Journal of Sol-Gel Science and Technology 36, 53–60, 2005c© 2005 Springer Science + Business Media, Inc. Manufactured in the United States.

Effects of the Chemical Modifier on the Thermal Evolutionof SrBi2Ta2O9 Precursor Powders

M.G. STACHIOTTI∗, R. MACHADO, A. FRATTINI, N. PELLEGRI AND O. DE SANCTISInstituto de Fısica Rosario, Facultad de Ciencias Exactas, Ingenierıa y Agrimensura, Universidad Nacional de

Rosario, 27 de Febrero, 210 Bis (2000) Rosario, Argentinamarcelo@ifir.edu.ar

machado@ifir.edu.ar

frattini@fceia.unr.edu.ar

pellegrii@fceia.unr.edu.ar

oski@fceia.unr.edu.ar

Received January 11, 2005; Accepted May 20, 2005

Abstract. The effects of the chelating agent on the thermal evolution of SrBi2Ta2O9 precursor powders wereinvestigated. The precursor solutions were prepared from non-hydrolyzing precursors of bismuth and strontium anda tantalum alkoxide. The utilization of diethanolamine or triethanolamine as chelating agent was found to producethe segregation of metallic bismuth in the as-prepared powders, which led to the formation of a multiphase system.On the other hand, acetoin, one of the α-hydroxyketones, showed outstanding characteristics for the low-temperaturesynthesis of SrBi2Ta2O9: elimination of residual organics at low temperature, an earlier onset of crystallization, andno segregation of secondary phases during the whole crystallization process.

Keywords: ferroelectric, SBT, precursor chemistry, stoichiometry, low-temperature crystallization

Introduction

Strontium Bismuth Tantalate (SBT) ferroelectric thinfilms have attracted considerable attention for the de-velopment of non-volatile ferroelectric random accessmemories (NV-FRAMs) due to its relatively low volt-age operation, low leakage current, fast switching andgood fatigue resistance with Pt bottom electrodes [1, 2].However, SBT films have some critical problems suchas high processing temperatures (>750◦C) to be ap-plied to semiconductor process and a low remanentpolarization which is insufficient for the high densityintegration of FRAMs. In addition, the control of stoi-chiometry in SBT films is still a tricky subject becauseof the lost of the bismuth component with diffusioninto the Pt bottom electrode or evaporation during heat-

∗To whom all correspondence should be addressed.

treatment. In order to circumvent these shortcomings,many strategies have been employed such as composi-tional changes and modifications of the synthesis con-ditions.

SBT films can be prepared by pulsed laser deposi-tion [3] , metal organic chemical vapor deposition [4]as well as by a variety of chemical solution depositiontechniques that cover from the simple metal organicdecomposition [5] to more sophisticated sol-gel routesmainly based on hydrolysis and condensation of metal-lic alkoxides [6–8].

Chemical Solution Deposition (CSD) techniques areinteresting routes for the evaluation of new materialsand novel strategies due to their versatility, simplic-ity and low temperature material synthesis. They allowprecise composition control, tailoring preferred struc-tures by the control of parameters [9]. However, as start-ing materials, alkoxides make the whole fabricating

54 Stachiotti et al.

process very susceptible to temperature and ambienthumidity and, for that reason, they must be handledunder inert atmosphere. Furthermore, for multioxidefilms such as SBT, the starting metal alkoxides and theselected strategy will determine the extent of intermix-ing of the metal species, the formation of a network ver-sus individual inorganic phases, the carbon content ofthe films, the temperature at which pyrolysis of organicspecies occurs, and the densification and crystallizationbehavior of the film [10].

In order to reduce the reactivity of metal alkoxides,complexing agents or stabilizers have been used whichconsequently increase the stability of the precursor so-lutions. Ta and Bi alkoxides were coordinated usingpyridine [11], alcohol-amine [12] and acetylacetone[13], and the metal complex compound became sta-ble. However, many of these strategies imply the useof more than one solvent or one chelating agent, whichmakes difficult the control of the process of gel poly-merization, delays the residue elimination and thus re-tards the crystallization. In order to overcome theseproblems, new precursor materials and solvents areused, thus stable precursor solution can be prepared toallow their handling in air, which makes the operationmore convenient. Recently, Kim et al. [14, 15] reporteda stable and simple chemical route for the fabricationof SBT thin films using non-hydrolyzing precursors ofbismuth and strontium and a tantalum alkoxide as start-ing materials. It was observed that an alkanolamine,as chelating agent, provided stability to the SBT solu-tion by retarding the hydrolysis and condensation rates.The films fabricated from this solution and annealedat 800◦C exhibited low leakage current density, lowvoltage saturation and high remanent polarization ascompared to those annealed at 700◦C. The improve-ment in properties with higher annealing temperaturewas ascribed to improved film density and crystal qual-ity. Differential Thermal Analisys measurements dis-played a broad exothermic peak between 400◦C and700◦C which was mainly attributed to crystallizationof the SBT gel powder. However, because of the largeweight loss observed in the Thermogravimetric Anal-ysis curve in this temperature range, the crystallizationpeak is overlapped with peaks arising from the decom-position of residual organics [15]. This indicates thatthe proposed chemical route is not appropriated for thefabrication of SBT films at temperatures close or below700◦C. The complete elimination of residual organicsat low temperature is a necessary condition for the low-temperature synthesis of SBT thin films.

The decomposition of the organics is a complex phe-nomena and it depends upon the chemistry of the pre-cursor solution. In particular, the selection of chelat-ing agents affects the sol stability, the removal of or-ganics and the crystallization temperature of the ox-ide film. Among the stabilizers, the alkanolamines,especially diethanolamine (DEA) and triethanolamine(TEA), are useful because they can effectively stabi-lize various alkoxides and shield uniform films of sim-ple oxides. However, they have high boiling pointswhich may suppress the low-temperature crystalliza-tion of oxide films. Very recently, the stabilization ef-fects of α-Hydroxyketones on metal alkoxides solu-tions (Ti, Zr, Nb, Ta) were investigated [16, 17]. It wasfound that acetoin, one of the α-Hydroxyketones, hasthe highest stabilization effect on those metal alkox-ides. Furthermore, for ziconium, acetoin led to a chelatecomplex which was able to reduce the crystallizationtemperature of ZrO2 [16]. In this paper we investi-gate the effects of acetoin, diethanolamine, and tri-ethanolamine as complexing or chelating agent [here-after called chemical modifier (CM)] in the preparationof SBT precursor solutions using non-hydrolyzing pre-cursors of bismuth and strontium and tantalum ethox-ide as starting materials. In particular, we examine theinfluence of the CM on the decomposition temperatureof organic species and the thermal evolution of phaseformation.

Experimental

SBT precursor solutions were prepared by us-ing strontium acetate [Sr(CH3COO)2·1/2 H2O, pu-rity 99.9%, Aldrich], bismuth nitrate pentahidrate[BI(NiO3)35H2O, purity 99.99%, Aldrich) and tan-talum ethoxide [Ta(OCH2CH3)5, purity 99.99%,Aldrich) as source materials, with methanol and glacialacetic as solvents [14, 15]. Acetoin (3-Hydroxy-2-butanone, CH3COCH(OH)CH3, A1, 795-1, Aldrich),diethanolamine ((CH2OHCH2)2NH, purity 99%,Aldrich) and triethanolamine ((CH2OHCH2)3N, purity98%, Aldrich) were used as CM.

For the preparation of the SBT solutions, tantalumethoxide was initially dissolved in methanol under ni-trogen atmosphere. The chemical modifier was addedto the tantalum-methanol solution (the molar ratio ofthe chemical modifier to tantalum ethoxide was R =10). Then, the strontium acetate was added. Separately,bismuth nitrate was dissolved in acetic acid. Finally, the

Effects of the Chemical Modifier on the Thermal Evolution of SrBi2Ta2O9 Precursor Powders 55

bismuth solution was added dropwise to the tantalum-strontium solution with continuous stirring. The metalprecursors were mixed to form a Bismuth-rich SBT so-lution with a target composition of Sr0.8Bi2.2Ta2O9 (itwas shown that films synthesized with Sr-deficient andBi-excess compositions show significantly improvedferroelectric properties). The final concentration of theprecursor solutions, controlled by varying the methanolcontent, was 0.5 M.

It is well known that acetic acid is not an inert sol-vent in alkoxide solutions; it might act as a chemicalmodifying agent by reacting with the alkoxide precur-sor, replacing some of the alkoxy groups, altering thehydrolysis and condensation characteristics of the pre-cursors. However, in this case, the presence of aceticacid leads to the production of by-products such as freealcohols and the formation of water and methyl acetateester. The alkanolamines or acetoin as CM induces twomajor effects such as steric hindrance and inductive ef-fects, which reduce the reaction of water with alkoxideprecursor and provide the stability of solutions withaging time.

To derive gel powders the parent solutions were keptovernight in an oven maintained at 60◦C under 100 Paof pressure. After solvent evaporation the residue washeating at 200◦C. The dried gel was crushed in a mor-tar pestle to get fine amorphous powders (the averageparticle size of the powder prepared using acetoin asCM is approximately 10 µm).

By means of a Netzsche STA 409 analyzer, simulta-neous Differential Thermal Analisys (DTA) and Ther-mogravimetric Analysis (TGA) measurements werecarried out on the as-obtained powder gels from roomtemperature to 800◦C in normal atmosphere. MgO cru-cibles and α-alumina reference were used. The sys-tem was calibrated against the melting point of Au(1063◦C). The heating and cooling rates were 5◦C/min.The temperatures of DTA events were determined bythe usual method, which uses the intersection of theDTA line base with the extension of the linear regionof the rising peak slope. The estimated uncertainty was±5◦C.

High temperature X-ray diffraction (HT-XRD) pat-terns were obtained in air atmosphere making useof an Anton-Paar HTK10 high-temperature chamber.The XRD data were collected on a Phillips PW1700diffractometer using CuKα radiation (1.5405 A) anda graphite monochromator (the step size being of 2θ

= 0.02◦ and a 5 seconds time per step). The powderswere analyzed from 200 to 800◦C. Before data collect-

ing, the powders were treated during 10 minutes at eachtemperature.

Results and Discussion

Figure 1 show DTA–TGA curves for powders pre-pared using acetoin, DEA and TEA as CM. Roughlyspeaking, the TGA curves show a constant weightloss assigned to the decomposition and burning of theresidual organic groups. The samples prepared withthe two alkanolamines display a considerable weightloss up to ∼ 500◦C, and no weight loss is observedabove this temperature. We note that this temperatureis 200◦C lower than the one reported by Kim et al. [15],where an alkanolamine was used as chemical modifier.These authors, however, reported neither which alka-nolamine was used nor its concentration. When acetoinis used as CM, however, no weight loss at tempera-tures >430◦C is observed, so the thermal decompo-sition of the gel is completed at a temperature 70◦Clower than the one corresponding to the alkanolaminesamples.

The DTA curves display intense exothermic peakscentered near the temperatures where the eliminationof the residual organic groups finishes. These peaksare attributed to the elimination of the last organicresidues, such as hydroxyls, with a simultaneous on-set of the crystallization. The acetoin sample exhibitsthe sharpest crystallization peak which is in the range410–510◦C. The DEA and TEA samples show broaderpeaks, with ending points at 605 and 620◦C, respec-tively. We point out that the acetoin sample exhibits asmall mass gain centered at 580◦C (this gain was notobserved in the blank test) that is attributed to oxy-gen absorption. A similar phenomenon was observedin ZrO2 powders [18]. The alkanolamine samples, how-ever, do not exhibit any appreciable mass gain at thistemperature. We point out that the DTA-TG curves forthe acetoin sample were measured twice in order tosecure reproducibility.

High temperature XRD spectra for the acetoin sam-ple are shown in Fig. 2. The spectra show no diffrac-tion peaks (except for the ones corresponding to theplatinum plate) until reaching 500◦C. At this temper-ature, broad peaks corresponding to the defective flu-orite phase are formed. So, the onset of crystallizationof the defective fluorite phase takes place at a temper-ature between 400 and 500◦C. The fluorite phase re-mains stable up to ∼600◦C, and then the transformation

56 Stachiotti et al.

Figure 1. DTA-TGA curves of SBT gel powders prepared with acetoin, diethanolamine (DEA) and triethanolamine (TEA) as chemical modifier.

to a tetragonal layer-perovskite SBT phase starts. Or-thorhombic SBT was obtained, after cooling, at roomtemperature. It is worth to note that no other crystallinephases than the ones corresponding to the intermediatefluorite and the final SBT structures appear during theoverall thermal evolution of the sample.

The samples prepared with DEA (Fig. 3) andTEA (Fig. 4) exhibit HT-XRD spectra remark-ably different from the ones corresponding to ace-toin. The spectra at 250◦C display several peaks(2θ = 22.32◦, 27.10◦, 37.83◦, 39.44◦, 44.49◦, 48.47◦

and 55.89◦) These peaks are clearly assigned to thepresence of metallic bismuth in the as-prepared pow-ders. Above 400◦C the spectra show peaks at 2θ =27.80◦, 31.83◦, 32.73◦, 46.20◦ and 55.49◦ that corre-spond to β-Bi2O3. The presence of bismuth crystals at250◦C and their oxidation at 400◦C are in consonancewith the DTA measurements. In fact, the DTA curvesfor the samples prepared with DEA and TEA both dis-

play endothermic peaks at 281◦C, which are assignedto the melting of metallic bismuth. At 500◦C, in addi-tion to the Bi2O3 phase, diffractions corresponding toSrTa2O6, at 2θ = 28.27◦, 29.29◦, 31.20◦, 31.63◦ and32.65◦, can be identified. This characteristic diffractionpattern remains until ∼750◦C, where the intermediateoxides disappear and the tetragonal SBT phase irruptstogether with some traces of β-Bi2O3. After coolingto room temperature, the samples exhibit diffractionscorresponding to orthorhombic SBT with the presenceof two additional peaks, one centered at 2θ = 28.16◦

and the other at 2θ = 29.55◦. The first one correspondsto the main peak of β-Bi2O3. Although β-Bi2O3is un-stable, the presence of strontium stabilizes this phaseat room temperature [19]. The second would be as-cribed to residual Ta2O5. In addition, after removingthe residual sample from the platinum plate of thehigh-temperature chamber, a reacted region on its sur-face was observed. Subsequent XRD analysis, shown

Effects of the Chemical Modifier on the Thermal Evolution of SrBi2Ta2O9 Precursor Powders 57

Figure 2. High temperature XRD spectra of SBT gel powders prepared with acetoin as chemical modifier. RT labels the spectrum measuredat room temperature.

Figure 3. High temperature XRD spectra of SBT gel powders prepared with diethanolamine (DEA) as chemical modifier. RT labels thespectrum measured at room temperature.

58 Stachiotti et al.

Figure 4. High temperature XRD spectra of SBT gel powders prepared with triethanolamine (TEA) as chemical modifier. RT labels thespectrum measured at room temperature.

in Fig. 5, detected the formation of the metallic com-pound Bi2Pt.

It was found that the combination of Hydroxyke-tones with amines showed unexpectedly strong stabi-lization effects on alkoxides [17]. So, we also inves-tigated the effects of binary combinations of acetoinand alkanolamines as CM. To this end we preparedthe following samples: AD [acetoin (R = 5) + DEA(R = 5)] and AT [acetoin (R = 5) + TEA (R = 5)].DTA-TGA measurement results are qualitatively sim-ilar to the ones reported for DEA and TEA. The mainquantitative differences to highlight are: (i) in the bi-nary samples the residual elimination finishes in abruptand short mass losses at ∼ 475◦C. (ii) the AD and theAT samples display crystallization peaks with endingpoints at 568 and 554, respectively. So, the binary sam-ples exhibit the crystallization peak on a wider temper-ature range compared with the acetoin sample, but ona narrower temperature range compared with the DEAand TEA samples. Regarding the thermal evolution ofphase formation, the HT-XRD spectra of the AT pow-der showed peaks corresponding to metallic bismuthand β-Bi2O3 at 250◦C and 400◦C, respectively. For theAD sample, metallic bismuth peaks were not detected

at 250◦C. However, a detailed inspection of its spec-trum at 400◦C indicated the presence of the β-Bi2O3

phase, so the no detection of metallic bismuth at 250◦Ccan be attributed to its low concentration.

The presence of metallic bismuth in the as-preparedSBT powders coming from precursor solutions whereDEA and TEA were used as chemical modifiers can beundoubtedly attributed to the presence of amine groups.The amine group is a powerful Lewis base which can re-duce the Bi3+ ion during the step of solvent evaporationand formation of the gel powder. Two facts corroboratethis assumption: (i) the absence of metallic bismuth inthe sample prepared with acetoin and (ii) the content ofmetallic bismuth enlarges for increasing power of thereductor agent (TEA is a reductor agent stronger thanDEA) and also for increasing concentration. The seg-regation of bismuth from the SBT precursor materialat a too early stage is responsible for the formation ofa multiphase system, where secondary phases such asβ-Bi2O3 appeared.

The presence of a transient bismuth excess will pro-duce undesirable effects in the ferroelectric propertiesof SBT. Recently, the effects of Bi excess on the mi-crostructure and electrical properties of Aurivillius thin

Effects of the Chemical Modifier on the Thermal Evolution of SrBi2Ta2O9 Precursor Powders 59

Figure 5. XRD spectrum of the reacted region observed in the platinum plate of the high-temperature chamber. The formation of Bi2Pt isdetected.

films have been investigated [20, 21]. These studiesdemonstrated that the presence of Bi2O3 and Bi2 Pt assecondary phases produces a strong degradation of theferroelectric properties of the film. Zurbuchen et. al.[22] demonstrated that transient bismuth excess duringfilm growth can lead to the formation of a Bi-rich phaseat the film-substrate interface which can alter the elec-trical properties of the film as a whole. Moreover, theyalso reported that, due to the volatility of bismuth andbismuth oxide species, SBT films that appear phase-pure by x-ray diffraction have microstructural defects,caused by transient bismuth nonstoichiometry, whichare ferroelectrically inactive.

Finally, we can infer from the presented results thatthe crystallization path for the acetoin and the alka-nolamine samples is completely different. In the for-mer, a transformation path amorphous → defective flu-orite → layer-perovskite is developed with increasingtemperature without segregation of transient bismuthand appearance of secondary phases. We already men-tioned that this sample exhibits a small mass gain at∼580◦C (see Fig. 1). This temperature is in the tem-perature range where the defective fluorite → layer-perovskite transformation takes place. A similar massgain was observed in the thermal evolution of ZrO2

gel powders [18], where an intermediate metastabletetragonal phase, stabilized by the presence of oxygen

vacancies, crystallizes from the amorphous phase. Asit is known, the defective fluorite phase has a cuasi-cubic structure with two of the eight nearest neighboroxygens for each Ta-ion missing, so that the resultingcoordination is a six-fold octahedron, rather that eight-fold (perfect fluorite structure). When oxygen diffu-sion mechanisms become operative, atmospheric oxy-gen atoms are incorporated into the defective fluoritestructure destabilizing and transforming this phase tothe layer-perovskite one. In the alkanoamine samples,the presence of oxides (Bi2O3,SrTa2O6) at a too earlystage and the absence of oxygen absorption would in-dicate that solid-state-reactions are leading the path tocrystallization.

Conclusions

We have investigated the effects of acetoin, di-ethanolamine, and triethanolamine as chemical mod-ifier in the preparation of SBT precursor solutions. Wefound that the use of alkanolamines produces the seg-regation of metallic bismuth in the as-prepared pow-ders which is responsible for the formation of a multi-phase system, where secondary phases such as β-Bi2O3

appeared. The presence of transient metallic bismuthduring film growth will lead to serious stoichiometry

60 Stachiotti et al.

problems with a subsequent degradation of the ferro-electric properties of SBT films. On the other hand,the utilization of acetoin as chemical modifier pro-duces (i) the elimination of residual compounds at alower temperature, (ii) an earlier onset of crystalliza-tion in a narrower temperature range, (iii) a transfor-mation path amorphous→ defective fluorite → layer-perovskite with increasing temperature without appear-ance of secondary phases, and (iv) no segregation ofmetallic bismuth in the as-prepared samples. Theseproperties indicate that acetoin is a promising chelatingagent for the low-temperature synthesis of SBT films.Investigations on the microstructure and ferroelectricproperties of thin films annealed at different tempera-tures are in progress.

Acknowledgments

The authors would like to thank Dr. Patricia Rivas formUniversidad Nacional de La Plata for technical sup-port in the DTA-TGA measurements. M.G.S. thankssupport from CIUNR. R.M. thanks Fundacion Jose-fina Pratts. This work was supported by Agencia Na-cional de Promocion Cientıfica y Tecnologica and Con-sejo Nacional de Investigaciones Cientıficas y Tecnicas(Argentina).

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