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Materials Chemistry and Physics 151 (2015) 308e311

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Transverse piezoelectric properties of {100} e Oriented PLZT[x/65/35]thin films

S. Laxmi Priya a, V. Kumar a, *, Fumiya Kurokawa b, Isaku Kanno b

a Centre for Materials for Electronics Technology (C-MET), Scientific Society, Department of Information Technology, Ministry of Communication andInformation Technology, Govt. of India, Athani (PO), Thrissur 680581, Kerala, Indiab Mechanical Engineering, Kobe University, Kobe 657-8501, Japan

h i g h l i g h t s

� Preferentially {100}-oriented thin films of PLZT[x/65/35]; x ¼ 6,7,8 deposited by solegel spin-coating method.� Evaluation of transverse piezoelectric coefficient, e31

* of piezoelectric thin films by Cantilever method.� StructureeProperty correlation in PLZT thin films.

a r t i c l e i n f o

Article history:Received 20 May 2014Received in revised form14 October 2014Accepted 29 November 2014Available online 5 December 2014

Keywords:Thin filmsSol-gel growthCrystal symmetryPiezoelectricity

* Corresponding author.E-mail addresses: vkumar@cmet.gov.in, vkumar10

http://dx.doi.org/10.1016/j.matchemphys.2014.11.0710254-0584/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Preferentially {100}-oriented thin films of lead lanthanum zirconate titanate, (Pb1�xLax) (Zr0.65Ti0.35)1�x/

4O3[PLZT(x/65/35)] with compositions near the morphotropic phase boundary (MPB) were prepared onsilicon substrates (111)Pt/Ti/SiO2/Si by solegel spin coating technique. The structural, micro structuraland electrical characteristics have been studied as a function of thin film composition. Crystallineorientation and microstructure of the thin films have been determined by X-ray diffraction, Scanningelectron microscopy, respectively. The transverse piezoelectric coefficient, e31* of the PLZT thin films havebeen evaluated by tip deflection of unimorph cantilevers. The influence of thin film composition on e31*

have been determined. PLZT (7/65/35) thin film exhibited the optimum dielectric and piezoelectriccharacteristics with a dielectric permittivity, εr ¼ 917; dielectric loss, tan d ¼ 0.03 and averagee31* ¼ �4.2 C/m2.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Micro electromechanical systems (MEMS) have been receivinggreater attention as an actively researched area since the needs formicro devices are increasing. MEMS using piezoelectric thin filmsare important for developing micro sensors and actuators [1e6].For MEMS applications like micro actuators, development of dense,crack-free thin films integrated on silicon substrates is essential.Thin film texture and composition are also significant from thepoint of device applications. Preferentially {100}-oriented thinfilms having compositions near MPB with rhombohedral structureare of interest for electromechanical applications requiring highstrains [7e9]. In this context thin films of PLZT (x/65/35); x ¼ 6, 7,and 8 are very attractive. For thin film deposition, solegel method

@yahoo.com (V. Kumar).

offers numerous advantages including excellent control ofcomposition, texture, uniform film thickness, and lower processingtemperatures. For MEMS applications, the in-plane transversepiezoelectric coefficient e31* is the most important parameter to beconsidered. Therefore the aim of the present work is the solegeldeposition of preferentially {100}-oriented (Pb1�xLax) (Zr0.65Ti0.35)O3 [PLZT(x/65/35)]; x ¼ 6, 7, and 8 thin films and the determinationof their transverse piezoelectric characteristics.

2. Experimental work

Stable precursor solutions corresponding to the compositions(Pb1�xLax) (Zr0.65Ti0.35)1�x/4O3, x ¼ 0.06, 0.07, 0.08 were preparedby solegel process as per the procedure adopted from our previouswork [10]. The reactants, lead acetate (99%, Merck, India),lanthanum acetyl acetonate hydrate (Sigma Aldrich, India) andzirconium acetyl acetonate (98%, Merck, India) were refluxed in

Fig. 1. Measurement system of the transverse piezoelectric properties of PLZT thinfilms.

S. Laxmi Priya et al. / Materials Chemistry and Physics 151 (2015) 308e311 309

ethanol. The clear solution obtained was mixed with a solution oftitanium (IV) isopropoxide (97%, Sigma Aldrich, India) in isopropylalcohol containing equimolar amounts of diethanolamine (98%,Merck, India). To obtain {100}-preferentially oriented films, TiO2layer having a thickness of about 10 nmwas first spin-coated on the(111) Pt/Ti/SiO2/Si substrate (Inostek, Korea), dried at 110 �C for15min and then subjected to annealing at 600 �C for 30min [11,12].Later SrTiO3 (ST) layer having a thickness of 45 nmwas coated overit under the same conditions. In addition to the substrate quality,the annealing conditions of the buffer layers are controlling thenucleation and growth steps and preferred orientation could beachieved by carefully controlling the annealing temperatures. Apartfrom this, the buffer layers also act as effective barrier layers pre-venting inter diffusion [13e15]. Finally, PLZT films were coatedabove the ST layers by spin-coating (Model: KW-4A, Chemattechnologies, USA) process at a speed of 4000 rpm for 25 s. Thecoated PLZT layers were dried at 110 �C for 15 min and thenannealed in two stages, initially at 400 �C for 10 min and then at600 �C for 20min. The crystallographic orientation of the depositedfilms were determined using an X-ray diffractometer (Model:D5005, Bruker, Germany). Raman spectra were recorded using aRaman Spectrometer (Model: DXR, Thermo scientific, USA). Goldtop electrodes of 350 nm thickness were deposited on the PLZTfilms using a Vacuum Coater (Model: 12A4D, Hind High Vacuum,India). Ferroelectric and dielectric properties were determinedusing a Piezoelectric evaluation system (Model: FT Analyser 2000,aixACCT, Germany). Leakage current measurements were recorded

Fig. 2. (A) XRD patterns and (B) Raman spectra of PLZT films with varying lanthanum

on PLZT thin films with positive voltage applied on the top goldelectrode and negative voltage on the bottom electrode.

For the measurement of piezoelectric properties of the PLZTfilms, rectangular beams of substrate with PLZT films were dicedout and transverse piezoelectric properties were measured asdescribed in our earlier studies [16,17]. The transverse piezoelectricproperties of the PLZT thin films were evaluated using unimorphcantilevers of PLZT/Si. The measurement system is depicted inFig. 1. Piezoelectric vibration was generated by applying sine wavevoltage between upper and bottom electrodes. Tip displacementwas measured using a laser Doppler vibrometer (Model: AT-3500,Graphtec, Japan) and a laser interferometer (Model: AT-1100,Graphtec, Japan). Micro structural analysis of thin films was car-ried out using a High Resolution Scanning Electron Microscope(HRSEM, Model: Genesis Apex 2, Ametek, Japan).

3. Results and discussion

The X-ray diffraction patterns of PLZT films are as shown inFig. 2A. It is seen that all the three films exhibit {100}-preferredorientation. The introduction of TiO2 and SrTiO3 bottom layers andthe modification of heat treatment conditions played a prominentrole in accomplishing the preferred crystallographic orientation[11,12]. From the Raman spectra (Fig. 2B), it is seen that thin films ofPLZT (x/65/35); x¼ 6, 7 are purely rhombohedral as evidenced by E,E’ modes at ~65 cm�1 and E mode at 285 cm�1. In the case of PLZT(8/65/35) thin film, the broad peak at 140 cm�1 [A1(1TO)] and326 cm�1 [A1(2TO)] [Fig. 2B (c)] are sensitive indications of thepresence of some fractions of tetragonal phase [18e21]. All the thinfilms exhibited dense columnar microstructure as revealed by theSEM of their cross-sections (Fig. 3).

Transverse piezoelectric properties of the thin films were eval-uated from the deflection of PLZT/Pt/Ti/SiO2/Si cantilevers byapplying unipolar sine wave voltage between upper and bottomelectrodes. In this experiment, we measured the tip displacementat the frequency of 700 Hz. From the piezoelectric displacement,the transverse piezoelectric coefficient e31* was evaluated as perequation (1) given below;

e*31 ¼ d31sE11;p þ sE12;p

y� h2s3s11;sð1� vsÞL2

d

V(1)

where d, V, L, h, vs and s11,s are the tip displacement, applied voltagebetween top and bottom electrodes, length of the cantilever,thickness, Poisson's ratio and the elastic compliance of the sub-strate respectively [22]. The subscripts “s” and “p” denote the

concentrations (a) 6/65/35, (b) 7/65/35, and (c) 8/65/35 on Pt/Ti/SiO2/Si substrate.

Fig. 3. HRSEM of the PLZT (a) 6/65/35, (b) 7/65/35, and (c) 8/65/35 thin films.

Fig. 4. Transverse piezoelectric response in {100} - oriented PLZT (a) (6/65/35), (b) (7/65/35), and (c) (8/65/35) thin films as a function of applied voltage.

S. Laxmi Priya et al. / Materials Chemistry and Physics 151 (2015) 308e311310

substrate and the piezoelectric films respectively. The elasticcompliance s11 is given by the reciprocal of the Young's modulus.The Young's modulus and Poisson's ratio of the <110 > Si, which isthe length direction of the cantilevers was reported as 168 GPa and0.066, respectively.

Fig. 5. Domains in {100} e oriented PLZT thin film having (A) rhombohedral and (B)tetragonal structure.

Transverse piezoelectric coefficient e31* determined from theabove equation are shown in Fig. 4. It is seen that maximumtransverse piezoelectric coefficient and least nonlinearity is ob-tained for PLZT (7/65/35) thin film. The nonlinearity in e31* de-creases in the order, PLZT (8/65/35) > PLZT (6/65/35) > PLZT (7/65/35). This is because both PLZT (6/65/35) and (7/65/35) thin filmsexhibit pure rhombohedral geometry. For {100}-oriented thin filmswith rhombohedral structure the domains a! are stable (Fig. 5A)[23]. In the case of PLZT (8/65/35) thin film, which has a MPBcomposition, large nonlinearity in e31* is attributed to the switchingof non-180

�domains b

!; c! also as shown in Fig. 5B. In the case of

PLZT (7/65/35) thin film, higher value of e31* may be due to fieldenforced Rhombohedral / Tetragonal phase transformation. Incontrast to the piezoelectric effect, the displacements of all do-mains (180

�and non-180

�) contribute to dielectric permittivity

[24]. Larger values of εr for the PLZT (8/65/35) films is consistentwith this (Table 1). The dielectric characteristics obtained are betterthan those reported [25,26]. Among these compositions, theleakage current density is minimum (~10�2 mA/cm2) for the PLZT(7/65/35) thin film (Fig. 7) indicating its suitability for micro actu-ator applications.

Slim P-E hysteresis loops of the PLZT thin films (Fig. 6A) arecharacteristic of high e La. substituted PZT films. Fig. 6B shows thestrain curves(SeE) of PLZT films by applying bipolar sine wavevoltage of ±30 V at 100 Hz. All the curves are butterfly shapedindicating domain switching of ferroelectricity.

4. Conclusions

Crystallographically {100}-preferentially oriented PLZT filmshaving thickness of 2 mmwere prepared on Pt/Ti/SiO2/Si substratesusing solegel spin coating technique. The TiO2 and ST bottom layersfacilitated preferred {100}-orientation of PLZT. {100}-oriented PLZT(6/65/35) and PLZT (7/65/35) thin films having rhombohedralsymmetry have stable domains and exhibit minimum nonlinearityin e31* . In the case of PLZT (8/65/35) thin films having MPB

Table 1Electrical characteristics of PLZT thin films.

PLZT [x/65/35]

Dielectricpermittivity εr

Dielectric losstan d

Transverse piezoelectriccoefficient je31* j (C/m2)

(6/65/35) 864 0.02 3.6(7/65/35) 917 0.03 4.2(8/65/35) 951 0.03 3.3

Fig. 6. (A) P-E hysteresis loop and (B) SeE curve under bipolar excitation in PLZT thin films (a) 6/65/35, (b) 7/65/35, and (c) 8/65/35.

Fig. 7. Leakage current of PLZT thin films (a) 6/65/35, (b) 7/65/35, and (c) 8/65/35 onTiO2/ST/Pt/Ti/SiO2/Si substrate.

S. Laxmi Priya et al. / Materials Chemistry and Physics 151 (2015) 308e311 311

composition, switching of non-180�domains contribute to large

nonlinearity in e31* . PLZT (7/65/35) thin films are found to exhibitthe optimum characteristics such as minimum nonlinearity in e31*

and minimum leakage current density and are considered suitablefor micro actuator applications.

Acknowledgements

One of the authors, Laxmi Priya S would like to acknowledgeUGC for the Junior Research Fellowship.

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