Rare Earth-Substituted Bi4Ti3O12 Nanocrystalline Films: Synthesis and Ferroelectric Characterization

M. S. Tomar*, S.L. Dussan-Devia, and O. Perales-Perez

University of Puerto Rico, Mayagüez, PR 00681-9044, *mtomar@uprm.edu

ABSTRACT

The recent interest for nanocrystalline ferroelectric materials is due to their potential applications in nonvolatile ferroelectric random access memory (NvFeRAM) systems. The present work is focused on the chemical-solution synthesis and characterization of bare and rare earth (Ce, Er or Pr)-doped Bi4Ti3O12 (BIT) nanocystalline thin films. The atomic fraction of the dopant species, ‘x’ = 0.55, was selected based on our previous works. Thin films were deposited by spin coating onto Pt /Si substrates followed by their annealing in air at 750°C. XRD analyses of the films revealed the formation of well-crystallized and nanostructures layered perovskite (Aurivillius-type). The average crystal size was estimated to be between 20-40 nm using the Debye-Scherrer’s equation for the (117) peak. AFM imaging confirmed those estimations. Depending on the type of dopant, capacitors fabricated with rare earth-doped BIT films with Pt as top electrode exhibited different ferroelectric behaviors.

Keywords: Layered-ferroelectric, Bi4Ti3O12, Chemical-solution, Nanostructured Thin Films, Rare Earth.

1 INTRODUCTION

The recent interest for nanocrystalline ferroelectric materials is due to their promising use in memory applications. Although several systems have been proposed as candidates for NvFeRAM applications, for instance Pb(Zr,Ti)O3 or SrBi2Ta2O9 [1], their poor behavior against fatigue and the low remnant polarization values limited their final use [2]. Recent studies suggest that doping of bismuth titanate (BIT) with rare-earth species could overcome those limitations [3]. BIT-based materials belong to the family of Aurivillius-type compounds [4] and consists of perovskite-like blocks [(A(n-1)xMx)BnO3n+1]2-

interleaved with (Bi2O2)2+ sheets. Here A can be mono, di or trivalent ions, or a mixture of them (e.g., Bi3+, Ba2+); whereas B represent the tetra-, penta-, or hexa-valent species (e.g., Ti4+, Nb5+), and M is a trivalent rare-earth element. The stoichiometry corresponding to n = 3 can be represented as Bi4-xMxTi3O12. Tomar et al. showed that partial replacement of Bi3+-site ions by La3+ ions improved the ferroelectric properties of thin films produced by

chemical-solution route [5]. The rationale behind this enhancing effect of rare-earth ions is still a matter of discussion. Uong Chon et al. indicated that substitution of rare earth for bismuth in Bi4Ti3O12 partly reduced tetravalent titanium to its trivalent state, which could induce a Jahn-Teller distortion [6]. This distortion could have been caused by the difference in ionic radii between the dopant and Bi species. Based on the above considerations, the present work investigated the effect of the type of lanthanide species on the structural and ferroelectric properties of doped-BIT thin films. Cerium, Erbium and Praseodymium ions were selected as dopants. Nanocrystalline thin films were produced by chemical-solution route. The atomic fraction of the dopant ion, ‘x’ = 0.55, was selected based on our previous works [5, 7].

2 EXPERIMENTAL

2.1 Materials

Salts of trivalent Bismuth, Cerium, Erbium and Praseodymium acetate and Titanium (IV) iso-propoxide [Ti(OCH(CH3)2)4] were used as precursors. Suitable weights of the precursor salts were dissolved in acid 2-ethilhexanoic [CH3(OH2)3CH (C2H5) CO2H], or acetic acid [CH3CO2H]. Obtained solutions were placed on a hot plate pre-heated to 200 °C to facilitate the solids dissolution and the formation of a viscous solution. The viscous solution was filtered through a 0.2 µm pore size syringe filters to eliminate any solid impurities. Substrates of Pt/TiO2/SiO2/Si were intensively washed to favor the adherence of the deposited film onto clean surfaces. 2.2 Synthesis of Nanocrystalline Thin Films

Bare and rare-earth-doped bismuth titanate nanocrystalline thin films were synthesized by solution chemistry route (Sol-Gel) and spin coating technique. The viscous solution bearing the lanthanide and titanium species was added dropwise onto Pt/TiO2/SiO2/Si substrate placed on the spin-coating system at 6000 rpm. Films thicknesses were controlled adjusting the speed and the time of spinning as well as the number of coating layers. Thin films were annealed in air at 750oC for one hour to develop the final Aurivillius structure.

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2.3 Characterization

Structural analyses of thin films were carried out in a Siemens D5000 x-ray diffractometer (XRD) using Cu-Kα radiation (0.15405nm). The films thicknesses were determined by the reflectance method with an optical filmetrics thickness analyzer F-series system. Atomic force microscope (AFM) di CP-II Veeco was used in tapping-mode to study the surface morphology of the nanocystalline thin films. The ferroelectric hysteresis loops were measured in a RT6000HVS probe unit (Radiant Tech) in virtual ground mode. Capacitance was measured with an impedance analyzer HP4294a. The leakage current was determined by using a 6517A electrometer/ high resistance meter under constant electric field.

3 RESULTS AND DISCUSSION 3.1 XRD Analyses

Figure 1 shows the XRD patterns of Bi3.45M0.55Ti3O12 (BMT) polycrystalline films, where M = Ce, Er, Pr. The average films thicknesses were 0.45 µm for all samples. All diffraction peaks corresponded to a single phase of bismuth layered-perovskite structure. Thin films exhibited the preferred (117) orientation in addition to (00ℓ)-type peaks. The average crystal size was estimated to be between 20-40 nm by using the Debye-Scherrer’s equation for the (117) peak. The excellent matching between BMT diffraction peaks with those of bare-BIT suggested that lanthanides substitution of Bi3+ ions would have not affected the formation of the basic host structure. Accordingly, it can be suggested that Ce3+, Er3+ and Pr3+ ions, with ion radii of 1.14 Å, 1.00 Å, and 1.13Å, respectively, can substitute Bi ions (1.03 Å) in host BIT structure. The orthorhombic distortion ‘b/a’ for BCeT, BErT and BPrT were 0.990, 1.013, 0.990, and 0.977, respectively.

Figure 1: XRD patterns of Ce-, Er- and Pr-doped BIT thin films. The atomic fraction of dopant species was 0.55 in all samples. The films were annealed at 750°C for 60 minutes.

3.2 AFM Observations

Typical 2D-AFM images of Bi3.45M0.55Ti3O12 thin films

are shown in Figure 2. The surface of the films did not present any evidence of cracking or defects (pitting or pores) indicating fairly uniform deposition with grains averaging 110 nm, 170 nm, and 90 nm in diameter for the Ce, Er and Pr systems, respectively. The differences between the XRD and the AFM sizes would suggest the polycrystalline nature of the deposited films. Films smoothness was verified by their low root mean square (RMS) surface roughness values. These values were 4 nm for BCeT, 11 nm for BErT and 12 nm for BPrT.

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Figure 2: 1µm x 1µm AFM images of films deposited onto Pt/TiO2/SiO2/Si. a) Bare BIT; b) BCeT; c) BErT, and d) BPrT. The dopant fraction ‘x’ was 0.55 in doped-BIT films.

The surface morphology information provided by AFM

analyses also suggests that the presence of dopant ions would have promoted nucleation rate and hence smaller sizes of the nanocrystalline aggregates, when compared to bare BIT deposits.

3.3 Ferroelectric Properties

These measurements were performed using a Pt-

ferroelectric-Pt capacitor configuration. The capacitors were fabricated depositing 3.14x10-4 cm2 Pt electrodes on films surface by dc sputtering masking technique. The polarization, PR, as a function of the applied electric field (P-E hysteresis curves), for bare and doped-BIT thin films are given in Figure 3. All measurements were carried out under a maximum applied field of 500 KV/cm. Polarization

(117

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values for BIT, BCeT, BErT and BPrT were 9 µC/cm2, 25.7 µC/cm2, 22.8 µC/cm2 and 50.3 µC/cm2, respectively, the corresponding coercive fields were 70 KV/cm, 210 KV/cm, 196 KV/cm and 145 KV/cm. As seen, the ferroelectric characteristics of the nanocrystalline thin films were strongly dependent on the type of substituted ion with same composition, ‘x’ = 0.55. Previous works demonstrated that those three doped-systems exhibited maximum PR and coercive field values around this particular ‘x’ value [7]. Evidently, the substitution of Bi3+ by rare-earth ions in the perovskite-layers structure has improved the ferroelectric properties of bare BIT dramatically. This phenomenon can be attributed to the rotation of TiO6 octahedra in the a-b plane accompanied with the shift of the octahedron along the a-axis in pseudo-perovskite layer [6]. As a result, both the Bi2O2 layer and TiO6 tetrahedron could have been considerably distorted. In the case of Ce- and Er-doped BIT films, the loops were not saturated even for an applied voltage as high as 20 V. Despite of this saturation problem, the corresponding polarization values were greater than the one for bare BIT structure. The best ferroelectric response was achieved when this base material was doped with Praseodymium. The large PR value in Pr-substituted BIT resulted comparable to the one for commercially used Pb(Zr,Ti)O3 films. Therefore, Pr-doped BIT nanocrystalline materials can be considered promising candidates for lead-free ferroelectric applications.

Figure 3: Ferroelectric response of bare BIT and Ce-, Er- and Pr-doped BIT thin films. The atomic fraction of the dopant species was 0.55 in all samples.

3.2 Dielectric Measurements

The relation between the dielectric constant and dielectric loss as a function of applied frequency for BMT films is shown in Figure 4. As observed, the dielectric constant gradually decreases with a rising frequency for all

films. The dielectric constants corresponding to 1 KHz were 559, 394, 383, and 615 for bare-BIT, BCeT, BErT, and BPrT thin films, respectively. Once again, the best characteristics were attained when Pr ions doped bare BIT. The dissipation factors (loss tag) were found to be around 6% and 20% for all films.

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0.70 Figure 4: Dielectric constant and dissipation factor (loss Tag) for Bi3.45M0.55Ti3O12 (M = Ce, Er or Pr) thin films as a function of applied frequency. In turn, the dependence between dielectric constant and dielectric loss with applied voltage is summarized by Figure 5.

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Figure 5: Dielectric constant and dielectric loss-voltage curves for doped thin films. (a) BCeT, (b) BErT, (c) BPrT.

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Dielectric constant-voltage curves resembled the shape of a butterfly, particularly for the Pr-doped film, which is typical of a ferroelectric behavior. The two maximums can be attributed to ferroelectric polarization reversals. They can also be related to the coercive filed Ec. In our case, the Ec values determined from these curves were smaller than the ones estimated from the P-E hysteresis loops. This discrepancy is attributed to the dependence of Ec with applied frequency and voltage.

3.4 Leakage Current Behavior

The variation of leakage current density with applied

electric field, for bare and doped films, is shown in Figure 6. As seen, ohmic behavior dominates the leakage current transport in low field region. It is also evident that incorporation of the rare-earth ions into bare BIT structure provoked a drop on the leakage current. Park et al. [8] reported the existence of oxygen vacancies in bare BIT caused by the instability of Bi in the A site. Therefore, rare earth dopants might have contributed to minimize those defects.

Figure 6: Room-temperature variation of the leakage current with applied field for bare, Ce-, Er- and Pr-doped thin films.

4 CONCLUDING REMARKS

We report the successful fabrication of Bi3.45M0.55Ti3O12, (M = Ce, Er or Pr), nanocrystalline thin films using the chemical-solution and spin-coating techniques. XRD characterization revealed the formation of well-crystallized layered-perovskite (Aurivillius-type) structure, which preferred orientation along (117) and (00ℓ) planes. AFM images showed smooth and pinhole-free films with fairly uniform grains averaging 150nm in size. Each grain consisted of nanosize individuals. Ferroelectric measurements were conducted on Bi3.45M0.55Ti3O12 thin films for a Pt-BMT-Pt configuration at room temperature.

The ferroelectric behavior of nanocrystalline films was improved by substituting Ce, Er or Pr by Bi in the host BIT structure. Best ferroelectric response was achieved in presence of 0.55 atomic fraction of Praseodymium. The corresponding PR was very high, 50.30 µC/cm2, in comparison with 9 µC/cm2 for bare BIT. The dielectric constant-voltage curves evidenced typical ferroelectric behavior. Furthermore, doping of BIT will also favor the drop in the leakage current density.

This material is based upon work supported by the

National Science Foundation Grant No. 0351449. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors. Thanks are due to Professor R.S. Katiyar (UPR, San Juan) for facilitating the measurements.

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10 1001E-8

1E-7

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1E-3

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1

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