1 mm=1 mm..

4 36 ( ) S.M. Zanetti et al.rmaterials Letters nates, can be observed. The intermediate fluorite phase can be formed by small spherical grains, as can be seen in Fig. b and c. These small fluorite grains were reported by Koiwa et al. wx 5 for SBT films. As the temperature increased, bigger grains of SBN phase can be observed, and at 6508C the SBN perovskite grains are well defined. The films exhibited somewhat porous grain structure with rounded grains of about 100 nm. These rounded grains tend to form elongated ones by coalescence as the temperature increases. This kind of elongated grains was also observed for SBT films heat-treated at 7008C, but with 50% excess of bismuth wx 6. The superficial roughness is about 10 nm and the thickness, measured by SEM, is 30 nm, as can be seem on Fig. 3. The SEM micrograph presents a microstructure of elongated grains. The cross section shows the Pt layer and a typical polycrystalline structure for the film, as well as the Au top electrode. 3.. Dielectric properties of SBN thin films For electrical and ferroelectric measurements were used a film with 30 nm thickness. The dielectric constant was measured between 100 Hz and 10 MHz whose value ranges from 104 to 85, while the dissipation factor ranges from 0.07 to in the measured range. The dielectric constant and dissipation factors at a frequency of 100 khz were 9 and 0.033, respectively. Fig. 4 illustrates the C V characteristic for SBN film in the MFM configuration as well as the hysteresis loop. At 100 khz frequency, the dc electric field applied was swept from a positive bias to negative bias and back again. Two peaks are clearly seen in this figure. The sample capacitance changes from pf with applied voltage. The presence of two peaks is attributed to ferroelectric domain switching w13 x. Hysteresis loops of SBN films were observed at 100 khz at room temperature. Fig. 4b presents a typical P E hysteresis loop of film annealed at 7008C for h, deposited on PtrSi substrate. The remanent polarization Ž P. r and coercive field Ec were 5.6 mcrcm and 100 kvrcm, re- spectively. These values are comparable with several reports for SBT thin films, prepared by other techniques w6 8,14x Ž as summarized in Table 1., although the temperature of heat treatment was 8008C. Better Fig. 3. SEM micrograph of surface and the cross section of SBN thin film prepared on PtrSiO rsi 100 at 7008C for h.

5 ( ) S.M. Zanetti et al.rmaterials Letters Ž. Ž. Fig. 4. Electric characteristics for SBN film heat-treated at 7008C for h, at 100 khz frequency: a C V curve; b hysteresis loop at room temperature. results are reported by Watanabe et al. w15x for Sr-deficient SBN films; however, films were also heat-treated at 8008C in oxygen. The treatment under oxygen flux is believed to improve the electrical properties of these materials. Kato et al. w16x prepared SBN films heat-treated at 7008C by sol gel; however, the reported ferroelectrical properties are not quite good.

6 38 ( ) S.M. Zanetti et al.rmaterials Letters Table 1 Electrical properties of SBT and SBN films according to literature Ž. r c Reference Material Method Temp. 8C P mcrcm E kvrcm wx 6 SBT Sol gel 800 O wx 7 SBT MOD 800 O n.r w1x SBT0.8N0. PLD 700 O n.r. 60 w13x S0.9 B.1N Sol gel 800 O n.r. w14x SBN Sol gel 700 n.r This work SBN Polymeric precursors n.r.snot reported. These results suggest that altering the stoichiometry or the annealing atmosphere may improve the electrical characteristics of SBN films at temperature not higher than 7008C. monium oxalate, and Dr. J.A. Eiras ŽDepartamento de Fısica-UFSCar., for the electrical measurements facilities. 4. Conclusion It was reported that the preparation of SBN ferroelectric film by a chemical aqueous solution method, at a temperature of 7008C, is compatible with Si technology. SBN films are already crystallized at 6508C. The perovskite phase comes from an intermediate fluorite phase of small spherical grains. For a film annealed in air at 7008C for h, the measured dielectric constant and dissipation factors at 100 khz were 9 and 0.033, and remanent polarization and coercive field were 5.6 mcrcm and 100 kvrcm, respectively. C V measurements of the SBN film in metal-ferroelectric-metal configuration presented two peaks, associated to ferroelectric domain switching. The film properties can be improved by altering the stoichiometry and the atmosphere of heat treatment. Acknowledgements This work has financial support from Brazilian research agencies FAPESP Ž Proc. 96r and., MCTrFINEPrPRONEX and CNPq. One of the authors is personally supported by FAPESP. The authors wish to thank CBMM-Cia. Brasileira de Mineraça e Metalurgia for supplying the niobium am- References wx 1 J. Robertson, C.W. Chen, W.L. Warren, C.D. Gutleben, Appl. Phys. Lett. 69 Ž 1. Ž wx B. Aurivillius, P.H. Fang, Phys. Rev. 16 Ž.Ž wx 3 J.-P. Han, T.P. Ma, Appl. Phys. Lett. 7 Ž 10. Ž wx 4 I. Koiwa, K. Tani, J. Mita, T. Iwabuchi, Jpn. J. Appl. Phys. 37 Ž.Ž , Part I. wx 5 I. Koiwa, Y. Okada, J. Mita, A. Hashimoto, Y. Sawada, Jpn. J. Appl. Phys. 36 Ž.Ž , Part I. wx 6 T. Hayashi, T. Hara, H. Takahashi, Jpn. J. Appl. Phys. 36 Ž. 1 Ž , Part I. wx 7 D.J. Yeon, J.D. Park, T.S. Oh, J. Korean Phys. Soc. 3 Ž S wx 8 T.-C. Chen, T. Li, X. Zhang, S.B. Desu, J. Mater. Res. 1 Ž. 8 Ž wx 9 S.M. Zanetti, E.R. Leite, E. Longo, J.A. Varela, J. Mater. Res. 13 Ž 10. Ž w10x M.P. Pechini, U.S. Patent no. 3,330,697, w11x S.M. Zanetti, E. Longo, J.A. Varela, E.R. Leite, Mat. Lett. 31 Ž w1x V. Boquet, S.M. Zanetti, C.R. Foschini, E.R. Leite, E. Longo, J.A. Varela, Innovative Processing and Synthesis of Ceramic, Glasses, and Composites, in: N.P. Bassal, K.V. Logan, J.P. Singh Ž Eds.., Ceramic Transactions, Vol. 85, The Am. Ceram. Soc., Westerville, OH, 1998, pp w13x S. Triebwasser, Phys. Rev. 118 Ž w14x S.B. Desu, D.P. Vijay, Mat. Sci. Eng. B 3 Ž w15x K. Watanabe, M. Tanaka, E. Sumitomo, K. Katori, H. Yogi, J.F. Scott, Appl. Phys. Lett. 73 Ž.Ž w16x K. Kato, C. Zheng, J.M. Finder, S.K. Dey, Y. Torii, J. Am. Ceram. Soc. 81 Ž.Ž