Dielectric properties and microstructure of SrTiO BaTiO. multilayer thin films prepared by a chemical route

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1 Ž. Thin Solid Films Dielectric properties and microstructure of SrTiO BaTiO multilayer thin films prepared by a chemical route F.M. Pontes a, E.R. Leite a,, E.J.H. Lee a, E. Longo a, J.A. Varela b a Department of Chemistry, Federal Uni ersity of Sao Carlos-UFSCar, Caixa Postal 676, Sao Carlos, SP, Brazil b Institute of Chemistry, UNESP, Caixa Postal 355, Araraquara, SP, Brazil Received 29 March 2000; received in revised form 30 November 2000; accepted 14 December 2000 Abstract Multilayer thin films with perovskite structures were produced by the polymeric precursor method. SrTiO BaTiO Ž STO BTO. multilayers were deposited on PtŽ 111. Ti SiO SiŽ substrates by the spin-coating technique and heated in air at 700 C. The microstructure and crystalline phase of the multilayered thin films were examined by field-emission scanning electron microscopy Ž FE-SEM., transmission electron microscopy Ž TEM., resolution-high transmission electron microscopy Ž HRTEM., atomic force microscopy Ž AFM. and X-ray diffraction. The multilayer thin films consisted of grainy structures with an approximate grain size of 60 nm. The multilayered thin films showed a very clear interface between the components. The multilayer thin films revealed dielectric constants of approximately 527 and loss tangents of 0.03 at 100 khz. The dielectric constant calculated for this multilayer film system is the value of the sum of each individual component of the film, i.e. the total value of the sum of each SrTiO Ž STO. and BaTiO Ž BTO. layer. The multilayer obtained by the polymeric precursor method, also showed a ferroelectric behavior with a remanent polarization of 2.5 C cm 2 and a coercive field of 30 kv cm. The multilayer films displayed good fatigue characteristics under bipolar stressing after application of switching cycles Published by Elsevier Science B.V. All rights reserved. Keywords: Multilayer thin films; Dielectric constant; Ferroelectric behavior; Microstucture 1. Introduction Many studies have been made on perovskite compounds, such as BaTiO 3, PbTiO 3, PbZrO 3, Pb1 xzrxtio 3, Pb1 xla xtio 3, among others, due to their potential application in memory cell capacitors Ž DRAMs. and nonvolatile memories Ž NVRAM Most of these studies deal with single phase thin films; however, multilayered films consisting of alternate layers of different materials, have recently been produced Corresponding author. Tel.: ; fax: address: derl@power.ufscar.br Ž E.R. Leite Although they have excellent ferroelectric properties, these thin films produced on metal electrodes suffer a serious degradation of remanent polarization after long switching cycles. The choice of electrode material is known to be a very important factor to reduce the degradation of remnant polarization 9,10. One alternative way to obtain fatigue-free, high quality ferroelectric films is to interpose a dielectric layer Ž paraelectric material. between the ferroelectric films and the bottom and top electrodes. Various deposition techniques have been used to prepare multilayer thin films. However, there are few reports on the preparation and electrical properties of SrTiO 3 and BaTiO3 multilayer films produced by the sol gel technique, using the polymeric precursor method $ - see front matter 2001 Published by Elsevier Science B.V. All rights reserved. Ž. PII: S

2 14,15. A recent approach has been to achieve longlasting reliability of ferroelectric layers, including resistance to fatigue and imprint, using SrTiO3 as a dielectric layer between the BaTiO3 ferroelectric films and electrodes. An improvement in the ferroelectric properties is, thus, expected for the multilayer system using buffer layers. This paper reports on the production of a crack-free, dense, multilayered thin film of compounds alternating in layers, and the film s microstructural and electrical characterization. 2. Experimental ( ) F.M. Pontes et al. Thin Solid Films To produce the multilayered films, two solutions were prepared for deposition by spincoating, one for the barium titanate layer and the other for the strontium titanate layer. The raw materials for the solution synthesis were Ti OCHŽ CH. Ž titanium isopropoxide., CH OHCH OH Ž ethylene glycol. 2 2, citric acid, barium and strontium carbonates. Barium and strontium carbonates and titanium isopropoxide were used as precursors for the citrate solution. Ethylene glycol and citric acid were used as polymerization complexation agents in the process. Titanium citrates were formed by dissolution of titanium isopropoxide in a citric acid water solution. After homogenization, a stoichiometric amount of BaCO3 and SrCO3 was added in different solutions while they were stirred. Barium precipitation was prevented by controlling the ph with ammonium hydroxide. The complete dissolution of the salts produced two clear solutions. Ethylene glycol was added to both solutions to promote polyesterification reactions. The solutions viscosity was controlled by the water content. PtŽ 111. Ti SiO SiŽ 100. wafers were used as substrates. Deposition of the first layer was made with the Sr-solution on the substrates, with spincoating at a 4600-rev. min rotation speed and 30 s deposition time. After the first deposition, the films were dried on a hot plate at 150 C for 20 min to remove residual solvents. The films were then heattreated in two stages, the first stage consisting of heating at 400 C for 2 h to promote the decomposition of organic materials, and the second stage consisting of heating the films to 700 C for crystallization. The same process was repeated for the second layer, using the Ba-solution. These procedures were repeated, alternating the materials, until the films had five layers, the first and last layers being SrTiO 3. Fig. 1 illustrates the preparation of the SrTiO BaTiO multilayer thin 2 films. The crystalline phases of multilayer thin films were identified by X-ray diffraction Ž XRD. Ž Cu K radiation. and the diffraction patterns recorded on a Siemens D5000 machine in a 2 configuration, using a graphite monochromator. Field-emission scan- Fig. 1. Flow chart for polymeric precursor processing of SrTiO BaTiO multilayer thin film. ning electron microscopy Ž FE-SEM. ŽJEOL, JSM633 0F. was used to observe the microstructure of the thin films. Film thickness was measured by a thin film cross-section analysis made by transmission electron microscopy Ž TEM.. Atomic force microscopy Ž AFM. was used to obtain a three-dimensional image reconstruction of the sample surface. These images allow for an accurate analysis of the sample surface and the quantification of very important parameters such as roughness and grain size. A Digital Instruments Multi- Mode Nanoscope IIIa was used. Cross-sections of the thin films were used for structural investigation by transmission electron microscopy Ž TEM. and resolution-high transmission electron microscopy Ž HRTEM.. The images presented below were made with a JEOL 3010-ARP machine at 300 kv. Cross-sectional specimens were prepared by cutting the thin film-covered substrate wafers into slices. Two of these slices were glued together face-to-face joining the film-covered surfaces. This sandwich structure was

3 262 ( ) F.M. Pontes et al. Thin Solid Films then cast in epoxy resin in such a way that the center of the samples contained the joint wafer surfaces in edgeon orientation. After mechanical grinding and dimpling, these specimens were ion-milled to perforation in a stage cooled by liquid nitrogen. To carry out the electrical measurements, 0.3-mm diameter Au electrodes were deposited by sputtering through a designed mask onto the film surfaces Ž1 1cm area. to form metal insulator metal capacitors Ž MIM.. The dielectric properties were measured as a function of frequency using a Hewlett-Packard Ž 4194A. impedance gain phase analyzer. The capacitance voltage characteristic was measured for MIM configuration using a small AC signal of 10 mv at 100 khz. The signal was applied across the sample, while the dc electric field was swept from positive bias to negative bias and back to positive bias Ž C V curves.. Dielectric constant and dielectric loss Ž tan. values were measured in a frequency range of 1 khz 10 MHz. Ferroelectric hysteresis and fatigue behavior were performed using Radiant Technologies RT6000HVS ferroelectric test system. Hysteretic loops were traced by means of the Charge program included in the software of the RT6000HVS in a virtual ground mode test device. All the measurements were taken at room temperature. 3. Results and discussion 3.1. Crystallographic structure Fig. 2 shows the XRD patterns of multilayered thin film after heat treatment at 700 C for 2 h. multilayered thin film shows typical XRD patterns with a polycrystalline perovskite structure, although no preferred orientation is observed. Diffraction peaks were observed in both SrTiO3 and BaTiO layers, but no Ž Ba, Sr. 3 TiO3 solid solution was detected in the XRD analysis, suggesting that the Fig. 2. X-Ray diffraction patterns of: Ž. a SrTiO3 single-phase thin film; Ž b. BaTiO single-phase thin film; and Ž c. 3 multilayer thin film. Fig. 3. Atomic force microscopy images of the multilayer thin film. Scan-size 2 2 m. SrTiO and BaTiO layers did not react at the interlay- ers upon heating to 700 C Surface morphology and microstructure AFM imaging was carried out in the contact mode, using a triangular shaped 200- m long cantilever with a spring constant of 0.06 N m. The scanning rate varied from 1 to 2 Hz and the applied force from 10 to 50 nn, depending on the sample tip interactions. The surface roughness Ž rms. was calculated using the equipment s software routine. The grain size of the film heat treated at 700 C was estimated to be approximately 60 nm and the surface roughness was 2.9 nm, Ž Fig. 3b.. The film s surface appeared to be densely packed, smooth and free of cracks or voids. Fig. 3b shows a three-dimensional AFM image of an SrTiO BaTiO thin film annealed at 700 C for 2 h. Fig. 4 shows typical FE-SEM micrographs of multilayer film prepared on Pt Ti SiO2 Si substrate at 700 C for 2 h, in which each layer displays a very dense, uniform, and sharp interface. The interfaces between SrTiO3 layers and BaTiO3 layers are distinct and of good quality in the cross-sectional micrograph. Fig. 5 shows the TEM pho-

4 ( ) F.M. Pontes et al. Thin Solid Films Fig. 4. FE-SEM micrographs of the multilayer thin film prepared at 700 C by the polymeric precursor method. tomicrograph of multilayered thin film of SrTiO 3 BaTiO3 at 700 C. The multilayered thin film is approximately 640 nm thick, with the layers apparently packed quite densely. The interface between the SrTiO3 layer and the substrate is clearly good, as shown in Fig. 5. This figure also shows the clear interface between the SrTiO and BaTiO layers Ž in agreement with FE-SEM, Fig. 4.. Such a layer structure with clear interface was observed by Ohya et al. 16 for PbTiO3 PbZrO3 multilayered thin films obtained by the sol gel method. Fig. 6 shows a HRTEM micrograph of the multilayer film in the BaTiO3 layer, indicating 6-nm domain walls. Fig. 5. TEM microphotograph of multilayer thin film. Cross section; Ž. 1 SrTiO layers, and Ž. 2 layers. Fig. 6. HRTEM micrograph of the multilayer film in the BaTiO 3 layer. Domains can be seen in this region Dielectric properties The dielectric behavior was measured on a thin film in the MIM configuration. Fig. 7 shows room temperature values for the dielectric constant and loss tangent as a function of the frequency. It can be observed that the dielectric constant shows a slight frequency dependency. The dielectric constant and loss tangent, at a frequency of 100 khz, were approximately 527 and 0.03, respectively, at room temperature. The values reported here are higher than reported by Hayashi and Tanaka 17 for sol gel prepared films. It can be seen from Fig. 7, however, that there is a tendency for a decrease in the dielectric constant and an increase in the dissipation factor at higher frequencies, possibly related to interfacial polarization of space charges 18,19. Similar dielectric constant vs. frequency behavior has also been observed for other thin films 20,21. Moreover, the dielectric constant value measured with different electrodes varied by less than 2%, indicating a good degree of uniformity in film thickness. Assuming the effective dielectric constant of the multilayer structure is a series connection model of poly-srtio3 and poly-batio3 layer dielectrics, the effective dielectric constant, eff, can theoretically be expressed as: ž / t t t t t eff STO BTO STO BTO STO Ž. Ž. t t t t t

5 264 ( ) F.M. Pontes et al. Thin Solid Films Fig. 7. Dielectric constant and dissipation factor as a function of frequency for SrTiO BaTiO thin film heat treated at 700 C. where t 1, t3 and t5 are the thicknesses of poly-srtio 3, while t2 and t4 are the thicknesses of poly-batio 3. Thus, the theoretical evaluation of eff, calculated based on multilayer capacitors, was found to be approximately 513, assuming that the relative dielectric constants of SrTiO3 and BaTiO3 are 475 and 600, respectively. These values were obtained by the preparation of films of individual SrTiO3 layers and of BaTiO3 obtained by the same method Žpolymeric precursor method.. As shown in Fig. 7, the dielectric constant was very close to those calculated assuming the series connection model of two components. The calculated dielectric constant has a difference lower than 3% those Ž 527. r experimental dielectric constant of the multi- layer film. The dielectric constant of the multilayer thin films can thus be explained by a series connection of individual components. The behavior observed here is consistent with that reported by Chang et al. 22 for multilayer structured Pb x Zr1 xtio 3 BaTiO3 Ž PZT BTO. thin films obtained by rf magnetron sputtering. The bias voltage dependence of the capacitance was also studied in thin film. Fig. 8 shows the capacitance voltage Ž C V. characteristic of the thin film. The C V Ž Fig. 8. characteristics measured for film showed the hysteresis resulting from the switching of ferroelectric domains caused by the presence of the BaTiO3 ferroelectric layer. The ferroelectric properties of the film were confirmed by Ž P E. hysteresis loop measurements. Fig. 9 shows a typical polarization electric field Ž P E. hysteresis loop at room temperature for a multilayer capacitor. The remanent polarization Ž P. r and the Ž. 2 coercive field Ec are approximately 2.5 C cm and 30 kv cm, respectively. These results agree with others obtained for BTO films prepared by the polymeric precursor method with remanent polarization Ž P. of r Fig. 8. Capacitance vs. applied voltage characteristics of multilayer thin film. approximately 2.2 C cm 2. Because the multilayer film presented ferroelectric characteristics, the fatigue test was carried out by applying a pulse width of 8.6 s, and a bipolar square wave of 14 V was used to fatigue the multilayer film in the Au MIM Pt sandwich structure. The results shown in Fig. 10 indicate that the film does not degrade within 10 9 switching cycles. In addition, single phase BaTiO3 thin film degrade within 10 8 switching cycles. On the other hand, in the multilayer film on platinum-coated silicon substrate, an increase of polarization is observed after switching cycles; although polarization decreased after switching cycles. There are several possible reasons for the behavior observed, among them a poling of the capacitor, increase of resistive leakage current and or charge-up at the film electrode interface. These phenomena have also been reported for thin films prepared by other techniques 23,24. These results confirm that the presence of the dielectric layer, i.e. SrTiO3 between the electrode and ferroelectric film, produces excellent resistance of the multilayer film against polarization fatigue. Hence, we believe that the use of the multilayer structure allows for the preparation of a fatigue-free ferroelectric film. Fig. 9. Typical P E hysteretic loop of the SrTiO BaTiO layer thin film. multi-

6 ( ) F.M. Pontes et al. Thin Solid Films Acknowledgements The authors wish to express their appreciation for the financial support of the Brazilian research funding institutions FAPESP, CNPq, FINEP and PRONEX and would like to thank the LNLS ŽLaboratorio Nacional de Luz Sıncroton. for supplying the HRTEM facilities. References Fig. 10. Fatigue characteristics of multilayer and single phase BaTiO3 thin films deposited on platinum coated silicon substrate. 4. Conclusions Multilayered perovskite thin films were successfully prepared on PtŽ 111. Ti SiO SiŽ substrate at 700 C by the spin-coating technique of the polymeric precursor method. A smooth, dense surface was observed by FE-SEM and AFM. AFM analysis revealed low surface roughness Ž 2.9 nm. for the multilayered films. The multilayer thin films consisted of small grains with an average 60-nm grain size. The interface in the resulting multilayered SrTiO3 and BaTiO3 films was very clearly evidenced by TEM. The dielectric constants and loss tangent of multilayer thin films with approximately 640-nm thickness Ž obtained by TEM. were approximately 527 and 0.03 at 100 khz, respectively. The dielectric constant of multilayered films was explained by the series connection of the component films. The experimental dielectric constants were very close to the theoretical ones Ž 513. r. The HRTEM image also shows the presence of domain ferroelectrics with domain walls of 6 nm. The multilayer obtained by the polymeric precursor method showed ferroelectric behavior with a remanent polarization of 2.5 C cm 2 and a coercive field of 30 kv cm. The multilayer films with SrTiO 3 dielectric layers between the ferroelectric film and electrodes showed negligible polarization fatigue of up to cycles. These results indicate that multilayers Ž SrTiO BaTiO. are a promising combination in the production of fatigue-free films. 1 H. Basantakumar Shama, H.N.K. Sama, A. Mansingh, J. Appl. Phys. 85 Ž S.-B. Xiong, S. Migita, H. Ota, S. Sakai, Mater. Lett. 38 Ž A. Li, C. Ge, P. Lu, D. Wu, S. Xiong, N. Ming, Mater. Lett. 31 Ž C.R. Cho, Mater. Sci. Eng. B 64 Ž C. Byun, J.W. Jang, Y.J. Cho, K.J. Lee, B.W. Lee, Thin Solid Films 324 Ž I. Kanno, T. Hayashi, M. Kameda, T. Kitakawa, T. Hirao, Jpn. J. Appl. Phys. 32 Ž H.N. Al-Shareef, O. Auciello, A.I. Kingon, J. Appl. Phys. 77 Ž T. Azuma, S. Takahashi, M. Kuwabara, Jpn. J. Appl. Phys. 32 Ž R. Ramesh, J. Lee, T. Sands, V.G. Keramidas, O. Auciello, Appl. Phys. Lett. 64 Ž S. Aggarwal, A.M. Dhote, R. Ramesh, W.L. Warren, G.E. Pike, D. Dimos, M.V. Raymond, J.T. Evans, Jr., Appl. Phys. Lett. 69 Ž S.-G.ap. Lee, Y.-H.ie Lee, Thin Solid Films 353 Ž M. Yoshimura, W. Suchanek, T. Watanabe, B. Sakurai, M. Abe, J. Mater. Res. 13 Ž J.C. Jiang, X.Q. Pan, W. Tian, C.D. Theis, D.G. Schlom, Appl. Phys. Lett. 74 Ž S.M. Zanetti, E.R. Leite, E. Longo, J.A. Varela, J. Mater. Res. 13 Ž V. Bouquet, E. Longo, E.R. Leite, J.A. Varela, J. Mater. Res. 14 Ž Y. Ohya, T. Ito, Y. Takahashi, Jpn. J. Appl. Phys. 33 Ž T. Hayashi, T. Tanaka, Jpn. J. Appl. Phys. 34 Ž P.M. Sutton, in: J.B. Birks, J.H. Schulman Ž Eds.., Progress in DielectricWiley, New York, 1960, p M. Tomozawa, in: M. Tomozawa, R.H. Doremus Ž Eds.., Treatise on Materials Science and Technology, Academic, New York, 1977, p P.C. Joshi, S.B. Desu, Thin Solid Films 300 Ž C.-H. Lu, C.-Y. Wen, Mater. Lett. 38 Ž L.H. Chang, W.A. Anderson, Thin Solid Films 303 Ž J.F.M. Cillessen, M.W.J. Prins, R.M. Wolf, J. Appl. Phys. 81 Ž H. Maiwa, N. Ichinose, K. Okazaki, Jpn. J. Appl. Phys. 33 Ž