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Materials Science and Engineering B 178 (2013) 1224– 1229

Contents lists available at ScienceDirect

Materials Science and Engineering B

jou rn al h om epa ge: www.elsev ier .com/ locate /mseb

hort communication

mprovement of the fatigue and the ferroelectric properties of PZTlms through a LSCO seed layer

ofia A.S. Rodrigues ∗, José P.B. Silva, Anatoli Khodorov, Javier Martín-Sánchez,. Pereira, M.J.M. Gomes

entre of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal

r t i c l e i n f o

rticle history:eceived 22 April 2013eceived in revised form 11 July 2013ccepted 13 August 2013vailable online 27 August 2013

eywords:ulsed laser deposition techniqueZT thin filmserroelectric properties

a b s t r a c t

The ability to optimizate the preparation of Lead Zirconate Titanate (PZT) films on platinized Si substrateby pulsed laser deposition was demonstrated. The effect of the modification of the interface film/electrodethrough the use of a (La,Sr)CoO3 (LSCO) seed layer on the remnant polarization, fatigue endurance andstress in PZT films was studied. An improvement on the ferroelectric properties was found with theusing of the LSCO layer. A remnant polarization (Pr) of 19.8 �C/cm2 and 4.4 �C/cm2 for films with andwithout the LSCO layer were found. In the same way the polarization fatigue decreases significantly afterdeposition of the LSCO layer between the film and substrate. Atomic force microscopy (AFM) imagesrevealed a different growth process in the films. Current–voltage (I–V) measurements showed that theuse of LSCO seed layer improves the leakage current and, on the other hand the conduction mechanisms in

atigue the film without LSCO, after the fatigue test, was found to be changed from Schottky to Poole–Frenkel. Thetrap activation energy (about 0.14 eV) determined from Poole–Frenkel mode agrees well with the energylevel of oxygen vacancies. The films stresses were estimated by XRD in order to explain the improvementon the structure and consequentially ferroelectric properties of the films. The model proposed by Dawberand Scott was found to be in agreement with our experimental data, which seems to predict that the

impo

oxygen vacancies play an

. Introduction

The presence of the spontaneous polarization at room tem-erature and the possibility to switch it by an applied field, makehe ferroelectric materials to be very attractive as a potentialon-volatile memory element [1–5]. For this type of memory,alled FeRAM, ferroelectrics with high remnant polarization shoulde used in meantime the leakage currents should be kept small.he leakage current J(V) is governed by conduction mechanisms,hat could be related to the film/electrode interface, like Scottkymission (SE) mechanism or to the defects at grain boundaries, likeoole–Frenkel (PE) and space-charge limited current (SCLS) [6].ue to either the presence of interface effects or several kinds of

raps in the bulk of the film, ferroelectrics thin films with metalliclectrodes are very complicated structures, where electrons, holesnd ions all contribute to the conductivity [7].

Lead zirconate titanate (PZT) films are very attractive materialsrom the ferroelectric point of view, due to its large values of theolarization. Because of compatibility with current Si integrated

∗ Corresponding author. Tel.: +351 965195485; fax: +351 253678981.E-mail addresses: sofiarodrigues@fisica.uminho.pt, sofia.asr@gmail.com

S.A.S. Rodrigues).

921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.mseb.2013.08.001

rtant role on fatigue.© 2013 Elsevier B.V. All rights reserved.

circuit processing, it is very important to grow PZT films on Sisubstrates covered with Pt used as a bottom electrode. But growingof high quality PZT films on such a substrate meets some difficultybecause of large lattice mismatch as well as poor chemical affinity.This leads to the appearing of minor non-ferroelectric phases likepyrochlore that, in order to avoid it, needs some special conditionsduring PZT film growing. Another problem is the formation ofdefects at the interface PZT film/Pt electrode. The later may causea strong degradation of ferroelectric properties when such acapacitor is kept under continuous cycling [8–11]. The defectscan pin the ferroelectric domain boundaries that prevent theirmoving in accordance with the applied electrical field [12], whichcan result in the failure of the device. When the device processingexposes capacitors to oxygen reduced ambient, it can cause oxygendepletion of the capacitor and the formation of oxygen vacancies(Vo), that are one of the most common mobile defects in PZTfilms [13,14]. The residual stresses also affect the properties,performance and long term stability of the films [13]. A range ofmethods are currently available to estimate the residual stressessuch as neutron diffraction, nano-indentation, X-ray diffraction,

and Raman spectroscopy. Many researches have shown that theexcessive residual compressive or tensile stress may cause filmdelamination from the substrate, surface crack in films, splinter-ing and adhesion problems of the film to the substrate [15,16].

and Engineering B 178 (2013) 1224– 1229 1225

Tso

sfecr[dL

tqsttpsaslgtcpl

2

LafatraswPamicv

(pt(pNtwsdwmaeuLr

Fig. 1. The optical transmittance spectra of PZT films deposited on glass at 650 ◦C

S.A.S. Rodrigues et al. / Materials Science

herefore, it is important to study and understand the origin oftresses and what is their influence on the mechanical propertiesf the films.

Perovskite electrodes being similar to PZT in chemistry andtructure, like LSCO, can provide a significant improvement inerroelectric properties and polarization fatigue. Moreover, suchlectrodes used as a seed layer can promote nucleation andrystallization of PZT film [9,17–20]. The type of charge carriersesponsible for conduction in LSCO films have been object of study21,22], where the grain boundaries and holes were considered toominate the transport behaviour, and the electrical conduction inSCO samples, i.e., it is a p-type conductor.

Pulsed laser deposition (PLD) technique is a physical deposi-ion method that has been successfully employed to grow highuality PZT films [23,24]. PLD is a powerful method in terms oftoichiometric transfer from the multicomponent oxide targets tohe growing film, high deposition rate and inherent simplicity forhe growth of multilayered structures. But the growth of pure PZTerovskite phase on non-perovskite substrates like MgO, platinizedilicon, etc. by PLD needs using high oxygen background pressurebout 200–400 mTorr [25–27]. This might cause a decrease in den-ity of grown films and still may be optimized using LCSO as a seedayer and bottom electrode with post-deposition annealing in oxy-en atmosphere as we will demonstrate in this paper. Changes ofhe fatigue resistance and ferroelectric properties of PZT films withomposition Zr/Ti of 55/45, produced by pulsed laser depositionrocess, upon the interfacial modification, through a LSCO seed

ayer, are also investigated.

. Experimental procedure

PZT films were deposited on Pt/Ti/SiO2/Si substrates by Pulsedaser Deposition (PLD) technique, using a KrF excimer laser with

source pulse wavelength of 248 nm. The deposition was per-ormed in O2 atmosphere (0.1 mbar) while the substrate was heatedt 700 ◦C by a quartz lamp. The laser was operated at 10 Hz andhe beam was focused through a 30 cm focal length lens onto aotating target at 45◦ angle of incidence with the energy density ofbout 6 J/cm2. Starting from ceramic targets, based on PZT compo-ition with ratio Zr/Ti 55/45, films with thickness of about 200 nmere produced directly on Pt/Ti/SiO2/Si substrate as well as on

t/Ti/SiO2/Si substrate with (La0.5,Sr0.5)CoO3 (LSCO) seed layer (ofbout 50 nm thickness) deposited in the same conditions usingultitarget carousel system. After deposition, an in situ anneal-

ng was performed, in oxygen atmosphere, in order to improve therystalline structure as well as to compensate the possible oxygenacancies formed during the film growth.

The structure of PZT films was studied by X-ray diffractionXRD) and atomic force microscopy (AFM) techniques. The XRDatterns were recorded by a Philips X’Pert X-ray diffractome-er, using Ni filtered CuK� radiation in a ranged from 20◦ to 45◦

with a step size of 0.02◦ and a measuring time of 1 s). The mor-hological characterization was carried out with a commercialanoscope III AFM system working in tapping mode. Based on lat-

ice parameters and the elastic constants of PZT, the films stressesere estimated from XRD measurements using the calculated d-

pacing in the stressed and unstressed states as is described inetail in our previously published work [28]. Top gold electrodes,ith a diameter of 1 mm, were evaporated through a shadowask. After the electrodes deposition an annealing at 200 ◦C, in

ir, during 30 min, was done to improve the interface between

lectrode and film. The electrical properties were evaluatedsing I–V measurements that were performed using a KEITH-EY 617 programmable electrometer at room temperature, foreverse and forward bias configurations. Polarization–electric field

at oxygen background pressures 0.10 (blue), 0.15 (green) and 0.20 mbar (red). (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

measurements (P–E) hysteresis loop acquired at 10 kHz were per-formed using a modified Sawyer–Tower circuit.

3. Results and discussion

One of the main advantages of PLD is the high kinetic energy(typically of about 50 kV) of ablated species. The enhanced surfacemobility of such species arrived on the substrate can be used toassist nucleation and growth of the films at lower temperatures.However, the growth of PZT films is usually performed at rela-tively high oxygen pressures of about 200–400 mTorr in order toavoid formation of non-ferroelectric pyrochlore phase. Whereas itis known [29], that when deposition is performed at high back-ground pressure above about 100 mTorr, most of the initial kineticand internal energy of the ablation plasma is quenched by multiplecollisions with the ambient gas on its journey from target to sub-strate, obviating one of the most important benefits of PLD. Hence,the use of the high background oxygen pressure allows avoidingpyrochlore phase formation but may cause a decrease in density ofgrown films as it will be illustrated below.

Fig. 1 shows the optical transmittance T spectra of PZT filmsdeposited on glass at 650 ◦C in the same conditions but at differ-ent oxygen background pressures. The oscillations in T come fromthe interference due to reflection from the top surface of the filmand the interface between the film and substrate. The well oscil-lating optical transmission evidences a flat surface and a uniformthickness of the films. The amplitude of the fringes is the measureof difference in refractive indexes of substrate and film [30]. As isseen from Fig. 1, starting with PO2 = 0.10 mbar the amplitude of thefringes starts decreasing that evidences the decrease in refractiveindex of PZT films.

Assuming the refractive index of glass to be 1.46 the refractiveN index of PZT films was calculated follow the “Envelop method”[31]. The value of N was 2.42 (at � = 800 nm) for PZT films depositedat PO2 values from 0.10 down to 0.010 mbar. This value is close tothe refractive index of bulk PZT and evidences an optimal densityof deposited PZT films at these oxygen pressures. N was calculatedto be 2.02 and 1.85 at PO2 = 0.15 mbar and 0.20, respectively anddecreased to 1.70 at further increase of oxygen pressures up to0.40 mbar showing obvious decrease in the density of the depositedfilms.

The similar behaviour was observed when PZT films weredeposited on platinized silicon substrate and in situ annealedin oxygen for 15 min. When PZT films were deposited at PO2 =0.10 mbar the intensity of XRD peaks increased nearly in 4 times

1226 S.A.S. Rodrigues et al. / Materials Science and E

5550454035302520

(Sub

stra

te)

(200

)

(111

)

(110

)

(100

)

2θ (degre e)

(a)(S

ubst

rate

)(c)

(b)

(Pyr

cl.)

(Pyr

cl.)

(Sub

stra

te)

(100

)

Inte

nsity

(arb

. uni

t)

(210

)(S

ubst

rate

)

(200

)

(111

)(S

ubst

rate

)

(110

)

Ffi

cibeolpfmfoossatp

mgsfiitldwLgsf

ttwpwfmcmo

ig. 2. X-ray diffraction patterns of (a) LSCO, (b) PZT film without LSCO and (c) PZTlm with a LSCO seed layer, all films deposited on Pt/Ti/TiO2/Si substrates.

ompare to films grown at PO2 = 0.20 and full width at half max-mum of (1 0 0) peak decreased from 0.22 to 0.13 showing theetter crystalline structure of films grown at PO2 = 0.10 mbar. How-ver, the presence of lead-deficient pyrochlore phase was alwaysbserved in the films deposited at PO2 = 0.10 mbar that relates toow oxidation reaction of lead and its evaporation. The poor incor-oration of lead may be caused by the difficulty of perovskite phaseormation on platinized silicon substrate because of large lattice

ismatch as well as poor chemical affinity. This is supported by theact that PZT films with perovskite structure can be easily grownn perovskite substrates while lead-deficient pyrochlore phase wasbserved when PZT were grown on MgO, Pt-coated Si and GaAsubstrates under similar conditions [25]. The stoichiometric perov-kite LSCO layer can be easily grown on platinized silicon substratet PO2 = 0.10 mTorr (Fig. 2a). The use it as a seed layer allowed uso grow dense pure perovskite PZT films at PO2 = 0.10 mTorr withost-deposition annealing in oxygen atmosphere as shown in Fig. 2.

Fig. 3 shows AFM images of: (a) Pt/Ti/SiO2/Si substrate (rootean square (rms) roughness value of about 1 nm); (b) PZT film

rown directly on Pt/Ti/SiO2/Si substrate (rms ∼ 14 nm); (c) LSCOeed layer grown on Pt/Ti/SiO2/Si substrate (rms ∼ 4 nm); (d) PZTlm grown on LSCO seed layer (rms ∼ 7 nm). Analysing these AFM

mages, it is clearly visible that the presence of a LSCO layer betweenhe PZT film and platinized silicon substrate induces a particu-arly process of film growth, where grains are homogeneouslyistributed thought the film surface. A rougher surface is obtainedhen the PZT film is grown directly on the substrate, without any

SCO seed layer, with the formation of larger clusters of grains. Arain size of 38 nm and 34 nm for PZT films with and without a LSCOeed layer respectively was calculated using the Scherrer’s formularom the XRD data.

Based on the lattice parameters and the elastic constants of PZThe films stresses were estimated from XRD measurements usinghe calculated d-spacing in the stressed and unstressed states likeas described in a previous work [28]. The results revealed theresence of tensile stresses of about 3 MPa and 75 MPa in case of PZTith and without a LSCO layer between film and substrate, before

atigue test, respectively. Because we have no epitaxial growth, the

ain factors causing an appearing of tensile stresses in PZT films

ould be a large grain size, more rich Ti content, difference in ther-al expansion coefficients (˛) of PZT and Si as well as excess of

xygen vacancies in the lattice sites of PZT [28]. Since our PZT films

ngineering B 178 (2013) 1224– 1229

have the same composition and grain size doesn’t differ a lot theobserved tensile strasses relate to two latter factors. In the caseof PZT ˛PZT ≈ 11 × 10−6 K−1, whereas ˛Si ≈ 3.6 × 10−6 K−1 [32]. So,during cooling after crystalization at 700 ◦C the Si substrate shrinksup slower that PZT causing the tensile strain in the film. The useof LSCO seed layer promotes decreasing of stresses in both cases.The strain caused by thermal coefficients mismatch reliefs and theoxygen stoichiometry on the bottom film interface is improved. So,the use of perovskite LCSO conductive electrode/seed layer havingwell matched structure and chemistry with perovskite PZT allowedgrowing of dense PZT films with pure perovskite phase by PLD atoxygen pressure of 0.1 mbar. Because PZT films are deposited onsimilar perovskite structure, nucleation and crystallization of PZTare easier and denser films with lower roughness and stresses havebeen successfully grown.

In order to study the conductivity of PZT thin films with andwithout LSCO seed layer, current–voltage measurements were per-formed in both reverse and forward bias configurations. The delaytime used in the I–V measurements was 10 s [33]. Fig. 4 shows thevariation of the current intensity with applied voltage at room tem-perature, for a voltage range of −5 V to 5 V. Analyzing Fig. 4, weobserved that the leakage current increased after fatigue tests inboth films, with and without LSCO, however the increase in thefilm deposited directly on the platinized substrate is more signif-icant than in the film with LSCO. This increase can be explainedby the formation of charge defects such as gaps of oxygen in thePZT film. Possibly, the oxygen vacancies are redistributed and piledup at the PZT/Pt interface through long-distance migration duringcycling under applied electrical field. Usually, the defects presentin PZT thin films contribute to the leakage current. In this sense, weattribute the lowering of the leakage current to a decrease of theoxygen vacancies in the PZT film due to the incorporation of oxy-gen from the underlying oxide-based LSCO layer. Obviously, this isnot observed in the case where the PZT is deposited directly on theplatinized silicon substrate. A slight shift on I–V curves of films withLSCO seed layer between film and substrate, to the right side, is alsoobserved. This may be attributed to the LSCO layer that works as ablocking layer for the charge carriers at the interface PZT/LSCO.

Analysing the conduction mechanisms we conclude that for thelow voltage regime, the Ohmic conduction dominates and the leak-age current increases linearly with the applied voltage in all thefilms. For higher voltages, with exception of PZT film after fatiguetests without LSCO seed layer, the Schottky thermionic emissionwas recognized as the predominant conduction mechanism basingon the fitting procedures (Fig. 5), that is attributed to the currentthrough the barrier at the film/electrode interface. In the case of PZTdeposited directly on the platinized silicon substrate, after severalcycles of polarization and depolarization the conduction mecha-nism was changed from Schottky to Poole–Frenkel, attributed tothe defects at grain boundaries, which is in agreement with theincrease of oxygen vacancies in this case. The PF current density isgiven by:

JPF = E exp

[− q

kT

(�PF −

√qE

�εεo

)](1)

where �PF is the barrier height. The slope (s) of the straight line inthe log (J/E) versus E1/2 plot is related to the previous Eq. (2) andthe y-intercept (b) was given by Eq. (3) that allow us to estimatedthe barrier height (�PF).

s =(

q)√

q(2)

kT �εεo

b = − q

kT�PF (3)

S.A.S. Rodrigues et al. / Materials Science and Engineering B 178 (2013) 1224– 1229 1227

F t/Ti/Sid

cwt(1

bpP

ig. 3. AFM surface morphology images of the (a) Pt/Ti/SiO2/Si substrate, (b) PZT/Peposited on Pt/Ti/SiO2/Si.

Taking into account the PF equation, the height of traps barriersalculated from Eq. (3) is �PF = 0.14 eV, which is in good agreementith the activation energy of oxygen vacancies (0.15 eV) [34]. From

he slope of the strait line (s) and using Eq. (2) the electric constantε) and the refractive index of film were estimated to be 3.95 and.99 respectively.

The ferroelectric properties of these films were also investigatedy P–E hysteresis loop measurements. The determined values forositive and negative remnant polarization and coercive field (Pr

+,r−, Ec

+, Ec−) are presented in Table 1. Fig. 6 shows the hysteresis

O2/Si film, (c) LSCO seed layer and (d) PZT/LSCO film, while the last two films also

loops measured for the films with and without LSCO seed layer.It should be noted that the hysteresis loop is slightly asymmetricon field axes, which was more pronounced in PZT film depositeddirectly on platinized silicon substrate. Also in this case the val-ues for remnant polarization are lower in comparison with the filmdeposited on LSCO seed layer. The field asymmetry may be asso-

ciated to the presence of internal fields [7]. As can been seen fromTable 1, the remnant polarization (Pr) values increase by four timesand the electric field (Ec) is a half time less, with the use of LSCOlayer. These results make the use of LSCO layer to be a good choice to

1228 S.A.S. Rodrigues et al. / Materials Science and Engineering B 178 (2013) 1224– 1229

6420-2-4-610-1210-1110-1010-910-810-710-610-510-410-310-210-1100

PZT/Substrate after fat igu e PZT/Substrate before fatigue PZT/LSCO before fat igue PZT /LSCO after fat igue

Cur

rent

(A)

Voltage (V)

Fig. 4. Current–voltage measurements of PZT film with and without LSCO seed layer,before and after fatigue.

y = 0,0015x - 3,4669R2 = 0,9991

-2,95

-2,90

-2,85

-2,80

-2,75

-2,70

500450400350300

E1/2(V/cm)1/2

Log

(J/E

) (A

/ V

cm)

Fig. 5. Linear fit for the Poole–Frenkel mechanism for the negative bias for PZT afterfatigue test, without LSCO seed layer.

-60 0 -40 0 -200 0 200 40 0 60 0-40

-30

-20

-10

0

10

20

30

40 PZT /LSCO/Sub strate PZT/S ubstrate

Pola

rizat

ion

(μC

/cm

2 )

F

bp

ts

106

10 7

10 8

0,4

0,5

0,6

0,7

0,8

0,9

1,0

PZT/Substrate PZT/LSCO/Substrate

P r(

N) /

P r(

0)

No. cycles

TD

Elec tric Field (kV/cm)

ig. 6. The hysteresis loops obtained for PZT films with and without LSCO seed layer.

uilt metal-ferroelectric-metal capacitors with good ferroelectric

roperties.

The improvement of ferroelectric properties of PZT filmshrough the deposition of the LSCO layer between film and sub-trate is directly related to the improvement that this layer causes

able 1etermined values for: positive and negative remnant polarization and coercive field (Pr

+

Samples Pr+ (�C/cm2) Pr

− (�

PZT/LSCO/substrate 19.8 −18.7

PZT/substrate 4.4 −4.3

Fig. 7. Fatigue endurance of PZT films with and without LSCO seed layer.

in the film structure. The presence of this LSCO electrode/seedlayer, improves the crystallinity of the films, presenting the desiredperovskite phase without secondary phases like pyrochlore. More-over, the films are denser with lower roughness and stresses thatautomatically confers better ferroelectric properties.

Fig. 7 shows the fatigue endurance of PZT films with and withoutthe LSCO interface modification. Interestingly increasing the num-ber of reversal cycles, the remnant polarization (Pr) of the PZT filmsis rather stable in the sample with LSCO as a seed layer, while itdecreases in PZT film without LSCO. After 108 cycles, the remnantpolarization of PZT film with LSCO decreases of about 15%, while itdecreases 58% in PZT film without seed layer. Rong et al. showed intheir recent study about the effects of PbO interfacial modificationin PZT films, that after 106 cycles the Pr values decrease 60% and 80%,in films with and without interfacial modification [9]. The fatiguebehaviour could be related to tensile stress presented in films andto the accumulation of oxygen vacancies at electrodes interfacesvia diffusion at the PZT/Pt interface under the electric field. Themigrated oxygen vacancies form an interfacial defect layer nearthe electrodes which efficiently reduces the electrical field acrossthe ferroelectric layer or inhibit the nucleation of the oppositelypolarized domains [10,35,36]. Therefore, by modifying the inter-face through the deposition of an LSCO layer between film andsubstrate, the films tension are significantly lower and the oxygenvacancies that appear by the diffusion of this element through theinterface will be reduced. Moreover, as we referred above, the LSCOlayer improves the structure of the film, causing a less number ofimperfections and reducing the film stress enhancing polarizationand films lifetime. Considering the oxygen vacancies as the mainfactor in the decrease of polarization with increasing the numberof cycles, a model proposed by Dawber and Scott in [37] was usedto fit the experimental values obtained for fatigue curve of PZT thinfilm without LSCO seed layer. The dependence of P(N), the switchedcharge per unit area P versus switching event number N, in ABO3perovskite structure ferroelectric thin films is given from Eq. (4):{ [ (

−3480 + 5.21 × 10−6)]

N}

P(N) = A exp −1.075 expT

× EA ×f

+ B (4)

, Pr− , Ec

+, Ec−).

C/cm2) Ec+ (kV/cm) Ec

− (kV/cm)

120 −130193 −230

S.A.S. Rodrigues et al. / Materials Science and E

106 10 7 10 81,5

2,0

2,5

3,0

3,5

4,0

4,5

Pr ( μ

C/c

m2 )

No. cycles

Experimental data Theoretical model

Fs

wtttd

4

bsDrAststfPTeshobefiwv

[[[[[

[[[[[

[

[[

[

[

[

[[[

[[

[[[

ig. 8. Results of model simulation and the fatigue experiment for PZT without LSCOeed layer.

here A is the difference between the Pr(0) and Pr(N), B ishe value of Pr(N), EA is the electric field assuming a deple-ion layer in the interface film/electrode. From Fig. 8, is possibleo see a good adjustment of this model to our experimentalata.

. Conclusion

The growing of PZT films by PLD can be optimized by decreasingackground oxygen pressure down to 0.10 mbar and using LSCOeed layer with post-deposition annealing in oxygen atmosphere.ense films with pure perovskite phase and relatively good fer-

oelectric properties for the polycrystalline PZT films were grown.FM characterization revealed that the PZT films grown on LSCOeed layer have lower surface roughness and well defined lit-le grains scattered all over the film surface. I–V measurementshowed that LSCO seed layer has decreased the leakage current andhe conduction mechanism in film without LSCO seed layer afteratigue test was changed from Schottky emission mechanism tooole–Frenkel, which is related to the increase of oxygen vacancies.he LSCO interfacial modification enhanced the ferroelectric prop-rties which can be related to the improvement achieved in terms oftructure and crystallinity, reducing the films residual stress whichas important effects on the films life. On the other hand the usef this layer as a seed layer for the growth of PZT films proved toe quite relevant in improving the resistance to fatigue. The fatigue

xperimental data for PZT film deposited directly on substrate andt result from the model proposed in Ref. [37], is in an agreementith the suggestion that the fatigue is mainly related to oxygen

acancies.

[

[[[

ngineering B 178 (2013) 1224– 1229 1229

Acknowledgements

This work was partially supported by the Portuguese Foundationfor Science and Technology (FCT) through the Pluriannual Projectof Centre of Physics of University of Minho. S.A.S.R. thanks FCT forthe financial support (grant SFRH/BD/30531/2006). J.P.B.S. thanksFCT for the financial support (grant SFRH/BD/44861/2008). J.M.S.thanks FCT for the financial support (grant SFRH/BPD/64850/2009).

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