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Materials Chemistry and Physics 110 (2008) 7–10

Materials science communication

Effect of substrates on phase formation in PMN-PT 68/32 thinfilms by sol–gel process

P. Kumar a,∗, Sonia b, R.K. Patel b, C. Prakash c, T.C. Goel d

a Department of Physics, National Institute of Technology, Rourkela 769008, Indiab Department of Chemistry, National Institute of Technology, Rourkela 769008, India

c DRDO Bhawan, Rajaji Marg, New Delhi 110011, Indiad BITS Pilani-Goa Campus, Goa 403720, India

Received 16 March 2007; received in revised form 29 September 2007; accepted 28 November 2007

bstract

PMN-PT 68/32 thin films have been prepared on Pt/Si, ITO coated glass, stainless steel and silicon substrates in the identical processingonditions by sol–gel process. Annealing temperature of 600 ◦C was ascertained by thermo gravimetric analysis (TGA) study of the dried sol–gel

owder of PMNT-PT 68/32 composition. X-ray diffraction (XRD) study showed ∼95% perovskite phase formation on Pt/Si and ITO coated glassubstrates. SEM micrographs showed the formation of sub micron size grains on Pt/Si and ITO coated glass substrates. Diffuse phase transitionith transition temperature (Tc) ∼190 ◦C was observed in 0.8 �m thick PMN-PT 68/32 films deposited on Pt/Si and ITO coated glass substrates. 2007 Elsevier B.V. All rights reserved.

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eywords: Thin films; Dielectric properties; Annealing; Coatings

. Introduction

High dielectric constant (εr), piezoelectric, pyroelectric andlectro-optic properties arouse scientific and technologicalnterests in ferroelectric materials [1–5]. These properties oferroelectric materials are making them useful in a significantumbers of industrial and commercial applications. The moreecent developments in the ferroelectric field is in medical ultra-onic composites, high displacement piezoelectric actuators,hotostriction and thin and thick films for piezoelectric andntegrated circuit applications, which have served to keep thendustry of ferroelectric based devices young, amidst its growing

aturity [6]. Morphotropic phase boundary (MPB) of PMN-PTystem exists between PMNT 70/30 and 65/35 compositions7]. PMN-PT thin films near MPB have attracted much attentionue to its excellent dielectric, piezoelectric and electromechan-cal properties [8–14]. Out of all these different properties,

iezoelectric properties of PMN-PT thin films have attractedonsiderable attention since it can add functionality to micro-abricated devices such as microsensors and actuators [15,16].

∗ Corresponding author.E-mail address: pvn77@rediffmail.com (P. Kumar).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.11.035

ol–gel technique has been used for the deposition of PMN-T thin films. This thin film deposition technique offers goodomogeneity, low processing temperature, low processing costnd ease of tailoring the properties by compositional variations17]. In addition, development of desired phase in ferroelectrichin films depends on the type of substrates used [18].

In this article, we report the development of varied perovskitehase% in sol–gel deposited PMN-PT 68/32 (an MPB compo-ition) thin films on different substrates. Dielectric propertiesf PMN-PT 68/32 thin films on Pt/Si and ITO coated glassubstrates have also been discussed.

. Experimental

Lead acetate trihydrate, Pb(C2H5O2)2.3H2O, magnesium ethoxide,g(OC2H5)2, niobium ethoxide Nb(OC2H5)5, and titanium isopropoxide

i(OCH(CH3)2)4 (all Aldrich) were used as starting materials with 2-ethoxyethanol and acetic acid as solvent and catalyst for PMN-PT 68/32

ransparent solution preparation. A transparent solution of PMN-PT 68/32 sto-chiometric composition with 15 mol% excess lead (to compensate lead lossuring annealing) was prepared using a route reported by us earlier [19]. This

olution was dried, crushed in a mortar pestle and from its thermo gravimetricnalysis (TGA) study annealing temperature was ascertained ∼600 ◦C. Sub-trates were ultrasonically cleaned first in methanol then in trychloroethylenend finally in acetone. First, a thin interlayer around 500 A of PbTiO3/PT waseposited by sol–gel technique because it easily crystallizes into pure perovskite

8 istry and Physics 110 (2008) 7–10

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P. Kumar et al. / Materials Chem

hase and facilitates subsequent perovskite phase crystallization [20]. Thin filmsere coated using a spin coater at 3000 rpm for 20 s. The films were then heat

reated at 300 ◦C in air for 2 min after each coating for solvent removal andnnealed at 600 ◦C for 20 min after each two coatings for perovskite phase for-ation. The final firing of the films was done at 650 ◦C for 2 h in air. Six, 12 and

6 nos. of single coatings were deposited for getting 0.3 �m, 0.6 �m and 0.8 �mhick films of PMN-PT 68/32 system on different substrates, respectively. Thetructure and morphology of the films were characterized using X-ray diffractionXRD) (Philips, PW 3020) with Cu K� (λ = 1.5405 A) and SEM (Cambridgetereo scan 360 scanning electron microscope) techniques, respectively. Bottomlectrodes were created by etching the films in 5% HF solution. Thicknesses ofhe films were measured using taly–step method. For dielectric measurementsf films, gold dots of 1 mm diameter were deposited by RF sputtering technique.ielectric constant (εr) as a function of temperature was measured at 1 kHz fre-uency by an impedance analyzer (HP Model 4192A) with an oscillation voltageevel of 100 mV.

. Results and discussions

Fig. 1 shows the TGA plot of the dried sol–gel powderf PMN-PT 68/32 composition. A weight loss of ∼10% wasbserved up to 200 ◦C, which is attributed to the evaporation ofbsorbed water and solvent. A large weight loss ∼16% near300 ◦C is due to the decomposition of acetyl groups and

onded alkyl groups and their combustion [21]. After 500 ◦C, nobvious weight loss is observed, suggesting all reactions withinol–gel powder are completed. According to TGA results, PMN-T 68/32 thin films are heat treated at 300 ◦C for 2 min after eachoating for the removal of residual organics and at 600 ◦C for0 min after each two coatings for perovskite phase formation.

Fig. 2 shows the X-ray diffraction patterns of 0.8 �m thickol–gel derived PMN-PT 68/32 films deposited on different sub-trates in the identical conditions. The diffraction peaks are sharpnd distinct, indicating good homogeneity and crystalline naturef the films. It can be seen that intensity of the most intense peakf pyrochlore phase and perovskite phase of the PMN-PT 68/32lms varies with the type of substrate used for thin film depo-ition. This indicates the variation of perovskite phase% in thinlms with the substrates. Perovskite phase% in the thin films is

alculated using the relation [22]:

erovskite phase% = [Iperov/(Iperov + Ipyro)]

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Fig. 1. TGA curve of sol–gel powder of PMN-PT 68/32 MPB composition.

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ig. 2. XRD patterns of PMN-PT 68/32 thin films on (a) Pt/Si, (b) ITO coatedlass, (c) stainless steel and (d) silicon substrates.

nd for 0.8 �m thick PMNT 68/32 films, it is found to be ∼95%n Pt/Si and ITO coated glass substrates, whereas 71% and9% perovskite phase is found on stainless steel and siliconubstrates, respectively. Low perovskite phase% on silicon sub-trate can be understood due to two reasons. First, the surface ofhe silicon is easily oxidized into amorphous silicon oxide by thexygen in oxide films or the ambient atmosphere. Secondly, mostf the cations in the oxide film easily diffuse into the silicon sub-trate or react with the silicon. Thus, the secondary phase formedy diffusions or reactions forming lead silicate offers impedi-ent to the perovskite phase development on silicon substrate

23]. Whereas, in case of stainless steel substrate, lattice mis-atch between substrate and PMN-PT thin films can give rise

o the presence of large internal stress between substrate–filmnterfaces, which can impede the evolution of perovskite phase.ow value of annealing temperature ∼600 ◦C for maximum per-vskite phase% on Pt/Si and ITO coated glass substrates cane understood as PT base template layer helps in reducing thennealing temperature of the films [20].

Fig. 3 shows the SEM micrographs of the 0.8 �m thick PMN-T 68/32 films on different substrates. These micrographs reveal

hat the thin films on Pt/Si and ITO coated glass substrates arerack free with the grain size of ∼50 nm and 20 nm, respectively.

P. Kumar et al. / Materials Chemistry and Physics 110 (2008) 7–10 9

ITO coated glass, (iii) stainless steel and (iv) silicon substrates.

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Table 1Variation of εr at room temperature with film thickness on ITO coated glass andPt/Si substrates

PMN-PT 68/32 thin films on εr of 0.3 �mthick film

εr of 0.6 �mthick film

εr of 0.8 �mthick film

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Fig. 3. SEM of PMN-PT 68/32 thin films on (i) Pt/Si, (ii)

he films on stainless steel and silicon substrates are show-ng development of some cracks in the films, which might beue to the large internal stress present between substrate–filmnterfaces [24]. Cracks in the films can also develop due to thearge lattice mismatch between the PMN-PT 68/32 thin filmsnd stainless steel and silicon substrates [25].

Values of εr at room temperature of 0.3 �m, 0.6 �m and.8 �m thick PMN-PT 68/32 films on ITO coated glass andt/Si substrates are given in Table 1. The increase of εr with

he film thickness is believed to be due to the extrinsic contri-

ution to the dielectric response [26]. The low value of εr inMN-PT 68/32 thin films compare to bulk ceramics of the sameomposition may be due to the presence of substrate clamp-ng. This substrate clamping develops due to the tensile strain

celh

Fig. 4. Variation of εr and tan δ with temperature of 0.8 �m thick PMN

TO coated glass substrates 500 750 950t/Si substrates 415 500 835

n the films [27,28]. In addition, thermal expansion mismatch,

hemical interaction between materials of the film and bottomlectrode leads to the formation of so-called “dead layer”. Thisayer has significantly lower dielectric constant and containsigh concentration of defects which, in turn, serve as an effective

-PT 68/32 films on (a) ITO coated glass and (b) Pt/Si substrates.

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rap centres for mobile charges and domain walls. The existencef such layer affects the dielectric properties of the films espe-ially for very thin films. Besides dead layer effect, the existencef slight pyrochlore phase in the PMN-PT films may also causehe decrease in the εr. The dielectric constant (εr) of pyrochlorehase of PMN system at room temperature is around 10 andith the increase in pyrochlore phase in PMN-PT system, theielectric constant decreases [29].

Variation of εr and tan δ with the increase in temperature of.8 �m thick PMN-PT 68/32 films on Pt/Si and ITO coated glassubstrates at 1 kHz are shown in Fig. 4(a) and (b). Room temper-ture values of εr and tan δ on ITO coated glass/(Pt/Si) substratesre 950/835 and 0.034/0.025, respectively. Dense microstructureshown in Fig. 3) may be one of the reasons for higher dielec-ric constant of PMN-PT 68/32 thin films on ITO coated glassubstrates than on Pt/Si substrates [30]. As can be seen fromig. 4(a) and (b), εr and tan δ of the PMN-PT 68/32 films onoth the substrates increases with the increase in temperature.hin films on both the substrates show diffuse phase transition190 ◦C. The diffuse phase transition in the thin films can be

ue to ultra fine particle size of the films [31]. This can also beue to the applications of different stress to different grains andariations in local stress. Higher tan δ near transition tempera-ure (Tc) in the thin films on Pt/Si substrate may be due to theresence of space charges [32].

. Conclusions

0.8 �m thick PMN-PT 68/32 films showed development ofaried perovskite phase% on Pt/Si and ITO coated glass, stain-ess steel and silicon substrates. The thin films on Pt/Si andTO coated glass substrates are crack free with grain size of50 nm and 20 nm, respectively. Dielectric constant of theMN-PT 68/32 thin films on Pt/Si and ITO coated glass sub-trates increases with the increase in film thickness. Diffusehase transition was observed for the thin films deposited ont/Si and ITO coated glass substrate.

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