Circular region areas, RMS roughnesses, and boundary lengths for all samples.
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
1.1. Crystal structure of BiFeO3
Bulk BiFeO3 shows a rhombohedral symmetry (point group
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 √3
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
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 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.
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 |
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 (
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.
3.2. Crystal structure
Figure 8 shows the
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.
3.4. Ferroelectric property
Figure 10 shows the leakage current density versus electric field (
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
Figures 13(a) and 13(b) show the
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.
Figures 15(a) and 15(b) show
μC/cm2 for the films of BFO, BFO-N, and BFO-O, respectively. In addition, the double coercive field (2
and the Pt electrode, as mentioned above.
4.3. Improvement of ferroelectric property of BiFeO3 thin films by electric filed application
To evaluate the effect of the electric field application,
Moreover, the leakage current is reduced at about 1 order of magnitude after the
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.
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