J Sol-Gel Sci Techn (2007) 42:265–269DOI 10.1007/s10971-006-0651-2

Characterization of zinc-modified lithium tantalate thin filmsfabricated by chemical solution deposition methodHiroshi Uchida · Katsumi Onofuji · Hiroshi Funakubo ·Seiichiro Koda

Published online: 9 February 2007C© Springer Science + Business Media, LLC 2007

Abstract Zinc-substituted lithium tantalate thin films werefabricated for improving the electrical resistivity by compen-sating the valence of lattice defects in LiTaO3 crystal. Thefilms with the chemical composition of (Li1.00−xZnx)TaO3

were fabricated on (111)Pt/TiO2/SiO2/(100)Si substrate by achemical solution deposition technique using metal-organicprecursors. Dense films consisting of a ilumenite-type crys-talline phase were deposited by spin coating on the sub-strates, followed by heat-treatment at 650◦C for 5 min in air.The leakage current density of the LiTaO3 film was reducedfrom approximately 10−4 to 10−6 A/cm2 by substitutingZn2+ ions for Li+ ions in the LiTaO3 films. Polarization–electric field hysteresis loop was improved significantly bypartial substitution of Zn2+ for Li+ ions, which is based onthe enhancement of electrical resistivity.

Keywords Sol-gel . Metal-organic precursor . Thin film .

Ferroelectric material . Lithium tantalate . Zinc . Leakagecurrent density . Polarization

1 Introduction

Thin films of ferroelectric materials are applicable for variouselectronic devices such as nonvolatile random access mem-ories, microactuators, piezoelectric sensors, light waveguide

H. Uchida (�) · K. Onofuji · S. KodaDepartment of Chemistry, Sophia University,Tokyo 102-8554, Japane-mail: uchidah@sophia.ac.jp

H. FunakuboDepartment of Innovative Engineered Materials,Tokyo Institute of Technology,Yokohama 226-8502, Japan

devices, etc. Active survey for novel ferroelectric materi-als is extensively performed in recent research because thematerials with large spontaneous/remanent polarization arerequired strongly for development of the high-performanceelectronic devices.

Multi-component oxides with ilumenite-type crystalstructure, which possess the unit-cell structure similar tothat of perovskite, are recognized as strong candidates forferroelectric materials with excellent polarization properties.They are recognized as excellent ferroelectric materials, aswell as piezo-, pyroelectric, and optoelectronic materials[1, 2]. For example, theoretical estimation indicated thatlithium tantalate (LiTaO3) and niobate (LiNbO3), kinds ofilumenite-type oxide, possess spontaneous polarization (Ps)of approximately 55 and 80 µC/cm2, which is as large asconventional perovskite ferroelectrics such as lead titanate(PbTiO3) and bismuth ferrite (BiFeO3) [3, 4]. Also, practi-cal evaluation for single crystal or polycrystalline LiNbO3

and LiTaO3 showed excellent values of spontaneous polar-ization of approximately 50 up to 80 µC/cm2 [5–7]. Rela-tively higher phase-transition temperature (891 and 1468 K,respectively) than other ferroelectrics would support the po-tential of these materials [8, 9]. However, thin-film ilumenite-type ferroelectrics exhibited extremely degraded electricproperties than the bulk materials. Appropriate polariza-tion properties have never been achieved whereas many re-searchers tried to fabricate ilumenite-type oxide films in pre-vious studies [10, 11]. These problems would be due to thedefect structure in ilumenite-type crystal lattice: In the caseof LiNbO3 and LiTaO3, evaporation of Li+ ions from thefilm surface and/or diffusion into the underlying layer dur-ing heat-treatment process could generate the defect structurebased on the Li-vacancy model [12, 13], which would disturbthe polarization switching and/or the electrical insulation ofthese materials. The degradation of the electrical properties

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based on the surface evaporation and/or diffusion could beobserved on the thin-film materials remarkably rather thanthe bulk materials.

We recognized that the charge compensation by ion-modification is generally an effective technique for recov-ering the degraded electric property owing to the lattice de-fects in ferroelectric crystals [13–15]. Some higher-valentcations, such as Mg2+, Zn2+ and Fe3+, are also proposedas candidates for the charge compensation on the ilumenite-type ferroelectrics [13, 16, 17], whereas it would requirethe combinatorial research with a large amount of time andeffort to determine the species and amount of the cationssuitable for the charge compensation. Authors recognize thatthe film deposition based on sol-gel technique would con-tribute successfully for the material survey because it canprovide the materials with various chemical compositions(i.e., various species and amount of elements) easier thanother film-deposition techniques. In the present study, ion-modified ilumenite LiTaO3 thin films were fabricated bychemical solution deposition (CSD) technique using sol-gelsolution. Zn2+ was selected as a cation for the charge com-pensation of the Li-vacancy in LiTaO3 crystal lattice. Theelectric and ferroelectric properties of the Zn2+-substitutedLiTaO3 thin films were also evaluated.

2 Experimental procedure

Zn2+-substituted LiTaO3 films were fabricated by CSDtechnique using metal-organic precursors. Lithium ac-etate [Li(CH3COO)], tantalum ethoxide [Ta(OC2H5)5], zincacetylacetonate [Zn(C5H7O2)2] and 2-methoxyethanol wereused as starting resources. Precursor solutions were pre-pared by mixing appropriate amounts of starting resourceswith a nominal chemical composition of (Li1.00−xZnx)TaO3

(x = 0–0.06, with 2% Li excess, concentration: approxi-mately 0.2 mol/dm3). The solutions were spin-coated on(111)Pt/TiO2/SiO2/(100)Si substrates at a rate of 3000 rpmfor 50 s, followed by a drying process at 150◦C for 1 minand a pyrolysis process at 400◦C for 1 min. These processeswere repeated until the film thickness reached approximately300 nm. The resulting films were then heat-treated for crys-tallization at 600–750◦C for 5 min in air. After film depo-sition by CSD, circular Pt top electrodes with diameters of∼100 µm were deposited on the films by electron-beamdeposition.

The crystal structure of the films fabricated on(111)Pt/TiO2/SiO2/(100)Si substrates was determined byan X-ray diffraction (XRD) 2θ – ω scan using a RigakuRINT2100 with CuKα radiation. The film thickness and mor-phology of the films were observed using a Hitachi S4500scanning electron microscope (SEM). The dielectric con-stant (εr) and the loss factor (tan δ) were measured using

a Hewlett Packard 4194A impedance/gain-phase analyzer.The polarization (P)-electric field (E) hysteresis curve wasexamined using a Toyo Corporation FCE ferroelectric testsystem. The leakage current densities were measured usinga Hewlett Packard 4155B electrometer/high-resistance me-ter with a 0.02 V step and a waiting time of 0.2 s for eachstep. The thermal behavior of the spin-coating solution wasevaluated using the powder sample by differential thermalanalysis (DTA) and thermogravimetry (TG) using a RigakuTG8120 DTA-TG analyzer. The powder samples were ob-tained by drying the spin-coating solution at 150◦C in air.

3 Results and discussion

Prior to the film deposition, thermal behavior of the spin-coating solution for LiTaO3 films was evaluated in order todetermine the heat-treatment temperatures (i.e., the drying,pyrolysis, and crystallization temperatures). Figure 1 showsthe DTA-TG curves for the spin-coating solution for thenon-substituted LiTaO3 film (x = 0) during a heating pro-cess measured at a heating rate of 10◦C/min. An endothermicreaction with weight loss was found at around 100◦C, whichwould correspond to the evaporation of 2-methoxyethanolsolvent remaining in the powder sample. Furthermore, inten-sive exothermic reactions together with weight loss occurredat around 300◦C and 600◦C. These reactions are recognizedas the decomposition (i.e., the incomplete combustion) of theorganic-groups in metal-organic precursors and the completecombustion of the residual carbon., respectively. No obviouschange in the heat flow and the weight loss was observedabove 650◦C. Spin-coating solutions for Zn2+-substitutedLiTaO3 films also exhibited almost the same behavior. Basedon these data, the authors determined the drying and pyroly-sis temperatures in this experiment to be 150◦C and 400◦C,respectively.

Fig. 1 DTA-TG curves for spin-coating solution of nonsubstitutedLiTaO3 (x = 0) during heating process measured at a heating rate of10◦C/min

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Fig. 2 X-ray diffraction patterns for nonsubstituted LiTaO3 films(x = 0) heat-treated at 550–700◦C. [•: LiTaO3, ×: Background fromsample stage]

Figure 2 shows XRD 2θ – ω patterns of the nonsubsti-tuted LiTaO3 films (x = 0) heat-treated at 550–700◦C. Nocrystalline phase was observed for the sample heat-treatedat 550◦C, while small peaks assigned to ilumenite LiTaO3

appeared at 600–700◦C. The peak intensity of LiTaO3 phaseincreased slightly with the heat treatment temperature. Theseresults indicate that the crystallization of LiTaO3 occurs ataround 600◦C, which corresponds well to the temperaturerange where complete decomposition was accomplished asindicated in Fig. 1. Although only small peaks of ilumeniteLiTaO3 phase were obtained for these films, the authors rec-ognized that the crystallization would saturate above 600◦Cbecause the dielectric properties of the resulting films werealso saturated above this temperature as well as the XRDpeak intensity; i.e., the dielectric constant, εr, and dielectricloss, tan δ, of non-substituted LiTaO3 films heat-treated at600, 650 and 700◦C were approximately 73, 77 and 77, 0.04,0.04 and 0.05, respectively, at a frequency of 10 kHz, whichare identical to those of bulk LiTaO3 samples [18, 19].

A typical cross-sectional SEM image of the non-substituted LiTaO3 film is shown in Fig. 3. The film heat-treated at 650◦C possessed dense microstructure with smoothsurface. Structural defects such as cracks and pores, werenot found. No obvious difference in the microstructure wasconfirmed among the samples fabricated at different temper-atures.

Zn2+-substituted LiTaO3 films were fabricated at 650◦C,where the thermal decomposition and crystallization are ac-complished. Figure 4 shows XRD 2θ – ω patterns of thefilms with Zn2+ contents of x = 0–0.06. The intensity ofXRD peaks of LiTaO3 phase decreased gradually with in-

Fig. 3 Cross-sectional SEM image for nonsubstituted LiTaO3 films(x = 0) heat-treated at a crystallization temperature of 650◦C. [(1)LiTaO3 film, (2) bottom Pt electrode and (3) Si substrate]

Fig. 4 X-ray diffraction patterns for nonsubstituted LiTaO3 and Zn2+-substituted films with various Zn2+ contents (x = 0–0.06). [•: LiTaO3,×: Background from sample stage]

creasing Zn2+ content, x. No other crystalline phases werefound up to x = 0.06.

Figure 5 shows the leakage current density (J)-electricfield (E) curves of the films with Zn2+ contents of x = 0–0.06.In the range of electric field from –600 to +600 kV/cm, theZn2+-substituted LiTaO3 films exhibited lower current den-sities than those of the non-substituted LiTaO3 film. Asym-metric J-E curves in positive and negative electric field wouldbe ascribed to the difference in thermal history between thetop and bottom Pt electrodes; i.e., the top Pt electrodes wereas-deposited while the bottom electrodes were heat-treatedat 650◦C with the LiTiO3 films. The relationship between thecurrent density at an electric field of ±200 kV/cm and theZn2+ content is summarized in Fig. 6. The current density

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Fig. 5 Leakage current density (J)-electric field (E) curves for nonsub-stituted LiTaO3 and Zn2+-substituted films with various Zn2+ contents(x = 0 – 0.06)

Fig. 6 Leakage current densities for nonsubstituted LiTaO3 and Zn2+-substituted films as a function of Zn2+ contents (x = 0 – 0.06), measuredat an electric field of plus/minus 200 kV/cm

was reduced from approximately 10−4 to 10−6 A/cm2 by theZn2+-substitution up to x = 0.02. The reduced current den-sity was maintained at x = 0.02–0.06. The improvement ofthe electrical resistivity would not be ascribed to the refine-ment of the microstructure but to the charge compensationof the lattice defects such as Li vacancy, as mentioned in theintroduction part.

Figure 7 shows the P-E hysteresis loops of the films withZn2+ contents of x = 0–0.06. Well-saturated P-E curves werehardly obtained for all samples. The hysteresis loop for thenon-substituted LiTaO3 film (x = 0) was distorted due to alarge current density. The distortion was improved by theZn2+-substitution with x = 0.02–0.06. Such improvementof P-E hysteresis loops would be ascribed to the chargecompensation of the lattice defects, which provides mainlythe enhancement of the electrical resistivity as shown inFigs. 5 and 6. The charge compensation would also havea possibility for releasing the domain pinning due to the

Fig. 7 Polarization (P)-electric field (E) hysteresis loops for nonsub-stituted LiTaO3 and Zn2+-substituted films with various Zn2+ contents(x = 0–0.06)

lattice defect, which would result in the enhancement of theremanent polarization [13–15]. From the result in Fig. 7, theauthors recognized that the Zn2+-substitution up to x = 0.02might enhance the remanent polarization of the LiTaO3 film.However, we also deduce that the remanent polarization weredecreased by the Zn2+-substitution over x = 0.02, whichwould be ascribed to the degraded crystallinity as discussedin Fig. 4.

The εr and tan δ values for x = 0, 0.02, 0.04 and 0.06 were77, 321, 276 and 188, and 0.04, 0.02, 0.03 and 0.03, respec-tively. The εr value was increased by the Zn2+-substitutionup to x = 0.02, while it decreased gradually thereafter. Theremarkable enhancement of the εr value at x = 0.02 occurredsimultaneously with the improvement of P-E hysteresis loopas shown in Fig. 7. This result suggests that the εr valuewould be also enhanced by the charge compensation of thelattice defect. The lowering of the εr value above x = 0.02might be due to the degradation in the crystallinity, which isalso similar to the phenomena as discussed in Fig. 7.

4 Conclusions

The films with the chemical composition of(Li1.00−xZnx)TaO3 were fabricated on (111)Pt/TiO2/SiO2/(100)Si substrate by CSD technique using metal-organic precursors. Dense films consisting of a ilumenite-type crystalline phase were deposited by spin coating onthe substrate, followed by heat-treatment at 650◦C for5 min in air. The leakage current density was reduced fromapproximately 10−4 to 10−6 A/cm2 by the Zn2+-substitution

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with Zn2+ content of x = 0.02, which would be based onthe charge compensation of Li-vacancy in LiTaO3 crystallattice. The Zn2+ substitution also had possibilities forimproving the P-E hysteresis behavior and enhancing theεr value. Although excessive Zn2+-substitution reduces thequality of LiTaO3 films due to degrading the crystallinity,ion modification using appropriate amount of Zn2+ ionwould enhance the polarization and dielectric propertiessimultaneously, as well as the electrical resistivity.

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