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Effect of annealing atmosphere on the ferroelectric properties of inkjet printed BiFeO3 thin films
Applied Surface Science 454 (2018) 233–238
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Effect of annealing atmosphere on the ferroelectric properties of inkjet printed BiFeO3 thin films ⁎
Jiao Lia, Na Shab, , Zhe Zhaoa,c,
T
⁎
a
School of Material Science and Engineering, Tianjin University, Tianjin 30072, China Shool of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China c School of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 20118, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: BiFeO3 thin films Inkjet printing Sol-gel Annealing atmosphere Ferroelectric properties
In this work, BiFeO3 thin films were deposited on FTO substrate by inkjet printing through a sol-gel route. The effect of annealing atmosphere (air, N2, O2, wet air, wet O2, wet N2) on the microstructure and surface property of the BiFeO3 thin films was characterized by XRD and FE-SEM at room temperature. Ferroelectric properties and leakage currents were investigated in detail to understand the link between annealing atmosphere and thin films quality. Pure perovskite phase and dense microstructure can be promised by ink-jet printing and the thickness of the films can be controlled by the number of layers printed. On the other hand, attributed to the pure BiFeO3 phase and better surface topography, the films annealed in wet air showed the lowest leakage current density and best ferroelectric properties. It was proposed that less Fe2+ resulted from the annealing process is the key for the better ferroelectric properties.
1. Introduction BiFeO3 (BFO), as an important multiferroic materials, with high ferroelectric Curie temperature (TC ∼ 1103 K) and high antiferromagnetic Neel temperature (TN ∼ 643 K) has been widely investigated for many years for its potential applications in various multifunctional devices [1,2]. Several researches have prepared BFO thin films with different methods, such as pulsed laser deposition (PLD) [3], chemical vapor deposition (CVD) [4], sputtering [5] and sol-gel based spin-coating [6], but these methods cannot fulfill the requirements for excellent ferroelectric properties, especially the films synthesized by sol-gel show unstable remnant polarization and severe current leakage. Compared with these traditional deposition methods, inkjet printing is a promising film fabrication method for the future printed and flexible electronics. It has advantages for its low cost, low temperature processing and low materials waste. Usually, dispersion of commercial or previously synthesized particles is the most frequent way to prepare inks for inkjet printing. However, films prepared by this way needs to be annealed at higher temperature to get acceptable density. Additionally, clogging is another popular problem that limits the application of inkjet printing. Therefore, preparing inks with proper rheological properties by sol-gel routes is a useful solution to avoid these two problems [7]. On the other hand, large leakage current density caused by the
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charged defects of the BFO thin films at room temperature is a main problem that limits its application. As generally suggested for BFO, the charged defects are divided into two types: one is p-type carriers due to Bi vacancies for the loss of Bi [8,9], the other is n-type conductivity due to the oxygen vacancies caused by the valance change of Fe3+ [10,11]. Although many studies have improved the ferroelectric properties of BFO by A-site or B-site doping or co-doping [12–14], annealing condition has not been investigated in detail for the purpose of improving the ferroelectric properties BFO thin films. In this study, the BFO inks were prepared by means of so-gel technology and then were printed on the FTO substrate to form dense BiFeO3 thin films. The effects of annealing atmosphere (air, nitrogen, oxygen, wet air, wet nitrogen and wet oxygen) on the structure and ferroelectric properties of these films were studied. 2. Experimental procedure The BFO inks were prepared by sol-gel method. An appropriate amount of iron (III) nitrate (Fe (NO3)3·9H2O, AR), bismuth nitrate (Bi (NO3)3·5H2O, AR) were dissolved together in ethylene glycol ((CH2OH)2, AR) to synthesize precursor solution. Tartaric acid (C4H6O6, AR) as the chelating solvent was added to the solution in 1:1 M ration to the precursors, and the mixture was stirred continuously at 60 °C for 30 min. Subsequently, appropriate polyethylene glycol 400
Corresponding authors at: School of Material Science and Engineering, Tianjin University, Tianjin 30072, China (Z. Zhao). E-mail addresses: [email protected] (N. Sha), [email protected] (Z. Zhao).
https://doi.org/10.1016/j.apsusc.2018.05.074 Received 10 October 2017; Received in revised form 3 May 2018; Accepted 11 May 2018 Available online 14 May 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Scientific, USA). Au top electrodes (Φ = 0.3 mm) were deposited on the BFO thin films surface through a shadow mask. Ferroelectric loops and leakage currents were tested by ferroelectric system (Multiferroic test system, Radiant Technologies, USA).
3. Results and discussion 3.1. Phase and microstructure analysis Fig. 2a shows the XRD patterns of the pure BFO thin films and 10% Bi-excess BFO (B1.1FO) thin films annealed at 550 °C in wet air and dry air, respectively. The XRD patterns indicate that the main peaks of all the thin films can be indexed as (0 1 2), (0 1 4), (1 1 0) and (0 2 4) plane. All the samples exhibit a distorted rhombohedral structure of space group R3c without detectable secondary or impurity phases. As shown in Fig. 2(b), no matter annealed in wet or dry air, the (0 1 2) diffraction peaks of B1.1FO thin films shift slightly towards the higher angle, which results from the decrease in the interplanar spacing in B1.1FO thin films. It is hard for Bi to form interstitial ions owing to the lager radius of the Bi ions, so we could consider that it is possible to form partial Schottky disorder due to the volatility of Bi [15]. And Bi vacancies accompanied with oxygen vacancies arise from the loss of Bi during annealing process. Thus, the observed decrease in lattice parameters is attributed to the decrease in the defects (especially Bi deficient) [16]. Generally, the addition of excess Bi is an efficient method to compensate Bi loss during high temperature annealing when Bi-contained ferroelectric films were prepared [17,18]. In our study, even all the samples were annealed at 550 °C which is relatively low among BiFeO3 thin film studies, 10% Biexcess was still effective to reduce Bi nonstoichiometric problem. In order to study the effect of annealing atmosphere on BFO thin films in detail, the B1.1FO thin films were annealed in wet air, dry air, wet N2, dry N2, wet O2, dry O2 (Fig. 2c and d). As shown in Fig. 2c, all these films exhibit (0 1 2) preferred orientation while the films annealed in dry N2 and dry O2 presented secondary phase (Bi2Fe4O9). It has been reported that the purity of BFO thin films depend on the annealing atmosphere due to bismuth volatilization [19,20]. And it is possibly that annealing in dry N2 and dry O2, even 10% Bi-excess cannot compensate to bismuth vacancies created due to Bi volatilization in our study. As shown in Fig. 2d, the (0 1 2) peaks of B1.1FO thin films annealed in dry N2 shift to smaller angle which is the same as the BFO thin films does (Fig. 2b) due to some Bi deficient. Meanwhile, the (0 1 2) peaks of B1.1FO thin films annealed in wet N2 and dry O2 shift slightly to smaller angle, which maybe results from the exist of some defects in the films. In addition, the relative intensity of the peaks of (0 1 2) peak has a tendency to increase when the films were annealed in wet atmosphere and it means that wet atmosphere is beneficial to crystallinity of films. Fig. 3 shows the surface morphology and cross-section FE-SEM results of all the films deposited on FTO substrate. As shown in Fig. 3i, the thickness detected from the figure for all the films is about 650 nm. In other word, the thickness of each layer prepared by inkjet printing is about 108 nm. Although all the films are well crystallized, there are still some small pores observed in the surface of the films. As can be seen in the images, BFO films (Fig. 3a and b) show clear grain boundaries and noticeable very fine sub-domain structures in most grains while B1.1FO films show clear polycrystalline features with less fine sub-domain structures. The reason for this difference might be linked with the local Bi loss during the sintering, which might lead to local crystal structure distortion. But this still needs more detailed study in the future. It seems that the excess Bi helps the grain to coalescence [15]. In addition, the average grain size of the B1.1FO films annealed in dry N2 (Fig. 3e) is larger than those of B1.1FO films annealed in other atmospheres. And this is possibly due to the increase in the defects in films (Fig. 2d) as discussed earlier. The increase in grain size could possibly influence the ferroelectric properties.
Fig. 1. (a) Picture of inkjet printer, (b) picture of print head.
(PEG400, AR) was added to alleviate films cracking and 2Methoxyethanol (C3H8O2, AR) was used to adjust the viscosity of the inks to meet the request for reliable printing. The final solution was stirred for 1 h at room temperature. The BFO inks were transparent and reddish brown sols. Generally, viscosity and surface tension were the most important parameters for the printability of the sols. The viscosity of the sols was 12 mPa·s measured by a rotational viscometer (Kinexus pro+, Malvern, UK), and the surface tension of the inks was about 32.39mN/m tested by a Du Nouy method with a surface tensiometer (BZY-201, Fangrui, Shanghai, China). All these parameters of the sols met the printing condition. Inks with proper rheological properties were deposited on FTO substrates by a modified inkjet printer Epson L800 (Fig. 1a) equipped with six channels. Each channel connected to 60 nozzles with inner diameter of 25.3 μm (Fig. 1b). In our study, we only used one channel for easier control over the pattern design and printing performance. The resolution and the ink volume of the printer was controlled at 1440 × 1440 ppi and 65%, respectively. After each inkjet printing, the wet films were dried at 200 °C for 10 min, and pre-annealed at 350 °C for another 10 min. The inkjet printing and heat treatment processes were repeated for six times to reach the desired thickness. Finally, the BFO films were annealed in tubular furnace at 550 °C for 30 min in different atmosphere. Films with 10% excess Bi (B1.1FO) were fabricated with the same method. And the 10 mol% of excess bismuth nitrate was added to compensate Bi loss during annealing process. The crystalline structure of all the films was checked up by X-ray diffraction (XRD, TD-3500, Tongda, Liaoning, China). The thickness and the surface morphology of the films were investigated by fieldemission scanning electron microscopy (FE-SEM, Gemini SEM 300, Carl Zeiss, Germany). The chemical states of the ions were measured by Xray Photoelectron Spectroscopy (XPS, ESCALAB 250, Thermo Fisher 234
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Fig. 2. (a) XRD patterns of BFO thin films and B1.1FO thin films annealed in wet air and dry air, (b) The magnified image of (0 1 2) peak for 2θ range of 21–24°, (c) XRD patters of B1.1 FO thin films annealed in different atmospheres, (d) The magnified image of (0 1 2) peak for 2θ range of 21–24°.
films and B1.1FO thin films annealed in wet air show one order of the magnitude lower than those of BFO thin films and B1.1FO thin films annealed in dry air. The results suggest that appropriate Bi compensation and annealing in moisture are effective ways to improve the ferroelectric properties of BFO thin films.
3.2. Ferroelectric properties 3.2.1. Effect of Bi compensation The detailed ferroelectric properties and leakage current densities of BFO thin films and B1.1FO thin films annealed in dry air and wet air are shown in Fig. 4a and 4b, respectively. As shown in Fig. 4a the saturated P-E hysteresis loops of BFO thin films and B1.1FO thin films annealed in wet air and dry air are obtained, but there still exist some leakage currents. It can be seen, the improved P-E hysteresis loops were observed in the B1.1FO thin films annealed in dry air and wet air (Fig. 4a), and the remnant polarization (2Pr) and the coercive electric field (2Ec) is about 23.7 μC/cm2, 37.8 μC/cm2 and 600 kV/cm, 546 kV/cm, respectively. However, the pure BFO films show poor ferroelectric properties because of the loss of Bi (agree with the result from Fig. 2b), and the 2Pr for BFO thin films annealed in dry air and wet air is about 6.13 μC/cm2, 13.67 μC/cm2, respectively. The films annealed in wet air show higher remnant polarization than those annealed in dry air. These improved ferroelectric properties of the B1.1FO thin films annealed in wet air are mainly attributed to the lower leakage current density. As shown in Fig. 4b, the excess of Bi compensate the Bi loss and reduce the number of defects, which lead to higher resistivity. It is shown that the leakage current density of the B1.1FO thin films decreased about one order of the magnitude compared with that of pure BFO thin films. In addition, the leakage current densities of BFO thin
3.2.2. Effect of annealing atmosphere The room temperature P-E hysteresis loops of B1.1FO films annealed in different atmospheres are shown in Fig. 5a. As can be seen, the B1.1FO films annealed in wet air and wet O2 exhibit strong ferroelectric properties with remnant polarization of 2Pr ∼ 37.8 μC/cm2 and ∼41.6 μC/cm2 and the coercive electric field of 600 kV/cm and 646 kV/cm, respectively. The B1.1FO films annealed in wet N2 display a smaller 2Pr ∼ 27.4 μC/cm2. Furthermore, the ferroelectric properties of the B1.1FO films annealed in dry air and dry O2 are better than that of the B1.1FO films annealed in dry N2. These results reveal that the B1.1FO films annealed in the O-rich atmosphere can enhance ferroelectric properties. In addition, all the B1.1FO films annealed in wet atmosphere show higher the remnant polarization than the B1.1FO thin films annealed in dry atmosphere does as discussed earlier (Fig. 4a). The remnant polarization in our study has improved greatly compared to that reported by Zhang et al. (2.3 μC/cm2) [12], Ren et al (∼2.6 μC/cm2) [21], Leu et al. (∼5 μC/cm2) [22] and Singh et al. (8.84 μC/cm2) [23]. However, compared with epitaxial films deposited on the substrates 235
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Fig. 3. SEM morphologies of BFO films and B1.1 FO films annealed in different atmospheres. (a) BFO-dry air, (b) BFO-wet air, (c) B1.1FO-dry air, (d) B1.1FO-wet air, (e) B1.1FO-dry N2, (f) B1.1FO-dry O2, (g) B1.1FO-wet N2, (h) B1.1FO- wet O2, (i) cross-sectional SEM image of the sample.
high leakage current density of the B1.1FO thin films annealed in dry N2 and dry O2 may be related to these non-stoichiometric defects. As generally suggested, both the bismuth volatility and the valance change of Fe3+ could lead to non-stoichiometry defects during material processing, such as drying and annealing of BFO thin films. The possible defect chemistry reactions for the formation of Bi and oxygen vacancies are suggested below [24]:
with buffer layers, this value is lower than the remnant polarization of that reported by Zhu et al. (55 μC/cm2) [3], Das et al. (71 μC/cm2) [5]. Considering the differences in the film fabrication methods and the types of substrate, it is possible to improve the ferroelectric properties of inkjet printed BFO thin films by further studies. As shown in Fig. 5b, the leakage current densities of all the B1.1FO films varied with the annealing atmosphere agreed with the result of ferroelectric properties. The B1.1FO thin films annealed in air and O2 show a much lower leakage current density than the B1.1FO thin films annealed in N2. According to the binary Bi2O3–Fe2O3 phase diagram, BFO is a linear compound, so any deviation from stoichiometry could lead to the appearance of secondary phase. As shown in Fig. 2c, the films annealed in dry N2 and dry O2 presented secondary phase, which means there were some non-stoichiometry defects in these films. The
X 2FeFe + OOX → 2FeFe′ + V..O +
1 O2 ↑ 2
× 2Bi×Bi + 3OO → 2VBi″ ′ + 3V..O + Bi2 O3 ↑
(1) (2)
These two reactions can be mostly promoted in flowing N2 atmosphere and thus lead to more serious deviation from stoichiometry and so more charge carriers (Bi and oxygen vacancies) in the annealed 236
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Fig. 4. (a) Ferroelectric P-E hysteresis loops of BFO films and B1.1FO films. (b) Leakage current densities of the BFO thin films and B1.1FO films.
Fig. 5. (a) Ferroelectric P-E hysteresis loops of B1.1FO films annealed in different atmospheres, (b) Leakage current densities of B1.1FO films annealed in different atmospheres.
films. As mentioned in SCLC mechanism later, the resistance properties are related to the density of the charge carriers and therefore the BFO thin films annealed in N2 show poor leakage current performance. Meanwhile, the leakage current density of the B1.1FO thin films annealed in wet atmosphere is always lower than that annealed in dry atmosphere. The leakage current densities of the B1.1FO annealed in wet air, wet O2, wet N2 are 2.96 × 10−6 A/cm2, 2.04 × 10−5A/cm2, 2.66 × 10−3 A/cm2 at 100 kV/cm, respectively, which is about two orders of the magnitude lower than that of B1.1FO thin films annealed in corresponding dry atmospheres. This may be due to the potential mechanism that the water molecules can reacted with oxygen vacancies as expressed by Eq. (3) [25,26]:
H2 O(gas) + V ··O + OOX ↔ 2OH·O
(3)
To further understand the leakage behavior, the electrical conduction mechanism of the samples annealed in different atmospheres was studied by plotting log J versus log E (Fig. 6.) According to previous results [24,27,28], different conduction mechanisms can be deduced by the slope od log J ∝ logE s curves. It can be found that all the curves for various samples can be fitted by three segmented straight lines in log Jlog E plots. In the low electric field region, the slope is close to 1, which means that the leakage current shows ohmic behavior originated from the thermal emission of electrons [24]. In the middle electric field, the slope is close to 2, which confirms space charge limited conduction (SCLC) mechanism. The SCLC mechanism correlates with the density of the defects and the defects in BFO materials are Bi deficient and oxygen vacancies [27]. In the higher electric field, the slope increase with the value being greater than 3, which may be originated from Pool-Frenkel
Fig. 6. log (J)-log (E) characteristics of the B1.1FO thin films annealed in different atmospheres.
emission [28]. From Fig. 6, the conduction mechanism of the B1.1FO thin films annealed in N2 is dominated by SCLC mechanism. However, Ohmic mechanism is the important conduction mechanism in the B1.1FO thin films annealed in O2 and air. This is due to more defects in the films annealed in N2. In order to identify the origin of the defects of the films, XPS of all 237
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Fig. 7. XPS spectra of the Fe ions in the B1.1FO films annealed in different atmospheres.
the samples was used to investigate the oxidation of Fe. Fig. 7 show the XPS spectra fitting analysis of the Fe2p, where binding energies were aligned with respect to the C1s peak (284.8 eV). The peak position of Fe 2p3/2 is around 710 eV, which means the coexistence of both Fe3+ and Fe2+ oxidation states. In fact, the fitting analysis to the peak revealed that the ratio of Fe3+/Fe2+ was about 1:1 in our films. However, the ratios of Fe3+/Fe2+ in the B1.1FO thin films annealed in wet air and wet O2 are calculated as 65.09/34.91 and 61.32/38.68, respectively, indicating that they have less Fe2+ ions. Generally, the existence of Fe2+ ions would cause a large structural distortion and increase the number of oxygen vacancies, thus leading to poor ferroelectric properties. The formation of oxygen vacancies is given below [24]: X 2FeFe + OOX → 2FeFe′ + V..O +
1 O2 ↑ 2
(1)
In other word, annealing in wet air and wet O2 benefit the decrease of oxygen vacancies in B1.1FO thin films, resulting in ferroelectric properties improvement. 4. Conclusions In summary, BFO thin films were prepared by inkjet printing on FTO substrate. The inks synthetized by sol–gel method can promise inkjet printing as a stable and reliable manufacturing technique for obtaining high quality thin films with controllable thickness. In addition, the effect of annealing atmosphere on the microstructure and ferroelectric properties of BFO thin films by inkjet printing has been investigated. The XRD and XPS results illuminate that annealing atmosphere may affect the presence of oxygen vacancies and valence of Fe in BFO thin films. The thin films annealed in wet air possess the best quality in terms of the ferroelectric properties (2Pr ∼ 37.8 μC/cm2, 2Ec ∼ 600 kV/cm) due to a lower leakage current density of 2.96 × 10−6 A/cm2 at 100 kV/cm. These results could be explained by the reduced number of Fe2+ which is related to oxygen vacancies. The study suggested that annealing in wet air could be a useful method to improve the ferroelectric properties of BFO thin films. Acknowledgments This work was supported by the National Key R & D Program of
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Full Length Article
Effect of annealing atmosphere on the ferroelectric properties of inkjet printed BiFeO3 thin films ⁎
Jiao Lia, Na Shab, , Zhe Zhaoa,c,
T
⁎
a
School of Material Science and Engineering, Tianjin University, Tianjin 30072, China Shool of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China c School of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 20118, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: BiFeO3 thin films Inkjet printing Sol-gel Annealing atmosphere Ferroelectric properties
In this work, BiFeO3 thin films were deposited on FTO substrate by inkjet printing through a sol-gel route. The effect of annealing atmosphere (air, N2, O2, wet air, wet O2, wet N2) on the microstructure and surface property of the BiFeO3 thin films was characterized by XRD and FE-SEM at room temperature. Ferroelectric properties and leakage currents were investigated in detail to understand the link between annealing atmosphere and thin films quality. Pure perovskite phase and dense microstructure can be promised by ink-jet printing and the thickness of the films can be controlled by the number of layers printed. On the other hand, attributed to the pure BiFeO3 phase and better surface topography, the films annealed in wet air showed the lowest leakage current density and best ferroelectric properties. It was proposed that less Fe2+ resulted from the annealing process is the key for the better ferroelectric properties.
1. Introduction BiFeO3 (BFO), as an important multiferroic materials, with high ferroelectric Curie temperature (TC ∼ 1103 K) and high antiferromagnetic Neel temperature (TN ∼ 643 K) has been widely investigated for many years for its potential applications in various multifunctional devices [1,2]. Several researches have prepared BFO thin films with different methods, such as pulsed laser deposition (PLD) [3], chemical vapor deposition (CVD) [4], sputtering [5] and sol-gel based spin-coating [6], but these methods cannot fulfill the requirements for excellent ferroelectric properties, especially the films synthesized by sol-gel show unstable remnant polarization and severe current leakage. Compared with these traditional deposition methods, inkjet printing is a promising film fabrication method for the future printed and flexible electronics. It has advantages for its low cost, low temperature processing and low materials waste. Usually, dispersion of commercial or previously synthesized particles is the most frequent way to prepare inks for inkjet printing. However, films prepared by this way needs to be annealed at higher temperature to get acceptable density. Additionally, clogging is another popular problem that limits the application of inkjet printing. Therefore, preparing inks with proper rheological properties by sol-gel routes is a useful solution to avoid these two problems [7]. On the other hand, large leakage current density caused by the
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charged defects of the BFO thin films at room temperature is a main problem that limits its application. As generally suggested for BFO, the charged defects are divided into two types: one is p-type carriers due to Bi vacancies for the loss of Bi [8,9], the other is n-type conductivity due to the oxygen vacancies caused by the valance change of Fe3+ [10,11]. Although many studies have improved the ferroelectric properties of BFO by A-site or B-site doping or co-doping [12–14], annealing condition has not been investigated in detail for the purpose of improving the ferroelectric properties BFO thin films. In this study, the BFO inks were prepared by means of so-gel technology and then were printed on the FTO substrate to form dense BiFeO3 thin films. The effects of annealing atmosphere (air, nitrogen, oxygen, wet air, wet nitrogen and wet oxygen) on the structure and ferroelectric properties of these films were studied. 2. Experimental procedure The BFO inks were prepared by sol-gel method. An appropriate amount of iron (III) nitrate (Fe (NO3)3·9H2O, AR), bismuth nitrate (Bi (NO3)3·5H2O, AR) were dissolved together in ethylene glycol ((CH2OH)2, AR) to synthesize precursor solution. Tartaric acid (C4H6O6, AR) as the chelating solvent was added to the solution in 1:1 M ration to the precursors, and the mixture was stirred continuously at 60 °C for 30 min. Subsequently, appropriate polyethylene glycol 400
Corresponding authors at: School of Material Science and Engineering, Tianjin University, Tianjin 30072, China (Z. Zhao). E-mail addresses: [email protected] (N. Sha), [email protected] (Z. Zhao).
https://doi.org/10.1016/j.apsusc.2018.05.074 Received 10 October 2017; Received in revised form 3 May 2018; Accepted 11 May 2018 Available online 14 May 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Scientific, USA). Au top electrodes (Φ = 0.3 mm) were deposited on the BFO thin films surface through a shadow mask. Ferroelectric loops and leakage currents were tested by ferroelectric system (Multiferroic test system, Radiant Technologies, USA).
3. Results and discussion 3.1. Phase and microstructure analysis Fig. 2a shows the XRD patterns of the pure BFO thin films and 10% Bi-excess BFO (B1.1FO) thin films annealed at 550 °C in wet air and dry air, respectively. The XRD patterns indicate that the main peaks of all the thin films can be indexed as (0 1 2), (0 1 4), (1 1 0) and (0 2 4) plane. All the samples exhibit a distorted rhombohedral structure of space group R3c without detectable secondary or impurity phases. As shown in Fig. 2(b), no matter annealed in wet or dry air, the (0 1 2) diffraction peaks of B1.1FO thin films shift slightly towards the higher angle, which results from the decrease in the interplanar spacing in B1.1FO thin films. It is hard for Bi to form interstitial ions owing to the lager radius of the Bi ions, so we could consider that it is possible to form partial Schottky disorder due to the volatility of Bi [15]. And Bi vacancies accompanied with oxygen vacancies arise from the loss of Bi during annealing process. Thus, the observed decrease in lattice parameters is attributed to the decrease in the defects (especially Bi deficient) [16]. Generally, the addition of excess Bi is an efficient method to compensate Bi loss during high temperature annealing when Bi-contained ferroelectric films were prepared [17,18]. In our study, even all the samples were annealed at 550 °C which is relatively low among BiFeO3 thin film studies, 10% Biexcess was still effective to reduce Bi nonstoichiometric problem. In order to study the effect of annealing atmosphere on BFO thin films in detail, the B1.1FO thin films were annealed in wet air, dry air, wet N2, dry N2, wet O2, dry O2 (Fig. 2c and d). As shown in Fig. 2c, all these films exhibit (0 1 2) preferred orientation while the films annealed in dry N2 and dry O2 presented secondary phase (Bi2Fe4O9). It has been reported that the purity of BFO thin films depend on the annealing atmosphere due to bismuth volatilization [19,20]. And it is possibly that annealing in dry N2 and dry O2, even 10% Bi-excess cannot compensate to bismuth vacancies created due to Bi volatilization in our study. As shown in Fig. 2d, the (0 1 2) peaks of B1.1FO thin films annealed in dry N2 shift to smaller angle which is the same as the BFO thin films does (Fig. 2b) due to some Bi deficient. Meanwhile, the (0 1 2) peaks of B1.1FO thin films annealed in wet N2 and dry O2 shift slightly to smaller angle, which maybe results from the exist of some defects in the films. In addition, the relative intensity of the peaks of (0 1 2) peak has a tendency to increase when the films were annealed in wet atmosphere and it means that wet atmosphere is beneficial to crystallinity of films. Fig. 3 shows the surface morphology and cross-section FE-SEM results of all the films deposited on FTO substrate. As shown in Fig. 3i, the thickness detected from the figure for all the films is about 650 nm. In other word, the thickness of each layer prepared by inkjet printing is about 108 nm. Although all the films are well crystallized, there are still some small pores observed in the surface of the films. As can be seen in the images, BFO films (Fig. 3a and b) show clear grain boundaries and noticeable very fine sub-domain structures in most grains while B1.1FO films show clear polycrystalline features with less fine sub-domain structures. The reason for this difference might be linked with the local Bi loss during the sintering, which might lead to local crystal structure distortion. But this still needs more detailed study in the future. It seems that the excess Bi helps the grain to coalescence [15]. In addition, the average grain size of the B1.1FO films annealed in dry N2 (Fig. 3e) is larger than those of B1.1FO films annealed in other atmospheres. And this is possibly due to the increase in the defects in films (Fig. 2d) as discussed earlier. The increase in grain size could possibly influence the ferroelectric properties.
Fig. 1. (a) Picture of inkjet printer, (b) picture of print head.
(PEG400, AR) was added to alleviate films cracking and 2Methoxyethanol (C3H8O2, AR) was used to adjust the viscosity of the inks to meet the request for reliable printing. The final solution was stirred for 1 h at room temperature. The BFO inks were transparent and reddish brown sols. Generally, viscosity and surface tension were the most important parameters for the printability of the sols. The viscosity of the sols was 12 mPa·s measured by a rotational viscometer (Kinexus pro+, Malvern, UK), and the surface tension of the inks was about 32.39mN/m tested by a Du Nouy method with a surface tensiometer (BZY-201, Fangrui, Shanghai, China). All these parameters of the sols met the printing condition. Inks with proper rheological properties were deposited on FTO substrates by a modified inkjet printer Epson L800 (Fig. 1a) equipped with six channels. Each channel connected to 60 nozzles with inner diameter of 25.3 μm (Fig. 1b). In our study, we only used one channel for easier control over the pattern design and printing performance. The resolution and the ink volume of the printer was controlled at 1440 × 1440 ppi and 65%, respectively. After each inkjet printing, the wet films were dried at 200 °C for 10 min, and pre-annealed at 350 °C for another 10 min. The inkjet printing and heat treatment processes were repeated for six times to reach the desired thickness. Finally, the BFO films were annealed in tubular furnace at 550 °C for 30 min in different atmosphere. Films with 10% excess Bi (B1.1FO) were fabricated with the same method. And the 10 mol% of excess bismuth nitrate was added to compensate Bi loss during annealing process. The crystalline structure of all the films was checked up by X-ray diffraction (XRD, TD-3500, Tongda, Liaoning, China). The thickness and the surface morphology of the films were investigated by fieldemission scanning electron microscopy (FE-SEM, Gemini SEM 300, Carl Zeiss, Germany). The chemical states of the ions were measured by Xray Photoelectron Spectroscopy (XPS, ESCALAB 250, Thermo Fisher 234
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Fig. 2. (a) XRD patterns of BFO thin films and B1.1FO thin films annealed in wet air and dry air, (b) The magnified image of (0 1 2) peak for 2θ range of 21–24°, (c) XRD patters of B1.1 FO thin films annealed in different atmospheres, (d) The magnified image of (0 1 2) peak for 2θ range of 21–24°.
films and B1.1FO thin films annealed in wet air show one order of the magnitude lower than those of BFO thin films and B1.1FO thin films annealed in dry air. The results suggest that appropriate Bi compensation and annealing in moisture are effective ways to improve the ferroelectric properties of BFO thin films.
3.2. Ferroelectric properties 3.2.1. Effect of Bi compensation The detailed ferroelectric properties and leakage current densities of BFO thin films and B1.1FO thin films annealed in dry air and wet air are shown in Fig. 4a and 4b, respectively. As shown in Fig. 4a the saturated P-E hysteresis loops of BFO thin films and B1.1FO thin films annealed in wet air and dry air are obtained, but there still exist some leakage currents. It can be seen, the improved P-E hysteresis loops were observed in the B1.1FO thin films annealed in dry air and wet air (Fig. 4a), and the remnant polarization (2Pr) and the coercive electric field (2Ec) is about 23.7 μC/cm2, 37.8 μC/cm2 and 600 kV/cm, 546 kV/cm, respectively. However, the pure BFO films show poor ferroelectric properties because of the loss of Bi (agree with the result from Fig. 2b), and the 2Pr for BFO thin films annealed in dry air and wet air is about 6.13 μC/cm2, 13.67 μC/cm2, respectively. The films annealed in wet air show higher remnant polarization than those annealed in dry air. These improved ferroelectric properties of the B1.1FO thin films annealed in wet air are mainly attributed to the lower leakage current density. As shown in Fig. 4b, the excess of Bi compensate the Bi loss and reduce the number of defects, which lead to higher resistivity. It is shown that the leakage current density of the B1.1FO thin films decreased about one order of the magnitude compared with that of pure BFO thin films. In addition, the leakage current densities of BFO thin
3.2.2. Effect of annealing atmosphere The room temperature P-E hysteresis loops of B1.1FO films annealed in different atmospheres are shown in Fig. 5a. As can be seen, the B1.1FO films annealed in wet air and wet O2 exhibit strong ferroelectric properties with remnant polarization of 2Pr ∼ 37.8 μC/cm2 and ∼41.6 μC/cm2 and the coercive electric field of 600 kV/cm and 646 kV/cm, respectively. The B1.1FO films annealed in wet N2 display a smaller 2Pr ∼ 27.4 μC/cm2. Furthermore, the ferroelectric properties of the B1.1FO films annealed in dry air and dry O2 are better than that of the B1.1FO films annealed in dry N2. These results reveal that the B1.1FO films annealed in the O-rich atmosphere can enhance ferroelectric properties. In addition, all the B1.1FO films annealed in wet atmosphere show higher the remnant polarization than the B1.1FO thin films annealed in dry atmosphere does as discussed earlier (Fig. 4a). The remnant polarization in our study has improved greatly compared to that reported by Zhang et al. (2.3 μC/cm2) [12], Ren et al (∼2.6 μC/cm2) [21], Leu et al. (∼5 μC/cm2) [22] and Singh et al. (8.84 μC/cm2) [23]. However, compared with epitaxial films deposited on the substrates 235
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Fig. 3. SEM morphologies of BFO films and B1.1 FO films annealed in different atmospheres. (a) BFO-dry air, (b) BFO-wet air, (c) B1.1FO-dry air, (d) B1.1FO-wet air, (e) B1.1FO-dry N2, (f) B1.1FO-dry O2, (g) B1.1FO-wet N2, (h) B1.1FO- wet O2, (i) cross-sectional SEM image of the sample.
high leakage current density of the B1.1FO thin films annealed in dry N2 and dry O2 may be related to these non-stoichiometric defects. As generally suggested, both the bismuth volatility and the valance change of Fe3+ could lead to non-stoichiometry defects during material processing, such as drying and annealing of BFO thin films. The possible defect chemistry reactions for the formation of Bi and oxygen vacancies are suggested below [24]:
with buffer layers, this value is lower than the remnant polarization of that reported by Zhu et al. (55 μC/cm2) [3], Das et al. (71 μC/cm2) [5]. Considering the differences in the film fabrication methods and the types of substrate, it is possible to improve the ferroelectric properties of inkjet printed BFO thin films by further studies. As shown in Fig. 5b, the leakage current densities of all the B1.1FO films varied with the annealing atmosphere agreed with the result of ferroelectric properties. The B1.1FO thin films annealed in air and O2 show a much lower leakage current density than the B1.1FO thin films annealed in N2. According to the binary Bi2O3–Fe2O3 phase diagram, BFO is a linear compound, so any deviation from stoichiometry could lead to the appearance of secondary phase. As shown in Fig. 2c, the films annealed in dry N2 and dry O2 presented secondary phase, which means there were some non-stoichiometry defects in these films. The
X 2FeFe + OOX → 2FeFe′ + V..O +
1 O2 ↑ 2
× 2Bi×Bi + 3OO → 2VBi″ ′ + 3V..O + Bi2 O3 ↑
(1) (2)
These two reactions can be mostly promoted in flowing N2 atmosphere and thus lead to more serious deviation from stoichiometry and so more charge carriers (Bi and oxygen vacancies) in the annealed 236
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Fig. 4. (a) Ferroelectric P-E hysteresis loops of BFO films and B1.1FO films. (b) Leakage current densities of the BFO thin films and B1.1FO films.
Fig. 5. (a) Ferroelectric P-E hysteresis loops of B1.1FO films annealed in different atmospheres, (b) Leakage current densities of B1.1FO films annealed in different atmospheres.
films. As mentioned in SCLC mechanism later, the resistance properties are related to the density of the charge carriers and therefore the BFO thin films annealed in N2 show poor leakage current performance. Meanwhile, the leakage current density of the B1.1FO thin films annealed in wet atmosphere is always lower than that annealed in dry atmosphere. The leakage current densities of the B1.1FO annealed in wet air, wet O2, wet N2 are 2.96 × 10−6 A/cm2, 2.04 × 10−5A/cm2, 2.66 × 10−3 A/cm2 at 100 kV/cm, respectively, which is about two orders of the magnitude lower than that of B1.1FO thin films annealed in corresponding dry atmospheres. This may be due to the potential mechanism that the water molecules can reacted with oxygen vacancies as expressed by Eq. (3) [25,26]:
H2 O(gas) + V ··O + OOX ↔ 2OH·O
(3)
To further understand the leakage behavior, the electrical conduction mechanism of the samples annealed in different atmospheres was studied by plotting log J versus log E (Fig. 6.) According to previous results [24,27,28], different conduction mechanisms can be deduced by the slope od log J ∝ logE s curves. It can be found that all the curves for various samples can be fitted by three segmented straight lines in log Jlog E plots. In the low electric field region, the slope is close to 1, which means that the leakage current shows ohmic behavior originated from the thermal emission of electrons [24]. In the middle electric field, the slope is close to 2, which confirms space charge limited conduction (SCLC) mechanism. The SCLC mechanism correlates with the density of the defects and the defects in BFO materials are Bi deficient and oxygen vacancies [27]. In the higher electric field, the slope increase with the value being greater than 3, which may be originated from Pool-Frenkel
Fig. 6. log (J)-log (E) characteristics of the B1.1FO thin films annealed in different atmospheres.
emission [28]. From Fig. 6, the conduction mechanism of the B1.1FO thin films annealed in N2 is dominated by SCLC mechanism. However, Ohmic mechanism is the important conduction mechanism in the B1.1FO thin films annealed in O2 and air. This is due to more defects in the films annealed in N2. In order to identify the origin of the defects of the films, XPS of all 237
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Fig. 7. XPS spectra of the Fe ions in the B1.1FO films annealed in different atmospheres.
the samples was used to investigate the oxidation of Fe. Fig. 7 show the XPS spectra fitting analysis of the Fe2p, where binding energies were aligned with respect to the C1s peak (284.8 eV). The peak position of Fe 2p3/2 is around 710 eV, which means the coexistence of both Fe3+ and Fe2+ oxidation states. In fact, the fitting analysis to the peak revealed that the ratio of Fe3+/Fe2+ was about 1:1 in our films. However, the ratios of Fe3+/Fe2+ in the B1.1FO thin films annealed in wet air and wet O2 are calculated as 65.09/34.91 and 61.32/38.68, respectively, indicating that they have less Fe2+ ions. Generally, the existence of Fe2+ ions would cause a large structural distortion and increase the number of oxygen vacancies, thus leading to poor ferroelectric properties. The formation of oxygen vacancies is given below [24]: X 2FeFe + OOX → 2FeFe′ + V..O +
1 O2 ↑ 2
(1)
In other word, annealing in wet air and wet O2 benefit the decrease of oxygen vacancies in B1.1FO thin films, resulting in ferroelectric properties improvement. 4. Conclusions In summary, BFO thin films were prepared by inkjet printing on FTO substrate. The inks synthetized by sol–gel method can promise inkjet printing as a stable and reliable manufacturing technique for obtaining high quality thin films with controllable thickness. In addition, the effect of annealing atmosphere on the microstructure and ferroelectric properties of BFO thin films by inkjet printing has been investigated. The XRD and XPS results illuminate that annealing atmosphere may affect the presence of oxygen vacancies and valence of Fe in BFO thin films. The thin films annealed in wet air possess the best quality in terms of the ferroelectric properties (2Pr ∼ 37.8 μC/cm2, 2Ec ∼ 600 kV/cm) due to a lower leakage current density of 2.96 × 10−6 A/cm2 at 100 kV/cm. These results could be explained by the reduced number of Fe2+ which is related to oxygen vacancies. The study suggested that annealing in wet air could be a useful method to improve the ferroelectric properties of BFO thin films. Acknowledgments This work was supported by the National Key R & D Program of
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