2004-2005 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Feng, SL

Song, ZT

Wang, Y

Applied Physics Letters

PolyU Institutional Repository

Microwave characterization of Pb,LaTiO3 thin films integratedon ZrO2/SiO2/Si wafers by sol-gel techniquesZ. T. Songa)Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hong Kong, China and The Research Center of Semiconductor Functional Film EngineeringTechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Science,Shanghai, ChinaY. Wang, H. L. W. Chan, and C. L. ChoyDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hong Kong, ChinaS. L. FengThe Research Center of Semiconductor Functional Film Engineering Technology, Shanghai Instituteof Micro-system and Information Technology, Chinese Academy of Science, Shanghai, China Science,Shanghai 200050, China(Received 18 May 2004; accepted 17 September 2004)Polycrystalline perovskite lead lanthanum titanate (PLT) thin films were prepared by a sol-gelmethod on ZrO2/SiO2/Si substrates. The structure of the films was studied by x-ray diffraction andscanning electron microscopy, and the microwave dielectric properties characterized on a networkanalyzer. A strong dependence of the dielectric constant of PLT films and, correspondingly, theresonance frequency of PLT-based interdigital capacitor on the sample preparation conditions wereobserved. They resulted from the structural transformation of PLT from a layered structure to auniform film as the annealing temperature was raised from 550 to 700 °C, suggesting a possibleway to modify the device performance by controlling the layered structure of the ferroelectricfilm. © 2004 American Institute of Physics. [DOI: 10.1063/1.1823038]While many of the ferroelectric/piezoelectric thin film-based devices may work at relatively low frequencies (usu-ally at megahertz or lower), in recent years ferroelectric andpiezoelectric devices working at higher frequencies [up togigahertz (GHz)] have also attracted strong interest. Typicalexamples of such devices include microwave phase shiftersand piezoelectric film-based biosensors for DNA and proteintests.1–6 Around the GHz range, the ferroelectric domainsusually cannot switch fast enough to keep up with the exter-nal ac field; consequently, the dielectric behaviors of the fer-roelectrics are dramatically different from their low fre-quency dielectric properties. The microwave characterizationof ferroelectric thin films is of significance both for funda-mental understandings and practical applications. Techni-cally, the high frequency dielectric measurement of ferro-electric films remains a challenge despite continued researchefforts in the last few decades on the development of varioustesting techniques and theoretical models for solving thetechnical problems. This may be the reason for the lack ofmicrowave characterization of many typical ferroelectric thinfilms, even though barium strontium titanate films, as a can-didate for tunable microwave devices, have been extensivelystudied in the last decade.7–9In this letter, the dielectric properties of lanthanum-doped lead titanate [nominal composition assPb0.85La0.10dTiO3 (PLT)] thin films integrated on bufferedsilicon were investigated at high frequencies up to the GHzrange. PLT is a very useful ferroelectric material that can beused for infrared micro-sensors, ferroelectric memories,electro-optical devices, and piezoelectric micro-sensors (forbiological applications).10–15 The structural development andcontrol, ferroelectric behavior, pyroelectric performance, andpiezoelectric properties of PLT have been extensively studiedin the literature.The samples for tests have a layered structure: PLTfilm/ZrO2/SiO2/Si. Both ZrO2 and SiO2 are the buffer lay-ers. The sample preparation was basically a multistep micro-fabrication process. On a (100)-oriented silicon wafer with a5.46 mm thermally grown SiO2 layer, ZrO2 film (200 nmthick) was deposited by sol-gel and spin coating techniquesfollowed by pyrolysis at 600 °C for 1 h in O2. On ZrO2, thePLT film was similarly coated layer by layer. By each spin-ning and drying, a ,50-nm-thick layer was formed. After 20layers were coated and dried, the whole stack was annealedin O2 at different temperatures (550, 600, 650, and 700 °C,all for 1 h). Details of the sample preparation have beendescribed elsewhere.15X-ray diffraction (XRD) revealed that the PLT film has apure perovskite phase and is randomly oriented, as shown inFig. 1. With increase of annealed temperature, the intensityof (100) peak had increasing trend. The microstructure of thesamples was observed under scanning electron microscopy(SEM). It was found that the film is very dense, smooth, andfree of cracks. The average grain size of PLT is about 50 nmin diameter. The cross-sectional micrographs of two samples(one annealed at 550 °C and another at 700 °C) are shownin Fig. 2. Apart from amorphous SiO2 and columnar-structured ZrO2, a layered structure is observed in the PLTlayer of the 550 °C-annealed sample [the parallel linesshown in Fig. 2(a)]. Quite obviously the layered structurewas formed because the final annealing temperatures550 °Cd was lower than the hydrolysis temperatures600 °Cd so that sufficient diffusion could not occur to elimi-nate the boundaries between the adjacent layers in PLT. Fora)Electronic mail: ztsong@itsvr.sim.ac.cnAPPLIED PHYSICS LETTERS VOLUME 85, NUMBER 20 15 NOVEMBER 20040003-6951/2004/85(20)/4696/3/$22.00 © 2004 American Institute of Physics4696the same reason, such layered structure was also observed inthe 600 °C-annealed sample (SEM picture not shown). Forthe sample annealed at 650 and 700 °C, the layer boundariesdisappeared and the whole PLT became a uniform singlelayer [Fig. 2(b)]. As will be demonstrated later in this letter,the structural evolution has led to very different dielectricproperties in the samples.The dielectric characterization was conducted at roomtemperature on the Au/PLT/ZrO2/SiO2/Si coplanar capaci-tors. On the PLT film, a gold layer was sputtered by magne-tron sputtering and then patterned by photolithography andwet chemical etching to form the interdigital electrodes(IDEs). The finger width of the IDEs is 3 mm, the separationbetween two adjacent fingers is 5 mm, and the finger lengthis 25 mm. The total number of fingers is 21 (refer to Ref. 15for the electrode configuration). The measurement of theseinterdigital capacitors (IDCs) was carried out on a vectornetwork analyzer 8720ES (Agilent, USA) together with amicoprobe station RF-1 (Cascade, USA). Before the mea-surement, the test system was carefully calibrated followingthe operation instruction. Then the reflection parameter S11of the IDCs was determined and from S11 the impedance wasobtained. The dielectric constant of PLT was extracted fromthe impedance by using a program that we developed basedon the Gevorgian’s model.8,16,17Figure 3 shows the frequency dependence of the dielec-tric constant of the PLT films annealed at different tempera-tures. In the whole frequency range, the sample annealed at ahigher temperature has a larger dielectric constant than thesample annealed at a lower temperature, as shown in Fig.3(a). At f =1 GHz, for example, the dielectric constant of thesamples annealed at 550, 600, 650, and 700 °C was found tobe 157, 158, 222, and 269, respectively. It is interesting tonote that the dielectric constant of the 550 °C-annealedsample is only about 40% of the dielectric constant of the700 °C-annealed sample (the difference is even larger atlower frequencies), while the XRD and SEM observation didnot show significant difference in the two samples on thecrystallinity and grain size. Thus we propose that the criticalfactor that causes the difference is the layered structure ofPLT. Just like many other interface layers in ferroelectricheterostructures, the interfaces inside the PLT films havesmaller dielectric constant, leading to the lowering of thedielectric constant of the whole PLT layer in the550 °C-annealed sample (and similarly in the600 °C-annealed one). For the sample annealed at 700 °C,the dielectric constant is larger because it dose not have suchinterfaces. For the same reason, the loss tangent of thesamples decreases as the annealing temperature increases, asshown in Fig. 3(b).When observed over a larger frequency range, an inter-esting relationship between the dielectric resonance and thesample structure can be established. As shown in Fig. 4,resonance peaks can be clearly identified in the loss tangentversus frequency plot [Fig. 4(b)], which correspond to abruptdecreases of the capacitance in the capacitance versus fre-quency graph [Fig. 4(a)]. The resonance frequencies fr of thesamples (annealed at 550, 600, 650, and 700 °C) are 6.2, 6.3,FIG. 2. Cross-sectional observation under SEM of PLT/ZrO2/SiO2/Si: (a)layered structure in PLT in the sample annealed at 550 °C, and (b) uniformPLT layer in the sample annealed at 700 °C.FIG. 1. X-ray diffraction of PLT integrated on ZrO2/SiO2/Si and annealedat different temperatures.FIG. 3. Frequency dependence of (a) the in-plane dielectric constant, and(b) the loss tangent of PLT films.Appl. Phys. Lett., Vol. 85, No. 20, 15 November 2004 Song et al. 469711.1, and 13.3 GHz, respectively. When the frequency de-pendence of the capacitance increases into the range of reso-nance, the capacitance of the device rapidly decreases andtends to zero, meanwhile, the loss tangent of the device rap-idly increases, then decreases until negative, but the curve offrequency dependence of the capacitance of the device endsthe resonance frequency. Therefore, after the resonance fre-quency, the negative loss tangent of no capacitance had nomeans. A correlation in the fr shift and the layered structureof PLT can also be established: A low annealing temperatureleads to a layered structure and results in a low fr and viceversa.18The experimental results indicate a strong dependence ofthe dielectric performance of the ferroelectric film and deviceon annealing temperature. Such dependence may have beenobserved in many other ferroelectric films.19 Nevertheless,the mechanism of such dependence in the literature is usuallyrelated to the grain size, crystallinity, and/or orientation ofthe ferroelectric film. In our work, however, the dominantfactor of the mechanism is the layered structure that exists insome of the samples and disappears in the others. Techni-cally, this result implies that control of the layered structuremay be another way to modify the dielectric property of theferroelectric films and the performance of the devices.In summary, we have fabricated and characterized themicrowave properties of perovskite PLT films onZrO2/SiO2/Si substrate. Depending on the annealing tem-perature, the in-plane dielectric constant of the PLT film at1 GHz was found to vary from ,160 to ,260. Correspond-ingly, the resonance frequency of the interdigital capacitancechanged from 6.1 to 13.3 GHz. The strong dependence ofthe dielectric properties of the films on the annealing condi-tions was explained as a result of the evolution of PLT froma layered structure to a uniform film as the annealing tem-perature was raised from 550 to 700 °C.This work was supported by the Center for Smart Mate-rials of The Hong Kong Polytechnic University, the NationalHigh Technology Development Program of China underGrant No. 2003AA302720 and 2004AA302G20, ShanghaiNanotechnology Promotion Center (0352nm016,0452nm012), Chinese Academy of Sciences Foundation, theFore-research of basic research project (2001CCA02800),and Science and Technology Council of Shanghai(03dz11009, 04JC14080).1A. F. Collings and F. Caruso, Rep. Prog. Phys. 60, 1397 (1997).2D. Galt, J. C. Price, J. A. Beall, and R. H. Ono, Appl. Phys. Lett. 63, 3078(1993).3M. J. Lancaster, J. Powell, and A. Porch, Supercond. Sci. Technol. 11,1323 (1998).4A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N.Setter, J. Electroceram. 11, 5 (2003).5K. M. Lakin, G. R. Kline, and K. T. McCarron, IEEE Trans. MicrowaveTheory Tech. 43, 2933 (1995).6R. Gabl, H. D. Feucht, H. Zeininger, G. Eckstein, M. Schreiter, and R.Primig, Biosens. Bioelectron. 19, 615 (2004).7S. W. Kirchoefer, J. M. Pond, A. C. Carter, W. Chang, K. K. Agarwal, J.S. Horwitz, and D. B. Chrisey, Microwave Opt. Technol. Lett. 18, 168(1998).8S. S. Gevorgian, T. Martinsson, P. L. J. Linner, and E. L. Kollberg, IEEETrans. Microwave Theory Tech. 44, 896 (1996).9H. D. Wu, Z. H. Zhang, F. Barnes, A. Kain, and J. D. Cuchiaro, IEEETrans. Appl. Supercond. 4, 156 (1994).10Z. G. Dong, M. R. Shen, and W. W. Cao, Appl. Phys. Lett. 82, 1449(2003).11S. J. Lee, K. Y. Kang, S. K. Han, M. S. Jang, B. G. Chae, Y. S. Yang, andS. H. Kim, Appl. Phys. Lett. 72, 299 (1998).12A. Patel and J. S. Obhi, GEC J. Res. 12, 141 (1995).13S. B. Majumder, M. Jain, and R. S. Katiyar, Thin Solid Films 402, 90(2002).14S. J. Kang, D. H. Chang, and Y. S. Yoon, Thin Solid Films 373, 53(2000).15Z. T. Song, N. Chong, H. L. W. Chan, and C. L. Choy, Appl. Phys. Lett.79, 668 (2001).16Y. L. Cheng, N. Chong, Y. Wang, J. Z. Liu, H. L. W. Chan, and C. L.Choy, Integr. Ferroelectr. 55, 939 (2003).17S. Gevorgian, E. Carlsson, S. Rudner, L. D. Wernlund, X. Wang, and U.Helmersson, IEE Proc., Part H: Microwaves, Antennas Propag. 143, 397(1996).18At the gigahertz frequency range, these devices cannot be regarded aspurely capacitive. Therefore in the equation fr=1/2p˛LC, L is not a con-stant value because the contribution of the devices may change with fre-quency [if L does not change, then fr of the high « material should besmaller than that of the low « material, opposite to the observation shownin Fig. 4(a)]. The reason for sample A having a smaller fr value thansample D, for example, can be explained to be a result of, first, the productof L and C of sample A being larger than the latter (in another world, theL value of sample A must be much larger than the L value of sample D).Because of the very complicated device structure and measurement circuit,however, we have not been able to have an equivalent circuit for an ac-curate and precise description of the behavior of our devices under thesemeasurement conditions. Second, with different annealed temperatures,PLT film having different dielectric property which are different dielectricconstant and range of resonance. From Fig. 4, the capacitance trend tozero which the resonant take place at. Therefore, the samples with differ-ent temperatures cannot get all data from 60 MHz to 20 GHz, they begin60 MHz and end of resonant frequency. What are the reasons for the rangeof resonance, etc.? Further investigation on this issue is still to be under-taken.19C. R. Cho, W. J. Lee, B. G. Yu, and B. W. Kim, J. Appl. Phys. 86, 2700(1999).FIG. 4. Dielectric resonance of IDE/PLT/ZrO2/SiO2/Si capacitors—thefrequency dependence of (a) the capacitance, and (b) loss tangent.4698 Appl. Phys. Lett., Vol. 85, No. 20, 15 November 2004 Song et al.

Microwave characterization of (Pb,La)TiO₃ thin films integrated on ZrO₂/SiO₂/Si wafers by sol-gel techniques

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