Preliminary results on sol-gel processing of 〈100〉 oriented Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 thin films using diol-based solutions

A novel sol-gel method is used here for the synthesis of air-stable and precipitate-free diol-based sols of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT). Sols containing a 15 mol% lead excess have been used for the preparation of PMN-PT thin films. The

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  J Sol-Gel Sci Techn (2007) 42:331–336DOI 10.1007/s10971-006-0203-9 Preliminary results on sol-gel processing of    100   orientedPb(Mg 1 / 3 Nb 2 / 3 )O 3 -PbTiO 3  thin films using diol-based solutions M. L. Calzada  ·  M. Alguer´o  ·  J. Ricote  ·  A. Santos  · L. Pardo Published online: 17 October 2006 C  Springer Science  +  Business Media, LLC 2006 Abstract  A novel sol-gel method is used here for the syn-thesis of air-stable and precipitate-free diol-based sols of 0.7Pb(Mg 1 / 3 Nb 2 / 3 )O 3 -0.3PbTiO 3  (PMN-PT). Sols contain-ing a 15 mol% lead excess have been used for the prepa-ration of PMN-PT thin films. The films were depositedonto (111)Pt/TiO 2  /SiO 2  /(100)Si substrates, and crystallisedin oxygen by Rapid Thermal Processing (RTP), using differ-ent temperatures and soaking times. Single perovskite filmsare obtained when treated at temperatures between 600 and700 ◦ C for 6 s. Those crystallised at other temperatures con-tain a secondary pyrochlore phase. This phase also appearsin the films treated at 650 ◦ C with soaking times longer than6 s. PMN-PT films with a  100  preferred orientation wereprepared by using a PbTiO 3  seeding layer onto the substrate.These PMN-PT films present relaxor-type electrical prop-erties. Dielectric permittivity,  ε T  33 , shows significant disper-sion. Its temperature dependence presents a broad maximumat 110–130 ◦ C, which position shifts towards higher temper-atures with frequency. Ferroelectric hysteresis loops showhigh values of saturation polarisation but very low rema-nence. The piezoelectric activity of the films has been testedby the measurement of their local piezoelectric hysteresisloops. Keywords  Thin films . Lead magnesium niobiumtitanate . Piezoelectrics M. L. Calzada (  ) · M. Alguer´o · J. Ricote · A. Santos · L. PardoInst. Ciencia de Materiales de Madrid (CSIC),Cantoblanco, 28049 - Madrid, Spaine-mail: 1 Introduction Leadmagnesiumniobiumtitanate,Pb(Mg 1 / 3 Nb 2 / 3 ) 1 −  x  Ti  x  O 3 (PMNT), is a crystalline solid solution between two per-ovskites:therelaxorferroelectricPb(Mg 1 / 3 Nb 2 / 3 )O 3  (PMN)with high dispersive permittivity ( > 20000 at room temper-ature), and the normal ferroelectric PbTiO 3  (PT) with a firstorder ferroelectric to paraelectric transition at  ∼ 490 ◦ C [1]. PMN and PT form a solid solution with a morphotropicphase boundary (MPB) near 35 mol% of PT [2]. By varying the molar ratio between PMN and PT, different PMN-PT(or PMNT) solid solutions can be obtained with differentproperties. Low PT concentrations in PMN-PT are relaxor ferroelectrics, while a number of ferroelectric phases appear for PT contents  ≥ 20 mol%. Ultra-high piezoelectric coeffi-cients ( d  33  > 2000 pC · V − 1 ) have been measured for singlecrystals, with a composition close to the MPB and along the  100   direction of the pseudocubic crystal [3 – 6]. These re- sultshaveattractedalargeinterestonPMN-PTmaterialsdueto their potential applications in actuators and transducers.In the case of PMN-PT thin films, the interest focuseson different issues such as their integration in miniaturizedelectromechanical systems (MEMS) while maintaining thelarge field induced strain and piezoelectric coefficients of the PMN-PT relaxor-ferroelectrics [7]. But the preparation of PMN-PT materials with the perovskite structure is com-plicated because of the tendency to form non-ferroelectricpyrochlorephases.SingleperovskitePMN-PTbulkceramicscannotbepreparedbydirectsolidstatereactionoftheoxides,being the columbite method the first route that succeededin the elimination of pyrochlore from ceramic powders[8]. In the case of the preparation of PMN-PT thin films ontoplatinised silicon substrates, different deposition techniqueshave been used [9 – 11]. Among these, Chemical Solution Springer   332 J Sol-Gel Sci Techn (2007) 42:331–336 Deposition (CSD), in which sol-gel is included [12], is a simple and low cost method that may be integrated with thesemiconductor technology [13]. Sol-gel derived PMN-PT thin films have mainly been prepared by the mehoxyethanolroute, based on the works of Gurkovich et al. [14] and Budd et al. [15]. But, problems related with the difficult handling of the starting reagents and sols (moisture-sensitivity,toxicity,  ... ), the homogeneity control of the sols (rapidhydrolysis and condensation) or the quality of the depositedfilm (roughness, porosity,  ... ) have encouraged the de-velopment of alternative sol-gel processes [16 – 21]. The conversionofthegellayerintoacrystallinefilmthroughheattreatment is also very important. The presence of pyrochlorephases, which have either been stabilised at relatively lowtemperatures ( < 600 ◦ C) or produced by lead loss duringannealing, is responsible for the degradation of the dielectricand ferroelectric properties of the crystalline PMN-PTfilms.In this work preliminary results are reported about thefabrication of PMN-PT thin films (PMN(70 mol%)-PT(30 mol%)) by a sol-gel method based on the diol-route[19, 22]. Air stable solutions are obtained, from which per- ovskite films can be prepared at 650 ◦ C. The use of a PbTiO 3 seeding layer onto the platinised silicon substrate induces apreferredorientationalongthe  100  directionperpendicular to the film substrate. Dielectric, ferroelectric and piezoelec-tric properties of these films are discussed. 2 Experimental method PMN and PT sols with nominal compositions of Pb(Mg 1 / 3 Nb 2 / 3 )O 3  and PbTiO 3 , respectively, weresynthesized by chemical methods based on the diol route[19, 22]. An air-stable and precipitate-free sol of PMN was synthesised using 1,3-propanediol, HO(CH 2 ) 3 OH,and acetic acid, CH 3 COOH, as solvents, and magnesiumethoxide, Mg(OC 2 H 5 ) 2 , niobium ethoxide, Nb(OC 2 H 5 ) 5 ,and lead acetate trihydrate, Pb(OOCH 3 ) 2 · 3H 2 O, as reagents.Mg(OC 2 H 5 ) 2  and Nb(OC 2 H 5 ) 5  were first refluxed for 8 h. under dried nitrogen atmosphere, in a mixture of HO(CH 2 ) 3 OH and CH 3 COOH, with a molar ratio of HO(CH 2 ) 3 OH/CH 3 COOH = 7/1. The molar ratios of Mg(II)/Nb(V) and of [Mg(II) + Nb(V)]/HO(CH 2 ) 3 OH wereof 1/2 and 1/10, respectively. After reflux, vacuum distilla-tion of the byproducts was carried out. A volume of distilledliquidequivalenttothe80vol%ofthetotalvolumeofetanol,C 2 H 5 OH, of the solution was distilled off. The resultingNb(V) – Mg(II) solwas stable in air. In another reaction flask,Pb(OOCH 3 ) 2 · 3H 2 O was refluxed under air in HO(CH 2 )OHfor 1 h., using a molar ratio of Pb(II)/HO(CH 2 )OH of 1/10and with an amount of Pb(OOCH 3 ) 2 · 3H 2 O that maintainsthe molar ratio of Pb(II)/Mg(II) = 3/1 in relation with theMg(OC 2 H 5 ) 2  of the PMN sol. Water was distilled off by vacuum distillation. The Pb(II) sol was added to theNb(V) – Mg(II) sol, obtaining a Pb(Mg 1 / 3 Nb 2 / 3 )O 3  (PMN)precursor sol that was stirred for 1 h in air. A PbTiO 3 (PT) precursor sol was synthesised following the procedurepreviously reported [19]. The concentrations of these PMN and PT sols were calculated by gravimetric methods asthe equivalent of Pb(Mg 1 / 3 Nb 2 / 3 )O 3  or PbTiO 3  per liter of solution. Calculated amounts of both sols were mixedand refluxed together for   ∼ 15 min to get a sol with anominal composition of 0.7Pb(Mg 1 / 3 Nb 2 / 3 )O 3  –0.3PbTiO 3 (PMN-PT). Larger reflux times lead to the formation of precipitate. To compensate for the lead loss during thethermal treatment of the films, a 15 mol% excess leadwas added to both PMN and PT sols. Figure 1 showsthe reaction scheme followed for the preparation of thesols.The PMN-PT sols were diluted to  ∼ 0.4 mol/L, using2-ethyl-1-hexanol, C 8 H 18 O. These sols were deposited byspin-coating onto (111)Pt/TiO 2  /SiO 2  /(100)Si substrates.Wetlayersweredriedat350 ◦ C/60sandcrystallisedbyRapidThermal Processing (RTP) in oxygen with a heating rate of  ∼ 200 ◦ C/s, using different crystallisation temperatures andsoaking times.A Jetstar 100T JIPELEC equipment was usedfor the RTP treatments. The use of rapid heating rates (here ∼ 200 ◦ C/s) up to get the temperature where the PMN-PTperovskite is stable avoids the stabilization of secondaryintermediate phases [23, 24], such as non-ferroelectric pyrochlores. This procedure was repeated several times toobtain PMN-PT films with thickness between 250–500 nm.To induce a   100   preferred orientation in the crystallinefilms,a  ∼ 50nmthickPbTiO 3  seedlayerwasdepositedfromthe PT solution, previous to the deposition of the PMN-PTfilms.Aliquots of the  ∼ 0.4 mol/L diluted PMN-PT sols weredried at  ∼ 100 ◦ C for 72 h for differential themal and ther-mogravimetricanalyses(DTA/TGA),betweenroomtemper-ature and 1000 ◦ C in an oxygen flux of 100 mL/min and aheating rate of 10 ◦ C/min. A Seiko DTA/TGA 320U modelapparatus was used for these analyses.Grazing incidence X-ray diffraction (GIXRD), with anincidence angle of   α = 2 ◦ , was used for monitoring the crys-tal phases developed in the films during their RTP treatment.X-ray diffraction (XRD) with Bragg-Brentano geometrywas used for the observation of the preferred orientation of the films. A Siemens D500 powder diffractometer with aCu anode was used in these studies. GIXRD patterns werecollected with this equipment using a 0.4 ◦ Soller slit and aLiF monochromator.Film surfaces were analysed by scanning electron mi-croscopy using a JSM-6335F NT Field Emission gun mi-croscope (FEG-SEM). Topography of the surfaces was alsostudied by scanning force microscopy (SFM). Springer   J Sol-Gel Sci Techn (2007) 42:331–336 333 Fig. 1  Scheme of synthesis of air-stable PMN-PT sols using a novel sol-gel process based on the diol route The temperature dependence of the dielectric permittiv-ity was measured with a HP 4284A precision LCR meter at0.05 V during cooling at 2 ◦ C min − 1 . Eight frequencies be-tween 1 and 125 kHz were scanned. Ferroelectric hysteresisloops were measured at room temperature and 200 Hz, witha modified Sawyer-Tower circuit and a Tektronix TDS520oscilloscope.The local piezoelectric characterisation of the PMN-PTfilms was carried out by piezoresponse force microscopy(PFM). A scanning force microscope Nanotec R  Electron-ica, with WSxM R  software, was used in this study, usingconductive Pt/Ir coated cantilevers with a force constant of 42 N/m and resonance frequency of 320 kHz, and applyingan ac voltage of 1 V at 50 kHz. 3 Results Figure 2 shows the DTA/TGA curves of the gel derived fromthe 0.4 mol/L diluted PMN-PT sol. A total weight loss of  ∼ 25 wt.% is measured in the thermal decomposition of thisgel. This weight loss is produced in a temperature inter-val between room temperature and  ∼ 450 ◦ C, where threeexothermic peaks at  ∼ 240,  ∼ 302 and  ∼ 372 ◦ C are de-tected. Over   ∼ 450 ◦ C, neither other energetic anomaly nor weightlossareobserved.Over   ∼ 800 ◦ C,asmallweightlossis produced due to lead evaporation.Figure 3 shows the GIXRD patterns of the crystallinePMN-PT thin films treated at temperatures between 500 and750 ◦ C (Fig. 3(a)) and with different soaking times for the temperature of 650 ◦ C (Fig. 3(b)). Results of Fig. 3(a) in- Fig. 2  TGA/DTA analyses of the PMN-PT gels, carried out in anoxygen flux and with a heating rate of 10 ◦ C/min dicate that the PMN-PT perovskite phase coexists with apyrochlore phase in the films treated at temperatures below600 ◦ C.SinglePMN-PTperovskitefilmsareonlyobtainedattemperaturesbetween600and700 ◦ C,whensoakingtimesof 6sareused.Over700 ◦ C,thedecompositionoftheperovskitefilm is produced, with the appearance of a pyrochlore phase.The degradation of the perovskite film is also observed witha progressive increase of the soaking time. The films treatedat 650 ◦ C for times longer than 6 s contain small amount of a pyrochlore secondary phase, the relative content of whichincreases as the soaking time increases (Fig. 3(b)). The use of a  ∼ 50 nm thick PbTiO 3  (PT) seeding layer onto the Pt/TiO 2  /SiO 2  /(100)Si substrates induces the for-mation of PMN-PT perovskite thin films with a preferredorientation along the  100  axis. Figure 3(c) shows the XRD patterns of the PT film used as seeding layer and of the Springer   334 J Sol-Gel Sci Techn (2007) 42:331–336 Fig. 3  (a) Grazing incidence X-ray diffraction (GIXRD) patterns of the PMN-PT films crystallised by RTP in oxygen at different temper-atures, using a soaking time of 6 s and a heating rate of 200 ◦ C/s. (b)GIXRD patterns of the PMN-PT films crystallised by RTP in oxy-gen at a temperature of 650 ◦ C with different soaking times. (c) X-raydiffraction(XRD)patternsofthePbTiO 3  seedinglayerandofthe  100  oriented PMN-PT film deposited onto this seeding layer  PMN-PT film prepared onto it. Note that both the PT and thePMN-PT grow with a  100  preferred orientation.Figure 4(a) shows the FEG-SEM image of the surface of  the  100  oriented PMN-PT thin film. Morphologically dif-ferentiated secondary phases are not observed in the image.Grain size is  ∼ 150 nm. The same size is observed from thetopographic SFM image (Fig. 4(b)). Figure 5 shows the temperature evolution of the dielectricpermittivity ( ε T  33 ) and the ferroelectric hysteresis loop. Thefilm has a single broad relaxor type maximum at temper-atures between 110 and 130 ◦ C, with a maximum value of  ε T  33  of   ∼ 2000 ε 0  at 115 ◦ C and 1 kHz. Room temperature  ε T  33 and loss tangent (tan  δ ) at this frequency are of   ∼ 1835 and ∼ 0.04, respectively. The film present a saturation polarisa-tion as high as  ∼ 20  µ C · cm − 2 , but a remnant polarisation of  ∼ 3  µ C cm − 2 .The piezoelectric activity of this  100  oriented PMN-PTfilm is demonstrated by the results of the Fig. 6, where local piezoelectric hysteresis loops are shown. Fig. 4  (a) SEM and (b) SFM images of the surface of the   100  oriented PMN-PT film 4 Discussion Major problems of the sol-gel routes used for the prepa-ration of thin films are toxicity and sensitivity to mois-ture of the chemicals used in the process. The so-calledmethoxyethanol route has been widely used in the fabrica-tion of films, also in PMN-PT thin films [25 – 29]. However, the moisture-sensitivity of the sols derived from this routeand the teratogenic character and high toxicity of this sol-vent encourage us to look for other sol-gel systems. Solsprepared by the methoxyethanol route are not stable in air,giving place to the formation of precipitate resulting fromthe hydrolysis reactions among the components of the solsand the moisture contained in air.As an alternative process for the PMN-PT system, thiswork reports the first results obtained by using a sol-gelsynthesis method based on the diol route. This route haspreviously been applied to the synthesis of air stable solscontaining Ti(IV) (PbTiO 3 , Pb(Zr   x  Ti 1 −  x  )O 3  precursor solu-tions) [19, 29] and Ta(V) (SrBi 2 Ta 2 O 9  precursor solutions)[22]. Nb(V) and Ta(V) are elements of the group Vb, and thus, they have very similar chemical properties. Therefore, Springer   J Sol-Gel Sci Techn (2007) 42:331–336 335 Fig. 5  (a) Variation of   ε T  33  with temperature in a  100  oriented PMN-PT film with a thickness of   ∼ 480 nm. Measurements were carried outat eight different frequencies between 1 and 125 kHz. (b) Ferroelectrichysteresis loop measured at 200 Hz in the former film it is expected that the reaction of niobium penta-ethoxidewithadiol(1,3-propanediol)shouldleadtoaniobiumpenta-glycolate compound with a similar structure to that reportedfor Ta(V) [22]. Moreover, this compound should have a low moisture sensitivity due to the steric hindrance of the fivediol groups bonded to Nb(V). Besides, this diol solvent isa non-toxic compound as compared with methoxyethanol[30]. Based on these ideas, a PMN sol was synthesized under driednitrogenatmosphere(duetothemoisture-sensitivityof the niobium ethoxide, magnesium ethoxide and lead acetatestarting reagents). After reaction, the PMN sol results tobe air stable and can be stored in air for months withoutprecipitateformationorappreciablegelation.ThePTsolwassynthesized as previously reported [19], also giving placeto an air stable sol. The mixture of both sols allows us toobtain air-stable and relatively low-toxic PMN-PT precursor solutions. Fig.6  Localpiezoelectrichysteresisloopsmeasuredin  100  orientedPMN-PT film with a thickness of   ∼ 250 nm IthasalsobeenreportedthatpreparationofPMN-PTcom-positions in the perovskite structure is not straightforward,since the formation of non-ferroelectric pyrochlore phaseoccurs at low temperatures and the degradation of the PMN-PTperovskiteathightemperaturesalsoproducespyrochlorephase[31 – 33].ThemolarratioofPMNtoPTinthesolidso- lution also determine the development of perovskite and/or pyrochlore in the material. In agreement with the results re-ported by Francis et al. [1] for PMN-PT films derived from the methoxyethanol sol-gel route, a single perovskite phaseis firstly observed by GIXRD in our films, with a nominalcompositionof0.7Pb(Mg 1 / 3 Nb 2 / 3 )O 3  –0.3PbTiO 3 ,at600 ◦ C.The films are single phase, within the XRD detection limits,up to 700 ◦ C, and over this temperature pyrochore and per-ovskite phases co-exist in the film. A similar behaviour isobserved with the increase of the soaking time at a constanttemperature of 650 ◦ C. Times over 6 s produce the conver-sionofthesingleperovskitefilmintoaperovskite/pyrochlorethin film. Therefore, single perovskite PMN-PT films couldbe here obtained with low thermal budgets (RTP in oxygenat 650 ◦ C for 6 s, with a heating rate of 200 ◦ C/s).Taking into account the possible applications of these films in MEMS, PMN-PT films with rombohe-dral/monoclinic structure and a high   100   preferred orien-tation are desired, for providing materials with very high d 33 piezoelectric coefficients, as is the case for single crystals. APbTiO 3  seeding layer with a good lattice matching with thePMN-PT perovskite phase has been used here for obtaining  100  orientation. This idea is based on previous works thatuse very thin layers for the nucleation of perovskite filmswith preferred orientations (i.e. Ti or TiO 2  layers for    111  Pb(Zr   x  Ti 1 −  x  )O 3  films [34] or PbTiO 3  layers for   100  PMN-PT films [28]). Figure 5(a) shows broad maxima of the  ε T  33  that shiftstowards higher temperatures with frequency, with  T  m Springer 
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