SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Effects of Ho and Ti ions individual doping and co-doping on the structural, electrical, and ferroelectric properties of the BiFeO3 thin films are reported. Pure BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by using a chemical solution deposition method. All thin films were crystallized in distorted rhombohedral structure containing no secondary or impurity phases confirmed by using an X-ray diffraction study. Changes in microstructural features, such as grain morphology and grain size distribution, for the doped samples were analyzed by a scanning electron microscopy. From the experimental results, a low electrical leakage (1.2 × 10−5 A/cm2 at 100 kV) and improved ferroelectric properties, such as a large remnant polarization (2Pr) of 52 μC/cm2 and a low coercive field (2Ec) of 886 kV/cm, were observed for the (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin film. Fast current relaxation and stabilization observed in the (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ imply effective reduction and neutralization of charged free carriers.

I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Perovskite based bismuth ferrite BiFeO3 (BFO) is one of the most promising lead free multiferroic materials. In which, the magnetic and the ferroelectric transition temperatures were reported to well above room temperature (643 and 1100 K, respectively).[1] Room-temperature multiferroic properties, giant ferroelectricity, and lead-free nature make BFO a superior candidate for next-generation devices including non-volatile memories, spintronics and lead-free piezoelectrics.[2-4] However, applications of BFO in electronic devices are limited due to the serious leakage-current problem, which related with impurity phases, boundaries, non-stoichiometry, ionic defects, and poor interfacial quality.[5, 6] Particularly, BFO thin film fabricated via a chemical solution deposition (CSD) method showed relatively large leakage and poor hysteresis behavior compared to other sophisticated techniques, such as laser ablation process, molecular beam epitaxy, and metal organic chemical vapor deposition.[7, 8] However, wet chemistry-based CSD technique has many advantages, such as uniformity of the molecules in precursor solution, film fabrication in ambient pressure, cost-effectiveness, and high throughput, which enable the fabrication of high-performance low-cost electronics.[8, 9] The presence of oxygen vacancies and the valence fluctuation (Fe2+/Fe3+) are believed as main factors for the large electrical leakage in the BFO thin film fabricated via a CSD method.[10-13] A facile way to improve the electrical and ferroelectric properties of the BFO thin film mainly relies on A- or B-site cationic substitutions.[12-16]

In general, dopings of rare earth (RE) metal (RE = La, Gd, Eu, Nd, Tb, etc.) ions to the Bi-site and transition-metal (TM = Co, Ni, Cu, Zn, Mg, Mn, Cr, Sc, Mo, Ti, V, etc.) ions to the Fe-site were suggested to reduce the leakage current density and to improve the ferroelectric properties.[13-24] The RE ions doping into Bi-site of BFO control Bi evaporation and stabilize the perovskite structure.[25] The conduction band of BFO is related to the d orbital electronic state of the Fe3+ ion, hence, the Fe-site doping by TM ions could influence physical properties by changing the electronic structure.[26] Doping of the TM ions also controls Fe2+/Fe3+ valence fluctuation through charge compensation.[25, 27] Substitution of smaller ionic radius RE ions into the Bi-site would induce more buckling in the Fe–O–Fe bond angle accompanying a smaller tolerance factor leading to a more insulating character.[26] The ionic radius of Ho (1.23 Å) with the coordination number of 12 is much smaller than that of Bi (1.36 Å); therefore, it can be readily substituted to the Bi-site of BFO. Substitution of smaller ionic radius Ho3+ into BFO is also expected to cause structural distortion.[20] Doping of Ti is an effective way to obtain stable perovskite phase with neutralized defect carriers.[17]

In this study, pure BFO, (Bi0.9Ho0.1)FeO3 (BHFO), Bi(Fe0.98Ti0.02)O3+δ (BFTO), and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ (BHFTO) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by using a CSD method. Doping effects on the microstructural properties of the thin films were investigated by using an X-ray diffraction (XRD) pattern and a scanning electron microscopy. The electrical and the ferroelectric properties for the thin films are systematically investigated and the results discussed in detail.

II. Experimental Procedure

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The raw materials used for the precursor solutions are bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], iron nitrate nonahydrate [Fe(NO3)3·9H2O], holmium nitrate pentahydrate [Ho(NO3)3·5H2O], and titanium isopropoxide {Ti[OCH(CH3)2]4}. Ethylene glycol (EG) [OH(CH2)2OH] and 2-methoxyethanol (2-MOE) [CH3O(CH2)2OH] were mixed together by constant stirring at 40°C in a water bath for 30 min and used as a solvent. Bismuth nitrate pentahydrate (5 mol% excess) was added to the mixed solvent and stirred for 30 min. To the above Bi-solution acetic acid (CH3CO2H) was added to catalyze the reaction and stirred for 30 min. Finally, iron nitrate nonahydrate was added to form the BFO precursor solution. For Ho doping, holmium nitrate pentahydrate was added into the Bi-solution containing acetic acid catalyst and stirred for 30 min. Iron nitrate nonahydrate was then added to form the BHFO precursor solution. The titanium solution was prepared separately by dissolving titanium isopropoxide into the 2-MOE within a glove box, and acetyl acetone (CH3COCH2COCH3) was used as a chelating agent. In the case of Ti doping, bismuth nitrate pentahydrate, acetic acid, and iron nitrate nonahydrate were added sequentially to the 2-MOE and EG-mixed solvent with 30 min stirring intervals. The resulting Bi-Fe solution was stirred for 2 h and the titanium solution was added to form the BFTO precursor solution. Similarly, for (Ho, Ti) co-doping, bismuth nitrate pentahydrate, acetic acid, holmium nitrate pentahydrate, and iron nitrate nonahydrate were added sequentially to the 2-MOE and EG-mixed solvent with 30 min stirring intervals. The resulting Bi–Ho–Fe solution was stirred for 2 h and the Ti solution was added to form the BHFTO precursor solution. The final solutions of BFO, BHFO, BFTO, and BHFTO were stirred for 3 h at room temperature. The concentrations of all solutions were adjusted to 0.1M.

All the thin films were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by using a spin coating method at a constant spinning rate of 3000 rpm for 20 s. After spin coating, the wet thin films were prebaked at 360°C for 10 min on a hot plate. The coating and the prebaking were repeated for 12 times to obtain the desired film thickness. Finally, all the thin films were subjected to conventional annealing at 550°C for 30 min under a nitrogen atmosphere for crystallization. Platinum electrodes with areas of 1.54 × 10−4 cm2 were deposited on the top surfaces of the thin films by ion sputtering through a metal shadow mask to form a capacitor structure.

The structures of the thin films were investigated by using an X-ray diffractometer (MiniFlex II; Rigaku, Tokyo, Japan). Surface morphologies and film thicknesses were analyzed by using a field-emission scanning electron microscope (MIRA II LMH; Tescan, Brno, Czech Republic). The ferroelectric hysteresis loops of the thin films were measured at a frequency of 1 kHz with triangular pulses by using a standardized ferroelectric test system (Radiant Technologies Inc., Precision LC, Albuquerque, NM). The leakage current densities of the thin films were measured by using an electrometer (6517A; Keithley, Cleveland, OH). The dielectric properties were analyzed by using a low-frequency impedance analyzer (4192A; HP, Minneapolis, MN).

III. Results and Discussions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The XRD patterns of the pure and the doped BFO thin films are shown in Fig. 1. X-rays with CuKα1 radiation (λ = 1.5418 Å) were used to obtain the diffraction patterns. From Fig. 1, all the thin films showed randomly oriented polycrystalline of distorted rhombohedral structure with R3c space group (JCPDS no. 72-2035). No secondary or impurity phases were found in all samples. This implies that the dopant concentrations do not reach the solubility limit of the BFO. There were no observable changes in the lattice parameters for the doped thin films owing to the smaller ionic radii of Ho and Ti ions.[28] However, small changes in the diffraction patterns corresponding to inline image planes were observed for the BHFO and the BHFTO thin films as shown in Fig. 1(a). From Fig. 1(a), overlapping of split peaks corresponding to inline image planes in the vicinity of 32° was clearly observed for the BHFO and the BHFTO thin films while there was no change in the BFTO thin film. These results indicate that the internal chemical pressure induced by the Ho doped into Bi-site would induce more distortion in perovskite without changing the original structure,[20, 29] while the concentration of the Ti into Fe-site is too low to affect the structure.

image

Figure 1. XRD patterns of the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films deposited on Pt(111)/Ti/SiO2/Si(100) substrates. (a) Magnified XRD patterns in the vicinity of 2θ = 32.0° to show the lattice distortion.

Download figure to PowerPoint

The surface morphological features of the BFO, BHFO, BFTO, and BHFTO thin films are shown in Fig. 2. From the top views of the surface morphologies, grains with wide pores between grains were observed for the BFO and BHFO thin films. Dense microstructures containing variable grain size with significantly reduced intergranualar porosity were observed in the BFTO and the BHFTO thin films. Among thin films, the BHFTO thin film showed densely packed microstructure with greatly reduced pores. The thickness of the each thin film was estimated approximately 350 nm from the insetted cross-sectional SEM images.

image

Figure 2. SEM morphologies of the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films with cross-sectional micrographs.

Download figure to PowerPoint

Figure 3(a) illustrates variations of leakage current density (J) as a function of applied electric field (E) for the thin films. The measured leakage current densities of the BFO, BHFO, BFTO, and BHFTO thin films were 1.4 × 10−2, 2.0 × 10−4, 3.1 × 10−5, and 1.2 × 10−5 A/cm2, respectively, at an applied electric field of 100 kV/cm. The measured leakage current densities of the BFTO and the BHFTO thin films are three orders lower than that of the BFO thin film. The BHFO thin film showed two orders lower than that of the pure BFO thin film. Presence of oxygen vacancies due to Bi evaporation, poor microstructure, and valence fluctuation of Fe2+/Fe3+ are related to the large electrical conduction in BFO thin film.[27, 30] The decrease in leakage current density in the doped thin films might be attributed to the effective reduction in free charge carriers through charge compensation.[25] Additions of RE elements are reported to stabilize the perovskite structure by neutralizing the oxygen vacancies generated by Bi evaporation during material preparation.[25, 31] In addition, the smaller ionic radius Ho doping leads to buckling of Fe–O–Fe bond angle in perovskite structure of the BFO and this crystallographic change has been reported to affect electrical transport properties.[26] Meanwhile, doping of the higher valence Ti ion was reported as an effective way for the neutralization of charged defects via charge compensation.[28] Among thin films, the BHFTO thin film showed the lowest leakage current density, which might be attributed to the combined effects of Ho and Ti ions in BFO. The leakage current densities of the BHFTO and the BFTO thin films were found to be of the same order and this might be attributed to the similar microstructural features of the BFTO and the BHFTO thin films. As shown in surface morphological studies, dense microstructures with significantly reduced pores between the grains were observed in the BHFTO and the BFTO thin films.

image

Figure 3. (a) Leakage current densities of the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films and (b) log(J)−log(E) characteristics of the thin films.

Download figure to PowerPoint

The conduction mechanisms observed in the BFO, BHFO, BFTO, and BHFTO thin films were studied by logarithmic plots of the leakage current density versus applied electric field [log(J) − log(E)] as shown in Fig. 3(b). For all thin films, the plots can be fitted well by linear segments with different slope values. At low electric field region, for all thin films, the slope value (S) ~1.0 indicates ohmic conduction mechanism, which is dominated by thermally stimulated free electrons.[32] Leakage current for ohmic conduction can be expressed as[32]

  • display math

where e is the electron charge, μ is the free carrier mobility, Ne is the density of the thermally stimulated electrons, and E is the applied electric field.

However, the change in the slope value from S~1 to S~2 with an increase in applied electric field implies the change in conduction mechanism from ohmic to space charge limited (SCL) conduction.[32] It means that the density of free electrons due to charge carrier injection becomes larger than the density of thermally stimulated electrons with an increase in the applied electric field.[32] The current density for SCL conduction is given by[32]

  • display math

where V is the applied voltage, εr is the static dielectric constant, ε0 is the permittivity of free space, d is the thickness of thin film, and θ is the ratio of the total density of free electrons to the trapped electrons.

In the case of the pure BFO thin film, an abrupt increase in the current with increase in the applied electric field, the slope value changes to greater than 4 (S > ~4) indicating the trap filled limited (TFL) conduction mechanism.[32] It implies that, at high electric field region, the applied voltage forces all the available traps to become filled and the excess charge carriers lead to a drastic increase in the current with the large slope value. The voltage at which abrupt increase in the current occurs is TFL voltage and is given by[32]

  • display math

where Nt is the total trap density. Thus, the leakage current observed in BFO originated from ohmic, SCL, and TFL conduction mechanisms, while Ho and Ti individually and co-doped thin films follow conductions via ohmic and SCL mechanisms.

The leakage current density–time measurements were carried out to examine current relaxation for the BFO, BHFO, BFTO, and BHFTO thin films as shown in Fig. 4(a). Generally, the presence of free charge carriers, such as oxygen defects, causes slow current relaxation and attains stabilization after a long time period; this phenomenon indicates large electrical leakage in the corresponding thin film.[33, 34] The variations of current relaxation time periods [extracted from Fig. 4(a)] for the pure and doped BFO thin films are shown in Fig. 4(b). From Fig. 4(b), required time periods for current relaxation and stabilization of the BFO, BHFO, BFTO, and BHFTO thin films were 55, 51, 51 and 37 ms, respectively. The presence of a large number of free charge carriers, such as oxygen vacancies, might cause slow current relaxation in the pure BFO thin film.[34] Compared to the pure BFO, the BHFO and BFTO thin films attain fast relaxation. The fast current relaxation and stabilization (with in 37 ms) of the BHFTO thin film implies significant reduction and neutralization of charge carriers by Ho and Ti ions co-doping into BFO.[34] This can be explained as follows.

image

Figure 4. (a) Current-time plots for the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films and (b) variations of current relaxation and stabilization time with respect to doping elements. The inset of Fig. 4(b) implies the slope values extracted from the two regions of the initial leakage current of Fig. 4(a).

Download figure to PowerPoint

The highly volatile Bi undergoes easy evaporation during material processing conditions, such as drying and annealing of BFO thin film, which leads to Bi3+ deficiency accompanied with oxygen vacancies. According to the Kröger–Vink notation, the formation of Bi and oxygen vacancies is given below[20, 35, 36]:

  • display math
  • display math

where inline image, inline image and inline image are the Bi, Fe, and oxygen ions in lattice, inline image and inline image indicate Bi and oxygen vacancies with three negative and two positive charges, and inline image represents the Fe2+ ion with one negative charge. The formation of Fe2+ leads to electron hopping between Fe2+ and Fe3+ through oxygen vacancies, which in turn increases the electrical leakage in BFO.[37] The bond dissociation energy of Ho–O (606 kJ/mol) is larger than that of Bi–O (343 kJ/mol), hence, the substitution of Ho controls the formation of oxygen vacancies and stabilizes the perovskite structure.[25] While, the co-doping of Ti4+ ions results in the formation of inline image for Fe2+ and inline image for Fe3+, which could eliminate the oxygen vacancies to attain charge neutralization in the perovskite structure.[28] Thus, co-doping of Ho and Ti results in a significant reduction in, and neutralization of, charged defect carriers. The inset of Fig. 4(b) shows the slope values extracted from two regions of initial leakage currents of Fig. 4(a). Though the BHFO and BFTO thin films attain stabilization through current relaxation with the same time period, the difference in slope values (for BHFO region I: −0.366 and region II −0.816 and for BFTO region I: −0.343 and region II: −0.706) implies different relaxation mechanisms.[33] This might be attributed to the effective reduction of defect carriers followed by Ho ion doping, besides charge compensation followed by Ti ion doping. The combined effects of Ho and Ti co-doping lead to a significant improvement in leakage properties. Therefore, improved electrical properties, such as low leakage current density and fast current relaxation, have been observed in the BHFTO thin film.

The ferroelectric polarization–electric field (P–E) hysteresis loops of the BFO, BHFO, BFTO, and BHFTO thin films are shown in Fig. 5. From the hysteresis analysis, both the pure BFO and the BHFO thin films showed highly irregular poor hysteresis behavior, whereas the BFTO and BHFTO thin films showed well-defined hysteresis loops. The 2Pr and 2Ec values for the BFTO and BHFTO thin films were 37 μC/cm2 and 808 kV/cm and 76 μC/cm2 and 952 kV/cm at a maximum applied electric field of 1023 and 1327 kV/cm, respectively. However, there are some conductive contribution in the ferroelectric hysteresis loops of the BFTO and BHFTO thin films. The additional conductive contributions associated with the ferroelectric hysteresis loops of the BFTO and BHFTO thin films have been cleaned by a standard ferroelectric hysteresis loop compensation.[38] The compensated hysteresis loops of the BFTO and BHFTO thin films are shown in Fig. 6. The compensated 2Pr and 2Ec values for the BFTO and BHFTO thin films were 33 μC/cm2 and 890 kV/cm at an applied electric field of 1023 kV/cm and 52 μC/cm2 and 886 kV/cm at an applied electric field of 1066 kV/cm, respectively.

image

Figure 5. Ferroelectric P–E hysteresis loops of the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films.

Download figure to PowerPoint

image

Figure 6. The hysteresis loops compensated by leakage contributions for the Bi(Fe0.98Ti0.02)O3+δ and the (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films.

Download figure to PowerPoint

Though the BHFO thin film exhibits low leakage current density, it showed poor ferroelectric properties. A similar result has already been observed for the Ho-doped BFO in bulk ceramics form.[39] The degradation of ferroelectric properties in the BHFO thin film might be attributed to poor microstructural features.[5, 6] Compared to the BFO and BHFO thin films, the BFTO thin film showed well-defined hysteresis loops with improved ferroelectric properties. The improvements to ferroelectricity in the BFTO thin film might be related to the decrease in leakage current density and the formation of a dense microstructure by Ti doping. Furthermore, the lowest unoccupied d-orbital in the Ti4+ ion (d0 electronic state) undergoes hybridization with the O 2p orbital. The resulting Ti 3d–O 2p hybridization has been reported to be essential for stabilizing ferroelectric distortion.[28] Among all, the BHFTO thin film showed the best ferroelectric properties with good stability against electrical breakdown. The improved properties in the BHFTO thin film are attributed to low leakage current density and distortion due to internal strain induced by co-doping elements. Hence, the key role of co-doping in improving the electrical and ferroelectric properties of the BFO thin film can be inferred clearly.

Frequency-dependent dielectric properties for the pure BFO, BHFO, BFTO, and BHFTO thin films were measured at room temperature by varying the applied frequencies from 4 kHz to 1 MHz as shown in Fig. 7. The measured dielectric constant (ε) values of the BFO, BHFO, BFTO, and BHFTO thin films were 58, 69, 78, and 83, respectively, at an applied frequency of 10 kHz. At the same applied frequency, the measured dielectric loss values for the pure BFO, BHFO, BFTO, and BHFTO thin films were 0.06, 0.04, 0.02, and 0.01, respectively. It is well known that there is a strong relationship between dielectric properties and microstructures. The relatively high dielectric constant and low dielectric loss of the BHFTO thin film might be related to the dense microstructure with significantly reduced intergranular pores.[5, 40] The high dielectric constant and low dielectric loss values of the co-doped thin film correlate with the large remnant polarization of the thin film.

image

Figure 7. Frequency-dependent dielectric properties of the BiFeO3, (Bi0.9Ho0.1)FeO3, Bi(Fe0.98Ti0.02)O3+δ, and (Bi0.9Ho0.1)(Fe0.98Ti0.02)O3+δ thin films.

Download figure to PowerPoint

IV. Conclusions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Pure BFO, BHFO, BFTO, and BHFTO thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by using a CSD method. The BHFTO thin film showed significant improvements in microstructural, electrical, and ferroelectric properties compared to Ho and Ti-doped BFO thin films. The BHFTO thin film exhibited well-defined ferroelectric hysteresis loops with large remnant polarization. The leakage current density of the BHFTO thin film was found to be 3 and 2 orders lower than those of the pure BFO and the BHFO thin films. Fast relaxation and stabilization of the leakage current of the BHFTO thin film are attributed to significant reduction and neutralization of charged free carriers by the combined effects of Ho and Ti ion doping into BFO.

Acknowledgments

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References

This work was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) (2010-0029634) and by Basic Science Research Program through the National Research Foundation of Korea (NRF) (2011-0004144) funded by the Ministry of Education, Science and Technology.

References

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussions
  6. IV. Conclusions
  7. Acknowledgments
  8. References
  • 1
    V. G. Bhide and M. S. Multani, “Mössbauer Effect in Ferroelectric-Antiferromagnetic BiFeO3,” Sol. Stat. Commun., 3 [9] 2714 (1965).
  • 2
    W. Eerenstein, N. D. Mathur, and J. F. Scott, “Multiferroic and Magneto-Electric Materials,” Nature, 442 [7104] 75965 (2006).
  • 3
    G. Catalan and J. F. Scott, “Physics and Applications of Bismuth Ferrite,” Adv. Mater., 21 [24] 246385 (2009).
  • 4
    T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M. P. Cruz, Y. H. Chu, C. Ederer, N. A. Spaldin, R. R. Das, D. M. Kim, S. H. Baek, C. B. Eom, and R. Ramesh, “Electrical Control of Antiferromagnetic Domains in Multiferroic BiFeO3 Films at Room Temperature,” Nat. Mater., 5 [10] 8239 (2006).
  • 5
    K. Y. Yun, M. Noda, M. Okuyama, H. Saeki, H. Tabata, and K. Saito, “Structural and Multiferroic Properties of BiFeO3 Thin Films at Room Temperature,” J. Appl. Phys., 96 [6] 3399403 (2004).
  • 6
    S. K. Singh, H. Ishiwara, and K. Maruyama, “Enhanced Polarization and Reduced Leakage Current in BiFeO3 Thin Films Fabricated by Chemical Solution Deposition,” J. Appl. Phys., 100 [6] 064102, 5pp (2006).
  • 7
    G. D. Hu, X. Cheng, W. B. Wu, and C. H. Yang, “Effects of Gd Substitution on Structure and Ferroelectric Properties of BiFeO3 Thin Films Prepared Using Metal Organic Decomposition,” Appl. Phys. Lett., 91 [23] 232909, 3pp (2007).
  • 8
    Y. Nakamura, S. Nakashima, and M. Okuyama, “BiFeO3 Thin Films Prepared by Chemical Solution Deposition with Approaches for Improvement of Ferroelectricity”; pp. 47996 in Ferroelectrics – Material Aspects, Edited by M. Lallart. InTech, INSA Lyon, Villeurbanne, 2011.
  • 9
    S. Habouti, R. K. Shiva, C.-H. Solterbeck, M. Es-Souni, and V. Zaporojtchenko, “La0.8Sr0.2MnO3 Buffer Layer Effects on Microstructure, Leakage Current, Polarization, and Magnetic Properties of BiFeO3 Thin Films,” J. Appl. Phys., 102 [4] 044113, 7pp (2007).
  • 10
    V. R. Palkar, J. John, and R. Pinto, “Observation of Saturated Polarization and Dielectric Anomaly in Magnetoelectric BiFeO3 Thin Films,” Appl. Phys. Lett., 80 [9] 162830 (2002).
  • 11
    S. K. Singh, H. Funakuba, H. Uchida, and H. Ishiwara, “Structural and Electrical Properties of BiFeO3 Thin Films,” Integr. Ferroelectr., 76 [1] 13946 (2005).
  • 12
    Y. Wang, Q. Jiang, H. He, and C.-W. Nan, “Multiferroic BiFeO3 Thin Films Prepared via a Simple Sol-Gel Method,” Appl. Phys. Lett., 88 [14] 142503, 3pp (2006).
  • 13
    M. D. Casper, M. D. Losego, and J. P. Maria, “Optimizing Phase and Microstructure of Chemical Solution-Deposited Bismuth Ferrite (BiFeO3) Thin Films to Reduce DC Leakage,” J. Mater. Sci., 48 [4] 157884 (2013).
  • 14
    J. K. Kim, S. S. Kim, W.-J. Kim, A. S. Bhalla, and R. Guo, “Enhanced Ferroelectric Properties of Cr-Doped BiFeO3 Thin Films Grown by Chemical Solution Deposition,” Appl. Phys. Lett., 88 [13] 132901, 3pp (2006).
  • 15
    S. K. Singh, K. Maruyama, and H. Ishiwara, “Reduced Leakage Current in La and Ni Co-Doped BiFeO3 Thin Films,” Appl. Phys. Lett., 91 [11] 112913, 3pp (2007).
  • 16
    N. M. Murari, R. Thomas, R. E. Melgarejo, S. P. Pavunny, and R. S. Katiyar, “Structural, Electrical, and Magnetic Properties of Chemical Solution Deposited BiFe1−xTixO3 and BiFe0.9Ti0.05Co0.05O3 Thin Films,” J. Appl. Phys., 106 [1] 014103, 5pp (2009).
  • 17
    T. Kawae, Y. Terauchi, H. Tsuda, M. Kumeda, and A. Morimoto, “Improved Leakage and Ferroelectric Properties of Mn and Ti Co-Doped BiFeO3 Thin Films,” Appl. Phys. Lett., 94 [11] 112904, 3pp (2009).
  • 18
    H. Ishiwara, “Impurity Substitution Effects in BiFeO3 Thin Films-From a Viewpoint of FeRAM Applications,” Curr. Appl. Phys., 12 [3] 60311 (2012).
  • 19
    C. M. Raghavan, J. W. Kim, H. J. Kim, W. J. Kim, and S. S. Kim, “Preparation and Properties of Rare Earth (Eu, Tb, Ho) and Transition Metal (Co) Co-Doped BiFeO3 Thin Films,” J. Sol-Gel. Sci. Technol., 64 [1] 17883 (2012).
  • 20
    F. Yan, M. O. Lai, L. Lu, and T. J. Zhu, “Enhanced Multiferroic Properties and Valence Effect of Ru-Doped BiFeO3 Thin Films,” J. Phys. Chem. C, 114 [15] 69948 (2010).
  • 21
    A. Lahmar, S. Habouti, M. Dietze, C.-H. Solterbeck, and M. Es-Souni, “Effects of Rare Earth Manganites on Structural, Ferroelectric, and Magnetic Properties of BiFeO3 Thin films,” Appl. Phys. Lett., 94 [1] 012903, 3pp (2009).
  • 22
    A. Z. Simões, R. F. Pianno, E. C. Aguiar, E. Longo, and J. A. Varela, “Effect of Niobium Dopant on Fatigue Characteristics of BiFeO3 Thin Films Grown on Pt Electrodes,” J. Alloys Comp., 479 [1–2] 2749 (2009).
  • 23
    S. Yasui, H. Uchida, H. Nakai, K. Nishida, H. Funakubo, and S. Koda, “Analysis for Crystal Structure of Bi(Fe, Sc)O3 Thin Films and Their Electrical Properties,” Appl. Phys. Lett., 91 [2] 022906, 3pp (2007).
  • 24
    D. H. Wang, W. C. Goh, M. Ning, and C. K. Ong, “Effect of Ba Doping on Magnetic, Ferroelectric, and Magnetoelectric Properties in Mutiferroic BiFeO3 at Room Temperature,” Appl. Phys. Lett., 88 [21] 212907, 3pp (2006).
  • 25
    Z. Hu, M. Li, Y. Yu, J. Liu, L. Pei, J. Wang, X. Liu, B. Yu, and X. Zhao, “Effects of Nd and High-Valence Mn Co-Doping on the Electrical and Magnetic Properties of Multiferroic BiFeO3 Ceramics,” Solid State Commun., 150 [23–24] 108891 (2010).
  • 26
    C.-H. Yang, D. Kan, I. Takeuchi, V. Nagarajan, and J. Seidel, “Doping BiFeO3: Approaches and Enhanced Functionality,” Phys. Chem. Chem. Phys., 14 [46] 1595362 (2012).
  • 27
    X. Qi, J. Dho, R. Tomov, M. G. Blamire, and J. L. M. Driscoll, “Greatly Reduced Leakage Current and Conduction Mechanism in Aliovalent-Ion-Doped BiFeO3,” Appl. Phys. Lett., 86 [6] 062903, 3pp (2005).
  • 28
    Y. Wang and C.-W. Nan, “Enhanced Ferroelectricity in Ti-Doped Multiferroic BiFeO3 Thin Films,” Appl. Phys. Lett., 89 [5] 052903, 3pp (2006).
  • 29
    X. Zhang, Y. Sui, X. Wang, Y. Wang, and Z. Wang, “Effect of Eu Substitution on the Crystal Structure and Multiferroic Properties of BiFeO3,” J. Alloys Comp., 507 [1] 15761 (2010).
  • 30
    G. L. Yuan, S. W. Or, H. L. W. Chan, and Z. G. Liu, “Reduced Ferroelectric Coercivity in Multiferroic Bi0.825Nd0.175FeO3 Thin Film,” J. Appl. Phys., 101 [2] 024106, 4pp (2007).
  • 31
    J. Liu, M. Li, L. Pei, B. Yu, D. Guo, and X. Zhao, “Effect of Ce Doping on the Microstructure and Electrical Properties of BiFeO3 Thin Films Prepared by Chemical Solution Deposition,” J. Phys. D Appl. Phys., 42 [11] 115409, 6pp (2009).
  • 32
    C. Wang, M. Takahashi, H. Fujino, X. Zhao, E. Kume, T. Horiuchi, and S. Sakai, “Leakage Current of Multiferroic (Bi0.6Tb0.3La0.1)FeO3 Thin Films Grown at Various Oxygen Pressures by Pulsed Laser Deposition and Annealing Effect,” J. Appl. Phys., 99 [5] 054104, 5pp (2006).
  • 33
    B. Nagaraj, S. Aggarwal, and R. Ramesh, “Influence of Contact Electrodes on Leakage Characteristics in Ferroelectric Thin Films,” J. Appl. Phys., 90 [1] 37582 (2001).
  • 34
    H. W. Jang, D. Ortiz, S.-H. Baek, C. M. Folkman, R. R. Das, P. Shafer, Y. Chen, C. T. Nelson, X. Pan, R. Ramesh, and C.-B. Eom, “Domain Engineering for Enhanced Ferroelectric Properties of Epitaxial (001) BiFeO3 Thin Films,” Adv. Mater., 21 [7] 81723 (2009).
  • 35
    A. Lahmar, K. Zhao, S. Habouti, M. Dietze, C.-H. Solterbeck, and M. Es-Souni, “Off-Stoichiometry Effects on BiFeO3 Thin films,” Solid State Ionics, 202 [1] 15 (2011).
  • 36
    G. L. Yuan and S. W. Or, “Multiferroicity in Polarized Single Phase Bi0.875Sm0.125FeO3 Ceramics,” J. Appl. Phys., 100 [2] 024109, 5pp (2006).
  • 37
    G. L. Yuan and S. W. Or, “Enhanced Piezoelectric and Pyroelectric Effects in Single-Phase Multiferroic Bi1−xNdxFeO3 (x = 0–0.15) Ceramics,” Appl. Phys. Lett., 88 [6] 062905, 3pp (2006).
  • 38
    C. Alemany, R. Jimenez, J. Revilla, J. Mendiola, and M. Calzada, “Pulsed Hysteresis Loops on Ferroelectric Thin Films,” J. Phys. D Appl. Phys., 32 [17] L7982 (1999).
  • 39
    N. Jeon, D. Rout, I. W. Kim, and S.-J. L. Kang, “Enhanced Multiferroic Properties of Single-Phase BiFeO3 Bulk Ceramics by Ho Doping,” Appl. Phys. Lett., 98 [7] 072901, 3pp (2011).
  • 40
    S. R. Das, P. Bhattacharya, R. N. P. Choudhary, and R. S. Katiyar, “Effect of La Substitution on Structural and Electrical Properties of BiFeO3 Thin Film,” J. Appl. Phys., 99 [6] 066107, 3pp (2006).