Introduction

Multiferroic materials, which exhibit simultaneously ferroelectric, ferromagnetic, antiferromagnetic, and ferroelastic behaviors, provide opportunities for potential applications in information storage, spintronic devices, and sensors [1]. Bismuth ferrite (BiFeO3) [BFO] is one such multiferroic material. BFO exhibits a distorted perovskite structure with rhombohedral symmetry. It belongs to the R3c space group with a unit cell parameter a = 0.5643 nm and a = 59.348° [2]. One of the striking features of BFO materials is the coexistence of ferroelectric (Tc = 1,123 K) and antiferromagnetic orderings (TN = 643 K) at room temperature due to a residual moment from a canted spin structure [3]. BFO is attracting great attention as a promising ferroelectric material for high-density FeREMs because of its large remanent polarization. However, BFO has serious problems as a ferroelectric material, having quite a large leakage current density, especially at room temperature. Therefore, dielectric breakdown occurs easily even at a low field, thereby indicating the difficulty in poling films. Furthermore, the highly electrically conductive nature of BFO makes it difficult to obtain excellent ferroelectric properties. To overcome this problem, various approaches have been proposed, including a substitution technique using Mn, Ti at the B-site, and/or La and Nd at the A-site [4, 5] and the formation of a solid solution with Pb(Zr, Ti)O3 and BaTiO3 compositions [6]. There are many reports on the reduction of the leakage current induced by doping and the formation of a solid solution. In these investigations, the capacitor structure formed from the metal-insulator-metal structure is used for current measurement. It should be noted that current measured in the capacitor structure includes contributions of the grain boundaries or a microstructure of the films. We have already reported on the good dielectric properties, especially the high remanent polarization and low leakage current densities of Pb(Zr0.52Ti0.48)O3 [PZT] heterolayered thin films, which were alternately spin-coated using Pb(Zr0.20Ti0.80)O3 and Pb(Zr0.80Ti0.20)O3 metal alkoxide solutions [7]. In this study, BFO/PZT multilayer thin films were prepared using the sol-gel method, which were spin-coated on the Pt/Ti/SiO2/Si substrate alternately using BFO and PZT metal alkoxide solutions. We also investigated the structural and dielectric properties of BFO/PZT multilayer thin films for application in electronic memory devices.

Experimental details

Using the sol-gel method, BFO and PZT with excess Pb-acetate 10 mol% precursor solutions were prepared from the starting materials Bi-nitrate pentahydrate [Bi(NO3)3·5H2O], Fe-nitrate nonahydrate [Fe(NO3)3·9H2O], Pb-acetate trihydrate [Pb(CH3CO2)2·3H2O], Zr n-propoxide [Zr(OCH2CH2CH3)4], and Ti iso-propoxide {Ti[OCH(CH3)2]4}, and the solvent 2-methoxyethanol. The PZT precursor solution was passed through a syringe filter and spin-coated on the Pt(200 nm)/Ti(10 nm)/SiO2(100 nm)/p-Si(100) substrates using a spinner operated at 3,000 rpm for 20 s to form the first layer. These PZT films were dried at 573 K for 30 min to remove the organic materials and sintered at 873 K for 30 min to crystallize them into a perovskite structure. The BFO precursor solution was then spin-coated and dried on the PZT films under the same conditions and sintered at 873 K for 10 min to form the second BFO layer. This procedure was repeated several times, fabricating BFO/PZT multilayer thin films. The crystalline structure of the BFO/PZT multilayer films was analyzed by X-ray diffraction [XRD], and surface and cross-sectional morphologies of the films were examined by scanning electron microscopy [SEM]. To measure the ferroelectric properties, Pt films were dc sputter-deposited on the BFO/PZT films as the top electrode with a diameter of 200 μm. The leakage current and polarization-electric field [P-E] hysteresis loops were analyzed using a ferroelectric test system (RT66B; Radiant Technologies, Inc., Albuquerque, NM, USA).

Results and discussion

Figure 1a, b, c shows the XRD patterns of PZT/BFO multilayer thin films. The XRD pattern was investigated through the GI-XRD method. All films showed the typical XRD patterns of a perovskite polycrystalline structure, and the second phase such as Bi2Fe4O3 or the preferred orientation was not observed. Generally, XRD patterns of PZT(52/48) thin films show a single peak for each diffraction angle. In addition, XRD patterns of BFO thin films show a single peak at 2θ = 22.5° and 46°. However, all PZT/BFO multilayer thin films showed that the XRD peak splits at each diffraction angle. This property may be understood in terms of the effect of the lower layer. The crystal growth of the upper BFO (or PZT) layer can be influenced by the lower PZT (or BFO) layers, and the crystallization behavior of the resultant film is controlled by choosing the initial layer or seeding layer.

Figure 1
figure 1

XRD patterns of PZT/BFO multilayer thin films.

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Figure 2 shows the cross section and surface SEM micrographs of PZT/BFO multilayer thin films. The average thickness of the film after one cycle of drying/sintering was approximately 45 to 55 nm, and the thickness of the PZT/BFO-6 film was 238 nm. All films consist of a fine grain structure with a relatively flat surface morphology, as shown in Figure 2a, b, c. PZT/BFO-4 and 6 films with a top layer of BFO shown in Figure 2d, f showed a dense grain structure. On the other hand, the PZT/BFO-5 film with a top layer of PZT shown in Figure 2e showed a fine and void-free grain structure. The average grain size increased with an increase in the number of coatings due to the increased number of heat treatments. The average grain sizes of the PZT/BFO-4 and 6 films were about 94 and 137 nm, respectively.

Figure 2
figure 2

SEM micrographs of PZT/BFO multilayer thin films. Cross section of the (a) PZT/BFO/PZT/BFO, (b) PZT/BFO/PZT/BFO/PZT, and (c) PZT/BFO/PZT/BFO/PZT/BFO films and surface morphologies of the (d) PZT/BFO/PZT/BFO, (e) PZT/BFO/PZT/BFO/PZT, and (f) PZT/BFO/PZT/BFO/PZT/BFO films.

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Figure 3 shows the dielectric constant and dielectric loss of the PZT/BFO multilayer thin films as a function of the measuring frequency from 1 kHz to 1 MHz. The relative dielectric constant decreased with an increase in the applied frequency, and the PZT/BFO multilayer thin films showed a typical frequency-dispersion property. The dielectric constant increased and the dielectric loss decreased with an increase in the number of coatings and an increase in the film thickness, and the PZT/BFO-6 film displays good results of 405 and 0.033%, respectively, at 1 kHz. PZT/BFO multilayer thin films exhibit a superior dielectric constant compared with a single-composition BFO film (166 at 1 kHz). According to a report by Wang et al. [1], the crystal structure of rhombohedral BFO thin films fabricated on a SrRuO3 [SRO] electrode changed into a monoclinic crystal structure due to the compressive stress imposed by the SRO electrode, which has an in-plane lattice parameter smaller than that of BFO. Furthermore, the magnitude of the ionic displacement relative to the centrosymmetric-strained perovskite structure was found to be extremely large. We believe that the rhombohedral crystal structure of the BFO film (a = 0.5634 nm) was distorted due to the large lattice mismatch and compressive stress imposed by the lower PZT(52/48) ceramic (a = 0.403 nm, c = 0.406 nm [8]). Therefore, the PZT/BFO multilayer thin film displayed good dielectric properties due to the large ionic displacements in the distorted perovskite structure. Dielectric loss decreased with an increase in the number of coatings. This phenomenon can probably be explained by the fact that the diffusion of Pb from the PZT film into the Pt bottom electrode [9] and the diffusion of Pb, Ti, Zr, Bi, and Fe at the interfaces of the PZT film and BFO film increase with an increase in the number of coatings, in other words, an increase in the number of annealing processes [7]. Therefore, interface layers formed between the PZT and BFO layers act as trap centers for the charges. However, further investigation and discussion are necessary to understand the dielectric properties of PZT/BFO multilayer films.

Figure 3
figure 3

Dielectric constant and dielectric loss of PZT/BFO multilayer thin films. (a) Dielectric constant and (b) dielectric loss of PZT/BFO multilayer thin films as a function of applied frequency.

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Figure 4 shows the hysteresis loops of PZT/BFO multilayer thin films. Well-saturated hysteresis loops could be obtained for all films. The remanent polarization and coercive field increased with an increase in the number of coatings. These properties can be understood in terms of the effect of grain size and the large ionic displacement in the distorted perovskite structure as discussed in Figure 3. Generally, by increasing film thickness, the stress induced from the substrate is reduced. However, in this study, despite an increase in film thickness, the coercive field increased with an increase in the number of coatings. This property can be understood in terms of the effect of interface layers formed between the PZT and BFO layers. The space-charge layer, which forms at the interfacial layer due to the diffusion of Pb, Ti, Zr, Bi, and Fe at the interfaces of the PZT and BFO films, acts to suppress the polarization rotation. The PZT/BFO-6 multilayer film shows a remanent polarization of 41.3 μC/cm2 and a coercive field of 15.1 MV/m.

Figure 4
figure 4

P-E hysteresis loops of PZT/BFO multilayer thin films. (a) PZT/BFO/PZT/BFO, (b) PZT/BFO/PZT/BFO/PZT, and (c) PZT/BFO/PZT/BFO/PZT/BFO films.

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Figure 5 shows the leakage current densities of PZT/BFO multilayer thin films with the applied voltage. Leakage current densities of PZT/BFO multilayer thin films decreased with an increase in the number of coatings, and these are much lower values than a pure BFO thin film [5]. These results suggest that the oxygen vacancies were greatly reduced, and the trap centers of carriers were formed at the interfaces between the BFO and PZT films, which increased with an increase in the number of coatings. The leakage current density of the PZT/BFO-6 multilayer thin film is less than 2.5 × 10-7 A/cm2 at 5 V. This value can be applied to memory devices. However, further investigation and discussion are necessary to understand the leakage current mechanism in PZT/BFO multilayer thin films.

Figure 5
figure 5

Leakage current density characteristics with an applied voltage for PZT/BFO multilayer thin films.

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Conclusions

In this study, PZT/BFO multilayer thin films were prepared using the sol-gel method, which were spin-coated on a Pt/Ti/SiO2/Si substrate alternately using PZT(52/48) and BFO alkoxide solutions. The average thickness of a film after one cycle of drying/sintering was approximately 45 to 55 nm. All PZT/BFO multilayer thin films show a dense and homogeneous grain structure with a relatively flat surface morphology. The dielectric properties such as dielectric constant, dielectric loss, remanent polarization, and leakage current density of PZT/BFO multilayer thin films were superior to those of single-composition BFO films, and those values for the PZT/BFO-6 film were 405, 0.033%, 41.3 μC/cm2 and 2.5 × 10-7 A/cm2, respectively, at 5 V. We believe that these properties of PZT/BFO multilayer films were caused by interface effects between the PZT and BFO films.