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Structural and electrical properties of Sm-substituted BiFeO3 thin films prepared by chemical solution deposition
Thin Solid Films 527 (2013) 126–132
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Structural and electrical properties of Sm-substituted BiFeO3 thin films prepared by chemical solution deposition Sushil Kumar Singh ⁎ MEMS Division, Solid State Physics Lab., Lucknow Road, Timarpur, Delhi-110054, India
a r t i c l e
i n f o
Article history: Received 25 February 2012 Received in revised form 21 November 2012 Accepted 22 November 2012 Available online 6 December 2012 Keywords: Multiferroics Bismuth ferrite Thin films Chemical solution deposition Structural properties Electrical properties Hysteresis loop
a b s t r a c t The structural and electrical properties of chemical-solution-deposited Bi1 −xSmxFeO3 (x = 0, 0.025, 0.05, 0.075, 0.1) thin films on Pt/Ti/SiOx/Si (100) substrates were investigated. Films up to 5 at.% Sm exhibited a single perovskite phase with rhombohedral structure, whereas films with 7.5 and 10 at.% Sm exhibited a distorted orthorhombic crystal structure. Atomic force microscopy of the films showed homogeneous and smooth surface. Films with 7.5 at.% Sm exhibited significant reduction in leakage current in the high electric field region and improved breakdown characteristic. The polarization vs. electric field (P–E) hysteresis loops were recorded in a 100 nm thick film with 3 V coercive voltages. Moreover, well saturated P–E hysteresis loops with high polarization (80 μC/cm 2) and low coercive field (300 kV/cm) were also recorded in 100 nm thick films with low coercive voltage (5 V). The Sm-substitution in BiFeO3 improved the fatigue endurance with no significant degradation in polarization even after 108 fatigue cycles. These results demonstrate that Sm-substituted BifeO3 films have potential for application in low voltage operational device. © 2012 Elsevier B.V. All rights reserved.
1. Introduction BiFeO3 (BFO) is one of the most widely studied multiferroics for potential device applications because of its existence as magnetoelectric multiferroic at room temperature, anti-ferromagnetic below ~ 370 °C [1], ferroelectric up to ~ 820 °C [2], and ferroelastic between 820 and 930 °C [3]. Thin films of BFO exhibit significant high values of electrical polarization (60–150 μC/cm 2) [4,5] as compared to widely used ferroelectric films such as PbZr1 − xTixO3, BaSrTiO3, and Bi3LaTi3O12. BFO crystallizes in the rhombohedral (a = 5.58 Å and α = 89.5°) crystal structure at room temperature with the space group R3c and G-type antiferromagnetism. Due to spin canting, it also exhibits ferromagnetism at room temperature [1,2,6]. Remnant polarization (Pr) values as large as 100 μC/cm 2 have also been reported for BFO films fabricated by pulsed laser deposition, RF sputtering, and chemical solution deposition (CSD) methods [7,8]. Ferroelectric thin films fabricated by CSD have advantages of homogeneous over a large area, reproducible and commercially used for fabrication of ferroelectric random access memory (FeRAM) devices. Device applications of BFO films have a constraint of high leakage current (low electrical resistivity). The possible reasons for this high leakage current are due to reduction of Fe 3+ ions to Fe 2+ and formation of oxygen vacancies for charge compensation. There has been controversy about high leakage current and Clark and Robertson have reported that the ambient band gap (2.9 eV) is too large for ⁎ Tel.: +91 1123903820; fax: +91 1123913609. E-mail addresses: [email protected], [email protected]. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.062
intrinsic semiconductor mechanisms [9]. In our previous studies [11–14], we thoroughly investigated Mn, Cr, and Ni-substitution at the Fe-site and La-substitution at the Bi site of BFO films for reducing the leakage current above the coercive field (Ec). We optimized the conditions for crystallizing single phase BFO films by CSD, i.e., the leakage current density was decreased to lower than 1 × 10−7 A/cm 2 in the lower electric field region (b 200 kV/cm) [10]. We observed that in BFO thin films, leakage current with high applied electric fields (>200 kV/cm) and remnant polarization were improved with small 2.5–7.5 at.% substitution of Mn, Cr, and Ni. However, in the case of La (20%)-substitution it showed improved leakage current density but with reduced polarization. The partial La-substituted BFO has already been intensively studied and this substitution shows significant enhancement in its multiferroic properties [11,15,16]. The ferroelectricity in BFO materials mainly arises from the Bi 6s lone pair electrons [17]. The chemical substitution at Bi-site with smaller ionic radii such as Sm 3+ than Bi 3+ of the perovskite structure is expected to cause more significant structure distortion and improved ferroelectric, piezoelectric and magnetic properties. The effect of Sm-substitution in BFO bulk as well as in thin films reportedly enhance the electrical and magnetic properties [18–21]. Yang et al. have reported improved leakage current density in BFO:Sm2O3 nano-composite [22]. The phase transition from rhombohedral to pseudo-orthorhombic structure was also observed with 14% Sm-substituted BFO films resulting in enhanced piezoelectric properties [23,24]. These studies prompted us to investigate the structural and electrical properties of Sm-substituted BFO thin films formed by CSD method.
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The major objectives of the present work are (1) to synthesize the Sm-substituted BFO thin films using the CSD method and (2) to investigate the structural and electrical properties of the resulting thin films. Our studies demonstrate that high saturated polarization (80 μC/cm 2) can be achieved in 100 nm-thick Sm-substituted BFO films with low coercive voltages (5 V). These films exhibited improved fatigue endurance with minimal degradation in polarization
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even after 10 8 fatigue cycles. These studies emphasize the potentials of Sm-substituted BFO for semiconductor device fabrication. 2. Experimental details Thin films of 300 nm thick Bi1 − xSmxFeO3 (x = 0, 0.025, 0.05, 0.075, and 0.1) were formed by chemical solution deposition on Pt/Ti/SiOx/Si(100) substrates. Chemical solutions with exact metal composition ratios at 0.2 mol/l concentration were spin-coated at 3000 rpm for 30 s, dried at 240 °C for 3 min, and pre-fired at 350 °C for 10 min in air followed by annealing at 550 °C for 10 min in nitrogen atmosphere. Electron beam evaporation was used to deposit Pt top electrodes of the following dimension 3.14 × 10 −4 cm 2 and 0.785 × 10 −4 cm 2 through a shadow mask. We used top electrodes of 3.14× 10−4 cm2 for current density vs. electric field (J–E) measurement and top electrodes of 0.785 × 10−4 cm2 for polarization vs. electric field (P–E) measurements. The crystalline structures of the films were investigated with a multipurpose X-ray diffractometer (X'Pert-Pro MPD, Philips) with Cu Kα radiations (40 kV and 30 mA). The surface morphologies of the films were studied using NanoNavi-SPA 400 atomic force microscope (AFM). Sampling Intelligent Scan (tapping) mode was used for acquiring the data. The electrical properties of the thin-film capacitors were measured at RT using a ferroelectric test system (Toyo Corp., FCE-1A/Fop-100V) and an HP4156A precision semiconductor parameter analyzer (Hewlett-Packard). 3. Result and discussions The synthesis of single-phase perovskite BFO is limited by the narrow deposition window constrained by annealing temperature, atmosphere and chemical substitution. During the synthesis of bulk BFO several impurity phases such as Bi2Fe4O9, Bi2O3, and Bi25FeO39 were observed due to the kinetics of phase formation [25]. Previous studies suggest that BFO is metastable with respect to Bi2Fe4O9, and Bi25FeO39 for 447 b T b 767 °C and a small impurity is sufficient to form the non-perovskite phases [26,27]. The thermodynamic stability of perovskite phase with respect to non-perovskite phases can be accounted in terms of tolerance factor, acid–base relations, chemical substitution and pressure. The crystallization temperature of BFO film is approximately 550 °C and BFO films annealed lower than 600 °C in air and N2, while O2 shows small amount of non-perovskite Bi2Fe4O9 phase. As the temperature was increased higher than 600 °C, the nonperovskite phase increases gradually in air and N2 and rapidly in O2 atmosphere. BFO films annealed in O2 atmosphere at 700 °C show sufficient amount of non-perovskite phase [28]. BFO films fabricated by CSD normally contain Bi2Fe4O9 non-perovskite phase. The formation of non-perovskite phase in the films can be attributed to the Bi loss due to volatilization during the annealing, which is more intensive in the O2 atmosphere. The formation of the non-perovskite phase can be suppressed by compensating Bi-atoms using the 5% Bi-excess precursor solution [28]. It was observed that high oxygen pressure
Table 1 Structural parameters of Bi1 −xSmxFeO3 (x = 0, 0.025, 0.05, 0.075 and 0.1) films with estimated error (SD).
Fig. 1. X-ray diffraction patterns of (a) Bi1−xSmxFeO3 (x = 0, 0.025, 0.05, 0.075 and 0.1) films on Pt/Ti/SiOx/Si(100) substrates annealed at 550 °C for 10 min in N2 atmosphere. Magnified view of (b) (012) peak and (c) (104/110) peaks.
Bi1−xSmxFeO3
Structural parameters
x=0 x = 0.025 x = 0.05 x = 0.075
a = 5.5982 Å a = 5.5788 Å a = 5.5778 Å a = 5.0353 Å b = 3.9453 Å c = 6.4449 Å a = 4.9846 Å b = 3.0905 Å c = 6.4753 Å
x = 1.0
α = 89.701° α = 89.723° α = 89.672° α = β = γ = 90°
SD = 0.008 Å SD = 0.11 Å SD = 0.04 Å SD = 0.0002 Å
α = β = γ = 90°
SD = 0.0001 Å
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increases the formation of non-perovskite phases in BFO thin films. The partial pressure of oxygen can be controlled by using nitrogen atmosphere which results in single perovskite phase BFO films with enhanced crystallinity [29]. It is expected that BFO thin films annealed in O2 atmosphere would maintain the oxygen stoichiometry and Fe 3+ valence state. In contrast, we observed a high leakage current density due to the existence of non-perovskite phase in oxygen atmosphere annealed
Fig. 2. X-ray diffraction patterns of Bi1−xSmxFeO3 (x=0, 0.025, 0.05, 0.075 and 0.1) films on Pt/Ti/SiOx/Si(100) substrates annealed at (a) 500 °C, (b) 575 °C, and (c) 600 °C for 10 min in N2 atmosphere.
BFO films. However, the BFO films annealed in N2 atmosphere exhibited a lower leakage current density owing to the presence of Fe 3+ valence states [30]. We have previously demonstrated that, BFO films annealed in N2 atmosphere at 550 °C and in air at 500 °C exhibited only perovskite phases [30]. In a similar study, Simoes et al. have also reported that BFO films annealed in air at 500 °C compensate the bismuth vacancies created due to bismuth volatilization, leading to the formation of relatively pure phase [31]. Chemical substitution reportedly influences the crystallization temperature of BFO films as 20% Cr-substitution reduced the crystalline temperature from 550 °C to 450 °C [32]. In the present study, we fabricated BFO and Sm-substituted BFO films using stoichiometric sol–gel solution and annealed films at 550 °C in N2 atmosphere to avoid the formation of non-perovskite phase. Fig. 1(a) shows the X-ray diffraction (XRD) patterns of Bi1−xSmxFeO3 (x=0, 0.025, 0.05, 0.075 and 0.1) films grown on Pt/Ti/SiO2/Si substrates. Fig. 1(b) and (c) shows the magnification of 21.5–23.5° and 31–33° respectively. Fig. 1(c) demonstrates that films up to 5 at.% Sm-substitution show a single perovskite phase (rhombohedral structure), whereas films with 7.5 and 10 at.% Sm-substitution show structural distortion from the parent rhombohedral structure. The XRD peaks for 0–5 at.% Sm-substituted films were indexed using a rhombohedral crystal structure with R3c space group and XRD patterns were analyzed using a program package [33] with least-square refinement for which the standard deviation (SD) was found to be minimum. Similarly, patterns of 7.5 and 10 at.% Sm-substituted films were indexed using an automated powder-indexing program, “Crysfire” [34] based on Rietveld refinement and the program suggested an orthorhombic crystal structure based on high figure of merit. The suggested structures from both the software packages were verified and refined using a Crysfire interactive program, “Chekcell”. Unfortunately, the space groups of 7.5 and 10 at.%-substituted films could not be determined because of the limited number of diffraction peaks. Table 1 lists the structural parameters obtained from the refinement with mean absolute discrepancy. We observed a systematic decrease in lattice parameter, a, with increasing Sm-substitution as the ionic radius of Sm (0.96 Å) is smaller than Bi (1.03 Å). With increasing Sm substitution the broadening of (012) diffraction peaks [Fig. 1(b)] was observed which might be due to the strain
Fig. 3. AFM images of BiFeO3 thin films on Pt/Ti/SiOx/Si(100) substrates.
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effect. The AFM images (cf. Fig. 4) also indicated the formation of smaller grains with increasing Sm-substitution. The clear splitting in the 104 and 110 diffraction peaks can also be seen in BFO films and depicted in Fig. 1(c). All the diffraction peaks in BFO films exactly matched with the rhombohedrally distorted perovskite structure with space group R3c. Sm-substitution in BFO reduced the splitting of the 104 and 110 diffraction peaks and showed a broadened single peak around 32°, which suggests that the rhombohedral distortion is reduced towards the orthorhombic structure with Sm-substitution. These results suggest that because of Sm-substitution induced distortion, tilted oxygen octahedral makes the rhombohedral phase unstable and consequently stabilize the orthorhombic phase shrinking lattice parameters as well as the overall volume of the unit cell. In a similar study, Daisuke Kan et al. have reported the structural transition behavior in rare-earth (Sm, Gd, Dy) substituted (001) epitaxial BFO films [24]. Using a scanning 2D XRD system the authors have reported only (001) and (002) reflections from the pure BFO films. However, extra diffraction spots 1/4{011} have been reported with Dy (7%) substitution. As Dy substitution increases, the 1/4 {011} superstructure spots disappeared and new superstructure spots 1/2{010} emerged. Dy-substitution beyond 6% in BFO films
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decreased the 1/4{011} spot intensity with concomitant increase in 1/2{010} spot intensity. These cumulative changes indicate structural changes from rhombohedral to the orthorhombic phase. As mentioned earlier, the phase transition depends upon the ionic radius of the rare earth dopant and structural transition shifts towards the larger composition values as the ionic radius of the dopant becomes larger. For Sm, structural transition has been reported beyond 14% substitution in BFO films [24]. This structural transition has been attributed to a hydrostatic pressure effect caused by the smaller radii of the isovalent rare earth ion substitution. Studying rare-earth element (La, Nd, Sm, Gd and Dy) substitution in BFO single crystals, Murashov et al. [35] have also reported the structural transitions to triclinic-rhombohedral and triclinic-orthorhombic phases with ~ 6– 10 and ~ 14–18% Sm-substitution respectively. This has been attributed to the distortion of the iron octahedral or bending of O–Fe–O and Fe–O–Fe bond angles. In contrast, single crystal with ≥ 18% Smsubstitution resulted in single-phase orthorhombic structure. In the current study, we used all the eight programs in the Crysfire suit for the 7.5 and 10 at.% Sm-substituted BFO films but no solution was found for the triclinic and rhombohedral structures. Because of the limited diffraction peaks, the presence of metastable structures
Fig. 4. AFM images of Bi1−xSmxFeO3 thin films on Pt/Ti/SiOx/Si(100) substrates: (a) x= 0.025, (b) x = 0.05, (c) x = 0.075, and (d) x = 0.1.
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cannot be ruled out. In the future, detailed study of crystal structure of rare-earth substituted BFO films is warranted. To further understand the influence of Sm-substitution on the perovskite phase formation in BFO films, we annealed Sm-substitute BFO films at various temperatures such as 450 °C, 500 °C, 550 °C, 575 °C, and 600 °C. Fig. 2 shows the XRD-pattern of Bi1 − xSmxFeO3 (x=0.025, 0.05, 0.075 and 0.1) films annealed at (a) 500 °C, (b) 575 °C, and (c) 600 °C. We observed crystalline phase formation above 500 °C whereas films annealed below 500 °C exhibited the amorphous phase. The perovskite phase was observed in up to 10% Sm-doped BFO films when annealed under N2 atmosphere at 500 and 550 °C. However, when films were annealed at 575 °C and 600 °C, secondary phase such as Bi2Fe4O9, Fe2O3, and Fe3O4 appeared and gradually decreased as Sm-substitution increased. In contrast, the Sm-doping in the BFO films suppressed the non-perovskite phase formation even at higher temperatures like 575 °C and 600 °C. These studies suggest that rare earth substitution in BFO at Bi-site suppresses the volatility of the Bi and provides chemical stability to the perovskite phase as well as lowers the temperature for perovskite phase formation. Fig. 3 shows the AFM micrographs of pure BFO films and Fig. 4 shows the AFM micrographs of Bi1 − xSmxFeO3 (x = 0.025, 0.05, 0.075, and 0.1) thin films on Pt/Ti/SiO2/Si(100) substrates. BFO thin films have granular structure with average diameter from 100 nm to 1 μm [14]. A decrease in the grain size was observed with increasing Sm-substitution in the BFO films and root mean square surface roughness on 5 × 5 μm 2 area was about 6–10 nm. These morphological changes induced by Sm substitution might be due to the different growth mechanisms including the nucleation process between BFO and Sm-substituted BFO films. It is well accepted that grain size of the BFO films was reduced with the rare-earth substitution (La, Nd) as well as with co-doping (La and Ni) [11,36,37]. These substitutions are expected to suppress the impurity phases, improve the surface morphology and reduce the leakage current in the films. Accordingly, we have also observed the reduction in grain size and modification in the leakage characteristic in Smsubstituted BFO films. The grain size dependent leakage current was reported for PZT films [38] and BiFeO3–BiCrO3 composite films [39]. Leakage current density is lower for small grained films which might be due to the inter-grain depletion of grain boundary limited conduction [40]. The local space charges near grain boundaries inhibit current flow and overlapped depletion regions of neighboring grain boundaries are substantial cause of low leakage current. In the case of large grain films, the depletion regions of the neighboring grain boundaries cannot overlap, which provides pathway for the charge transport and result in the high leakage current. Our unpublished results suggest that the leakage current in BFO thin film flows through not only grain boundary but also grain itself. These results suggest that the grain size in BFO films plays a crucial role and leakage current of BFO strongly depends on its microstructure of the films. Fig. 5(a) shows the J–E characteristics of 300 nm thick BFO and 7.5 at.% Sm-substituted BFO films. Fig. 5(b) shows the J–E characteristics of 300 and 600-nm-thick Pt/BiFeO3/Pt thin film capacitors measured at room temperature. The leakage current density for 300 and 600 nm thick BFO film is low on the order of 10 −7 A/cm 2 in the lower electric field region up to 100 kV/cm and 250 kV/cm respectively and it increased sharply after the limit is crossed. This sharp increase in the current density is the major constraint for measuring polarization property of BFO films at room temperature. Similar leakage current behaviors were also observed with up to 5 at.% Sm-substituted films and hindered us in measuring the polarization properties at RT. However, we observed low leakage current density for 7.5 and 10 at.% Sm-substituted BFO film capacitors and able to measure polarization properties at RT. At 300 kV/cm, the leakage current density in pure BFO and 7.5 at.% Sm-substituted BFO thin films were found to be in the range of 10 −1 A/cm 2 and 10 −4 A/cm 2 respectively. It is more than 2 orders of magnitude reduction in the leakage
Fig. 5. J–E characteristics of (a) 300 nm thick Bi1−xSmxFeO3 (x = 0, and 0.075) films and (b) 300 and 600 nm thick BiFeO3 films.
current in 7.5 at.% Sm-substituted BFO thin films. These results indicate that Sm-substitution results in significant reduction in leakage current in the high electric field region and improves breakdown characteristic. The exact reasons for the observed low leakage current in 7.5 and 10 at.% Sm-substituted BFO films are not yet elucidated. These observations could be partially explained by increased structural stability
Fig. 6. P–E hysteresis loops of 300 nm thick 7.5 at.% Sm-substituted BFO film measured at 20 kHz.
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(orthorhombic structure) which would suppress the conversion of Fe 3+ to Fe 2 states. The J–E characteristics also suggest that the Sm-doped BFO films exhibit linear over wide range of electric fields and improved breakdown characteristics. Fig. 6 shows the P–E hysteresis loops of 300 nm thick 7.5 at.% Sm-doped BFO film at 20 kHz frequencies. The Pr and coercive field (Ec) values in 7.5 at.% Sm-doped BFO film with an applied electric field of 1500 kV/cm were found to be ~ 75 μC/cm 2 and ~ 300 kV/cm, respectively. The degradation of the P–E hysteresis loops is not significant for frequencies higher than 10 kHz. However, the loop measured at 1 kHz still contains some leakage component. The physical properties of BFO thin films strongly depend on the impurity ion species, which influences the structural modifications of the films. The structural modification in BFO films induced by 7.5 at.% Sm-substitution enhanced the electric breakdown characteristics. This modification enabled us to measure the well saturated P–E hysteresis loops in 100 nm thick films. The Sm doping also reduced the coercive field and made poling of the films easier compared to undoped films. Fig. 7 demonstrates the well saturated P–E hysteresis loops of 100 nm thick 7.5 at.% Sm-substituted BFO films with 5 V applied field measured at 50 kHz. These results demonstrate the potentials of Sm-substituted BFO films for low voltage operational devices. Fig. 8 shows the fatigue characteristics of 7.5 at.% Sm-substituted BFO film: (a) during 10 8 fatigue cycles and (b) P–E hysteresis loops before and after 10 8 fatigue cycles. We applied bipolar square switching pulses of 800 kV/cm in amplitude and 100 kHz in frequency to fatigue the films as well as 20 kHz triangular shaped pulses of 800 kV/cm in amplitude to measure the remnant polarization. We observed the fatigueless behavior in BFO films until 10 5 switching cycles. The polarization increased afterwards and after 10 7 switching cycles, breakdown took place due to the high leakage current in the
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Fig. 8. (a) Polarization vs. fatigue cycle measurements of 7.5 at.% Sm-substituted BiFeO3 film during 108 fatigue cycles and (b) P–E hysteresis loops of 7.5 at.% Sm-substituted BiFeO3 films, before and after 108 fatigue cycles at 100 kHz.
films [41]. However, we could measure the fatigueless behavior in 7.5% Sm-doped BFO films until 10 8 switching cycles as demonstrated in Fig. 8(a). The polarization measurements of 7.5% Sm-doped BFO films before and after 10 8 fatigue cycles are shown in Fig. 8(b). No significant degradation in polarization was observed even after 10 8 fatigue cycles. 4. Conclusion In conclusion, Sm-substituted BFO thin films were deposited on Pt/Ti/SiO2/Si(100) substrates by chemical synthesis and both structural as well as electrical properties were investigated. 5 at.% Sm-substitution exhibited rhombohedral crystal structure like pure BFO films, while 7.5 and 10 at.% Sm-substitution showed possibly orthorhombic crystal structure. The 7.5 at.% Sm-substituted BFO films showed leakage current density almost 2 orders of magnitude lower than pure BFO film at 100 kV/cm applied electric field. The doped films exhibited high polarization (~ 75 μC/cm 2) and low coercive field (300 kV/cm) when P–E hysteresis loops were drawn at 20 kHz with the maximum electric field of 1500 kV/cm. These films had improved fatigue endurance with no significant degradation in polarization even after 10 8 fatigue cycles. Well saturated P–E hysteresis loops were recorded in a 100 nm-thick film with 5 V coercive voltages. The electrical properties in BFO films are enhanced due to the structural modification with Sm-substitution and the results highlight the potential applications of these films in low voltage operational devices. Acknowledgment Fig. 7. (a) P–E hysteresis loops of 100 nm 7.5 at.% Sm-substituted BFO film measured with varying coercive fields at 50 kHz and (b) polarization vs. coercive voltage.
This project was funded by SSPL, DRDO (Task 24). SK is a recipient of international collaborator research project by MSL, Tokyo Tech, Japan.
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The author wishes to acknowledge Dr. Ratnakar Palai, Patroica University for useful discussion. The author also acknowledges Dr. Amita Gupta, Sc-G and Director, SSPL for continuous support. References [1] G. Smolenskii, V. Yudin, E. Sher, Y.E. Stolypin, Sov. Phys. - JETP 16 (1963) 622. [2] G. Smolenskii, V. Isupov, A. Agranovskaya, N. Kranik, Sov. Phys. Solid State 2 (1961) 2651. [3] R. Palai, R.S. Katiyar, H. Schmid, P. Tissot, S.J. Clark, J. Robertson, S.A.T. Redfern, G. Catalan, J.F. Scott, Phys. Rev. B 77 (2008) 014110. [4] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [5] K.Y. Yun, D. Ricinschi, T. Kanashima, M. Okuyama, Appl. Phys. Lett. 89 (2006) 192902. [6] F. Kubel, H. Schmid, Acta Crystallogr. B 46 (1990) 698. [7] J. Li, J. Wang, M. Wuttig, R. Ramesh, N. Wang, B. Ruette, A.P. Pyatakov, A.K. Zvezdin, D. Viehland, Appl. Phys. Lett. 84 (2004) 5261. [8] Y.H. Lee, J.M. Wu, Y.L. Chueh, L.J. Chou, Appl. Phys. Lett. 87 (2005) 172901. [9] S.J. Clark, J. Robertson, Appl. Phys. Lett. 90 (2007) 132903. [10] S.K. Singh, K. Maruyama, H. Ishiwara, J. Appl. Phys. 100 (2006) 064102. [11] S.K. Singh, K. Maruyama, H. Ishiwara, J. Phys. D Appl. Phys. 40 (2007) 2705. [12] S.K. Singh, K. Maruyama, H. Ishiwara, Appl. Phys. Lett. 88 (2006) 262908. [13] S.K. Singh, K. Sato, H. Maruyama, H. Ishiwara, Jpn. J. Appl. Phys. Lett. 45 (2006) L1087. [14] S.K. Singh, R. Palai, H. Maruyama, H. Ishiwara, Electrochem. Solid-State Lett. 11 (2008) G43. [15] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, S.G. Lee, Appl. Phys. Lett. 85 (2005) 222903. [16] S.R. Das, P. Bhattacharya, R.N.P. Choudhary, R.S. Katiyar, J. Appl. Phys. 99 (2006) 066107. [17] P. Baettig, C.F. Schelle, R. LeSar, U.V. Waghmare, N.A. Spaldin, Chem. Mater. 17 (2005) 1376. [18] G.L. Yuan, Siu Wing Or, J. Appl. Phys. 100 (2006) 024109.
[19] D. Maurya, H. Thota, A. Garg, B. Pandey, P. Chand, H.C. Verma, J. Phys. Condens. Matter 21 (2009) 026007. [20] Z. Zhong, Y. Sugiyama, H. Ishiwara, Jpn. J. Appl. Phys. 49 (2010) 041502. [21] S.K. Singh, C.V. Tomy, T. Era, M. Itoh, H. Ishiwara, J. Appl. Phys. (2012) 102801. [22] H. Yang, H. Wang, G.F. Zou, M. Jain, N.A. Suvorova, D.M. Feldmann, P.C. Dowden, R.F. DePaula, J.L. MacManus-Driscoll, A.J. Taylor, Q.X. Jia, Appl. Phys. Lett. 93 (2008) 142904. [23] S. Fujino, M. Murakami, V. Anbusathaiah, S.H. Lim, V. Nagarajan, C.J. Fennie, M. Wuttig, L.S. Riba, I. Takeuchi, Appl. Phys. Lett. 92 (2008) 202904. [24] D. Kan, L. Palova, V. Anbusathaiah, C.J. Cheng, S. Fujino, V. Nagarajan, K.M. Rabe, I. Takeuchi, Adv. Funct. Mater. 20 (2010) 1108. [25] R. Thomas, J.F. Scott, D.N. Bose, R.S. Katiyar, J. Phys. Condens. Matter 22 (2010) 423201. [26] M. Valant, A.K. Axelsson, Neil Alford, Chem. Mater. 19 (2007) 5431. [27] S.M. Selbach, M.A. Einarsrud, T. Grande, Chem. Mater. 21 (2009) 169. [28] S.K. Singh, H. Funakuba, H. Uchida, H. Ishiwara, Integr. Ferroelectr. 76 (2005) 139. [29] K.Y. Yun, M. Noda, M. Okuyama, J. Korean Phys. Soc. 42 (2003) S1153. [30] S.K. Singh, H. Ishiwara, Jpn. J. Appl. Phys 45 (4B) (2006) 3194. [31] A.Z. Simoes, C.S. Riccardi, M.L.D. Santos, F.G. Garcia, E. Longo, J.A. Varela, Mater. Res. Bull. 44 (2009) 1747. [32] Z. Zhong, S.K. Singh, Y. Sugiyama, H. Ishiwara, Jpn. J. Appl. Phys. 48 (2009) 101402. [33] R. Palai, J. Phys. D Appl. Phys. 38 (2005) 1190. [34] R. Shirley, The CRYSFIRE System for Automatic Powder Indexing, , 2002. [35] V.A. Murashov, D.N. Rakov, V.M. Ionov, I.S. Dubenko, Y.V. Titos, Ferroelectrics 162 (1994) 11. [36] T. Kawae, H. Tsuda, A. Morimoto, Appl. Phys. Express 1 (2008) 051601. [37] S.K. Singh, K. Maruyama, H. Ishiwara, Appl. Phys. Lett. 91 (2007) 112913. [38] S.H. Hu, G.J. Hu, X.J. Meng, G.S. Wang, J.L. Sun, S.L. Guo, J.H. Chu, N. Dai, J. Cryst. Growth 260 (2004) 109. [39] S.K. Singh, S. Shanthy, H. Ishiwara, J. Appl. Phys. 108 (2010) 054102. [40] H. Hu, S.B. Krupanidhi, J. Mater. Res. 9 (1994) 1484. [41] Z. Zhong, S.K. Singh, K. Maruyama, H. Ishiwara, Jpn. J. Appl. Phys. Lett. 47 (2008) 2230.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structural and electrical properties of Sm-substituted BiFeO3 thin films prepared by chemical solution deposition Sushil Kumar Singh ⁎ MEMS Division, Solid State Physics Lab., Lucknow Road, Timarpur, Delhi-110054, India
a r t i c l e
i n f o
Article history: Received 25 February 2012 Received in revised form 21 November 2012 Accepted 22 November 2012 Available online 6 December 2012 Keywords: Multiferroics Bismuth ferrite Thin films Chemical solution deposition Structural properties Electrical properties Hysteresis loop
a b s t r a c t The structural and electrical properties of chemical-solution-deposited Bi1 −xSmxFeO3 (x = 0, 0.025, 0.05, 0.075, 0.1) thin films on Pt/Ti/SiOx/Si (100) substrates were investigated. Films up to 5 at.% Sm exhibited a single perovskite phase with rhombohedral structure, whereas films with 7.5 and 10 at.% Sm exhibited a distorted orthorhombic crystal structure. Atomic force microscopy of the films showed homogeneous and smooth surface. Films with 7.5 at.% Sm exhibited significant reduction in leakage current in the high electric field region and improved breakdown characteristic. The polarization vs. electric field (P–E) hysteresis loops were recorded in a 100 nm thick film with 3 V coercive voltages. Moreover, well saturated P–E hysteresis loops with high polarization (80 μC/cm 2) and low coercive field (300 kV/cm) were also recorded in 100 nm thick films with low coercive voltage (5 V). The Sm-substitution in BiFeO3 improved the fatigue endurance with no significant degradation in polarization even after 108 fatigue cycles. These results demonstrate that Sm-substituted BifeO3 films have potential for application in low voltage operational device. © 2012 Elsevier B.V. All rights reserved.
1. Introduction BiFeO3 (BFO) is one of the most widely studied multiferroics for potential device applications because of its existence as magnetoelectric multiferroic at room temperature, anti-ferromagnetic below ~ 370 °C [1], ferroelectric up to ~ 820 °C [2], and ferroelastic between 820 and 930 °C [3]. Thin films of BFO exhibit significant high values of electrical polarization (60–150 μC/cm 2) [4,5] as compared to widely used ferroelectric films such as PbZr1 − xTixO3, BaSrTiO3, and Bi3LaTi3O12. BFO crystallizes in the rhombohedral (a = 5.58 Å and α = 89.5°) crystal structure at room temperature with the space group R3c and G-type antiferromagnetism. Due to spin canting, it also exhibits ferromagnetism at room temperature [1,2,6]. Remnant polarization (Pr) values as large as 100 μC/cm 2 have also been reported for BFO films fabricated by pulsed laser deposition, RF sputtering, and chemical solution deposition (CSD) methods [7,8]. Ferroelectric thin films fabricated by CSD have advantages of homogeneous over a large area, reproducible and commercially used for fabrication of ferroelectric random access memory (FeRAM) devices. Device applications of BFO films have a constraint of high leakage current (low electrical resistivity). The possible reasons for this high leakage current are due to reduction of Fe 3+ ions to Fe 2+ and formation of oxygen vacancies for charge compensation. There has been controversy about high leakage current and Clark and Robertson have reported that the ambient band gap (2.9 eV) is too large for ⁎ Tel.: +91 1123903820; fax: +91 1123913609. E-mail addresses: [email protected], [email protected]. 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.062
intrinsic semiconductor mechanisms [9]. In our previous studies [11–14], we thoroughly investigated Mn, Cr, and Ni-substitution at the Fe-site and La-substitution at the Bi site of BFO films for reducing the leakage current above the coercive field (Ec). We optimized the conditions for crystallizing single phase BFO films by CSD, i.e., the leakage current density was decreased to lower than 1 × 10−7 A/cm 2 in the lower electric field region (b 200 kV/cm) [10]. We observed that in BFO thin films, leakage current with high applied electric fields (>200 kV/cm) and remnant polarization were improved with small 2.5–7.5 at.% substitution of Mn, Cr, and Ni. However, in the case of La (20%)-substitution it showed improved leakage current density but with reduced polarization. The partial La-substituted BFO has already been intensively studied and this substitution shows significant enhancement in its multiferroic properties [11,15,16]. The ferroelectricity in BFO materials mainly arises from the Bi 6s lone pair electrons [17]. The chemical substitution at Bi-site with smaller ionic radii such as Sm 3+ than Bi 3+ of the perovskite structure is expected to cause more significant structure distortion and improved ferroelectric, piezoelectric and magnetic properties. The effect of Sm-substitution in BFO bulk as well as in thin films reportedly enhance the electrical and magnetic properties [18–21]. Yang et al. have reported improved leakage current density in BFO:Sm2O3 nano-composite [22]. The phase transition from rhombohedral to pseudo-orthorhombic structure was also observed with 14% Sm-substituted BFO films resulting in enhanced piezoelectric properties [23,24]. These studies prompted us to investigate the structural and electrical properties of Sm-substituted BFO thin films formed by CSD method.
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The major objectives of the present work are (1) to synthesize the Sm-substituted BFO thin films using the CSD method and (2) to investigate the structural and electrical properties of the resulting thin films. Our studies demonstrate that high saturated polarization (80 μC/cm 2) can be achieved in 100 nm-thick Sm-substituted BFO films with low coercive voltages (5 V). These films exhibited improved fatigue endurance with minimal degradation in polarization
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even after 10 8 fatigue cycles. These studies emphasize the potentials of Sm-substituted BFO for semiconductor device fabrication. 2. Experimental details Thin films of 300 nm thick Bi1 − xSmxFeO3 (x = 0, 0.025, 0.05, 0.075, and 0.1) were formed by chemical solution deposition on Pt/Ti/SiOx/Si(100) substrates. Chemical solutions with exact metal composition ratios at 0.2 mol/l concentration were spin-coated at 3000 rpm for 30 s, dried at 240 °C for 3 min, and pre-fired at 350 °C for 10 min in air followed by annealing at 550 °C for 10 min in nitrogen atmosphere. Electron beam evaporation was used to deposit Pt top electrodes of the following dimension 3.14 × 10 −4 cm 2 and 0.785 × 10 −4 cm 2 through a shadow mask. We used top electrodes of 3.14× 10−4 cm2 for current density vs. electric field (J–E) measurement and top electrodes of 0.785 × 10−4 cm2 for polarization vs. electric field (P–E) measurements. The crystalline structures of the films were investigated with a multipurpose X-ray diffractometer (X'Pert-Pro MPD, Philips) with Cu Kα radiations (40 kV and 30 mA). The surface morphologies of the films were studied using NanoNavi-SPA 400 atomic force microscope (AFM). Sampling Intelligent Scan (tapping) mode was used for acquiring the data. The electrical properties of the thin-film capacitors were measured at RT using a ferroelectric test system (Toyo Corp., FCE-1A/Fop-100V) and an HP4156A precision semiconductor parameter analyzer (Hewlett-Packard). 3. Result and discussions The synthesis of single-phase perovskite BFO is limited by the narrow deposition window constrained by annealing temperature, atmosphere and chemical substitution. During the synthesis of bulk BFO several impurity phases such as Bi2Fe4O9, Bi2O3, and Bi25FeO39 were observed due to the kinetics of phase formation [25]. Previous studies suggest that BFO is metastable with respect to Bi2Fe4O9, and Bi25FeO39 for 447 b T b 767 °C and a small impurity is sufficient to form the non-perovskite phases [26,27]. The thermodynamic stability of perovskite phase with respect to non-perovskite phases can be accounted in terms of tolerance factor, acid–base relations, chemical substitution and pressure. The crystallization temperature of BFO film is approximately 550 °C and BFO films annealed lower than 600 °C in air and N2, while O2 shows small amount of non-perovskite Bi2Fe4O9 phase. As the temperature was increased higher than 600 °C, the nonperovskite phase increases gradually in air and N2 and rapidly in O2 atmosphere. BFO films annealed in O2 atmosphere at 700 °C show sufficient amount of non-perovskite phase [28]. BFO films fabricated by CSD normally contain Bi2Fe4O9 non-perovskite phase. The formation of non-perovskite phase in the films can be attributed to the Bi loss due to volatilization during the annealing, which is more intensive in the O2 atmosphere. The formation of the non-perovskite phase can be suppressed by compensating Bi-atoms using the 5% Bi-excess precursor solution [28]. It was observed that high oxygen pressure
Table 1 Structural parameters of Bi1 −xSmxFeO3 (x = 0, 0.025, 0.05, 0.075 and 0.1) films with estimated error (SD).
Fig. 1. X-ray diffraction patterns of (a) Bi1−xSmxFeO3 (x = 0, 0.025, 0.05, 0.075 and 0.1) films on Pt/Ti/SiOx/Si(100) substrates annealed at 550 °C for 10 min in N2 atmosphere. Magnified view of (b) (012) peak and (c) (104/110) peaks.
Bi1−xSmxFeO3
Structural parameters
x=0 x = 0.025 x = 0.05 x = 0.075
a = 5.5982 Å a = 5.5788 Å a = 5.5778 Å a = 5.0353 Å b = 3.9453 Å c = 6.4449 Å a = 4.9846 Å b = 3.0905 Å c = 6.4753 Å
x = 1.0
α = 89.701° α = 89.723° α = 89.672° α = β = γ = 90°
SD = 0.008 Å SD = 0.11 Å SD = 0.04 Å SD = 0.0002 Å
α = β = γ = 90°
SD = 0.0001 Å
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increases the formation of non-perovskite phases in BFO thin films. The partial pressure of oxygen can be controlled by using nitrogen atmosphere which results in single perovskite phase BFO films with enhanced crystallinity [29]. It is expected that BFO thin films annealed in O2 atmosphere would maintain the oxygen stoichiometry and Fe 3+ valence state. In contrast, we observed a high leakage current density due to the existence of non-perovskite phase in oxygen atmosphere annealed
Fig. 2. X-ray diffraction patterns of Bi1−xSmxFeO3 (x=0, 0.025, 0.05, 0.075 and 0.1) films on Pt/Ti/SiOx/Si(100) substrates annealed at (a) 500 °C, (b) 575 °C, and (c) 600 °C for 10 min in N2 atmosphere.
BFO films. However, the BFO films annealed in N2 atmosphere exhibited a lower leakage current density owing to the presence of Fe 3+ valence states [30]. We have previously demonstrated that, BFO films annealed in N2 atmosphere at 550 °C and in air at 500 °C exhibited only perovskite phases [30]. In a similar study, Simoes et al. have also reported that BFO films annealed in air at 500 °C compensate the bismuth vacancies created due to bismuth volatilization, leading to the formation of relatively pure phase [31]. Chemical substitution reportedly influences the crystallization temperature of BFO films as 20% Cr-substitution reduced the crystalline temperature from 550 °C to 450 °C [32]. In the present study, we fabricated BFO and Sm-substituted BFO films using stoichiometric sol–gel solution and annealed films at 550 °C in N2 atmosphere to avoid the formation of non-perovskite phase. Fig. 1(a) shows the X-ray diffraction (XRD) patterns of Bi1−xSmxFeO3 (x=0, 0.025, 0.05, 0.075 and 0.1) films grown on Pt/Ti/SiO2/Si substrates. Fig. 1(b) and (c) shows the magnification of 21.5–23.5° and 31–33° respectively. Fig. 1(c) demonstrates that films up to 5 at.% Sm-substitution show a single perovskite phase (rhombohedral structure), whereas films with 7.5 and 10 at.% Sm-substitution show structural distortion from the parent rhombohedral structure. The XRD peaks for 0–5 at.% Sm-substituted films were indexed using a rhombohedral crystal structure with R3c space group and XRD patterns were analyzed using a program package [33] with least-square refinement for which the standard deviation (SD) was found to be minimum. Similarly, patterns of 7.5 and 10 at.% Sm-substituted films were indexed using an automated powder-indexing program, “Crysfire” [34] based on Rietveld refinement and the program suggested an orthorhombic crystal structure based on high figure of merit. The suggested structures from both the software packages were verified and refined using a Crysfire interactive program, “Chekcell”. Unfortunately, the space groups of 7.5 and 10 at.%-substituted films could not be determined because of the limited number of diffraction peaks. Table 1 lists the structural parameters obtained from the refinement with mean absolute discrepancy. We observed a systematic decrease in lattice parameter, a, with increasing Sm-substitution as the ionic radius of Sm (0.96 Å) is smaller than Bi (1.03 Å). With increasing Sm substitution the broadening of (012) diffraction peaks [Fig. 1(b)] was observed which might be due to the strain
Fig. 3. AFM images of BiFeO3 thin films on Pt/Ti/SiOx/Si(100) substrates.
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effect. The AFM images (cf. Fig. 4) also indicated the formation of smaller grains with increasing Sm-substitution. The clear splitting in the 104 and 110 diffraction peaks can also be seen in BFO films and depicted in Fig. 1(c). All the diffraction peaks in BFO films exactly matched with the rhombohedrally distorted perovskite structure with space group R3c. Sm-substitution in BFO reduced the splitting of the 104 and 110 diffraction peaks and showed a broadened single peak around 32°, which suggests that the rhombohedral distortion is reduced towards the orthorhombic structure with Sm-substitution. These results suggest that because of Sm-substitution induced distortion, tilted oxygen octahedral makes the rhombohedral phase unstable and consequently stabilize the orthorhombic phase shrinking lattice parameters as well as the overall volume of the unit cell. In a similar study, Daisuke Kan et al. have reported the structural transition behavior in rare-earth (Sm, Gd, Dy) substituted (001) epitaxial BFO films [24]. Using a scanning 2D XRD system the authors have reported only (001) and (002) reflections from the pure BFO films. However, extra diffraction spots 1/4{011} have been reported with Dy (7%) substitution. As Dy substitution increases, the 1/4 {011} superstructure spots disappeared and new superstructure spots 1/2{010} emerged. Dy-substitution beyond 6% in BFO films
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decreased the 1/4{011} spot intensity with concomitant increase in 1/2{010} spot intensity. These cumulative changes indicate structural changes from rhombohedral to the orthorhombic phase. As mentioned earlier, the phase transition depends upon the ionic radius of the rare earth dopant and structural transition shifts towards the larger composition values as the ionic radius of the dopant becomes larger. For Sm, structural transition has been reported beyond 14% substitution in BFO films [24]. This structural transition has been attributed to a hydrostatic pressure effect caused by the smaller radii of the isovalent rare earth ion substitution. Studying rare-earth element (La, Nd, Sm, Gd and Dy) substitution in BFO single crystals, Murashov et al. [35] have also reported the structural transitions to triclinic-rhombohedral and triclinic-orthorhombic phases with ~ 6– 10 and ~ 14–18% Sm-substitution respectively. This has been attributed to the distortion of the iron octahedral or bending of O–Fe–O and Fe–O–Fe bond angles. In contrast, single crystal with ≥ 18% Smsubstitution resulted in single-phase orthorhombic structure. In the current study, we used all the eight programs in the Crysfire suit for the 7.5 and 10 at.% Sm-substituted BFO films but no solution was found for the triclinic and rhombohedral structures. Because of the limited diffraction peaks, the presence of metastable structures
Fig. 4. AFM images of Bi1−xSmxFeO3 thin films on Pt/Ti/SiOx/Si(100) substrates: (a) x= 0.025, (b) x = 0.05, (c) x = 0.075, and (d) x = 0.1.
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cannot be ruled out. In the future, detailed study of crystal structure of rare-earth substituted BFO films is warranted. To further understand the influence of Sm-substitution on the perovskite phase formation in BFO films, we annealed Sm-substitute BFO films at various temperatures such as 450 °C, 500 °C, 550 °C, 575 °C, and 600 °C. Fig. 2 shows the XRD-pattern of Bi1 − xSmxFeO3 (x=0.025, 0.05, 0.075 and 0.1) films annealed at (a) 500 °C, (b) 575 °C, and (c) 600 °C. We observed crystalline phase formation above 500 °C whereas films annealed below 500 °C exhibited the amorphous phase. The perovskite phase was observed in up to 10% Sm-doped BFO films when annealed under N2 atmosphere at 500 and 550 °C. However, when films were annealed at 575 °C and 600 °C, secondary phase such as Bi2Fe4O9, Fe2O3, and Fe3O4 appeared and gradually decreased as Sm-substitution increased. In contrast, the Sm-doping in the BFO films suppressed the non-perovskite phase formation even at higher temperatures like 575 °C and 600 °C. These studies suggest that rare earth substitution in BFO at Bi-site suppresses the volatility of the Bi and provides chemical stability to the perovskite phase as well as lowers the temperature for perovskite phase formation. Fig. 3 shows the AFM micrographs of pure BFO films and Fig. 4 shows the AFM micrographs of Bi1 − xSmxFeO3 (x = 0.025, 0.05, 0.075, and 0.1) thin films on Pt/Ti/SiO2/Si(100) substrates. BFO thin films have granular structure with average diameter from 100 nm to 1 μm [14]. A decrease in the grain size was observed with increasing Sm-substitution in the BFO films and root mean square surface roughness on 5 × 5 μm 2 area was about 6–10 nm. These morphological changes induced by Sm substitution might be due to the different growth mechanisms including the nucleation process between BFO and Sm-substituted BFO films. It is well accepted that grain size of the BFO films was reduced with the rare-earth substitution (La, Nd) as well as with co-doping (La and Ni) [11,36,37]. These substitutions are expected to suppress the impurity phases, improve the surface morphology and reduce the leakage current in the films. Accordingly, we have also observed the reduction in grain size and modification in the leakage characteristic in Smsubstituted BFO films. The grain size dependent leakage current was reported for PZT films [38] and BiFeO3–BiCrO3 composite films [39]. Leakage current density is lower for small grained films which might be due to the inter-grain depletion of grain boundary limited conduction [40]. The local space charges near grain boundaries inhibit current flow and overlapped depletion regions of neighboring grain boundaries are substantial cause of low leakage current. In the case of large grain films, the depletion regions of the neighboring grain boundaries cannot overlap, which provides pathway for the charge transport and result in the high leakage current. Our unpublished results suggest that the leakage current in BFO thin film flows through not only grain boundary but also grain itself. These results suggest that the grain size in BFO films plays a crucial role and leakage current of BFO strongly depends on its microstructure of the films. Fig. 5(a) shows the J–E characteristics of 300 nm thick BFO and 7.5 at.% Sm-substituted BFO films. Fig. 5(b) shows the J–E characteristics of 300 and 600-nm-thick Pt/BiFeO3/Pt thin film capacitors measured at room temperature. The leakage current density for 300 and 600 nm thick BFO film is low on the order of 10 −7 A/cm 2 in the lower electric field region up to 100 kV/cm and 250 kV/cm respectively and it increased sharply after the limit is crossed. This sharp increase in the current density is the major constraint for measuring polarization property of BFO films at room temperature. Similar leakage current behaviors were also observed with up to 5 at.% Sm-substituted films and hindered us in measuring the polarization properties at RT. However, we observed low leakage current density for 7.5 and 10 at.% Sm-substituted BFO film capacitors and able to measure polarization properties at RT. At 300 kV/cm, the leakage current density in pure BFO and 7.5 at.% Sm-substituted BFO thin films were found to be in the range of 10 −1 A/cm 2 and 10 −4 A/cm 2 respectively. It is more than 2 orders of magnitude reduction in the leakage
Fig. 5. J–E characteristics of (a) 300 nm thick Bi1−xSmxFeO3 (x = 0, and 0.075) films and (b) 300 and 600 nm thick BiFeO3 films.
current in 7.5 at.% Sm-substituted BFO thin films. These results indicate that Sm-substitution results in significant reduction in leakage current in the high electric field region and improves breakdown characteristic. The exact reasons for the observed low leakage current in 7.5 and 10 at.% Sm-substituted BFO films are not yet elucidated. These observations could be partially explained by increased structural stability
Fig. 6. P–E hysteresis loops of 300 nm thick 7.5 at.% Sm-substituted BFO film measured at 20 kHz.
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(orthorhombic structure) which would suppress the conversion of Fe 3+ to Fe 2 states. The J–E characteristics also suggest that the Sm-doped BFO films exhibit linear over wide range of electric fields and improved breakdown characteristics. Fig. 6 shows the P–E hysteresis loops of 300 nm thick 7.5 at.% Sm-doped BFO film at 20 kHz frequencies. The Pr and coercive field (Ec) values in 7.5 at.% Sm-doped BFO film with an applied electric field of 1500 kV/cm were found to be ~ 75 μC/cm 2 and ~ 300 kV/cm, respectively. The degradation of the P–E hysteresis loops is not significant for frequencies higher than 10 kHz. However, the loop measured at 1 kHz still contains some leakage component. The physical properties of BFO thin films strongly depend on the impurity ion species, which influences the structural modifications of the films. The structural modification in BFO films induced by 7.5 at.% Sm-substitution enhanced the electric breakdown characteristics. This modification enabled us to measure the well saturated P–E hysteresis loops in 100 nm thick films. The Sm doping also reduced the coercive field and made poling of the films easier compared to undoped films. Fig. 7 demonstrates the well saturated P–E hysteresis loops of 100 nm thick 7.5 at.% Sm-substituted BFO films with 5 V applied field measured at 50 kHz. These results demonstrate the potentials of Sm-substituted BFO films for low voltage operational devices. Fig. 8 shows the fatigue characteristics of 7.5 at.% Sm-substituted BFO film: (a) during 10 8 fatigue cycles and (b) P–E hysteresis loops before and after 10 8 fatigue cycles. We applied bipolar square switching pulses of 800 kV/cm in amplitude and 100 kHz in frequency to fatigue the films as well as 20 kHz triangular shaped pulses of 800 kV/cm in amplitude to measure the remnant polarization. We observed the fatigueless behavior in BFO films until 10 5 switching cycles. The polarization increased afterwards and after 10 7 switching cycles, breakdown took place due to the high leakage current in the
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Fig. 8. (a) Polarization vs. fatigue cycle measurements of 7.5 at.% Sm-substituted BiFeO3 film during 108 fatigue cycles and (b) P–E hysteresis loops of 7.5 at.% Sm-substituted BiFeO3 films, before and after 108 fatigue cycles at 100 kHz.
films [41]. However, we could measure the fatigueless behavior in 7.5% Sm-doped BFO films until 10 8 switching cycles as demonstrated in Fig. 8(a). The polarization measurements of 7.5% Sm-doped BFO films before and after 10 8 fatigue cycles are shown in Fig. 8(b). No significant degradation in polarization was observed even after 10 8 fatigue cycles. 4. Conclusion In conclusion, Sm-substituted BFO thin films were deposited on Pt/Ti/SiO2/Si(100) substrates by chemical synthesis and both structural as well as electrical properties were investigated. 5 at.% Sm-substitution exhibited rhombohedral crystal structure like pure BFO films, while 7.5 and 10 at.% Sm-substitution showed possibly orthorhombic crystal structure. The 7.5 at.% Sm-substituted BFO films showed leakage current density almost 2 orders of magnitude lower than pure BFO film at 100 kV/cm applied electric field. The doped films exhibited high polarization (~ 75 μC/cm 2) and low coercive field (300 kV/cm) when P–E hysteresis loops were drawn at 20 kHz with the maximum electric field of 1500 kV/cm. These films had improved fatigue endurance with no significant degradation in polarization even after 10 8 fatigue cycles. Well saturated P–E hysteresis loops were recorded in a 100 nm-thick film with 5 V coercive voltages. The electrical properties in BFO films are enhanced due to the structural modification with Sm-substitution and the results highlight the potential applications of these films in low voltage operational devices. Acknowledgment Fig. 7. (a) P–E hysteresis loops of 100 nm 7.5 at.% Sm-substituted BFO film measured with varying coercive fields at 50 kHz and (b) polarization vs. coercive voltage.
This project was funded by SSPL, DRDO (Task 24). SK is a recipient of international collaborator research project by MSL, Tokyo Tech, Japan.
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The author wishes to acknowledge Dr. Ratnakar Palai, Patroica University for useful discussion. The author also acknowledges Dr. Amita Gupta, Sc-G and Director, SSPL for continuous support. References [1] G. Smolenskii, V. Yudin, E. Sher, Y.E. Stolypin, Sov. Phys. - JETP 16 (1963) 622. [2] G. Smolenskii, V. Isupov, A. Agranovskaya, N. Kranik, Sov. Phys. Solid State 2 (1961) 2651. [3] R. Palai, R.S. Katiyar, H. Schmid, P. Tissot, S.J. Clark, J. Robertson, S.A.T. Redfern, G. Catalan, J.F. Scott, Phys. Rev. B 77 (2008) 014110. [4] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [5] K.Y. Yun, D. Ricinschi, T. Kanashima, M. Okuyama, Appl. Phys. Lett. 89 (2006) 192902. [6] F. Kubel, H. Schmid, Acta Crystallogr. B 46 (1990) 698. [7] J. Li, J. Wang, M. Wuttig, R. Ramesh, N. Wang, B. Ruette, A.P. Pyatakov, A.K. Zvezdin, D. Viehland, Appl. Phys. Lett. 84 (2004) 5261. [8] Y.H. Lee, J.M. Wu, Y.L. Chueh, L.J. Chou, Appl. Phys. Lett. 87 (2005) 172901. [9] S.J. Clark, J. Robertson, Appl. Phys. Lett. 90 (2007) 132903. [10] S.K. Singh, K. Maruyama, H. Ishiwara, J. Appl. Phys. 100 (2006) 064102. [11] S.K. Singh, K. Maruyama, H. Ishiwara, J. Phys. D Appl. Phys. 40 (2007) 2705. [12] S.K. Singh, K. Maruyama, H. Ishiwara, Appl. Phys. Lett. 88 (2006) 262908. [13] S.K. Singh, K. Sato, H. Maruyama, H. Ishiwara, Jpn. J. Appl. Phys. Lett. 45 (2006) L1087. [14] S.K. Singh, R. Palai, H. Maruyama, H. Ishiwara, Electrochem. Solid-State Lett. 11 (2008) G43. [15] D. Lee, M.G. Kim, S. Ryu, H.M. Jang, S.G. Lee, Appl. Phys. Lett. 85 (2005) 222903. [16] S.R. Das, P. Bhattacharya, R.N.P. Choudhary, R.S. Katiyar, J. Appl. Phys. 99 (2006) 066107. [17] P. Baettig, C.F. Schelle, R. LeSar, U.V. Waghmare, N.A. Spaldin, Chem. Mater. 17 (2005) 1376. [18] G.L. Yuan, Siu Wing Or, J. Appl. Phys. 100 (2006) 024109.
[19] D. Maurya, H. Thota, A. Garg, B. Pandey, P. Chand, H.C. Verma, J. Phys. Condens. Matter 21 (2009) 026007. [20] Z. Zhong, Y. Sugiyama, H. Ishiwara, Jpn. J. Appl. Phys. 49 (2010) 041502. [21] S.K. Singh, C.V. Tomy, T. Era, M. Itoh, H. Ishiwara, J. Appl. Phys. (2012) 102801. [22] H. Yang, H. Wang, G.F. Zou, M. Jain, N.A. Suvorova, D.M. Feldmann, P.C. Dowden, R.F. DePaula, J.L. MacManus-Driscoll, A.J. Taylor, Q.X. Jia, Appl. Phys. Lett. 93 (2008) 142904. [23] S. Fujino, M. Murakami, V. Anbusathaiah, S.H. Lim, V. Nagarajan, C.J. Fennie, M. Wuttig, L.S. Riba, I. Takeuchi, Appl. Phys. Lett. 92 (2008) 202904. [24] D. Kan, L. Palova, V. Anbusathaiah, C.J. Cheng, S. Fujino, V. Nagarajan, K.M. Rabe, I. Takeuchi, Adv. Funct. Mater. 20 (2010) 1108. [25] R. Thomas, J.F. Scott, D.N. Bose, R.S. Katiyar, J. Phys. Condens. Matter 22 (2010) 423201. [26] M. Valant, A.K. Axelsson, Neil Alford, Chem. Mater. 19 (2007) 5431. [27] S.M. Selbach, M.A. Einarsrud, T. Grande, Chem. Mater. 21 (2009) 169. [28] S.K. Singh, H. Funakuba, H. Uchida, H. Ishiwara, Integr. Ferroelectr. 76 (2005) 139. [29] K.Y. Yun, M. Noda, M. Okuyama, J. Korean Phys. Soc. 42 (2003) S1153. [30] S.K. Singh, H. Ishiwara, Jpn. J. Appl. Phys 45 (4B) (2006) 3194. [31] A.Z. Simoes, C.S. Riccardi, M.L.D. Santos, F.G. Garcia, E. Longo, J.A. Varela, Mater. Res. Bull. 44 (2009) 1747. [32] Z. Zhong, S.K. Singh, Y. Sugiyama, H. Ishiwara, Jpn. J. Appl. Phys. 48 (2009) 101402. [33] R. Palai, J. Phys. D Appl. Phys. 38 (2005) 1190. [34] R. Shirley, The CRYSFIRE System for Automatic Powder Indexing, , 2002. [35] V.A. Murashov, D.N. Rakov, V.M. Ionov, I.S. Dubenko, Y.V. Titos, Ferroelectrics 162 (1994) 11. [36] T. Kawae, H. Tsuda, A. Morimoto, Appl. Phys. Express 1 (2008) 051601. [37] S.K. Singh, K. Maruyama, H. Ishiwara, Appl. Phys. Lett. 91 (2007) 112913. [38] S.H. Hu, G.J. Hu, X.J. Meng, G.S. Wang, J.L. Sun, S.L. Guo, J.H. Chu, N. Dai, J. Cryst. Growth 260 (2004) 109. [39] S.K. Singh, S. Shanthy, H. Ishiwara, J. Appl. Phys. 108 (2010) 054102. [40] H. Hu, S.B. Krupanidhi, J. Mater. Res. 9 (1994) 1484. [41] Z. Zhong, S.K. Singh, K. Maruyama, H. Ishiwara, Jpn. J. Appl. Phys. Lett. 47 (2008) 2230.