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Synthesis and Characterization of PMN-PT Thin Films Prepared by a NewChemical RouteM. L. Santiago a; M. G. Stachiotti b; R. Machado b; N. Pellegri a; O. de Sanctis a

a Laboratorio de Materiales Cerámicos, FCEIA, IFIR, UNR, Rosario, Argentina b Instituto de Física Rosario,Rosario, Argentina

Online Publication Date: 01 January 2008

To cite this Article Santiago, M. L., Stachiotti, M. G., Machado, R., Pellegri, N. and de Sanctis, O.(2008)'Synthesis andCharacterization of PMN-PT Thin Films Prepared by a New Chemical Route',Ferroelectrics,370:1,85 — 93

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Ferroelectrics, 370:85–93, 2008Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150190802384427

Synthesis and Characterization of PMN-PT ThinFilms Prepared by a New Chemical Route

M. L. SANTIAGO,1 M. G. STACHIOTTI,2,∗ R. MACHADO,2

N. PELLEGRI,1 AND O. DE SANCTIS1

1Laboratorio de Materiales Ceramicos, FCEIA, IFIR, UNR, Av. Pellegrini 250,(2000) Rosario, Argentina2Instituto de Fısica Rosario, UNR, 27 de Febrero 210 Bis, (2000) Rosario,Argentina

Thin films of lead magnesium niobate-lead titanate (PMN-PT) have been synthesizedby a new and simple chemical solution deposition route, using Acetoin (3-Hydroxy 2-butanone) as chelating agent and methanol as solvent. The starting materials were Nio-bium ethoxide, Titanium butoxide, Magnesium ethoxide and Lead acetate. The films weredeposited on Pt/Ti/SiO2/Si substrates by spin-coating. By a multilayer process, homoge-nous and crack-free films with thickness up to 1 μm were obtained. The microstructureand surface morphology of the films were characterized by X-ray diffraction and atomicforce microscopy (AFM) techniques. Films with a predominant perovskite phase and av-erage grain size of ∼200 nm were obtained. The dielectric and ferroelectric propertiesare reported.

Keywords Piezoelectrics; PMN-PT; thin films; precursor chemistry

1. Introduction

Ultralarge piezoelectric responses have been observed in single crystal solid solu-tions of relaxor ferroelectric perovskites and PbTiO3: the specific systems being[Pb(Mg1/3Nb2/3)O3]1−x [PbTiO3]x (PMN-PT) and [Pb(Zn1/3Nb2/3)O3]1−x [PbTiO3]x

(PZN-PT). Optimized electromechanical properties are observed for compositions just onthe rhombohedral side of the morphotropic phase boundary. In the case of PMN-PT, thex = 0.3 (70PMN-30PT) composition near the morphotropic phase boundary has excel-lent dielectric and piezoelectric properties. Piezoelectric d33 coefficients of 2500 pC/Nandunipolar strains as large as 1.7% have been observed when the crystals were measuredalong (100) type directions [1]. For that reason, PMN-PT thin films have attracted muchattention as a candidate material for a number of applications in microelectronics and micro-electromechanical systems (MEMS), such as multilayer capacitors, ultrasonic transducers,microsensors and microactuators.

One of the major challenges for thin film processing is depositing pyrochlore-freePMN-PT on platinum-coated silicon substrates. It is known that pure perovskite PMN-PTceramics are difficult to obtain unless powders are prepared using the columbite method [2].

Received September 3, 2007; in final form March 12, 2008.∗Corresponding author. E-mail: [email protected]

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However it is difficult to use this method for thin film processing. Accordingly, the growthand processing of PMN–PT thin films have remained a difficult task, due to its tendencytowards the formation of pyrochlore phase, which deteriorates the dielectric and piezoelec-tric properties of the films. In fact, earlier reported works already stated the difficulty in theperovskite phase formation on platinum-coated silicon substrates [3, 4]. To overcome thisproblem, the utilization of other substrates, or the use of template layers to aid the perovskitephase formation has been explored. However, since platinum-coated silicon substrates offereasy integrability into semiconductor devices, research towards formation of PMN–PT onthose substrates holds great importance.

PMN-PT thin films have been obtained using different chemical and physical methods.The most often used physical processes are metalorganic vapour deposition (MOCVD)[5, 6] physical vapour deposition (PVD) including sputtering [7–9] and pulsed laser ablation(PLD) [10–15]. In the case of chemical preparation, the most commonly used method isthe chemical solution deposition (CSD), such as the sol–gel route [16–21]. Among thesevarious preparation techniques, CSD is promising because it provides high purity, largedeposition area, and easy composition control.

Solution preparation is a crucial step in the CSD technique. It generally involves theuse of metal-organic compounds that are dissolved in a common solvent. Solution pro-cesses based on the use of 2-methoxyethanol are perhaps the most widely used of anyof the CSD routes, primarily due to the ability of this solvent to solubilize a variety ofstarting reagents. Previous works involving CSD synthesis of PMN-PT thin films are allbased on the 2-methoxyethanol route [16–22]. Although this route offers an excellent con-trol and reproducibility of process chemistry, the preparation of the precursor solutionsinvolve distillation and refluxing strategies which difficult the solution preparation process.The chelate route constitutes an alternative solution synthesis approach which also utilizesalkoxide compounds as starting reagents [23]. Unlike true sol-gel routes, the chelate routerelies on the molecular modification of the alkoxide compounds through reactions withother reagents, namely chelating agents which provide stability to the precursor solutions.Compared to the 2-methoxyethanol process, chelate processes offer the advantages of rela-tively simple and rapid solution synthesis. The goal of the present study is the developmentof a chelate route for the synthesis of PMN-PT thin films. We show that stable PMN–PTprecursor solutions were prepared by selecting appropriate starting reagents. Homoge-nous and crack-free films, with thickness up to 1 μm, were deposited on Pt/Ti/SiO2/Sisubstrates. The quality of the films was analyzed by X-ray diffraction and atomicforce microscopy (AFM) techniques. The dielectric and ferroelectric properties are alsoreported.

2. Experimental

The choice of precursors, solvents and chelating agents is very important for producinghigh-quality thin films using CSD processing based on a chelate route. Regarding thechelating agent, acetic acid, acetylacetone, or amine compounds are the most commonlyused. We have shown, however, that the use of alkanolamines as chelating agent producesthe segregation of metallic bismuth in as-prepared SrBi2Ta2O9 (SBT) powders [24]. Theamine group is a powerful Lewis base which reduces the Bi3+ ion during the evaporation ofsolvent and formation of the gel powder. Consequently, the utilization of amine compoundsas chelating agent in the preparation of PMN-PT films could lead to the segregation ofmetallic Pb during the synthesis. This fact, together with the high volatility of lead and leadoxide species, could generate serious stoichiometry problems.


Synthesis and Characterization of PMN-PT Thin Films [779]/87

α-Hydroxyketones have emerged as good chelating agents due to their stabilizationeffects on metal alkoxides solutions [25, 26]. In particular, it was found that acetoin (3-Hydroxy 2-butanone) has the highest stabilization effect on Ti, Zr, Ta and Nb metal alkox-ides. Evenmore, we showed recently that, in comparison with amine compounds, the uti-lization of acetoin as chelating agent in the preparation of SBT thin films produces theelimination of residual organic compounds at a lower temperature, an earlier onset of crys-tallization in a narrower temperature range and no segregation of metallic bismuth. Thederived films presented a good microstructure [27, 28]. For that reason, the present routefor the synthesis of PMN-PT thin films is based on the utilization of acetoin as chelatingagent.

PMN-PT precursor solutions were prepared by using Niobium ethoxide (Nb(OC2H5)5),Titanium butoxide (Ti(OC2H5)4), Magnesium ethoxide (Mg(OC2H5)2) and Lead acetatetrihydrate (Pb(C2H3O2)2.3H2O) as source materials, with methanol as solvent. Acetoin(CH3COCH(OH)CH3) was used as chelating agent. Commercial alkoxides were used intheir as-received state. The alkoxide compounds (Niobium ethoxide, Titanium butoxideand Magnesium ethoxide) were initially dissolved in methanol under nitrogen atmosphere.Acetoin was added to each alkoxide-methanol solution (the molar ratio of the chelatingagent to the alkoxide was R = 4). The three solutions were then mixed to form an Mg/Nb/Ticomplex solution. Separately, Lead acetate trihydrate was dissolved in a mixture of methanoland acetic acid. The acetic acid was added to facilitate the solubility of the Lead reagent.Finally, the lead solution was added dropwise to the complex solution with continuousstirring. To compensate for lead loss during the heat treatment, 5 mol% excess lead was addedto the precursor solution. The metal precursors were mixed to form a PMN-PT solutionwith a target composition of [Pb(Mg1/3Nb2/3)O3]0.7 [PbTiO3]0.3 (70PMN-30PT). The finalconcentration of the precursor solutions, controlled by varying the methanol content, was0.3 M. A flow chart for the chemical solution processing of PMN-PT precursors is presentedin Fig. 1. The precursor solutions remained stable against phase segregation by precipitationfor at least three month at room temperature in ambient conditions. The 0.3 M PMN-PTsolutions were suitable for the deposition of uniform and crack-free layers by spin coating,with a thickness of ∼100 nm per layer. The film layers were deposited onto Pt/Ti/SiO2/Sisubstrates (Radiant Technologies Inc.) by a spin coating process at 3000 rpm for 30 s in aClean Bench. After deposition, each layer was dried at 200◦C for 3 min and pyrolyzed at400◦C for 6 min. Additional layers were spin coated to build up the desired thickness. Themultilayer coating was rapidly heat-treated by quickly placing it into a preheated furnace at adesignated temperature for 6 minutes, and then quenched in air. Two different crystallizationtemperatures were analysed: 700 and 800◦C. The flow diagram for the thermal treatmentof the samples is represented in Fig. 2.

Structures and crystal orientations of the films were measured using an X-ray diffrac-tometer with Cu Kα radiation (Philips X-Pert Pro) and a graphite monochromator (the stepsize being of 2θ = 0.02◦ and a 1 second time per step). A grazing incidence configuration(GIXRD) was used. Morphology, thickness and roughness of the thin films were analyzedusing Atomic Force Microscopy techniques (NanoTec ELECTRONICA). Contact modewas selected as equipment working configuration and the images were obtained in air atroom temperature. A SiN tip supported by a silicon cantilever of 0.76 Nm−1 spring constantwas selected. For electrical measurements, Pt top electrodes, each with a nominal area of7.8 10−5 cm2 were deposited through a shadow mask by sputtering. The dielectric constantand loss factor were measured with a HP 4192A LZ impedance analyser. Ferroelectrichysteresis loop measurements were conducted with a Sawer-Tower circuit at a frequencyof 50 Hz.



Figure 1. Schematic diagram for the preparation of PMN-PT (70/30) precursor solution.

3. Results and Conclusions

We have prepared dense PMN-PT thin films with an average thickness of 500 nm, usingthe spin-coating deposition method. As-annealed films were found to be dense, free ofcracks and well adhered to the substrates. The films also showed very smooth surfaces

Figure 2. Flow diagram for the thermal treatment of the PMN-PT thin films.



Figure 3. AFM topographic images of PMN-PT films annealed at 700◦C (a) and 800◦C (b).

as observed through an optical microscope. The topography and the surface roughnesswere analyzed by AFM technique. Figure 3 shows the topography of the PMN-PT filmsannealed at 700◦C (left panel) and 800◦C (right panel). At 700◦C the surface clearly showsthe presence of two phases. This fact will be further reconfirmed by the X-ray results. Thelarger grains correspond to the PMN-PT perovskite phase, while the smaller ones reflectfrom the presence of a spurious pyrochlore phase. When the heating treatment temperaturewas increased to 800◦C, the perovskite phase has increased due to ongoing nucleationand grain growth processes. The microstructure examination showed fine sized grains withpractically no presence of pyrochlore phase. It was also found that the grain size obviouslyincreased with increasing annealing temperature, 120 nm and 200 nm for samples annealedat 700◦C and 800◦C, respectively.

Figure 4 shows the XRD patterns of the PMN-PT films crystallized at 700◦C and800◦C. Peak indexing for the perovskite phase is done on the basis of the pseudocubicunit cell. As can be observed, the PMN-PT perovskite phase crystallized even at 700◦C,although a little amount of pyrochlore phase was detected. The formation of the secondarypyrochlore phase decreases with increasing annealing temperature. An approximation ofthe quantitative perovskite phase formation can be estimated from the relative intensitiesof the perovskite (110) peak and the pyrochlore (222) peak at 2θ = 29.27◦ : I(110)perovskite/[(110)perovskite + I(222)pyrochlore] . The results showed that 82% and 95% of perovskitephase were obtained at 700◦C and 800◦C, respectively. The comparison with x-ray diffrac-tion patterns (not shown here) of PMN-PT powders, prepared from the same precursorsolution, indicates that the films do not present any preferential crystal orientation. So,although the Pt substrate has a strong (111) texture, randomly oriented PMN-PT films wereobtained.

The dielectric properties of the films were investigated in terms of the dielectric con-stant and loss factor (tanδ), which were measured as a function of frequency in a range100 Hz–100 kHz using a HP 4192A LZ impedance analyser. Figure 5 shows the low fielddielectric constant (ε) and tanδ as a function of frequency. It shows that both dielectricconstant and the dielectric loss steadily decrease with the increase in frequency. This strongfrequency dispersion is a characteristic property of relaxor ferroelectrics. The loss factoris lower than 0.1 for frequencies below 30 kHz, but it rises considerably above this value.The dielectric constant and loss factor were found to increase with the increase in annealingtemperatures, which may be attributed to the increase in grain size and density of the filmsas was observed in AFM studies.

The ferroelectric hysteresis loops were measured by a Sawyer-Tower setup at a signalfrequency of 50 Hz. The loops for the films at the two annealing temperatures are illustrated



Figure 4. GIXRD patterns of PMN-PT films at 700◦C and 800◦C.

Figure 5. Dielectric constant (ε) and loss factor (tanδ) as a function of frequency for PMN-PT filmsannealed at different temperatures.



Figure 6. Polarization versus applied field characteristics of Pt/PMN-PT/Pt capacitors for annealedat 700◦C and 800◦C.

in Fig. 6. As to be expected the values of remanent polarization increase with increasingannealing temperatures. The remanent polarization (2Pr ) and the coercive field are approx-imately 18 μC/cm2 and 35 KV/cm for the film treated at 800◦C. The film annealed a 700◦Creveals a similar coercive field but a much smaller 2Pr value (≈ 6 μC/cm2).

Finally, we mention that we have explored the possibility of obtaining thicker filmsusing the present chelate route. Ten layers were deposited on a platinum-coated siliconsubstrate by spin coating at 3000 rpm for 30 s. Dense and crack-free films were obtainedafter sintering at 800◦C for 6 min. Fig. 7 shows a topography image and the corresponding

Figure 7. AFM 3D topographic image of a made step and the corresponding profile for a 10 layersPMN-PT film annealed at 800◦C.



section analysis profile for the 10-layer PMN-PT film. The resulting film thickness was∼1 μm.

In summary, we have developed a new chelate route for the synthesis of PMN-PT thinfilms. This approach offers the advantage of a simple and rapid solution synthesis. In com-parison with the 2-methoxyethanol, distillation and refluxing strategies are not required. Thederived PMN-PT films present a predominant perovskite phase, and a good microstructurewith grain size of about 120 nm and 200 nm for samples annealed at 700◦C and 800◦C,respectively. The dielectric and ferroelectric properties are comparable of those of PMN-PTthin films prepared with other methods [6, 8, 13, 16, 18]. Several processing conditions,such as a precise control of composition and a more adequate heat treatment must be furtheroptimized in order to minimize the amount of pyrochlore phase in the films.

Acknowledgments

The authors are thankful to Prof. Jose Antonio Eiras for helping us with the electricalcharacterization of the films. This work was supported by ANPCyT and CONICET. M.G.S.thanks support from CIUNR. M.L.S. thanks Fundacion Josefina Pratts.

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