2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Chong, N

Choy, CL

Liu, Z

Ming, N

Wong, KH

Zhu, X

Applied Physics Letters

PolyU Institutional Repository

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 18 6 MAY 2002Epitaxial growth and planar dielectric properties of compositionally gradedBa1ÀxSrxTiO3 thin films prepared by pulsed-laser depositionXinhua Zhua)Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong Kong, People’s Republic of ChinaNui Chong, Helen Lai-Wah Chan, Chung-Loong Choy, and Kin-Hung WongDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong Kong, People’s Republic of ChinaZhiguo Liu, and Naiben MingNational Laboratory of Solid State Microstructures, Department of Physics, Nanjing University,Nanjing 210093, People’s Republic of China~Received 16 November 2001; accepted for publication 2 March 2002!We have heteroepitaxially deposited compositionally graded (Ba12xSrx)TiO3 ~BST! thin films withincreasing x from 0.0 to 0.25 on ~100!-oriented MgO substrates using pulsed-laser deposition. Thecompositional gradients along the depth in the graded films were characterized by Rutherfordbackscattering spectroscopy. By using surface interdigital electrodes, the planar dielectric responseof epitaxial graded BST films was measured as a function of frequency, temperature, and dc appliedvoltage. At room temperature, the dielectric constant of the graded BST film was about 450 with adielectric loss, tan d of 0.007 at 100 kHz. Measurements varying the dc bias voltage showedhysteresis of the dielectric response and a tunability of 25% at an applied electric field of 80 kV/cm.The graded BST films undergo a diffuse phase transition with a broad and flat profile of thecapacitance versus temperature. Such behavior of the dielectric response in graded BST films isattributed to the presence of the compositional and/or residual strain gradients in the epitaxial gradedfilms. With such a graded structure, it is possible to a build a dielectric thin-film capacitor with alow-temperature dependence of the capacitor over a broad temperature regime. © 2002 AmericanInstitute of Physics. @DOI: 10.1063/1.1475367#(Ba12xSrx)TiO3 ~BST! ferroelectric thin films exhibit alarger electric field dependent dielectric constant, and haveattracted considerable attention for applications in active mi-crowave electronics such as microwave phase shifters, fre-quency agile filters, and tunable high-Q resonators.1–3 Pres-ently, for tunable device applications, coplanar electrodeconfigurations are frequently used in tunable microwave de-vices that utilize ferroelectric films. The coplanar tuning con-figurations are more compatible with planar microwave cir-cuitry because they are readily designed to have capacitancevalues that provide good impedance matching to the rest ofthe circuit.4 However, there is a concern in the practical ap-plications of tunable microwave resonators based on a singlecomposition of BST films that the resonant frequency de-pends strongly on the temperature changes because the di-electric constant («r) of a single compositional BST film~e.g. Ba0.5Sr0.5TiO3! is highly temperature dependent. Suchpoor temperature stability of the resonant frequency wouldresult in the carrier signal drifting in and out of resonance onhot and cold days.5 It would therefore be important to com-pensate its temperature coefficient of dielectric constant~TCK! without decreasing its high «r . To do so, we havedeveloped compositionally graded BST thin films, in whichthe Curie temperature is tailored by the compositional gradi-a!Also at: National Laboratory of Solid State Microstructures, Department ofPhysics, Nanjing University, Nanjing 210093, P. R. China; electronic mail:xhzhu@public1.ptt.js.cn3370003-6951/2002/80(18)/3376/3/$19.00ents to meet the requirements of low TCK value and high «rdielectric. Such graded materials are highly preferred in theapplications where the capacitance of a device should onlybe weakly dependent on temperature. Furthermore, for themicrowave dielectric applications, oriented grain or epitaxi-ally grown films are also highly required. In this work, wereport on the epitaxial growth of compositionally gradedBST films with increasing x from 0.0 to 0.25, which weredeposited directly on MgO ~100! single-crystal substrates bypulsed-laser ablation. The planar dielectric properties of ep-itaxial graded BST films were also measured through surfaceinterdigital electrodes ~IDEs! as a function of frequency,temperature, and applied dc bias.The compositionally graded BST thin films were grownlayer by layer on MgO ~100! single-crystal substratesby pulsed-laser deposition ~PLD!. The PLD system usedto grow graded BST films has been described previouslyin more detail.6 The stoichiometric sintered ceramic targetsof BaTiO3 , Ba0.9Sr0.1TiO3 , Ba0.8Sr0.2TiO3 , and Ba0.75Sr0.25TiO3 were used in this work for the film deposition. Thegraded BST films with a bottom layer of pure BaTiO3 and asurface layer of Ba0.75Sr0.25TiO3 were grown on MgO ~100!single crystals at 700 °C in an oxygen ambient pressure of200 mTorr to a total film thickness of 450 nm. After deposi-tion, the graded BST films were in situ annealed at 700 °Cfor 30 min, and then cooled down to room temperature.X-ray diffraction indicated that only the (h00) peaks ofgraded BST films were observed, suggesting a ~100!-6 © 2002 American Institute of Physics3377Appl. Phys. Lett., Vol. 80, No. 18, 6 May 2002 Zhu et al.preferred growth orientation. Rocking curve and f scanswere also used to examine the misalignment of the latticeand epitaxial growth of BST graded thin films. The resultsshowed that the full width half maximum of BST ~200! rock-ing curve was 0.41°, and the requisite four-fold symmetrywas also observed in the scans of the off-axis ~220! planes inthe graded BST crystal lattice, indicating that the gradedfilms were grown epitaxially on MgO ~100! substrates.Atomic force microscopy was also used to analyze the sur-face morphology of the graded films. A columnar grain struc-ture was observed. The mean size of the grains was 100 nm,and the root-mean-square surface roughness was in 3–8 nmrange. The compositional gradients in the film thickness di-rection were examined by Rutherford backscattering spec-troscopy ~RBS!.The planar dielectric properties of the graded films weremeasured by using IDEs technique. The IDEs were patternedby conventional photolithography. A 100 nm-thick Cr/Au bi-layer was deposited on the surface of the graded BST filmsby magnetron sputtering. The IDEs were formed after stan-dard lift-off techniques. The coplanar capacitor consists of 80pairs fingers that are 1.5 mm long, 10 mm wide, and space 5mm apart. Electrical connection to a Hewlett Packard HP4194A test fixture was made by using coaxial tables. Thefrequency response of the graded BST films was character-ized at room temperature, in the frequency range of102 – 107 Hz by a HP 4194A impedance analyzer by applyinga small signal of 100 mV amplitude. The dielectric constantof the graded films was extracted from the capacitance usingthe conformal mapping technique.7 Temperature dependenceof the planar dielectric properties was measured by placingsamples in a closed-cycle refrigerator. The measurementswere carried out on automated system, whereby a HP 4194Aimpedance analyzer and the temperature-control box werecontrolled by a computer. The experimental data were col-lected continuously at frequency between 103 and 106 Hz.The temperature range was between 220 °C and 130 °C, andthe maximum rate of cooling the samples was less than 1 °C/min. The capacitance–voltage (C – V) measurement was per-formed at room temperature by applying a small signal of100 mV and 1 MHz frequency across the sample while thedc bias voltage was swept from a negative bias to positivebias. The dielectric tunability was deduced from the C – Vcharacteristics.The compositional gradient in the graded BST thin filmcharacterized by RBS is shown in Fig. 1. The RBS spectrumshown in Fig. 1 was measured with a 2.9 MeV He1 ionbeam at a tilt of 5° and a scattering angle of 170°. As shownin Fig. 1, the RBS data clearly confirmed the expected in-creasing Ba content and the decreasing Sr content toward thebottom of the graded film, which in the graded structure ispure BaTiO3 . The measured compositional gradients of Baand Sr elements in the graded films along the thickness di-rection were consistent with that of stoichiometric ceramictargets during laser ablation. The thickness of the graded thinfilm determined by RBS data was also in agreement with thatobtained from cross sectional scanning electron microscopyimages.The frequency response of the graded BST films depos-ited on a MgO ~100! substrate is shown in Fig. 2. It can beseen that the measured capacitance exhibited minimal disper-sion as a function of the frequency. Using the conformalmapping technique, the dielectric constant («r) at 100 kHzof the graded films was calculated to about 450. The dielec-tric loss (tan d) showed no noticeable dispersion at frequen-cies up to 1 MHz. The loss tangent at 100 kHz was about0.007. The temperature dependence of the dielectric constantof the graded BST film is shown in Fig. 3. A broadeningtransition peak at about 90 °C was observed, that correspondsto the tetragonal-cubic phase transition. The maximum tem-perature, where the value is frequency dependent in the fre-quency range of 103 – 106 Hz, shifts toward higher tempera-ture with increasing frequency. The broadening andflattening of the peak is typical of diffuse transitions as ob-served in some polycrystalline BST and BaTiO3 thin films.8,9In addition, the profile of the dielectric constant versus tem-perature for the graded BST film experienced linear charac-teristics with a slope of 1.57/K in the temperature range from220 °C to 90 °C. Furthermore, the slope value could be tai-lored by changing the volume fraction of each layer. Theinset in Fig. 3 is the corresponding relative change of «r ,with respect to temperature of the graded thin film. As can beseen in Fig. 3 with such a graded structure, it is possible tobuild a dielectric thin-film capacitor with a low-temperaturedependence of the capacitor over a broad temperature re-gime. The feature of the flat profile of the dielectric constantFIG. 1. RBS spectrum of graded BST thin film deposited on a MgO ~100!substrate. The composition of the bottom layer is pure BaTiO3 .FIG. 2. Variation of the coplanar capacitance and dielectric loss tangent ofgraded BST thin film deposited on a MgO ~100! substrate as a function ofthe measured frequency. The measurement was performed on 80 pairs fin-gers of IDEs.3378 Appl. Phys. Lett., Vol. 80, No. 18, 6 May 2002 Zhu et al.versus temperature for graded BST thin films may be ex-plained by the presence of the compositional and/or residualstrain gradients in the epitaxial film as a result of the partialcoherence of the film with the substrate.10 Figure 4 shows thecapacitance versus surface electric-field characteristics forgraded BST thin film deposited on a MgO ~100! substrate.The measurement was carried out at room temperature and 1MHz. The dielectric constant at zero bias was calculated tobe 440, and a decrease in the dielectric constant from ;440to ;330 was observed with increasing dc bias voltage. Thus,an applied field of 80 kV/cm results in a tunability of 25%.This value is comparable to that of polycrystalline BST thinfilms11 ~reported to be 25% at an applied electric field of50–100 kV/cm!, and to that of epitaxial BaTiO3 thin films~30% at the field of ;70 kV/cm!,12 but it is smaller than thatreported by Carlson et al.13 @about 50%–60% of annealedepitaxial (Ba0.4Sr0.6)TiO3 film at 260 K and a field of 57kV/cm#. The main reason is that the epitaxial(Ba0.4Sr0.6)TiO3 film reported by Carlson et al.13 was ex situpostannealed at high temperatures of ;1100 °C in a quartztube furnace under flowing O2 at pressure of 50–100 mTorr,and such a high temperature postanneal treatment would bebeneficial to improving the grain size14 and crystallinity15 ofFIG. 3. Temperature dependence of the coplanar dielectric constant, « r ofgraded BST films measured at different frequencies. Inset is the correspond-ing relative change of dielectric constant with respect to temperature of thegraded thin film.FIG. 4. Variation of capacitance of graded BST films as a function of sur-face electric field at room temperature and 1 MHz. The measurement wasperformed on 80 pairs fingers of IDEs.the film. The present data are only from the preliminary testfor the graded films. Improvements in the film quality byoptimizing the PLD processing parameters are expected toincrease the dielectric properties, and the design of thegraded film with layers of high Sr contents would exhibit anessentially broader plateau region of the dielectric constantmaximum than the isotropic thin films. Impedance-bias mea-surements also show hysteresis indicating that the gradedfilms are ferroelectric.In conclusion, compositionally graded BST films wereepitaxially deposited layer by layer on a MgO ~100! substrateby PLD. The compositional gradients in the graded BST thinfilms along the depth were examined by RBS. The dielectricresponse of the epitaxial graded BST films was characterizedby surface IDEs as a function of frequency, temperature, andapplied dc bias. The dielectric constant at room temperaturewas about 450 with a dielectric loss, tan d, of 0.007 at 100kHz. An applied electric field of 80 kV/cm resulted in a 25%modulation of the dielectric constant. A broad and flat profileof dielectric constant versus temperature was observed ingraded BST films. Such dielectric behavior can be attributedto the presence of the compositional and/or strain gradientsin the epitaxial graded films.This work is financially supported by the Center forSmart Materials of The Hong Kong Polytechnic Universityand ASD through the project ‘‘Characterization of Smart Ma-terials’’ ~Project No. 1.11.37.A060!, and partially funded bythe National Natural Science Foundation of China~59832050!, the opening project of National Laboratory ofSolid State Microstructures, and by a grant for State KeyProgram for Basic Research of China. One of the authors~X.H.Z.! would like to acknowledge the Smart MaterialsCenter of Hong Kong Polytechnic University for his finan-cial support and also Keli fellowship from Nanjing Univer-sity.1 J. P. Goot, S. Trolier-McKinstry, and D. G. Schlom, Proceedings of theTenth IEEE International Symposium on Applications of Ferroelectrics,ISAF’96, 1996, Vol. 1, p. 333.2 V. K. Varadan, D. K. Gohdgaonkar, V. V. Varadan, J. F. Kelly, and P.Glikerdas, J. Microwave 35, 116 ~1992!.3 F. A. Miranda, R. R. Romanofsky, F. W. Van Keuls, C. H. Mueller, R. E.Treece, and T. V. Rivkin, Integr. Ferroelectr. 17, 231 ~1997!.4 D. C. DeGoot, J. A. Beall, R. B. Marks, and D. A. Rudman, IEEE Trans.Appl. Supercond. 5, 2272 ~1995!.5 R. J. Cava, J. Mater. Chem. 11, 54 ~2001!.6 X. H. Zhu, S. G. Lu, C. L. Choy, H. L. W. Chan, and K. H. Wong, Integr.Ferroelectr. 36, 73 ~2001!.7 S. S. Gevorgian, T. Martinsson, P. L. J. Linner, and E. L. Kollberg, IEEETrans. Microwave Theory Tech. 44, 896 ~1996!.8 V. M. Mukhortov, Y. S. Nikitin, M. G. Rdchenko, V. A. Aleshin, Y. I.Golovko, and V. P. Dudkevich, Sov. Phys. Tech. Phys. 31, 805 ~1986!.9 Z. Surowiak, E. Nogas, A. M. Margolin, I. N. Zakharchenko, and S. V.Biryukov, Ferroelectrics 115, 21 ~1991!.10 J. Chen, L. A. Wills, B. W. Wessels, D. L. Schultz, and T. J. Marks, J.Electron. Mater. 22, 701 ~1993!.11 S. Bilodeau, R. Carl, and P. C. Van Buskirk, Mater. Res. Soc. Symp. Proc.415, 219 ~1996!.12 B. H. Hoerman, G. M. Ford, L. D. Kaufman, and B. W. Wessels, Appl.Phys. Lett. 73, 2248 ~1998!.13 C. M. Carlson, T. V. Rivkin, P. A. Parilla, J. D. Perkins, D. S. Ginley, A. B.Kozyrev, V. N. Oshadchy, and A. S. Pavlov, Appl. Phys. Lett. 76, 1920~2000!.14 M. P. McNeal, S. J. Jang, and R. E. Newnham, J. Appl. Phys. 83, 3288~1998!.15 M. S. Tsai, S. C. Sun, and T. Y. Tseng, J. Appl. Phys. 82, 3482 ~1997!.

Epitaxial growth and planar dielectric properties of compositionally graded (Ba[sub 1-x]Sr[sub x])TiO₃ thin films prepared by pulsed-laser deposition

Epitaxial growth and planar dielectric properties of compositionally graded (Ba[sub 1-x]Sr[sub x])TiO₃ thin films prepared by pulsed-laser deposition

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