Improvement of the dielectric and ferroelectric properties in superlattice structureof Pb ( Zr,Ti ) O 3 thinfilmsgrownbyachemical solutionroute F. M. Pontes, E. Longo, E. R. Leite, and J. A. Varela Citation: Applied Physics Letters 84, 5470 (2004); doi: 10.1063/1.1751623 View online: http://dx.doi.org/10.1063/1.1751623 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/26?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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Leitea) LIEC-CMDMC, Department of Chemistry, UFSCar, Via Washington Luiz, km 235, CP-676, CEP-13565-905, Sa˜o Carlos, Sa˜o Paulo, Brazil J. A. Varela Institute of Chemistry, UNESP, Araraquara, Sa˜o Paulo, Brazil ~Received 25 November 2003; accepted 23 March 2004; published online 17 June 2004! Making heterolayered perovskite materials constitutes an approach for the creation of better dielectric and ferroelectric properties. In the experiment reported here, heterolayered PZT40/PZT60 films were grown on Pt/Ti/SiO2 /Si ~100! by a chemical solution deposition. The dielectric constant of the heterolayered thin film was significantly enhanced compared with that of pure PZT40 and PZT60 thin films. A dielectric constant of 701 at 100 kHz was observed for a stacking periodicity of six layers having a total thickness of 150 nm. The heterolayered film exhibited greater remanent polarization than PZT60 and PZT40 films. The values of remanent polarization were 7.9, 18.5, and 31 mC/cm2, respectively, for pure PZT60, PZT40, and heterolayered thin films, suggesting that the superior dielectric and ferroelectric properties of the heterolayered thin film resulted from a cooperative interaction between the ferroelectric phases made from alternating tetragonal and rhombohedral phases of PZT, simulating the morphotropic phase boundary of this system. ©2004 American Institute of Physics.@DOI: 10.1063/1.1751623# For many years a great deal of interest has focused on the practical applications of perovskite-type compounds, including BaTiO3 , PbTiO3 , PbZrO3, Pb12xZrxTiO3 , Pb12xLaxTiO3 , due to their potential to be used as memory cell capacitors ~DRAMs! and nonvolatile memories ~FRAM!.1,2 Traditionally, most of these studies have dealt with single-phase thin films, but an important technology that has emerged recently involves heterolayered or superlat- ticed thin films consisting of alternate layers of materials, phases, structures, and composition at gradient structures.3 First and foremost, this type of structure has been identified as possessing physical properties different from those of bulk materials, causing it to become a subject of intensive scien- tific research. It offers opportunities for potential application in high-density, nonvolatile memories and in other technolo- gies. In recent years, this superlattice technology has taken a revolutionary step forward in response to the challenging requirements of high-density memories. Heterolayered thin films with composition gradients normal to the substrate sur- face offer a preferable alternative approach, since they dis- play properties superior to those of conventional ferroelectric thin films.4,5 Various deposition techniques have been used to prepare heterolayered thin films with such superior properties.6–8 The most recent approach can make films with high dielectric constant and large remanent polarization, as well as long reliability of the ferroelectric layers with fatigue and imprint resistance.9 Recent research has concentrated on a number of heterolayered thin films whose layers include BaTiO3 , SrTiO3 , PbTiO3 , Pb12xCaxTiO3 , Ba12xSrxTiO3 , and PbZrxTi12xO3.10–13 The phase diagram of the PbZrxTi12xO3 solid solution system is particularly complex, containing a large number of structural transformations. At room temperature, the PZT properties are strongly influenced by the Zr:Ti ratio, exhibiting useful dielectric, ferroelectric, and piezoelectric properties.14,15 The current interest in PZT is ascribed to its interesting physical properties16,17 particu- larly for those with composition close to the morphotropic phase boundary~MPB! aroundx;0.45– 0.5, where all the properties are enhanced.18 The morphotropic phase boundary ~MPB! that separates rhombohedral Zr-rich PZT from tetrag- onal Ti-rich PZT is not well defined, since it appears to be associated with a region of phase coexistence whose width depends on the compositional homogeneity and on the pro- cessing conditions employed in its preparation. To overcome processing difficulties and chemical homogeneity problems, artificially engineered thin film heterostructures or superlat- tices with tetragonal and rhombohedral phase PZT offer a good alternative for tailoring the morphotropic-like phase boundary~MPB!. This approach to multilayered heterostruc- tures with MPB is expected to modulate and maximize the performance of the electrical properties of PZT thin films, potentially leading to a wealth of applications in electronic devices. Wanget al.11 have reported the enhancement of the dielectric constant in multilayered Pb(Zr0.2Ti0.8)O3 / Pb(Zr0.8Ti0.2)O3 thin films prepared by rf magnetron sputter- ing. Xu et al.6 reported on polycrystalline multilayered BaTiO3 /SrTiO3 thin films. A dielectric constant of 527 at 100 kHz was reported recently in multilayered SrTiO3 /BaTiO3 thin films, and was explained as the sum of each individual thin film using a series connection model.19 This letter reports the fabrication of a crack-free, dense, heterolayered PZT thin film with dielectric and ferroelectric properties characteristic of the MPB. However, its superior performance was artificially controlled and prepared by a chemical solution route. Figure 1 shows the schematic rep resentation of the ferroelectric structure considered here. The multilayered PZT thin films were grown on Pt/Ti/SiO2 /Si substrate by chemical solution routes. Two so-a!Electronic mail: derl@power.ufscar.br APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 26 28 JUNE 2004 54700003-6951/2004/84(26)/5470/3/$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 12:29:26 http://dx.doi.org/10.1063/1.1751623 lutions were prepared with nominal compositions of PbZr0.4Ti0.6O3 ~PZT40! and PbZr0.6Ti0.4O3 ~PZT60! for deposition by spin coating. The method employed in the preparation of chemical solutions for PZT thin films has been reported elsewhere.20 The first deposition layer was made with the PZT40 solution on the substrates, using spin coating at a 4600 rpm 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 heat treated in two stages. The first stage consisted of heating to 400 °C for 2 h todecompose the organic mate- rials. In the second stage, the films were heated to 700 °C for crystallization. The process was repeated for the second layer using the PZT60 solution. Alternating materials were depos- ited until the film consisted of six layers, with the first and last being PZT40 and PZT60 layers, respectively. The structure of the heterolayered thin films was ana- lyzed by x-ray diffraction~XRD! in the u–2u scan mode, recorded on a Rigaku diffractometer~D/max-2400! using Cu Ka radiation. The microstructure and thickness of the hetero- layered thin film were examined using, respectively, an atomic force microscope~AFM! ~Digital Instruments, Nano- scope IIIa! and a high-resolution transmission electron mi- croscope~HRTEM! ~JEOL 3010 operated at 300 kV!. The electrical properties of the Au/PZT40/PZT60/Pt capacitor structure were investigated. To prepare the capacitor cells, dots of Au were deposited on the multilayered thin films by evaporation through a shallow mask, on an area of 4.9 31022 mm2. The capacitor’s dielectric properties were measured by an HP4192A impedance/gain phase analyzer, while the ferroelectric properties were verified using a Radi- ant Technology RT6000HVS apparatus in virtual ground mode. The XRD patterns shown in Fig. 2 indicate that the het- erolayered thin films were polycrystalline in nature and had a perovskite structure. The XRD patterns consists of two sets, one for PZT40 and the other for PZT60, so the~101!/~110!, ~002!/~200!, and ~112!/~211! peaks are displayed separately in the heterolayered thin film. This result indicates that the tetragonal and rhombohedral phases coexist in the heterolay- ered thin film. The inset in the Fig. 2 clear shown the sym- metry signature of the PZT40 as begin of tetragonal nature and PZT60 as begins of rhombohedral nature. In addition, analysis by micro-Raman revealed clearly the tetragonal and rhombohedral nature of the PZT40 and PZT60, respectively. Wang et al. found a similar behavior in PZT20 and PZT80 thin films with heterolayered structures. Xuet al.6 and Pon- tes et al.19 reported that BaTiO3 /SrTiO3 heterolayered thin films did not diffuse together and did not form a solid solu- tion. It should be noted that the XRD patterns are composed of a BaTiO3 phase and a SrTiO3 phase. Figure 3 shows the cross section of the PZT heterolay- ered thin film analyzed by HRTEM. The thickness of the thin films after one drying/annealing cycle was approximately 25 nm. As can be seen in the cross-sectional micrograph, the interface between the PZT40 layer and the substrate is dis- tinct, presenting a good quality@see inset in Fig. 3~b!#. A total of six layers were deposited, varying the stacking se- quences to achieve a total thickness of about 150 nm. The surface morphology of the PZT60 layer, which was the top layer, was found to be very smooth, pinhole-free, and con- tinuous, devoid of cracks or rosette-like structures. The av- erage surface roughness of the heterolayered thin film was 3 nm and the average grain size varied from 50 to 60 nm; see inset in Fig. 2. Figure 4 shows the dielectric constant and dielectric loss as a function of frequency at room temperature. As can be seen, the dielectric constant of the heterolayered thin film was greater than that of single-phase thin films. Many studies have shown dielectric constant enhancement in heterolayered thin films associated with capacitors in series. To confirm the validity of capacitor series models, we have assumed the effective dielectric constant of the heterolayered structure to be a series connection model of poly-PZT40 and polyPZT60 dielectric layers, whose effective dielectric constant, FIG. 1. Schematic representation of the ferroelectric structure considered here. FIG. 2. X-ray diffractograms of~a! PZT40 thin film, ~b! PZT60 thin film, and~c! heterolayered PZT thin film. The inset shown a limited region x-ray data fitted to~101! and~110! reflections, as well as AFM micrograph~131 mm!. FIG. 3. ~a! Cross-sectional HRTEM micrograph of the heterolayered PZT thin film; The inset shows the film/film/substrate interface 15PZT40; 25PZT60. 5471Appl. Phys. Lett., Vol. 84, No. 26, 28 June 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 12:29:26 «eff , can be expressed theoretically as 1 «eff 5S t1 «PZT40 1 t2 «PZT60 1 t3 «PZT40 1 t4 «PZT60 1 t5 «PZT40 1 t6 «PZT60 D /~ t11t21t31t41t51t6!, ~1! wheret1 , t3 , and t5 are the thicknesses of poly-PZT40 and t2 , t4 , andt6 are the thicknesses of poly-PZT60. Thus, based on the heterolayered capacitors,«eff was calculated theoreti- cally to be about 481, assuming that the relative dielectric constants of PZT40 and PZT60 are 512 and 452, respec- tively, at a frequency of 100 kHz. These values were ob- tained from layers of PZT40 and PZT60 films prepared in- dividually. As shown in Fig. 4, the dielectric constant value was much higher than the latter (« r;701 at a frequency of 100 kHz!, and was calculated by assuming the series connec- tion model of two components. The calculated dielectric con- stant value showed a difference of more than 45% compared to the experimental dielectric constant value of the hetero- layered thin film. This suggests that heterolayered thin film cannot be explained as the sum of each individual thin film ~PZT40 and PZT60! using a series connection capacitor model. The behavior observed here is consistent with that reported by Wanget al.11 for heterolayer-structured PZT20/ PZT80 thin films obtained by rf magnetron sputtering. The hysteresis loops of single-layer PZT40, PZT60, and PZT heterolayered thin films are shown in Fig. 5. The hys- teresis loop of the single-layer PZT60~rhombohedral phase! and PZT40~tetragonal phase! thin films showed small rem- anent polarization values ofPr;7.9 and 18.5mC/cm2, re- spectively. On the other hand, the heterolayered PZT thin film exhibited a higher remanent polarization value than the other single-phase thin films withPr;31mC/cm2. In addi- tion, the remanent polarization value obtained here was found to be greater than the remanent polarization of 15 mC/cm2 reported by Wanget al.11 These findings demon- strate that inserting PZT60~rhombohedral phase! layers be- tween PZT40~tetragonal phase! layers improves the dielec- tric and ferroelectric properties of heterolayered thin film. These results can be interpreted as a cooperative interaction between two ferroelectric phases~tetragonal and rhombohe- dral! of similar energy. This cooperative effect, simulating MPB, allows for optimization of the reorientation of domains during the poling process.7,18 Due to the polycrystalline na- ture of the processed thin film, the stress effects caused by film/substrate mismatching can be disregarded. Thus, the in- fluence of this effect on the dielectric and ferroelectric prop- erties should be minimal. Consequently, we believe that the properties of heterolayered thin films made from alternating tetragonal and rhombohedral phases of PZT mimic those of PZT ceramic at the morphotropic phase boundary. In summary, the use of heterolayered thin films for achieving better performance than homogenous single lay- ered thin films is well established. We have shown here that heterolayered PZT thin films prepared by a chemical solution route provide remarkable improvement to the dielectric and ferroelectric properties. The enhancement might be inter- preted as the MPB effect, similar to the MPB effect in bulk PZT ceramics. Our results also demonstrate that the chemical solution route is an effective method for tailoring heterolay- ered structures to obtain specific properties or materials. 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