Sol-gel deposition and piezoelectric properties of {110}-oriented Pb ( Zr 0.52 Ti 0.48 )O 3 thin filmsD. Ambika, Viswanathan Kumar, Hideyuki Imai, and Isaku Kanno Citation: Applied Physics Letters 96, 031909 (2010); doi: 10.1063/1.3293446 View online: http://dx.doi.org/10.1063/1.3293446 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of pulse poling on the piezoelectric property of Pb(Zr0.52,Ti0.48)O3 thin films AIP Advances 4, 117116 (2014); 10.1063/1.4901912 Growth and properties of gradient free sol-gel lead zirconate titanate thin films Appl. Phys. Lett. 90, 062907 (2007); 10.1063/1.2472529 Structure and properties of W O 3 -doped Pb 0.97 La 0.03 ( Zr 0.52 Ti 0.48 ) O 3 ferroelectric thin filmsprepared by a sol-gel process J. Appl. Phys. 98, 034104 (2005); 10.1063/1.1999834 Electrical properties of sol-gel-derived Pb ( Zr 0.52 Ti 0.48 ) O 3 thin films on a PbTiO 3 -coated stainlesssteel substrate Appl. Phys. Lett. 81, 4805 (2002); 10.1063/1.1528288 Dielectric and piezoelectric properties of sol–gel derived lead magnesium niobium titanate films with differenttextures J. Appl. Phys. 89, 568 (2001); 10.1063/1.1324685

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Sol-gel deposition and piezoelectric properties of ˆ110‰-orientedPb„Zr0.52Ti0.48…O3 thin films

D. Ambika,1 Viswanathan Kumar,1,a� Hideyuki Imai,2 and Isaku Kanno2

1Centre for Materials for Electronics Technology (C-MET), (Department of Information Technology,Scientific society, Ministry of Communication and Information Technology, Govt. of India), Athani (PO),Thrissur, 680 771 Kerala, India2Micro Engineering, Kyoto University, Kyoto 606-8501, Japan

�Received 15 October 2009; accepted 21 December 2009; published online 22 January 2010�

Pb�ZrxTi1−x�O3 �PZT� thin films of thickness 2.0 �m were fabricated on silicon substrates�111�Pt/Ti/SiO2/Si using a sol-gel spin-coating technique. PZT films on substrates with a strontiumtitanate bottom layer are preferentially �110�-oriented with a columnar structure. The PZT filmsexhibit good dielectric properties with a dielectric permittivity, �r=1545 and dielectric loss, tan �=0.04. Excellent piezoelectric characteristics are also exhibited by the �110�-oriented films with anaverage effective transverse piezocoefficient, e31

� of −8.4 C /m2. The influence of film texture andcomposition on the transverse piezocoefficient have also been studied. © 2010 American Instituteof Physics. �doi:10.1063/1.3293446�

Ferroelectric thin films based on lead zirconate titanate�PZT� have received significant attention in view of theirapplications in microelectromechanical systems �MEMS�. Inpiezomicroactuators, fabrication of piezoelectric thin filmswith thickness in the range 0.5–2 �m are necessary to real-ize maximum displacement and force.1 PZT films of thick-ness in this range have been reported to be fabricated bytechniques such as aerosol deposition,2 Chemical solutiondeposition �CSD� �Refs. 3–5� and sputtering.6 For microac-tuator applications, the geometry of the thin film structuremakes their effective in-plane transverse piezoelectric coef-ficient, e31 the most important parameter to be considered.The important factors which influence e31 are the film thick-ness, density, film texture, and the compositional gradientacross the film thickness.1,7 Chemical solution deposition�CSD� methods like Sol-gel and metal organic decomposi-tion, not only are economical but also are promising for pro-ducing dense microstructures.8 However during CSD thelarge shrinkage of the films during annealing and the thermalmismatch with the silicon substrate often lead to the forma-tion of cracks and/or high porosities thereby limiting theirapplications in microactuators. One of the approaches forminimizing the shrinkage stresses during thin film crystalli-zation is the increase in the inorganic content in the precursorsolutions.9 SrTiO3 �ST� has been reported to act as an effec-tive seed layer for the low-temperature crystallization of PZTthin films.10 Thus, in conjunction with the use of ST templatelayers, a careful control of the solution chemistry also as-sumes significance in the context of fabrication of dense,crack-free PZT thin films. In this study, we report the fabri-cation of �110�-oriented PZT thin films of high-quality byspin-coating technique and also evaluate their transverse pi-ezoelectric coefficient e31 by measuring the piezoelectric vi-bration of PZT/Si. unimorphs.

The ST bottom layer �50 nm� was formed by spin coat-ing the precursor solution on �111�Pt/Ti/SiO2/Si substrates asreported earlier.8 The PZT precursor solution was preparedas per the procedure described earlier.11 The PZT �52/48�

films with an MPB composition having a 10 mol % excesslead was spin coated on to a ST precoated substrate at aspeed of 4000 rpm for 25 s. After coating each layer, the filmwas dried at 110 °C for 10 min and then subjected to anintermediate anneal at a temperature of 400 °C for 15 minbefore being finally annealed at a temperature of 650 °C for15 min. This process was repeated till a PZT film thicknessof about 2.0 �m. was achieved. The crystalline phases andorientation of the PZT thin films were characterized by x-raydiffractometer �D5005, Bruker, Germany�. Gold top elec-trodes �0.6 mm diameter� were deposited on to PZT thinfilms using rf sputtering. Capacitance and dielectric losses atroom temperature were measured using an impedance ana-lyzer �HP 4294A, Agilent, USA� at 1 kHz. FerroelectricPolarization-Electric field �P-E� hysteresis loops were mea-sured using a TF Analyzer 2000 �aixACCT, Germany�. Forthe measurement of the piezoelectric properties, rectangularspecimens of dimensions �11.68�1.5� mm2 were dicedfrom the substrates with the PZT films. The transverse piezo-electric properties of the PZT thin films were evaluated usingunimorph cantilevers of PZT/Si. The details of the measure-

a�Electronic addresses: vkumar10@yahoo.com and vkumar@cmet.gov.in.

FIG. 1. �Color online� XRD pattern of �a� ST bottom layer and the evolutionof �110�–orientation in PZT as a function of increasing film thickness �b�0.5 �m, �c� 1.0 �m, and �d� 2.0 �m. on ST/Pt/Ti/SiO2/Si. Inset shows theXRD of the PZT film without ST intermediate layer on Pt �111�/Ti/SiO2/Sisubstrate.

APPLIED PHYSICS LETTERS 96, 031909 �2010�

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ments have been described in the previous reports.12,13 Ap-plication of sine wave voltage between upper and bottomelectrodes generates the deflection by the transverse inverse-piezoelectric effect, and the tip displacement was measuredusing a laser Doppler vibrometer �Graphtec, AT-3500� and alaser interferometer �Graphtec, AT-1100�.

The x-ray diffraction �XRD� patterns of the PZT thinfilms shown in Fig. 1 indicate the presence of only the per-ovskite phase. Usually, PZT films deposited on Pt�111�/Ti/SiO2/Si are polycrystalline or exhibit either �100� or�111�-orientation.14–17 Preferred �110�-orientation has beenreported in sol-gel deposited PbTiO3-based thin films.18 Thehigh degree of �110�-orientation of PZT film in this case isattributed to the influence of the ST bottom layer. The bottomST layer shows a preferred �110�-orientation �Fig. 1�a�� andacts as a seed for the nucleation of PZT during annealing.The crystal orientation is controlled by the growth step. Thecross-sectional high resolution scanning electron micrographfor the PZT film on the substrate reveals a dense columnarstructure as shown in Fig. 2. The P-E hysteresis loop �Fig.3�a�� with a remnant polarization �2Pr=23.0 �C /cm2� ischaracteristic of a �110�-oriented PZT film.19 The thin filmhas a dielectric permittivity, �r=1545, dielectric loss, tan �=0.04 and a leakage current density of 2.8�10−7 A /cm2 atE=100 kV /cm �Fig. 3�b�� which demonstrates their highquality. Transverse piezoelectric properties of the PZT thinfilms were evaluated from the deflection of PZT/Si. cantile-vers by applying unipolar sine wave voltage. Prior to thedeflection measurements, we observed the resonant fre-quency of the test sample because the deflection characteris-

tics were analyzed on the basis of the static deflection of anideal unimorph cantilever model.20 In this experiment, wemeasured the tip displacement at the frequency 6500 Hz.Figure 4�a� shows the tip displacement, due to the transversepiezoelectric effect, of the PZT thin film as a function ofapplied unipolar voltage. The nonlinearity is attributed to thedomain wall motion. From the tip displacements shown inFig. 4�a�, the transverse piezoelectric coefficient e31

� wasevaluated, as described earlier.12,13

e31� =

d31

s11,pE � −

hs2

3s11,sL2

�

V,

where �, V, L, h, and s11 are the tip displacement, appliedvoltage between top and bottom electrodes, length ofthe cantilever, thickness, and the elastic compliance,respectively.20 The subscripts of “s” and “p” denote the sub-strate and the piezoelectric film, respectively. From the aboveequation we evaluated the piezoelectric properties of PZTthin films and the voltage dependence of e31

� is also shown inFig. 4�a�. The transverse piezoelectric compliance e31

� of thePZT thin film can reach upto a value of −10.1 C /m2 at E=100 kV /cm. The average value of −8.4 C /m2. for e31

� ob-tained in this case is close to the value of −9.6 C /m2

expected21 for a PZT of composition near MPB �e31

=−93.5 C /N / �13.8−4.07� m2 /N=−9.6 C /m2�. Figure4�b� shows the tip displacement of the cantilever as a func-tion of a bipolar sine wave voltage of �60 V at 100 Hz. Forbipolar excitation, the effects of polarization reversal is alsoincluded in the deflection. We could obtain the clear butterflycurve representing domain switching of ferroelectricity. Thehigh value of e31

� obtained in this case is attributed to thepreferred �110�-orientation and the dense columnar micro-structure �Fig. 2�. The influence of film orientation on e31

� ofPZT thin films may be explained from the polarization direc-tions for each structure. In PZT of MPB composition, tetrag-onal and rhombohedral phases coexist. In the case of �110�-oriented films with tetragonal structure, as shown in Fig.5�a�, there are two types of polarization directions. The firsttype �a�� makes an angle of 45° and the second type �b�� anangle of 90° whereas in the rhombohedral phase, the firsttype �c�� makes an angle of 35°15� and the second type �d��an angle of 90° with the film normal. The �110� polarizationvector, �e��, lying in the �110� plane is along a direction be-tween the �111� and �100� polar axis making it easy for thepolarization to rotate from one direction to another �Figs.5�a� and 5�b��. It is expected that the residual stress in the

FIG. 2. HRSEM of the cross section of PZT thin film on ST/Pt/Ti/SiO2/Sishowing columnar grain growth.

FIG. 3. �Color online� �a� P-E hysteresis loop and �b� leakage current ofPZT thin film on ST/Pt/Ti/SiO2/Si.

FIG. 4. �Color online� �a� Tip displacement of PZT/Si. unimorph cantileverand �e31

� � as a function of applied unipolar voltage and �b� tip displacementunder bipolar voltage excitation.

031909-2 Ambika et al. Appl. Phys. Lett. 96, 031909 �2010�

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PZT film is minimal as a result of the minimization of thethermal mismatch with the substrate by the intermediate STlayer and also due to the relieving of the stress upon coolingthrough the Curie temperature through the formation of �110�texture. This facilitates polarization rotation leading to en-hanced field-induced strain.

In summary, we have fabricated by sol-gel spin-coatingtechnique crack-free and dense �110�-oriented PZT filmshaving thickness of 2.0 �m on silicon substrates. The inter-mediate ST layers were found to promote �110�-preferredorientation in PZT. The �110�-oriented PZT films exhibitedexcellent dielectric and piezoelectric properties. The averagevalue of the effective transverse piezocoefficient of the PZTthin film, e31

� =−8.4 C /m2 indicated their suitability for ap-plications in micro electromechanical systems. The relation-ship between film texture and the transverse piezocharacter-istics have also been established.

This work was supported by Department of Science andTechnology �DST, Govt. of India� through Grant No. SR/S3/ME/041/2008. One of the authors D.A. is grateful to Council

of Scientific and Industrial Research �CSIR, Govt. of India�for the senior research fellowship.

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FIG. 5. �Color online� A Possible orientations of polarization vectors in�110�-oriented �a� tetragonal and �b� rhombohedral PZT film.

031909-3 Ambika et al. Appl. Phys. Lett. 96, 031909 �2010�

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