BiFeO3 Thin Films Prepared by Chemical Solution Deposition with Approaches for Improvement of Ferroelectricity

1. Introduction

Bismuth ferrite (BiFeO₃) is one of the most promising lead-free ferroelectric materials. Both bulk and thin film forms have been investigated and found a remarkably large ferroelectric polarization as well as G-type antiferromagnetism, often called it as multiferroics having ferroelectricity, magnetism, and ferroelasticity. There are several thin film fabrication methods including pulsed laser deposition, metal organic chemical vapour deposition, sputtering, and chemical solution deposition for this material. In this chapter, we describe the basic property of BiFeO₃ and the general outline of chemical solution deposition.

1.1. Crystal structure of BiFeO₃

Bulk BiFeO₃ shows a rhombohedral symmetry (point group R3c) with lattice parameters of a = 0.396 nm and α = 89.6° at room temperature (RT). Its transition temperature was investigated by Roginskaya et al. and a structural change showed at around 1100 K (Roginskaya et al., 1963). A detailed crystalline structure above the transition temperature has been investigated by Palai et al.. They reported that the transition temperature from rhombohedral to orthorhombic symmetry was at around 1100 K as well as orthorhombic to cubic symmetry at around 1200 K (Palai et al., 2008).

1.2. Electrical property of BiFeO₃

Rhombohedral BiFeO₃ has a ferroelectric polarization along [111]. Polarization of the single crystal was measured and showed small polarizations of 6.1 μC/cm² along [111] and 3.5 μC/cm² along [100] at 80 K (Teague et al., 1970). These small polarizations are regarded as low sample-quality now. Recent reports showed saturated polarizations of ~60 μC/cm² with single crystals as well as ~40 μC/cm² with ceramics measured at RT (Lebeugle et al., 2007; Shvartsman et al., 2007).

Recent study has also been focused on BiFeO₃ thin films. A saturated polarization of 55 μC/cm² along [001] was reported from epitaxial films on SrRuO₃/SrTiO₃ (001) substrates measured at RT (Wang et al., 2003; Li et al., 2004). Films also showed ferroelectric polarizations of 80 μC/cm² on SrTiO₃ (110) and 100 μC/cm² on SrTiO₃ (111), having an equivalent relationship between √3P ₍₀₀₁₎, √2P ₍₁₀₁₎, and P ₍₁₁₁₎ (Li et al., 2004). Moreover, a giant ferroelectric polarization beyond 150 μC/cm² has been reported from polycrystalline BiFeO₃ thin films with a tetragonal structure measured at 80 K (Yun et al., 2004). Recent theoretical calculation showed a polarization of about 100 μC/cm² in a rhombohedral structure as well as about 150 μC/cm² in a tetragonal structure. These values showed a good agreement with the experimental ones (Ederer et al., 2006; Ricinschi et al., 2006).

1.3. General outline of chemical solution deposition

Chemical solution deposition (CSD) is one of the thin film fabrication methods, and it includes spin-coating, drying and annealing processes. Precursor solution is deposited onto a substrate by a spin-coating process. After the spin-coating process, a film dying process is carried out to evaporate the solvent and decompose metal-organic compounds in the precursor. An amorphous film is obtained at this stage. These processes are repeated several times to obtain a desired film thickness. For the film crystallization, an annealing process is carried out. It is usually carried out by a rapid thermal annealing (RTA) equipment to crystallize and densify the film. Higher heating rate usually decomposes metal organic compounds quickly and then desired oxide films with a higher density can be obtained (Schwartz, 1997).

There are some advantages for CSD; (i) uniformity of the molecules in precursor solutions and thin films, (ii) control of the film thickness by changing the solution concentration or the coating speed, (iii) control of the composition ratio by mixing solutions, (iv) film fabrication in ambient pressure, (v) synthesis of a non-equilibrium phase by the low-temperature process. However, there are some disadvantages for this method; (i) possibility of cracks in a film fabrication process, (ii) contamination which results in a difficulty of the manufacturing process, (iii) films with low-coherency comparing with other thin film fabrication methods such as pulsed laser deposition, chemical vapor deposition, and molecular beam epitaxy.

1.4. Precursor solutions for BiFeO₃

Precursor solutions for the CSD method are distinctly important. They consist of metal organic compounds and solvent which determine process parameters such as drying and annealing temperatures, film thickness per one spin-coating process, and coating affinity to the substrates. In this chapter, BiFeO₃ thin films were prepared by CSD with precursor solutions using 2-ethylhexanoate bismuth [Bi(OCO(CH)(C₂H₅)C₄H₉)₃] and trisacetylacetonato iron [Fe(C₅H₇O₂)₃] as metal organic materials, and toluene as a solvent.

2. Ferroelectric property of BiFeO₃ thin films prepared by CSD with controlling Bi/Fe ratio in the precursor solution

In this section, we demonstrate the BiFeO₃ thin film growth with controlling Bi/Fe ratio of the precursor solutions. Composition ratio affects the crystal growth and the electric property of the films. We obtain both good crystallinity and ferroelectric polarization of 85 μC/cm² with films using 10 mol% Bi-excess solution ( Nakamura et al., 2007 ; Nakamura et al., 2008).

2.1. Film preparation by CSD with controlling Bi/Fe ratio

BiFeO₃ thin films were deposited on a Pt (200 nm)/TiO₂ (40 nm)/SiO₂ (600 nm)/Si substrate by CSD using precursor solutions of different Bi/Fe ratios: 10 mol% Fe-excess (10%Fe-ex.), stoichiometric, 5 mol% Bi-excess (5%Bi-ex.), 10 mol% Bi-excess (10%Bi-ex.), and 20 mol% Bi-excess (20%Bi-ex.). The precursor solution was spin-coated at 3000 rpm for 30 s and dried at 250°C for 5 min in air. These processes were repeated 20 times to obtain a film thickness of 250 nm. Then the films were annealed at 450 °C for 15 min in nitrogen atmosphere using the RTA equipment. For electrical measurement, Pt top electrodes with a diameter of 190 μm and a thickness of 100 nm were formed on the films by rf sputtering at RT. We confirmed by inductively coupled plasma (ICP) analysis that the composition ratios of BiFeO₃ thin films were the same as the precursor solutions.

2.2. Crystal structure

Figure 1 shows θ–2θ scans of the XRD patterns of the BiFeO₃ thin films with different Bi/Fe ratios. All the films show polycrystalline perovskite phase mainly. However, the 10%Fe-ex. BiFeO₃ film has small amount of a Bi₂Fe₄O₉ phase and the 20%Bi-ex. film shows a Bi₂O₃ phase. This indicates that excessive Fe or Bi compounds in the precursor solution tend to form impurity phase. Comparing the peak intensities corresponding to the (010) and (110) planes, the 10%Bi-ex. and 20%Bi-ex. films show higher diffraction intensities, indicating that they are crystallized well. This result suggests that Bi compounds in the precursor solution contribute the promotion of the film crystallization but that 5 mol% excess Bi is insufficient for the crystallization of such films.

2.3. Surface texture and raman spectrum

Figures 2(a-e) show the atomic force microscope (AFM) images of BiFeO₃ films taken in a 20 × 20 μm² area. All the BiFeO₃ thin films show a rosette structure, which consists of circular regions with an uneven texture and outer regions with a flat surface. These structures were also reported in PbZrO₃ thin films prepared by the sol–gel method (Alkoy et al., 2005). Table I shows the percentages of circular regions, RMS roughnesses and total boundary lengths surrounding the circular regions evaluated from Figs. 2(a-e). As can be seen in Table I, the percentage of circular region and RMS roughness tend to increase with an increase in Bi/Fe ratio. On the other hand, the total boundary length is ~200 μm and seems to have no systematic dependence.

Figure 1.

XRD θ–2θ patterns of BiFeO₃ thin films prepared using 10 mol%Fe-excess (10%Fe-ex.), stoichiometric, 5 mol% Bi-excess (5%Bi-ex.), 10 mol% Bi-excess (10%Bi-ex.), and 20 mol% Bi-excess (20%Bi-ex.) precursor solutions.

Figure 3(a) shows an AFM image of the 10%Bi-ex. BiFeO₃ thin film with white circles marking the measurement location of Raman spectroscopy. A laser with a 0.7 μm spot size and an excitation wavelength of 515 nm was applied to the film surface labelled ”Circular region” and “Outer region” in Fig. 3(a). Figure 3(b) shows the Raman spectra measured at RT in each measurement location shown in Fig. 3(a). The spectrum measured in BiFeO₃ ceramic is also shown as a reference. As shown in Fig. 3(b), the spectrum measured in the circular region is almost similar to that of BiFeO₃ ceramic consisting of polycrystalline grains. On the other hand, the spectrum measured in the outer region has a broad shape, found frequently in amorphous materials, and is different from that of BiFeO₃ ceramic.

Figure 2.

× 20 μm² surface AFM images for the BiFeO₃ films of (a) 10%Fe-ex., (b) stoichiometric, (c) 5%Bi-ex., (d) 10%Bi-ex., and (e) 20%Bi-ex.. Each film shows a rosette structure, which consists of circular regions and outer regions.

Sample	Circular region area; (%)	RMS roughness; (nm)	Boundary length; (mm)
10%Fe-ex.	46.0	3.6	238
Stoichiometric	48.1	5.1	225
5%Bi-ex.	41.1	5.6	196
10%Bi-ex.	85.9	4.8	165
20%Bi-ex.	53.7	6.8	208

Table 1.

Circular region areas, RMS roughnesses, and boundary lengths for all samples.

The same results are also obtained in BiFeO₃ thin films prepared using the other precursor solutions with different Bi/Fe ratios. These results indicate that the circular regions have a BiFeO₃ crystalline phase, while the outer regions have an amorphous BiFeO₃ phase. Moreover, it can be considered that each phase exists from the top to the bottom of the film in a vertical direction because the excitation can sufficiently penetrate up to the bottom of the film. Consequently, the BiFeO₃ thin films of 10%Bi-ex. and 20%Bi-ex. have more circular regions and show good crystallinity, as shown in Fig. 1. This tendency is also observed in the Pb(Zr,Ti)O₃ (PZT) thin film prepared by the sol–gel method. Excessive Pb compounds in the precursor solution promote the formation of PZT and lead to show more circular regions (Alkoy et al., 2005). From Figs. 1 and 2, however, the 20%Bi-ex. film shows a Bi₂O₃ phase, and the area of circular regions does not seem to increase so much compared with that in the 10%Bi-ex. film. This result suggests that excessive Bi compounds in the precursor solution are more reactive, thereby they promote the formation of BiFeO₃. However, the major amount of Bi compounds tends to form Bi₂O₃ as well as BiFeO₃, therefore the circular region does not seem to increase so much.

2.4. Ferroelectric property

Figure 4(a) shows the leakage current density versus electric field (J–E) property of the BiFeO₃ thin films measured at RT. A comparatively larger leakage current is obtained in the films that contain more Bi. Figure 4(b) shows P–E hysteresis loops of the BiFeO₃ thin film measured at RT with a scanning frequency of 20 kHz. The 10%Bi-ex. BiFeO₃ thin film shows more squareness in hysteresis loop than the other films. The remanent polarizations (P _r) for the maximum applied electric field of 1.2 MV/cm are 30, 38, 28, 85, and 53 μC/cm² for the films of 10%Fe-ex., stoichiometric, 5%Bi-ex., 10%Bi-ex., and 20% Bi-ex., respectively.

Figure 4.

a) J–E characteristics of BiFeO₃ thin films measured at RT. (b) P–E hysteresis loops measured at RT.

Figure 5.

Leakage current of BiFeO₃ thin films at 240 kV/cm as function of (a) amount of excess Bi, (b) percentage of circular region area, (c) RMS roughness, and (d) boundary length.

Figure 6.

Remanent polarization of BiFeO₃ thin films as function of (a) amount of excess Bi and (b) percentage of circular region area.

2.5. Relationship between surface texture and ferroelectricity

To investigate the influences of the surface texture and Bi/Fe ratio on the leakage current of BiFeO₃ thin films, we consider the amount of excess Bi, percentage of circular region area, RMS roughness, and boundary length at the surface between crystal and amorphous phases, as shown in Table I. Figures 5(a-d) show the leakage current measured at 240 kV/cm versus (a) amount of excess Bi, (b) circular region area, (c) RMS roughness, and (d) boundary length. As shown in Figs. 5(a-c), leakage current tends to exponentially increase with an increase in the amount of excess Bi, circular region area, and RMS roughness although some scattering of the data is observed in Fig. 5(c). On the other hand, the length between the circular regions and the outer regions does not seem to affect the leakage current as shown in Fig. 5(d). These results suggest that the BiFeO₃ thin film prepared using the Bi excess precursor solution tends to have more circular regions that have BiFeO₃ crystals and to have a larger RMS roughness. From these leakage trends, leakage current mainly passes through circular regions consisting of crystalline BiFeO₃ rather than through outer amorphous region, and that current is increased by a rough surface. We further investigate the influences of the surface texture and Bi/Fe ratio on the ferroelectric polarization of BiFeO₃ thin films. We plot the amount of excess Bi and percentage of circular region area that has BiFeO₃ crystals, as shown in Fig. 3(b). Figures 6(a) and 6(b) show the remanent polarization measured at RT versus (a) amount of excess Bi and (b) circular region area. The remanent polarization increases with an increase in Bi ratio below the 10%Bi-ex. BiFeO₃ film. However, the 20%Bi-ex. film decreases its remanent polarization because of the mixed phase of BiFeO₃ and Bi₂O₃. As shown in Fig. 6(b), the remanent polarization linearly increases with an increase in the percentage of the circular region area. From the extrapolated line in Fig. 6(b), fully crystallized BiFeO₃ thin films are expected to show 100 μC/cm². According to leakage and polarization plots in Fig. 5 and Fig. 6, a 10 mol% Bi-excess solution gives BiFeO₃ thin films the best ferroelectric property with more circular regions.

3. Insertion effect of Bi-excess layer on BiFeO₃ thin films

In section 2, Bi-excess solution, or precursor solution with excessive Bi compounds, promotes film crystallization, leading to a good ferroelectricity. In this section, we demonstrate the insertion effect of Bi-excess layer to the stoichiometric BiFeO₃ thin films to improve the crystal growth and ferroelectricity of the films ( Nakamura et al., 2007 ; Nakamura et al., 2008).

3.1. Insertion effect

Insertion effect, inserting Bi-excess BiFeO₃ layer to the film, is aiming to promote the crystal growth of the film and to obtain a good ferroelectricity. There are some reports that ferroelectric thin films prepared by CSD show a non-crystalline layer at the interface between the thin film and the electrode. Such a layer is reported as an interfacial layer which degrades the ferroelectric property of the film (Grossmann et al., 2002). These reports suggest that the low crystallinity part is concentrated at the interface between the film and the electrode. To improve the low crystallinity part, an insertion layer promoting crystal growth will be effective.

In our BiFeO₃ thin films, a thin film with stoichiometric solution shows low crystallinity with a small polarization, and a film with 10 mol% Bi-excess solution shows high crystallinity and a large polarization. Thus an insertion layer with 10 mol% Bi-excess solution is expected to be effective. To investigate the insertion effect of Bi-excess layers, three types of thin films were prepared on Pt/TiO₂/SiO₂/Si substrates, as shown in Fig. 7: stoichiometric BiFeO₃ thin film with Bi-excess top layer (Bi-T), bottom layer (Bi-B), and top and bottom layer (Bi-TB). Then the films were annealed at 450 °C for 15 min in a nitrogen atmosphere using the RTA process. For the electrical measurement, Pt top electrodes with a diameter of 190 μm were formed by rf sputtering.

Figure 7.

Schematic models of BiFeO₃ thin films inserting Bi-excess top and bottom layer (Bi-TB), top layer (Bi-T), and bottom layer (Bi-B).

3.2. Crystal structure

Figure 8 shows the θ–2θ scans of XRD patterns of the BiFeO₃ thin films with Bi-excess top and bottom layer (Bi-TB), top layer (Bi-T), and bottom layer (Bi-B). These results show that all the films exhibit mainly polycrystalline perovskite single phase without nonperovskite phases such as Bi₂Fe₄O₉ and Bi₂O₃. Comparing peak intensities corresponding to (010) and (110) planes, the crystallinity of Bi-TB is the best. Bi-T is the second best, followed by Bi-B. This result indicates that the Bi-excess top layer improves the crystallization in the annealing process. Moreover, this tendency suggests that the crystallization is produced from the surface to the bottom using the RTA process. As for the difference between Bi-TB and Bi-T, crystallinity of BiFeO₃ film can be enhanced by inserting the Bi-excess layer on the top surface and the bottom.

Figure 8.

XRD θ–2θ patterns of Bi-TB, Bi-T, and Bi-B BiFeO₃ thin films.

3.3. Surface texture and raman spectrum

Figures 9(a-c) show the AFM images of BiFeO₃ films taken over a 10 × 10 μm² area. As can be seen in Fig. 9(a), Bi-TB forms more grains than the others. On the other hand, Bi-T and Bi-B form finer grains as well as larger grains, as shown in Figs. 9(b) and 9(c). In addition, Bi-T seems to form larger grains than the film of Bi-B. The surface RMS roughness is estimated as 7.7, 6.7, and 5.0 nm for the films of Bi-TB, Bi-T, and Bi-B, respectively. The number of grains and the surface roughness increase with increasing crystallinity, comparing Fig. 9 with Fig. 8. To investigate the difference between finer and larger grains, Raman spectroscopy was carried out. A laser with a 0.7 μm spot size irradiated the points labelled A–C, which form large grains, and D–F, which form fine grains, as shown in Figs. 9(a-c). Figures 9(d) and 9(e) show Raman spectra measured at RT. These figures also include the spectrum measured in BiFeO₃ ceramic, as a reference. As shown in Fig. 9(d), the spectra measured in the areas A–C are almost the same as the spectrum of BiFeO₃ ceramic. On the other hand, the spectra measured in the areas D–F are different from the spectrum of BiFeO₃ ceramic, as shown in Fig. 9(e). These results indicate that the areas A–C have good BiFeO₃ crystals while the areas D–F seem to be amorphized. Moreover, the area at which the BiFeO₃ crystal spectrum was observed is the largest in Bi-TB. This result relates that Bi-TB crystallizes the best, comparing Figs. 9 and 8.

Figure 9.

× 10 μm² surface AFM images with markings of the typical locations of Raman spectroscopy for the films of (a) Bi-TB, (b) Bi-T, and (c) Bi-B, respectively. Areas A–C form large grains, while areas D–F form fine grains. (d) and (e) Raman spectra measured at RT for the locations shown in Figs. 9(a) –9(c).

3.4. Ferroelectric property

Figure 10 shows the leakage current density versus electric field (J–E) of BiFeO₃ thin films measured at (a) RT and (b) 80 K. When the electric field is lower than 300 kV/cm, the leakage currents are almost unchanged among three types of films both at RT and 80 K. This suggests that the amorphous phase of the surface limits the conduction in the case of lower electric field, as mentioned in Abe et al. (Abe et al., 1993). On the other hand, when the electric field is higher than 300 kV/cm at 80 K, the leakage current becomes large for the film of Bi-TB. Therefore, it is suggested that the amorphous phase includes defects that limit the carrier emission at the interface, and the leakage current increases at higher electric fields in the Bi-TB film. In the case of Bi-T and Bi-B, the amorphous phase might suppress the leakage current at high electric field. Figure 11 shows ferroelectric polarization versus electric field (P–E) hysteresis loops of BiFeO₃ thin film at (a) RT and (b) 80 K, respectively. At RT, the remanent polarizations (P _r) for maximum applied electric field of 1.0 MV/cm are 55, 26, and 17 μC/cm² for the films of Bi-TB, Bi-T, and Bi-B, respectively. In addition, the coercive field of Bi-TB is 385 kV/cm, which is the lowest in the three types of films. At 80 K, the remanent polarizations for maximum applied electric field of 2.0 MV/cm are 65, 46, and 32 μC/cm² for the films of Bi-TB, Bi-T, and Bi-B, respectively. The remanent polarization of Bi-TB is about twice that of the film prepared by stoichiometric solution (28 μC/cm² at RT, and 38 μC/cm² at 80 K). These results show that BiFeO₃ thin film of Bi-TB gives the best ferroelectric property among the three types of films, which is attributed to the good crystallinity of the BiFeO₃ film, comparing Figs. 11 and 8.

Figure 10.

Leakage current characteristics of BiFeO₃ thin films measured at (a) RT and (b) 80 K.

Figure 11.

P–E hysteresis loops of BiFeO₃ thin films measured at (a) RT and (b) 80 K.

4. Improvement of ferroelectricity of BiFeO₃ thin films by postmetallization annealing and electric field application

In this section, we describe the postmetallization annealing and electric field application by using 10 mol% Bi-excess BiFeO₃ thin film which shows good ferroelectricity in section 2. These are the ways to improve ferroelectricity of BiFeO₃ thin films. Postmetallizaton annealing is the electrode annealing process to reduce the leakage current which has already reported in several thin film materials such as BaTiO₃, (Ba,Sr)TiO₃, (Pb,Sr)TiO₃, and PZT after the deposition of top electrodes (Lee et al., 2004; Joo et al., 1997; Chung et al., 2001; Thakoor, 1994). Electric field application is to apply a high electric field to reverse its polarization reversal easily. It is typically carried out in bulk materials such as PZT (Kamel et al., 2007). These two approaches are expected to be effective to improve ferroelectric properties of BiFeO₃ thin films (Nakamura et al., 2009).

4.1. Film preparation methods

BiFeO₃ thin films were deposited on a Pt/TiO₂/SiO₂/Si substrate by CSD using 10mol % Bi-excess precursor solution. Spin-coating and drying processes were the same as in chapter 2. These processes were repeated 20 times to obtain a film thickness of 250 nm. Then, the films were treated by the RTA process at 450 °C for 20 min in nitrogen atmosphere. For the electrical measurement, Pt top electrodes were formed on the BiFeO₃ film by rf sputtering. After the deposition of Pt top electrodes, the sample was divided into three pieces and labelled as BFO, BFO-N, and BFO-O, respectively. Then the postmetallization annealing was carried out for 5 min at 300 °C in nitrogen atmosphere for BFO-N, and oxygen atmosphere for BFO-O by the RTA process. Finally, the following three films were obtained; BFO (as prepared film without postmetallization annealing), BFO-N (the film with the annealing in nitrogen atmosphere), and BFO-O (the film with the annealing in oxygen atmosphere).

4.2. Improvement of ferroelectric property of BiFeO₃ thin films by postmetallization annealing

Figure 12(a) shows the θ-2θ patterns of the XRD of the BiFeO₃ thin films with and without the postmetallization annealing. All the films consist mainly of polycrystalline perovskite phase, but a Bi₂O₃ phase is slightly observed. Evaluating the diffraction peak intensity of each film, it does not change among three films. This result indicates that the crystallinity of BiFeO₃ does not change by the postmetallization annealing. We note that the intensity of an observed Bi₂O₃ phase is much smaller than the BiFeO₃ phase and its intensity does not change after the postmetallization annealing. Therefore, it is enough to evaluate the dielectric property of BiFeO₃ and the annealing effect of the electrode. There may be a possibility of a peak shift due to a strain relaxation between the BiFeO₃ film and the Pt electrode by the postmetallization annealing, however, it is hard to observe the strain relaxation from such a small 190 μm diameter dot electrode because an incident X-ray beam width is about 2 mm, making it difficult to analyze the crystalline property of the small area. To confirm the strain relaxation, a 100 nm thick Pt film was deposited on the whole BiFeO₃ film surface and then the postmetallization annealing was carried out. Figure 12(b) shows the XRD patterns near (010) peak before and after the Pt deposition and the postmetallization annealing in nitrogen atmosphere. The peak intensity is decreased due to the deposition of the Pt film, but the peak shift is not observed. This result suggests that a clear strain relaxation does not occur near the interface between the BiFeO₃ film and the Pt electrode after the postmetallization annealing.

Figure 12.

a) XRD θ-2θ scans of as-prepared (BFO), N₂ annealed (BFO-N), and O₂ annealed (BFO-O) BiFeO₃ thin films. (b) Expanded scans near the (010) peak before and after the Pt deposition and the postmetallization annealing. A 100 nm thick of Pt film was deposited on the whole surface of the BiFeO₃ film.

Figures 13(a) and 13(b) show the J-E characteristics of BiFeO₃ thin films measured at (a) 80 K and at (b) RT. The leakage current is suppressed in both the BFO-N and BFO-O films at 80 K. This suppression is also observed at RT. Joo et al. reported that the leakage current is suppressed by the postmetallization annealing in oxygen atmosphere due to the reduction of oxygen vacancies in a film (Joo et al., 1997). Contrary to their result, our result suggests that the postmetallization annealing improves the contact between the BiFeO₃ film and the Pt electrode or the reduction of defects near the interface between the BiFeO₃ film and the Pt electrode as suggested in Pt/PZT/Pt capacitors, rather than the compensation of oxygen vacancies in the BiFeO₃ film.

Figure 13.

Current density-electric field (J-E) characteristics of BiFeO₃ thin films measured at (a) 80 K and at (b) RT.

Figures 14(a) and 14(b) show frequency dependences of the dielectric constant and the dielectric loss tan δ measured at (a) 80 K and at (b) RT. The dielectric constant and the loss tangent of BiFeO₃ thin films measured at 80 K are found to be 185 and 0.061, 172 and 0.045, and 186 and 0.048 for the BFO, BFO-N, and BFO-O film with a measuring frequency of 1 MHz, respectively. In addition, the frequency variability of from 10³ to 10⁶ Hz is 20.5% BFO, 16.9% BFO-N, and 18.3% BFO-O. The reduction in the frequency variability and the dielectric loss may be due to the reduction in the leakage current as shown in Fig. 13(a). The same tendencies are also observed in the BiFeO₃ films measured at RT, as shown in Fig. 13(b). The frequency variability from 10³ to 10⁶ Hz is 30.6% BFO, 23.3% BFO-N, and 23.4% BFO-O measured at RT. These results indicate that the frequency variability of and the dielectric loss are successfully reduced by the postmetallization annealing.

Figure 14.

Dielectric constant-frequency (ε-F) characteristics of BiFeO₃ thin films measured at (a) 80 K and at (b) RT.

Figures 15(a) and 15(b) show P-E hysteresis loops of BiFeO₃ thin films measured at (a) 80 K and at (b) RT with a scanning frequency of 20 kHz. The remanent polarizations (P _r) measured at 80 K under the maximum applied electric field of 1.2 MV/cm are 91, 87, and 89

μC/cm² for the films of BFO, BFO-N, and BFO-O, respectively. In addition, the double coercive field (2E _c) is reduced at about 90 kV/cm in the nitrogen-annealed film. The slight reduction of coercive field is also observed at RT, as shown in Fig. 15(b). This reduction of the coercive field may be due to the improvement of the contact between the Pt electrode and the BiFeO₃ film or the reduction in defects near the interface between BiFeO₃ film

and the Pt electrode, as mentioned above.

Figure 15.

P-E hysteresis loops measured at (a) 80 K and at (b) RT under 20 kHz triangular scanning voltage of as-prepared (BFO), N₂ annealed (BFO-N), and O₂ annealed (BFO-O) BiFeO₃ thin films.

4.3. Improvement of ferroelectric property of BiFeO₃ thin films by electric filed application

To evaluate the effect of the electric field application, P-E hysteresis loops were measured at 80 K in a following order; the first measurement was carried out by the applied voltage from 5 to 70 V, which corresponds to 0.2 MV/cm to 2.8 MV/cm, and then, the second measurement was carried out from 70 to 5 V with 1 kHz triangular wave. The temperature was set at 80 K to reduce the thermal effect and to apply high electric field. This measurement was carried out for the BFO-N film because it has the best insulation and ferroelectric characteristics among three films. Figures 16(a-d) show hysteresis changes in BFO-N film before (black lines) and after (red lines) applying the electric field of 0.4, 1.2, 2.0, and 2.8 MV/cm corresponding 10, 30, 50, and 70 V, respectively at 80 K. After applying the electric field of 2.8 MV/cm, which corresponds to 70 V, the shape of hysteresis loop is dramatically changed in the second measurement. In the sequence from Figs. 16(a-d), the remanent polarizations of the first measurement are 0.19, 29.9, 74.5, and 104 μC/cm² under the maximum field of 0.4, 1.2, 2.0, and 2.8 MV/cm, respectively. In addition, the remanent polarizations of the second measurement are 17.4, 84.4, 97.6, and 106.5 μC/cm² under the maximum field of 0.4, 1.2, 2.0, and 2.8 MV/cm, respectively.

Figure 16.

P-E hysteresis of BiFeO₃ thin films measured at 80 K under the maximum field of (a) 0.4 MV/cm (10 V), (b) 1.2 MV/cm (30 V), (c) 2.0 MV/cm (50 V), and (d) 2.8 MV/cm (70 V), respectively. Hysteresis loops were measured from 5 to 70 V the first measurement and then measured from 70 to 5 V the second measurement.

Moreover, the leakage current is reduced at about 1 order of magnitude after the P-E measurement, as shown in Fig. 17. Dependences of P _r and E _c on the applied electric field are shown in Fig. 18. P _r and E _c obtained in the first measurement are gradually increased with increase in the electric field, and then, the second measurement keeps large P _r value even in lower electric field. The third measurement from 5 to 70 V was carried out and values of P _r and E _c are almost the same as the second data, as shown in Fig. 18. Kohli et al. reported that the hysteresis of the PZT thin film was changed by applying pulse electric field because of the removal of 90° domain pinning (Kohli et al., 1998). Okamura et al. reported that the wake-up phenomenon, which shows an increase in the remanent polarization by applying switching pulses and removing the locked polarizations in the SrBi₂Ta₂O₉ film (Okamura et al., 2000). The improvement of ferroelectricity is possibly due to the relaxation of pinned domains or locked polarizations in the BiFeO₃ film by applying high electric field.

Figure 17.

Current density-electric field (J-E) characteristics of BiFeO₃ thin films measured at 80 K. The measurement was carried out before and after the P-E measurement shown in Fig. 5 (d).

Figure 18.

Applied field dependences of remanent polarization and coercive field at 80 K for the first, the second, and the third measurement.

5. Conclusion

We describe BiFeO₃ thin films prepared by CSD with several approaches to improve its ferroelectricity. Controlling Bi/Fe ratio in the precursor solution contributes the promotion of the film crystallization and shows a large polarization of 85 μC/cm² with 10 mol% Bi-excess solution. Insertion of the 10 mol% Bi-excess layer to the stoichiometric BiFeO₃ films also promotes the film crystallization, leading to the improvement of the ferroelectricity. Ferroelectric property of films using 10 mol% Bi-excess solution can further improve by the postmetallization annealing as well as the electric field application. These are the effective methods to improve ferroelectricity of BiFeO₃.

Acknowledgments

The authors thank Takaaki Nakamura, Hideo Fukumura, and Professor Hiroshi Harima of Kyoto Institute of Technology for conducting Raman spectroscopy.