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Structural and ferroelectric properties of bismuth ferrite thin films deposited by direct current reactive magnetron sputtering
Structural and ferroelectric properties of bismuth ferrite thin films deposited by direct current reactive magnetron sputtering Aleksandras Iljinas, Vytautas Stankus PII: DOI: Reference:
S0040-6090(15)01011-1 doi: 10.1016/j.tsf.2015.10.035 TSF 34725
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
31 May 2015 15 October 2015 15 October 2015
Please cite this article as: Aleksandras Iljinas, Vytautas Stankus, Structural and ferroelectric properties of bismuth ferrite thin films deposited by direct current reactive magnetron sputtering, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.10.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Structural and ferroelectric properties of bismuth ferrite thin
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films deposited by direct current reactive magnetron sputtering
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Aleksandras Iljinas*, Vytautas Stankus
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Department of Physics, Kaunas University of Technology, Studentų Street 50, LT-51368, Kaunas, Lithuania Email: [email protected]
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Abstract
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High quality bismuth ferrite thin films were deposited using in situ layer-by-layer reactive direct current magnetron sputtering. The optimal formation parameters were found in order to achieve the best structural and ferroelectric quality of perovskite thin films without post annealing. Films were deposited on platinized silicon (Pt/Ti/SiO2/Si) substrates at 400-600 C temperature using Ti2O seed layer. It was shown that the microstructure and ferroelectric properties depend on deposition temperature. Thin films, formed at 450-550 C temperature, have dense columnar structure and flat surface. Hysteresis measurements show that all investigated films exhibit ferroelectric properties. The highest coercive field of Ec=210 kV/cm and of Pr=115 C/cm2 was obtained for film deposited at 550 oC. Thin films characterize a leakage current which is conditioned by the space charge limited conduction mechanism. It was shown that the ferroelectric properties are very sensitive to stoichiometry of bismuth ferrate films. Coercive field dependence on frequency measurements shows that two regimes of domain wall motion are presented. Keywords: multiferroics; dc magnetron sputtering; thin films; ferroelectrics
1. Intoduction
BiFeO3 (BFO) thin films with perovskite structure have lots of attention during last years. BFO is assigned to multiferroic material group. Multiferroics have combined two (or
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more) properties of ferroelectric and ferromagnetic, simultaneously [1]. Therefore, multiferroics have interesting correlation between polarization and magnetization process, as well as strain in the materials. Multiferroics are characterized as having magnetoelectric effect that is to say electric field induces magnetization, and magnetic field induces electric polarization. One multiferroic is not necessarily equal to the other multiferroic, therefore, these materials have attracted interest in material science for potential applications. BFO is ferroelectric material below Curie temperature TC ~ 1100 K temperature [1, 2], it’s Neel temperature TN ~ 370 ºC [2, 3]. BFO films display a greater remnant polarization (Pr) and lower energy band gap than regular ferroelectric materials (PZT) [2]. Ferroelectric properties of BFO thin films depend on substrates materials, deposition method, measurement frequencies, at al. The high remnant polarization (Pr ~ 90 μC/cm2, 1 kHz) was observed in the BFO thin films, deposited on indium tin oxide coated glass as the substrate, using radio frequency magnetron sputtering [2]. Some 1
ACCEPTED MANUSCRIPT investigations assert that the weak ferromagnetism in BFO nanostructured films was observed [3, 4]. A lot of different deposition techniques are used for BiFeO3 thin films formation: RF magnetron sputtering [2, 5-10], pulsed DC magnetron sputtering [11],
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pulsed laser deposition [12-14], sol-gel technique [15-17], chemical solution deposition
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[18, 19] and low-energy cluster beam deposition [3]. Many formation processes till now
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have problems with films density, crystal structure, stoichiometric, ferroelectric properties. The properties of BFO thin films can be affected greatly by various oxide seed layer (few nm of thickness) [20]. Reactive magnetron layer-by-layer deposition method can be one of
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the most perspective methods for the formation of ferroelectric thin films. This method allows getting large area thin films with the same thickness and stoichiometry. However, the information about the BFO thin film growing conditions and properties deposited by
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reactive direct current magnetron sputtering is lacking. To obtain optimal deposition conditions of this method will allow forming perovskite BiFeO3 films with desirable properties for mass production.
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Therefore, the aim of this work was to synthesize BFO thin films in situ using layer-
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by-layer direct current reactive magnetron sputtering and to obtain the optimal parameters for the synthesis of the highest quality morphology and ferroelectric properties bismuth
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ferrite thin films. The dependence of structure and electrical properties on deposition temperature and composition of thin films were investigated.
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2. Experiment
The bismuth ferrite thin films were deposited on platinized silicon substrate by reactive magnetron layer-by-layer deposition in O2 gas environment (p = 1.33 Pa) at various substrate temperatures. Multilayer system Pt/Ti/SiO2/Si was used as the substrate with the thickness of Pt, TiO2 and SiO2 layer of 200 nm, 50 nm, and 1 µm, respectively. The SiO2 thin film on Si (100) substrate was grown by thermal oxidation method. The Pt and TiO2 layers deposited by magnetron sputtering at room temperature were (111) oriented. The seeding Ti2O layer of 5 nm thickness was deposited on Pt/Ti/SiO2/Si at 650 C temperature before starting the BFO thin film synthesis. The deposition of BiFeO3 was realized using substrate periodic and parallel to cathodes motion over the magnetrons (layer-by-layer). The period of motion was chosen of 3 s for the possibility of approximately one atomic layer deposition of each oxide. Bi and Fe disc targets of 3 inch diameter (Kurt J. Lesker Company, 99.999% and 99.95% purity) were used there. The distance between magnetron plane and substrate moving planes was kept for 65 mm. The samples were heated during 2
ACCEPTED MANUSCRIPT this magnetron deposition at (400 to 600) C. Bi2O3 and Fe2O3 deposition rates were 11 nm/min and 6 nm/min, respectively. The total time of deposition process was 60 min. The
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total thickness was chosen and determined as 0.60±0.01 m. The thickness was determined
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using Linnik micro-interferometer and tested with scanning electron microscope (SEM) cross-section view. Formation of the sample was finished by magnetron sputtering of the
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aluminium top electrodes of 200 nm thickness. Aluminium film was deposited through the mask. The top electrodes diameter on fabricated structures was 1.25 mm. The samples were analyzed using SEM (RAITH-e-LiNE, Raith GmbH). The crystallographic structure
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of thin films was investigated by X-ray diffraction (XRD) (Bruker D8 series diffractometer) using monochromatic CuKα radiation with Bragg-Brentano geometry. The average size of thin films crystallites was determined from the peak broadening by the
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single line and multiple line analysis. Sawyer and Tower method for polarization–electric field (P-E) loop measurements was used for hysteresis loops measurements.
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Multifunctional data acquisition NI USB-6361 device (National Instruments) was used for
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faster scientific research and analysis.
3. Results and discussions
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The crystalline nature of BiFeO3 thin films strongly depends on the substrate temperature when films are grown by sputtering. The samples were heated during this magnetron deposition at (400 to 600) C. This temperature interval was chosen because it
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is optimal for perovskite phase formation. The perovskite phase at 400C and lower substrate temperature has not obtained via too low formation energy [21, 22]. The samples, deposited at higher temperatures than 550 oC, show no perovskite phase because of high volatility of Bi2O3 and therefore bismuth deficiency in thin films. The XRD patterns of deposited thin films on 450 oC, 500 oC, 550 oC substrate temperatures are shown in Fig. 1. The analysis of the crystal structure was well identified as crystallized rhombohedral space group (R-3m) pure nanocrystall bismuth ferrite (BiFeO3) phase. The diffraction peaks were indexed using PDF card No. 72-2112. It is seen that temperature influences crystallinity and texture of thin films. The increase of deposition temperature from 450 oC to 550 oC affects the crystallite sizes growth (from 45 nm to 65 nm). Growing of (101) and (200) orientation peaks intensities was observed there too.
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ACCEPTED MANUSCRIPT Fig. 2 shows the SEM images of surface and cross-section samples, deposited at 450 C, 500 C, and 550 C temperatures. It is seen that columnar growth at all deposition temperatures was obtained. The diameter of columns depends on deposition temperature
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and is increasing to 80 nm at 450 C (Fig. 2a, 2b), 140 nm at 500 C (Fig. 2c, 2d), and 180
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nm at 550 C (Fig. 2e, 2f). It is well known that density of columns depends on two
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factors: nucleation’s centres density and surface migration of adatoms. Increase of temperature enhances the surface migration and agglomeration of adatoms to the centres. The initial column growth to width until columns are contacting is the result of this
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process. In opposite, due to too low temperature, the surface diffusion of adatoms is limited and nucleation centres density is growing. Therefore, grains grow along flow of deposited atoms and columns are more narrow.
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Few hysteresis loops were obtained at voltages between 5 V and 25 V in order to give significant results, concerning hysteresis properties. P–E hysteresis loops were obtained
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with the initial frequency of 50 Hz and at an applied maximum field of 400 kV/cm (Fig. 3).
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The BFO films exhibit not full saturated P–E hysteresis loops at 25 V, because of the low breakdown voltage of 28V - 35 V and leakage currents. Nevertheless, the BFO film shows the very high remnant polarization (Pr) of 115 C/cm2 compared with other reports (1.3
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C/cm2 [17], 1.8 C/cm2 [23], 7.5 C/cm2 [22], 60 C/cm2 [24] and 90 C/cm2 [25]) at relatively low coercive field Ec 210 kV/cm. It should be noted that deposition temperature influences the parameters of hysteresis slightly. So, remnant polarization slightly increases
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when deposition temperature and grain size increase: 105 C/cm2 at 80 nm, 110 C/cm2 at 140 nm, and 115 C/cm2 at 180 nm. Coercive field also increases, when grain size increases: 210 kV/cm at 80 nm, 220 kV/cm at 140 nm, and 230 kV/cm at 180 nm. It can be explained that grain size influences domain size and domain wall density in thin film and, therefore, changes ferroelectric properties. However, the breakdown voltage decreases with grain size decreasing: 35 V at 180 nm, 31 V at 140 nm, and 28 V at 80 nm. The breakdown typically starts through defect and grain boundaries. As we see in SEM images (Fig. 2), the intergrain space increases with grain size decreasing. All hysteresis loops are rather wide and have no saturation. A leakage current in thin films is the reason why saturation not obtained. Measurement of current-voltage characteristic of one sample (deposited at 550 C, 50:50) shows that the linear ln J vs ln E curves of bismuth ferrate thin film indicate a power law relation J Ewhere J is the leakage current density, E is the applied electric field and α is the nonlinearity coefficient, 4
ACCEPTED MANUSCRIPT which is defined as the slope of ln J – ln E plot. The linearity of the slopes of the ln J - ln E plot is characteristic for the space charge limited conduction (SCLC), corresponding to the shallow trap square law [26, 27]. An obtained value of α ~ 2.3 was shown in Fig. 4. The
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deviation from the theoretical α = 2 is due to the large concentration of structural disorders,
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such as oxygen vacancies, large amount of intercrystal spacing due to small sizes of
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crystallites, and other defects present in the polycrystalline thin films. Few samples with slightly different ratio of Bi/Fe (55:45) % and (45:55) % were deposited at 550 C temperature in order to investigate the dependence of properties on
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variation of stoichiometry. The XRD patterns of deposited thin films are shown in Fig. 5. It is shown that small stoichiometry variation does not change the phase of bismuth iron oxide drastically. Just intensity of (010) and (200) peaks are very weak.
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It should be noted that no significant difference of the surface morphology of samples with different stoichiometry was obtained. Therefore, we conclude that in our case
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morphology depends only on deposition temperature. However, hysteresis measurements showed the strong differences between
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ferroelectric properties of these films. Fig. 6 shows P–E hysteresis loops of BiFeO3 thin films of different stoichiometry deposited at 550 C temperature. It is seen that changing
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Bi/Fe ratio to (45:55) slightly reduce the remnant polarization to 105 C/cm2 with no changes of coercive field. But changing Bi/Fe ratio to (55:45) ferroelectric properties change drastically. It is seen that almost no saturation is observed and remnant polarization
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decrease till 50 C/cm2. Coercive field decreases also to 80 kV/cm. It is seen that BFO thin film ferroelectric properties strongly depend on stoichiometry and that 5 at. % Fe deficiency can strongly disimprove the quality. The deficiency of Fe in thin film leads to defects formation in perovskite lattice. Therefore the amount of perovskite phase in thin films is less. This relates with XRD measurements in Fig. 5. We saw that the BiFeO3 peaks are less intense at ratio Bi:Fe (55:45), although total thickness of all films is the same. It was found that the P-E hysteresis loop coercive field strongly depends on frequency. The measurements were performed from 20 Hz to 1 kHz frequency. From these measurements we can extract the frequency dependence of coercive field. Fig. 7 shows the plot of ln EC vs ln f. It can be seen that coercive field can be described by a power law of the form EC fβ, as predicted from ferroelectric switching domain wall motion limited to Ishibashi-Orihara model [28]. D/k, where D is the domain dimension and coefficient k is approximately equal to 6. There is crossover frequency (at ~70 Hz) between different 5
ACCEPTED MANUSCRIPT regimes (a and b). coefficient was calculated from our measurements in (a) interval as 1a=0.41, 2a=0.49,3a=0.74 and in (b) interval as 1b=0.07, 2b=0.08,3b=0.28. Such
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results according to Ishibashi-Orihara model show that domain wall motion mechanisms in (a) and (b) intervals can be associated with two regimes. The crossover effects were
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obtained in PZT [29, 30] and BFO [31] thin films, although exponential coefficient was
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different and depends on materials type, grain size, and thickness. Y.J. Shin et al. [31] reported crossover disappearance in BFO thin films by changing substrate type. Therefore this effect is not well investigated. As coefficient is related to domain dimension, we
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think that obtained crossover and reducing of in (b) interval can be related to domains splitting at obtained frequency to smaller sizes and that lead to reducing of domain wall
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motion speed. 4. Conclusions
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Perovskite phase of BiFeO3 thin films (600 nm thickness) were prepared using in situ
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layer-by-layer DC reactive magnetron sputtering. The optimal parameters were found in order to achieve the highest structural quality of perovskite thin films without post
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annealing. It was obtained that the 550 C substrate temperature and Bi:Fe (50:50) composition lead to the formation of dense and uniform bismuth ferrate thin film with desirable ferroelectric properties. It was shown that microstructure strongly depends on
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deposition temperature. The films exhibit columnar growth at all investigated deposition temperatures. The increased deposition temperature (from 450 C to 550 C) increases the diameter of columns (from 80 nm to 180 nm), the crystallite sizes (from 45 nm to 65 nm) and breakdown voltage values (from 28 V to 35 V). The synthesized BFO film shows the very high remnant polarization (Pr) of 115 C/cm2 at relatively low coercive field Ec 210 kV/cm. Remnant polarization and coercive field slightly increased with the increase of deposition temperature. Thin film exhibits a leakage current which is conditioned by the space charge limited conduction mechanism. Ferroelectric properties of BFO thin films strongly depend on ratio of Bi:Fe elements. P-E hysteresis loop coercive field strongly depends on frequency and exhibit crossover frequency (at ~70 Hz) between different domain wall motion regimes. Obtained crossover and reducing of coefficient can be related to domains splitting at obtained frequency to smaller sizes which reduces domain wall motion speed. 6
ACCEPTED MANUSCRIPT Acknowledgment
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This research was funded by a grant (No. MIP-069/2013) from the Research Council of Lithuania.
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References
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[1] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, Viehland D, Vaithyanathan V, Schlom DG, Waghmare UV, Spaldin NA, Rabe KM, Wuttig M, Ramesh R, Epitaxial BiFeO 3 Multiferroic Thin Film Heterostructures. Science 2003;299:1719. [2] Chen M, Ding J, Qiu J, Yuan N, Effect of film thickness and bottom electrode material on the ferroelectric and photovoltaic properties of sputtered polycrystalline BiFeO3 films. Mater. Lett. 2015;139:325. [3] Zhao S, Ma Z, Xing W, Ma Y, Bai A, Yun Q, Chen J, Enhanced ferromagnetism of clusterassembled BiFeO3 nanostructured films. Thin Solid Films 2014;570, Part B:351. [4] Wang W, Li N, Chi Y, Li Y, Yan W, Li X, Shao C, Electrospinning of magnetical bismuth ferrite nanofibers with photocatalytic activity. Ceram. Int. 2013;39:3511. [5] Yan H, Deng H, Ding N, He J, Peng L, Sun L, Yang P, Chu J, Influence of transition elements doping on structural, optical and magnetic properties of BiFeO3 films fabricated by magnetron sputtering. Mater. Lett. 2013;111:123. [6] Chiu S-J, Liu Y-T, Lee H-Y, Yu G-P, Huang J-H, Strain enhanced ferroelectric properties of multiferroic BiFeO3/SrTiO3 superlattice structure prepared by radio frequency magnetron sputtering. Thin Solid Films 2013;539:75. [7] Deng H, Zhang M, Li T, Wei J, Chu S, Du M, Yan H, Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films. J. Alloy. Compd. 2015;639:235. [8] Fan F, Luo B, Duan M, Xing H, Jin K, Chen C, Ferroelectric domain switching investigation of BiFeO3 thin film on Pt/Ti/SiO2/Si substrate. Appl. Surf. Sci. 2012;258:7412. [9] Kaya S, Lok R, Aktag A, Seidel J, Yilmaz E, Frequency dependent electrical characteristics of BiFeO3 MOS capacitors. J. Alloy. Compd. 2014;583:476. [10] Peng Z, Wang Y, Liu B, Evidence of interface dominated photovoltaic effect of Pt-sandwiched polycrystalline BiFeO3 thin film capacitors. Mat. Sci. Semicon. Proc. 2015;35:115. [11] Ternon C, Thery J, Baron T, Ducros C, Sanchette F, Kreisel J, Structural properties of films grown by magnetron sputtering of a BiFeO3 target. Thin Solid Films 2006;515:481. [12] Ahmed T, Vorobiev A, Gevorgian S, Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates. Thin Solid Films 2012;520:4470. [13] Ahn Y, Seo J, Lim D, Son JY, Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates. Curr. Appl. Phys. 2015;15:584. [14] Maeng WJ, Son JY, Highly (111)-oriented multiferroic BiFeO3 thin film on a glass substrate. J. Cryst. Growth. 2013;367:24. [15] Lin Z, Cai W, Jiang W, Fu C, Li C, Song Y, Effects of annealing temperature on the microstructure, optical, ferroelectric and photovoltaic properties of BiFeO3 thin films prepared by sol–gel method. Ceram. Int. 2013;39:8729. [16] Liu K, Cai W, Fu C, Lei K, Xiang L, Gong X, Preparation and electric properties of BiFeO 3 film by electrophoretic deposition. J. Alloy. Compd. 2014;605:21. [17] Ren Y, Zhu X, Zhang C, Zhu J, Zhu J, Xiao D, High stable dielectric permittivity and low dielectric loss in sol–gel derived BiFeO3 thin films. Ceram. Int. 2014;40:2489. [18] Dong W, Guo Y, Guo B, Liu H, Li H, Liu H, Enhanced photovoltaic properties in polycrystalline BiFeO3 thin films with rhombohedral perovskite structure deposited on fluorine doped tin oxide substrates. Mater. Lett. 2012;88:140. [19] Yan J, Gomi M, Hattori T, Yokota T, Song H, Effect of excess Bi on structure and ferroelectric properties of polycrystalline BiFeO3 thin films. Thin Solid Films 2013;542:150. 7
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[20] Wang JN, Li WL, Li XL, Fei WD, Decreased crystallization temperature and improved leakage properties of BiFeO3 thin films induced by Bi2O3 seed layer. Curr. Appl. Phys. 2013;13:2070. [21] Silva J, Reyes A, Esparza H, Camacho H, Fuentes L, BiFeO3: A Review on Synthesis, Doping and Crystal Structure. Integr. Ferroelectr. 2011;126:47. [22] Zhang G-j, Cheng J-r, Chen R, Yu S-w, Meng Z-y, Preparation of BiFeO3 thin films by pulsed laser deposition method. T. Nonferr. Metal. Soc. 2006;16, Supplement 1:s123. [23] Xue X, Tan G, Hao H, Ren H, Comparative study on substitution effects in BiFeO3 thin films fabricated on FTO substrates by chemical solution deposition. Appl. Surf. Sci. 2013;282:432. [24] Ahn Y, Seo J, Yeog Son J, Jang J, Ferroelectric domain structures and thickness scaling of epitaxial BiFeO3 thin films. Mater. Lett. 2015;154:25. [25] Ishiwara H, Impurity substitution effects in BiFeO3 thin films—From a viewpoint of FeRAM applications. Curr. Appl. Phys. 2012;12:603. [26] Nasyrov KA, Gritsenko VA, Transport mechanisms of electrons and holes in dielectric films. Phys. Usp. 2013;56:999. [27] Rose A, Space-Charge-Limited Currents in Solids. Phys. Rev. 1955;97:1538-1544. [28] Ishibashi Y, Orihara H, A theory of D-E hysteresis loop. Integr. Ferroelectr. 1995;9:57-61. [29] Yang SM, Jo JY, Kim TH, Yoon JG, Song TK, Lee HN, Marton Z, Park S, Jo Y, Noh TW, Ac dynamics of ferroelectric domains from an investigation of the frequency dependence of hysteresis loops. Phys. Rev. B 2010;82:174125. [30] Karthik J, Damodaran AR, Martin LW, Epitaxial Ferroelectric Heterostructures Fabricated by Selective Area Epitaxy of SrRuO3 Using an MgO Mask. Adv. Mater. 2012;24:1610-1615. [31] Shin YJ, Jeon BC, Yang SM, Hwang I, Cho MR, Sando D, Lee SR, Yoon JG, Noh TW, Suppression of creep-regime dynamics in epitaxial ferroelectric BiFeO3 films. Scientific Reports 2015;5:10485.
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ACCEPTED MANUSCRIPT Fig. 1. XRD diagrams of BiFeO3 thin films deposited at 450 C, 500 C and 550 C temperatures. Fig. 2. Surface and cross-section SEM images of BiFeO3 thin films deposited at (a, b) 450 C, (c, d) 500 C and (e, f) 550 C temperatures. P–E hysteresis loops of thin film deposited at 550 C temperature with applied voltage
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Current–voltage characteristics of BiFeO3 thin film, deposited at 550oC and plot of log j vs.
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Fig. 4. log E.
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between 5 V and 25 V at 50 Hz.
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Fig. 5. XRD diagrams of BiFeO3 thin films of different stoichiometry deposited at 550 oC substrate temperature. Fig. 6. P–E hysteresis loops of BiFeO3 thin films of different stoichiometry deposited at 550 C temperature. Fig. 7. The EC values as a function of frequency.
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Highlights
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BiFeO3 thin films were synthesized using in situ reactive DC magnetron sputtering.
High remnant polarization Pr=115 C/cm2 was obtained.
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Films formed at 450-550 C temperature have dense columnar structure.
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Ec dependence on frequency shows that two regimes of domain wall motion are presented
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S0040-6090(15)01011-1 doi: 10.1016/j.tsf.2015.10.035 TSF 34725
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
31 May 2015 15 October 2015 15 October 2015
Please cite this article as: Aleksandras Iljinas, Vytautas Stankus, Structural and ferroelectric properties of bismuth ferrite thin films deposited by direct current reactive magnetron sputtering, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.10.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Structural and ferroelectric properties of bismuth ferrite thin
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films deposited by direct current reactive magnetron sputtering
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Aleksandras Iljinas*, Vytautas Stankus
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Department of Physics, Kaunas University of Technology, Studentų Street 50, LT-51368, Kaunas, Lithuania Email: [email protected]
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Abstract
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High quality bismuth ferrite thin films were deposited using in situ layer-by-layer reactive direct current magnetron sputtering. The optimal formation parameters were found in order to achieve the best structural and ferroelectric quality of perovskite thin films without post annealing. Films were deposited on platinized silicon (Pt/Ti/SiO2/Si) substrates at 400-600 C temperature using Ti2O seed layer. It was shown that the microstructure and ferroelectric properties depend on deposition temperature. Thin films, formed at 450-550 C temperature, have dense columnar structure and flat surface. Hysteresis measurements show that all investigated films exhibit ferroelectric properties. The highest coercive field of Ec=210 kV/cm and of Pr=115 C/cm2 was obtained for film deposited at 550 oC. Thin films characterize a leakage current which is conditioned by the space charge limited conduction mechanism. It was shown that the ferroelectric properties are very sensitive to stoichiometry of bismuth ferrate films. Coercive field dependence on frequency measurements shows that two regimes of domain wall motion are presented. Keywords: multiferroics; dc magnetron sputtering; thin films; ferroelectrics
1. Intoduction
BiFeO3 (BFO) thin films with perovskite structure have lots of attention during last years. BFO is assigned to multiferroic material group. Multiferroics have combined two (or
AC
more) properties of ferroelectric and ferromagnetic, simultaneously [1]. Therefore, multiferroics have interesting correlation between polarization and magnetization process, as well as strain in the materials. Multiferroics are characterized as having magnetoelectric effect that is to say electric field induces magnetization, and magnetic field induces electric polarization. One multiferroic is not necessarily equal to the other multiferroic, therefore, these materials have attracted interest in material science for potential applications. BFO is ferroelectric material below Curie temperature TC ~ 1100 K temperature [1, 2], it’s Neel temperature TN ~ 370 ºC [2, 3]. BFO films display a greater remnant polarization (Pr) and lower energy band gap than regular ferroelectric materials (PZT) [2]. Ferroelectric properties of BFO thin films depend on substrates materials, deposition method, measurement frequencies, at al. The high remnant polarization (Pr ~ 90 μC/cm2, 1 kHz) was observed in the BFO thin films, deposited on indium tin oxide coated glass as the substrate, using radio frequency magnetron sputtering [2]. Some 1
ACCEPTED MANUSCRIPT investigations assert that the weak ferromagnetism in BFO nanostructured films was observed [3, 4]. A lot of different deposition techniques are used for BiFeO3 thin films formation: RF magnetron sputtering [2, 5-10], pulsed DC magnetron sputtering [11],
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pulsed laser deposition [12-14], sol-gel technique [15-17], chemical solution deposition
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[18, 19] and low-energy cluster beam deposition [3]. Many formation processes till now
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have problems with films density, crystal structure, stoichiometric, ferroelectric properties. The properties of BFO thin films can be affected greatly by various oxide seed layer (few nm of thickness) [20]. Reactive magnetron layer-by-layer deposition method can be one of
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the most perspective methods for the formation of ferroelectric thin films. This method allows getting large area thin films with the same thickness and stoichiometry. However, the information about the BFO thin film growing conditions and properties deposited by
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reactive direct current magnetron sputtering is lacking. To obtain optimal deposition conditions of this method will allow forming perovskite BiFeO3 films with desirable properties for mass production.
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Therefore, the aim of this work was to synthesize BFO thin films in situ using layer-
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by-layer direct current reactive magnetron sputtering and to obtain the optimal parameters for the synthesis of the highest quality morphology and ferroelectric properties bismuth
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ferrite thin films. The dependence of structure and electrical properties on deposition temperature and composition of thin films were investigated.
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2. Experiment
The bismuth ferrite thin films were deposited on platinized silicon substrate by reactive magnetron layer-by-layer deposition in O2 gas environment (p = 1.33 Pa) at various substrate temperatures. Multilayer system Pt/Ti/SiO2/Si was used as the substrate with the thickness of Pt, TiO2 and SiO2 layer of 200 nm, 50 nm, and 1 µm, respectively. The SiO2 thin film on Si (100) substrate was grown by thermal oxidation method. The Pt and TiO2 layers deposited by magnetron sputtering at room temperature were (111) oriented. The seeding Ti2O layer of 5 nm thickness was deposited on Pt/Ti/SiO2/Si at 650 C temperature before starting the BFO thin film synthesis. The deposition of BiFeO3 was realized using substrate periodic and parallel to cathodes motion over the magnetrons (layer-by-layer). The period of motion was chosen of 3 s for the possibility of approximately one atomic layer deposition of each oxide. Bi and Fe disc targets of 3 inch diameter (Kurt J. Lesker Company, 99.999% and 99.95% purity) were used there. The distance between magnetron plane and substrate moving planes was kept for 65 mm. The samples were heated during 2
ACCEPTED MANUSCRIPT this magnetron deposition at (400 to 600) C. Bi2O3 and Fe2O3 deposition rates were 11 nm/min and 6 nm/min, respectively. The total time of deposition process was 60 min. The
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total thickness was chosen and determined as 0.60±0.01 m. The thickness was determined
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using Linnik micro-interferometer and tested with scanning electron microscope (SEM) cross-section view. Formation of the sample was finished by magnetron sputtering of the
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aluminium top electrodes of 200 nm thickness. Aluminium film was deposited through the mask. The top electrodes diameter on fabricated structures was 1.25 mm. The samples were analyzed using SEM (RAITH-e-LiNE, Raith GmbH). The crystallographic structure
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of thin films was investigated by X-ray diffraction (XRD) (Bruker D8 series diffractometer) using monochromatic CuKα radiation with Bragg-Brentano geometry. The average size of thin films crystallites was determined from the peak broadening by the
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single line and multiple line analysis. Sawyer and Tower method for polarization–electric field (P-E) loop measurements was used for hysteresis loops measurements.
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Multifunctional data acquisition NI USB-6361 device (National Instruments) was used for
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faster scientific research and analysis.
3. Results and discussions
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The crystalline nature of BiFeO3 thin films strongly depends on the substrate temperature when films are grown by sputtering. The samples were heated during this magnetron deposition at (400 to 600) C. This temperature interval was chosen because it
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is optimal for perovskite phase formation. The perovskite phase at 400C and lower substrate temperature has not obtained via too low formation energy [21, 22]. The samples, deposited at higher temperatures than 550 oC, show no perovskite phase because of high volatility of Bi2O3 and therefore bismuth deficiency in thin films. The XRD patterns of deposited thin films on 450 oC, 500 oC, 550 oC substrate temperatures are shown in Fig. 1. The analysis of the crystal structure was well identified as crystallized rhombohedral space group (R-3m) pure nanocrystall bismuth ferrite (BiFeO3) phase. The diffraction peaks were indexed using PDF card No. 72-2112. It is seen that temperature influences crystallinity and texture of thin films. The increase of deposition temperature from 450 oC to 550 oC affects the crystallite sizes growth (from 45 nm to 65 nm). Growing of (101) and (200) orientation peaks intensities was observed there too.
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ACCEPTED MANUSCRIPT Fig. 2 shows the SEM images of surface and cross-section samples, deposited at 450 C, 500 C, and 550 C temperatures. It is seen that columnar growth at all deposition temperatures was obtained. The diameter of columns depends on deposition temperature
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and is increasing to 80 nm at 450 C (Fig. 2a, 2b), 140 nm at 500 C (Fig. 2c, 2d), and 180
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nm at 550 C (Fig. 2e, 2f). It is well known that density of columns depends on two
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factors: nucleation’s centres density and surface migration of adatoms. Increase of temperature enhances the surface migration and agglomeration of adatoms to the centres. The initial column growth to width until columns are contacting is the result of this
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process. In opposite, due to too low temperature, the surface diffusion of adatoms is limited and nucleation centres density is growing. Therefore, grains grow along flow of deposited atoms and columns are more narrow.
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Few hysteresis loops were obtained at voltages between 5 V and 25 V in order to give significant results, concerning hysteresis properties. P–E hysteresis loops were obtained
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with the initial frequency of 50 Hz and at an applied maximum field of 400 kV/cm (Fig. 3).
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The BFO films exhibit not full saturated P–E hysteresis loops at 25 V, because of the low breakdown voltage of 28V - 35 V and leakage currents. Nevertheless, the BFO film shows the very high remnant polarization (Pr) of 115 C/cm2 compared with other reports (1.3
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C/cm2 [17], 1.8 C/cm2 [23], 7.5 C/cm2 [22], 60 C/cm2 [24] and 90 C/cm2 [25]) at relatively low coercive field Ec 210 kV/cm. It should be noted that deposition temperature influences the parameters of hysteresis slightly. So, remnant polarization slightly increases
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when deposition temperature and grain size increase: 105 C/cm2 at 80 nm, 110 C/cm2 at 140 nm, and 115 C/cm2 at 180 nm. Coercive field also increases, when grain size increases: 210 kV/cm at 80 nm, 220 kV/cm at 140 nm, and 230 kV/cm at 180 nm. It can be explained that grain size influences domain size and domain wall density in thin film and, therefore, changes ferroelectric properties. However, the breakdown voltage decreases with grain size decreasing: 35 V at 180 nm, 31 V at 140 nm, and 28 V at 80 nm. The breakdown typically starts through defect and grain boundaries. As we see in SEM images (Fig. 2), the intergrain space increases with grain size decreasing. All hysteresis loops are rather wide and have no saturation. A leakage current in thin films is the reason why saturation not obtained. Measurement of current-voltage characteristic of one sample (deposited at 550 C, 50:50) shows that the linear ln J vs ln E curves of bismuth ferrate thin film indicate a power law relation J Ewhere J is the leakage current density, E is the applied electric field and α is the nonlinearity coefficient, 4
ACCEPTED MANUSCRIPT which is defined as the slope of ln J – ln E plot. The linearity of the slopes of the ln J - ln E plot is characteristic for the space charge limited conduction (SCLC), corresponding to the shallow trap square law [26, 27]. An obtained value of α ~ 2.3 was shown in Fig. 4. The
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deviation from the theoretical α = 2 is due to the large concentration of structural disorders,
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such as oxygen vacancies, large amount of intercrystal spacing due to small sizes of
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crystallites, and other defects present in the polycrystalline thin films. Few samples with slightly different ratio of Bi/Fe (55:45) % and (45:55) % were deposited at 550 C temperature in order to investigate the dependence of properties on
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variation of stoichiometry. The XRD patterns of deposited thin films are shown in Fig. 5. It is shown that small stoichiometry variation does not change the phase of bismuth iron oxide drastically. Just intensity of (010) and (200) peaks are very weak.
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It should be noted that no significant difference of the surface morphology of samples with different stoichiometry was obtained. Therefore, we conclude that in our case
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morphology depends only on deposition temperature. However, hysteresis measurements showed the strong differences between
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ferroelectric properties of these films. Fig. 6 shows P–E hysteresis loops of BiFeO3 thin films of different stoichiometry deposited at 550 C temperature. It is seen that changing
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Bi/Fe ratio to (45:55) slightly reduce the remnant polarization to 105 C/cm2 with no changes of coercive field. But changing Bi/Fe ratio to (55:45) ferroelectric properties change drastically. It is seen that almost no saturation is observed and remnant polarization
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decrease till 50 C/cm2. Coercive field decreases also to 80 kV/cm. It is seen that BFO thin film ferroelectric properties strongly depend on stoichiometry and that 5 at. % Fe deficiency can strongly disimprove the quality. The deficiency of Fe in thin film leads to defects formation in perovskite lattice. Therefore the amount of perovskite phase in thin films is less. This relates with XRD measurements in Fig. 5. We saw that the BiFeO3 peaks are less intense at ratio Bi:Fe (55:45), although total thickness of all films is the same. It was found that the P-E hysteresis loop coercive field strongly depends on frequency. The measurements were performed from 20 Hz to 1 kHz frequency. From these measurements we can extract the frequency dependence of coercive field. Fig. 7 shows the plot of ln EC vs ln f. It can be seen that coercive field can be described by a power law of the form EC fβ, as predicted from ferroelectric switching domain wall motion limited to Ishibashi-Orihara model [28]. D/k, where D is the domain dimension and coefficient k is approximately equal to 6. There is crossover frequency (at ~70 Hz) between different 5
ACCEPTED MANUSCRIPT regimes (a and b). coefficient was calculated from our measurements in (a) interval as 1a=0.41, 2a=0.49,3a=0.74 and in (b) interval as 1b=0.07, 2b=0.08,3b=0.28. Such
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results according to Ishibashi-Orihara model show that domain wall motion mechanisms in (a) and (b) intervals can be associated with two regimes. The crossover effects were
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obtained in PZT [29, 30] and BFO [31] thin films, although exponential coefficient was
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different and depends on materials type, grain size, and thickness. Y.J. Shin et al. [31] reported crossover disappearance in BFO thin films by changing substrate type. Therefore this effect is not well investigated. As coefficient is related to domain dimension, we
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think that obtained crossover and reducing of in (b) interval can be related to domains splitting at obtained frequency to smaller sizes and that lead to reducing of domain wall
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motion speed. 4. Conclusions
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Perovskite phase of BiFeO3 thin films (600 nm thickness) were prepared using in situ
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layer-by-layer DC reactive magnetron sputtering. The optimal parameters were found in order to achieve the highest structural quality of perovskite thin films without post
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annealing. It was obtained that the 550 C substrate temperature and Bi:Fe (50:50) composition lead to the formation of dense and uniform bismuth ferrate thin film with desirable ferroelectric properties. It was shown that microstructure strongly depends on
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deposition temperature. The films exhibit columnar growth at all investigated deposition temperatures. The increased deposition temperature (from 450 C to 550 C) increases the diameter of columns (from 80 nm to 180 nm), the crystallite sizes (from 45 nm to 65 nm) and breakdown voltage values (from 28 V to 35 V). The synthesized BFO film shows the very high remnant polarization (Pr) of 115 C/cm2 at relatively low coercive field Ec 210 kV/cm. Remnant polarization and coercive field slightly increased with the increase of deposition temperature. Thin film exhibits a leakage current which is conditioned by the space charge limited conduction mechanism. Ferroelectric properties of BFO thin films strongly depend on ratio of Bi:Fe elements. P-E hysteresis loop coercive field strongly depends on frequency and exhibit crossover frequency (at ~70 Hz) between different domain wall motion regimes. Obtained crossover and reducing of coefficient can be related to domains splitting at obtained frequency to smaller sizes which reduces domain wall motion speed. 6
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This research was funded by a grant (No. MIP-069/2013) from the Research Council of Lithuania.
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References
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[1] Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, Viehland D, Vaithyanathan V, Schlom DG, Waghmare UV, Spaldin NA, Rabe KM, Wuttig M, Ramesh R, Epitaxial BiFeO 3 Multiferroic Thin Film Heterostructures. Science 2003;299:1719. [2] Chen M, Ding J, Qiu J, Yuan N, Effect of film thickness and bottom electrode material on the ferroelectric and photovoltaic properties of sputtered polycrystalline BiFeO3 films. Mater. Lett. 2015;139:325. [3] Zhao S, Ma Z, Xing W, Ma Y, Bai A, Yun Q, Chen J, Enhanced ferromagnetism of clusterassembled BiFeO3 nanostructured films. Thin Solid Films 2014;570, Part B:351. [4] Wang W, Li N, Chi Y, Li Y, Yan W, Li X, Shao C, Electrospinning of magnetical bismuth ferrite nanofibers with photocatalytic activity. Ceram. Int. 2013;39:3511. [5] Yan H, Deng H, Ding N, He J, Peng L, Sun L, Yang P, Chu J, Influence of transition elements doping on structural, optical and magnetic properties of BiFeO3 films fabricated by magnetron sputtering. Mater. Lett. 2013;111:123. [6] Chiu S-J, Liu Y-T, Lee H-Y, Yu G-P, Huang J-H, Strain enhanced ferroelectric properties of multiferroic BiFeO3/SrTiO3 superlattice structure prepared by radio frequency magnetron sputtering. Thin Solid Films 2013;539:75. [7] Deng H, Zhang M, Li T, Wei J, Chu S, Du M, Yan H, Nonvolatile unipolar resistive switching behavior of amorphous BiFeO3 films. J. Alloy. Compd. 2015;639:235. [8] Fan F, Luo B, Duan M, Xing H, Jin K, Chen C, Ferroelectric domain switching investigation of BiFeO3 thin film on Pt/Ti/SiO2/Si substrate. Appl. Surf. Sci. 2012;258:7412. [9] Kaya S, Lok R, Aktag A, Seidel J, Yilmaz E, Frequency dependent electrical characteristics of BiFeO3 MOS capacitors. J. Alloy. Compd. 2014;583:476. [10] Peng Z, Wang Y, Liu B, Evidence of interface dominated photovoltaic effect of Pt-sandwiched polycrystalline BiFeO3 thin film capacitors. Mat. Sci. Semicon. Proc. 2015;35:115. [11] Ternon C, Thery J, Baron T, Ducros C, Sanchette F, Kreisel J, Structural properties of films grown by magnetron sputtering of a BiFeO3 target. Thin Solid Films 2006;515:481. [12] Ahmed T, Vorobiev A, Gevorgian S, Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates. Thin Solid Films 2012;520:4470. [13] Ahn Y, Seo J, Lim D, Son JY, Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates. Curr. Appl. Phys. 2015;15:584. [14] Maeng WJ, Son JY, Highly (111)-oriented multiferroic BiFeO3 thin film on a glass substrate. J. Cryst. Growth. 2013;367:24. [15] Lin Z, Cai W, Jiang W, Fu C, Li C, Song Y, Effects of annealing temperature on the microstructure, optical, ferroelectric and photovoltaic properties of BiFeO3 thin films prepared by sol–gel method. Ceram. Int. 2013;39:8729. [16] Liu K, Cai W, Fu C, Lei K, Xiang L, Gong X, Preparation and electric properties of BiFeO 3 film by electrophoretic deposition. J. Alloy. Compd. 2014;605:21. [17] Ren Y, Zhu X, Zhang C, Zhu J, Zhu J, Xiao D, High stable dielectric permittivity and low dielectric loss in sol–gel derived BiFeO3 thin films. Ceram. Int. 2014;40:2489. [18] Dong W, Guo Y, Guo B, Liu H, Li H, Liu H, Enhanced photovoltaic properties in polycrystalline BiFeO3 thin films with rhombohedral perovskite structure deposited on fluorine doped tin oxide substrates. Mater. Lett. 2012;88:140. [19] Yan J, Gomi M, Hattori T, Yokota T, Song H, Effect of excess Bi on structure and ferroelectric properties of polycrystalline BiFeO3 thin films. Thin Solid Films 2013;542:150. 7
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ACCEPTED MANUSCRIPT Fig. 1. XRD diagrams of BiFeO3 thin films deposited at 450 C, 500 C and 550 C temperatures. Fig. 2. Surface and cross-section SEM images of BiFeO3 thin films deposited at (a, b) 450 C, (c, d) 500 C and (e, f) 550 C temperatures. P–E hysteresis loops of thin film deposited at 550 C temperature with applied voltage
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Current–voltage characteristics of BiFeO3 thin film, deposited at 550oC and plot of log j vs.
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between 5 V and 25 V at 50 Hz.
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Fig. 5. XRD diagrams of BiFeO3 thin films of different stoichiometry deposited at 550 oC substrate temperature. Fig. 6. P–E hysteresis loops of BiFeO3 thin films of different stoichiometry deposited at 550 C temperature. Fig. 7. The EC values as a function of frequency.
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Highlights
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BiFeO3 thin films were synthesized using in situ reactive DC magnetron sputtering.
High remnant polarization Pr=115 C/cm2 was obtained.
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Films formed at 450-550 C temperature have dense columnar structure.
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Ec dependence on frequency shows that two regimes of domain wall motion are presented
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