Very large dielectric constant of highly oriented Pb 1x Ba x TiO 3 thin films prepared by chemical deposition F. M. Pontes, E. R. Leite, G. P. Mambrini, M. T. Escote, E. Longo, and J. A. Varela Citation: Applied Physics Letters 84, 248 (2004); doi: 10.1063/1.1637150 View online: http://dx.doi.org/10.1063/1.1637150 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/2?ver=pdfcov Published by the AIP Publishing This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. 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M. Pontes, E. R. Leite,a) G. P. Mambrini, M. T. Escote, and E. Longo LIEC–CMDMC, Department of Chemistry, UFSCar, Via Washington Luiz, km 235, CP-676, CEP-13565-905, Sa˜o Carlos, S. P., Brazil J. A. Varela Institute of Chemistry, UNESP, Araraquara, S. P., Brazil ~Received 19 August 2003; accepted 6 November 2003! Highly ~100! oriented Pb0.8Ba0.2TiO3 /LaNiO3 structures were grown on LaAlO3(100) substrates by using a wet, soft chemical method and crystallized by the microwave oven technique. The Au/PBT/LaNiO3 /LaAlO3 capacitor shows a hysteresis loop with remnant polarization,Pr , of 15mC/cm2, and coercive field,Ec , of 47 kV/cm at an applied voltage of 3 V, along with a dielectric constant over 1800. Atomic force microscopy showed that Pb0.8Ba0.2TiO3 is composed of large grains about 300 nm. The experimental results demonstrated that the microwave preparation is rapid, clean, and energy efficient. Therefore, we demonstrated that the combination of the soft chemical method with the microwave process is a promising technique to grow highly oriented thin films with excellent dielectric and ferroelectric properties, which can be used in various integrated device applications. ©2004 American Institute of Physics.@DOI: 10.1063/1.1637150# The development of ferroelectric thin film technology has been receiving great investments due to the possibility of achieving a greater miniaturization of the integrated elec- tronic circuits used nowadays.1 Many thin films with a perovksite-type structure have been extensively investigated for a variety of integrated device applications. However, it is a generally acknowledged fact that ferroelectric oxide thin films with perovskite-type structure, deposited on Pt/Ti/SiO2 /Si substrates, present serious fatigue problems af- ter a great number of polarization switching cycles.2 There- fore, much research has been extensively undertaken in order to achieve not only fatigue-free ferroelectric thin films, but also films with high dielectric constant. In order to achieve these characteristics, electrodes based on metallic oxides such as LaNiO3 , SrRuO3, BaPbO3, and YBa2Cu3O72x , re- placing the electrodes based on noble metals like platinum, and the growth of epitaxial or highly oriented thin films, are an alternative approach to reach better properties.3,4 Among them, the most widely used metallic oxide used in electrodes has been LaNiO3 .5,6 The current interest in ferroelectric thin films, epitaxially or highly oriented grown, is due to the fact that they present better properties than polycrystalline thin films.7,8 In addition, attempts have been made to enhance the crystallization ability of ferroelectric thin films and metallic oxide electrodes. This has stimulated a search for other tech- niques for the annealing process, in which conventional an- nealing~furnace! and rapid thermal annealing processes are utilized nowadays. Another approach utilizes the microwave frequency source of energy that is being developed as a way to process materials and has opened an opportunity to en- hance crystallization with a lower annealing processing time, since it decreases the interfacial reactions between ferroelec- tric thin films and electrodes and also improves the control over the crystallographic orientation of the thin films.9–11 Several synthesizing methods are available for the growth of ferroelectric thin films and metallic oxide elec- trodes, such as metalorganic chemical vapor deposition,12 pulse laser deposition,13 magnetron sputtering,14 and the most common of them involves chemical methods such as sol–gel.15 Among those methods, wet soft chemical methods have a better potential for technological applications, be- cause of their precise control of composition and homogene- ity, and good conformality.16,17 Baoet al.18 reported LaNiO3 thin films grown on thermally oxidized silicon substrates by the sol–gel technique with the subsequent deposition of (Pb,La)TiO3 thin films also prepared by sol–gel. Wang et al.19 reported the preparation of the highly~100! oriented Ba0.9Sr0.1TiO3 /LaNiO3 heterostructure grown on Si~100! by chemical solution routes. However, there is no report on Pb0.8Ba0.2TiO3 thin films on LaNiO3 bottom electrodes. Moreover, in this current work both Pb0.8Ba0.2TiO3 and LaNiO3 thin films are highly oriented. In the present work, we report on the preparation of highly ~100! oriented Pb0.8Ba0.2TiO3 and metallic oxide LaNiO3 thin films with excellent structural, microstructural, dielectric, and ferroelectric properties on LaAlO3 substrates by the soft chemical method, and a domestic microwave oven was used to crystallize these thin films. The LaNiO3 and Pb0.8Ba0.2TiO3 thin films were prepared by a wet, soft chemical method, as described elsewhere.17,20 The LaNiO3 thin films were spin coated on~100! LaAlO3 substrates by a commercial spinner operating at 7200 revolutions/min for 30 s~spin coater KW-4B, Chemat Tech- nology!. Each annealing layer was dried at 150 °C for 5 min, and then prefired at 300 °C for 6 h in a conventional oven. After the prefiring, each layer was crystallized in a micro- wave oven at 700 °C for 10 min. Using the same procedure, the Pb0.8Ba0.2TiO3 thin films were deposited by spinning the precursor solution on the LaNiO3 /LaAlO3 structure. In this case, the thin films were annealed at 600 °C for 10 min in the microwave oven. Through this process, we have obtained thickness values of about 250 nm for LaNiO3 and 300 nm fora!Electronic mail: derl@power.ufscar.br APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 2 12 JANUARY 2004 2480003-6951/2004/84(2)/248/3/$22.00 © 2004 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:29:38 http://dx.doi.org/10.1063/1.1637150 Pb0.8Ba0.2TiO3 , reached by repeating the spin-coating and heating treatment cycles. The microwave oven used was a simple domestic model similar to that described in Ref. 9. The structure of the LaNiO3 and Pb0.8Ba0.2TiO3 thin films was analyzed by x-ray diffraction in theu–2u mode scan, recorded on a Rigaku diffractometer~D/max- 2400! using CuKa radiation. The microstructure and the thickness of the thin films were examined using atomic force microscopy ~AFM! ~Digital Instruments, Nanoscope IIIa! and scanning electron microscopy~Zeiss, DSM940A!, re- spectively. The electric properties were measured by an Au/Pb0.8Ba0.2TiO3 /LaNiO3 /LaAlO3 capacitor structure. The upper electrodes of Au for the electrical measurements were prepared by evaporation through a shadow mask with 4.931022 mm2 dot area. Dielectric and ferroelectric proper- ties of the capacitor were measured by an HP4192A impedance/gain phase analyzer and a Radiant Technology RT6000HVS in a virtual ground mode, respectively. Figure 1 shows the x-ray diffraction patterns for~a! ~100! LaAlO3 substrate,~b! LaNiO3 /LaAlO3 structure, and ~c! Pb0.8Ba0.2TiO3 /LaNiO3 /LaAlO3 structure. It can be seen that the LaNiO3 thin film electrode annealed at 700 °C for 10 min is highly ~100! oriented and crystallizes into the perov- skite single phase@Fig. 1~b!#. Figure 1~c! shows the Pb0.8Ba0.2TiO3 thin film deposited on the LaNiO3 /LaAlO3 structure, where the intensities of the~100! and ~200! peaks are stronger than those of the~101! and~110! peaks, indicat- ing that the Pb0.8Ba0.2TiO3 thin film is highly ~100! oriented and only displays the perovskite phase. Therefore, both thin films are highly~100! oriented, due the following factors; the enhanced crystallization and interfacial growth improvement of the thin films, which were significantly influenced by the microwave treatment, as well as by the matching of the lat- tice parameters between the LaNiO3 and Pb0.8Ba0.2TiO3 thin films. In Fig. 1~b! the intensity of the~100! peak of the substrate is much weaker than the one related to the~200! peak. This is in agreement with the same order of peak in- tensity observed in the pure substrate, Fig. 1~a!. In Fig. 1~c! there is no substrate peak at all. It may be considered that the thickness of the Pb0.8Ba0.2TiO3 /LaNiO3 structure might play some role in explaining this. The inset in Fig. 1 shows the AFM surface morphologies obtained in the contact mode of the~a! LaAlO3 substrate,~b! LaNiO3 /LaAlO3 structure, and ~c! Pb0.8Ba0.2TiO3/ LaNiO3/LaAlO3 structure. It was found that the surface of the LaAlO3 substrate is very smooth, with an average surface roughness of 0.07 nm. For the LaNiO3 thin film electrode annealed at 700 °C for 10 min, a smooth, pinhole-free, and continuous surface morphology with no cracks was ob- served. The average surface roughness value of the LaNiO3 thin film electrode on the LaAlO3 substrate is 10 nm and the average grain size is in the range from 120 to 160 nm@see the inset of Fig. 1~b!#. The surface morphology of the Pb0.8Ba0.2TiO3 thin film annealed at 600 °C for 10 min on the LaNiO3 /LaAlO3 structure shows large grains with a size of about 300 nm with an average surface roughness of 12 nm @see the inset of Fig. 1~c!#. Figure 2 shows the frequency dependence of the dielec- tric constant and dielectric loss of the Pb0.8Ba0.2TiO3 thin film on the LaNiO3 bottom electrode. The dielectric constant shows a slight decrease and the dielectric loss slowly in- FIG. 1. X-ray diffractograms of~a! the LaAlO3(100) substrate;~b! LaNiO3 thin film on the LaAlO3(100) substrate; and~c! the Pb0.8Ba0.2TiO3 thin film on LaNiO3 /LaAlO3(100). Inset shows the AFM micrographs (131 mm) of the ~a! substrate;~b! LaNiO3 thin film; and ~c! Pb0.8Ba0.2TiO3 thin film. S 5Substrate peak. Magnified sections of the x-ray diffractograms, emphasiz- ing the peaks~d! in the 2u region from 22° to 24°~e! in the 2u region from 45° to 50°. FIG. 2. Frequency dependence of the dielectric constant and dielectric loss of the Pb0.8Ba0.2TiO3 thin film with a Au/PBT/LaNiO3 /LaAlO3(100) con- figuration. 249Appl. Phys. Lett., Vol. 84, No. 2, 12 January 2004 Pontes et al. This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 186.217.234.225 On: Tue, 14 Jan 2014 12:29:38 creases with increasing frequency. The dielectric constant value at the frequency of 100 kHz is 1804, which is much higher than the one reported by Giridharan and Jayavel,21 about 118, for sol–gel derived Pb0.8Ba0.2TiO3 thin films on platinum electrodes. In addition, the dielectric constant value obtained in this work is also rather larger than those from other thin films on LaNiO3 bottom electrodes reported in the literature.22,23Baoet al.18 reported a dielectric constant value of 1468 at a frequency of 1 kHz and the remnant polarization and the coercive field were about 4.24mC/cm2 and 23.2 kV/ cm, respectively, for a Pb0.75La0.25TiO3 thin film prepared by sol–gel on the LaNiO3 bottom electrode. Wanget al.19 re- lated a dielectric constant value of about 650 at a frequency of 1 kHz and the remnant polarization and the coercive field were 10.8mC/cm2 and 96 kV/cm, respectively, for Ba0.9Sr0.1TiO3 thin films on the LaNiO3 bottom electrode. Notice that the dielectric constant value for the Pb0.8Ba0.2TiO3 thin film deposited on the LaNiO3 electrode is much higher than those for films directly deposited on platinum and LaNiO3 bottom electrodes prepared by other techniques. This enhancement may be due to the improve- ment between the Pb0.8Ba0.2TiO3 thin film and the LaNiO3 electrode interface, which could be a result of the formation of a highly oriented Pb0.8Ba0.2TiO3 /LaNiO3 /LaAlO3 struc- ture, as well as to the large grain size of the Pb0.8Ba0.2TiO3 thin films. This supports our idea that microwave oven treat- ments suppress the formation of a very low dielectric con- stant layer at the thin film/electrode interface, which is the main cause of the lower value of the dielectric constant re- lated to many ferroelectric thin films.18,24 Pb0.8Ba0.2TiO3 thin films on the LaNiO3 bottom elec- trode exhibit ferroelectric characteristics, as can be seen from the hysteresis loop shown in Fig. 3~a!. The Pb0.8Ba0.2TiO3 thin film showed good ferroelectricty, and the remnant polar- ization and coercive field were 15mC/cm2 and 47 kV/cm, respectively. Figure 3~b! shows the electric field dependence of the capacitance or dielectric constant for the Pb0.8Ba0.2TiO3 thin film. A dielectric constant–electric field hysteresis loop is also observed, which reflects the ferroelec- tric behavior. This corroborates the result of theP–E hys- teresis loop in Fig. 3~a!. The small value of the coercive field and the high value of the remnant polarization obtained for the Pb0.8Ba0.2TiO3 thin film on the LaNiO3 bottom electrode can be interpreted as the result of highly~100! oriented thin films. However, for achieving these properties, the contribu- tion of the large grain size of Pb0.8Ba0.2TiO3 might also oc- cur, in which the domain walls in the thin films with larger grains are easier to switch under a lower external field, re- sulting in the small value of the coercive field. In conclusion, highly ~100! oriented Pb0.8Ba0.2TiO3 / LaNiO3 structures have been grown on LaAlO3(100) sub- strates by a wet, soft chemical method and annealed by a microwave oven process. This study indicates that the Pb0.8Ba0.2TiO3 thin film on the LaNiO3 bottom electrode was shown to have excellent structural, microestructural, and electrical properties. The dielectric constant is above 1800, making it a very attractive candidate for many applications. Moreover, the remarkable improvement in all the properties suggests that the wet, soft chemical method, combined with annealing by the microwave oven process, is an alternative approach to obtaining thin films with a quality comparable to the best thin films, suitable for integrated device applica- tions, and processed by conventional methods. The authors gratefully acknowledge the financial support of the Brazilian financing agencies FAPESP, CNPq/ PRONEX, and CAPES. 1A. Roelofs, T. Schneller, K. Szot, and R. 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