Improved Ferroelectric and Leakage Properties of Ce Doped in BiFeO 3 Thin Films

Ce dopedBiFeO 3 thin filmswith a perovskite structurewere prepared using solution-gelationmethod. It shows that the ferroelectric properties have been enhanced after dopingCe.The enhanced ferroelectric properties are attributed to the structural transformation and the reduced leakage current after doping rare metal of Ce. It has been found that the phase structures of the films transfer from rhombohedral symmetry structure to the coexistence of the tetragonal and orthorhombic symmetry structure. And Fe ions have been reduced, which leads to the decreased leakage for Ce doped BiFeO 3 thin films.The present work can provide an available way to improve the ferroelectric and leakage properties for multiferroic BiFeO 3 based thin films.


Introduction
Multiferroic materials exhibit ferroelectric, ferromagnetic, and ferroelasticity properties simultaneously in a certain temperature range.The single phase BiFeO 3 (BFO) materials with perovskite structure have aroused wide concerns due to its high curie temperature (T N ) of 1103 K and Neil temperature (T C ) of 643 K [1], which make it a most promising candidate in ferroelectric memory storage and magnetoelectric devices [2][3][4][5].However, for BiFeO 3 materials with a rhombohedrally distorted perovskite structure belonging to the space group R3c, it is difficult to gain a large saturation and remnant polarization due to the higher leakage current arising from defects such as impurity phases and oxygen vacancies [6,7].Several investigations have been carried out to prove that it is an effective approach for rare earth iron doping at Bi site to overcome the technical barrier and improve the ferroelectric and leakage properties of BiFeO 3 materials [8][9][10].Evidently, donor and acceptor dopants have contrary effects on modulating the charged defects.Thus, it becomes necessary to further understand how they affect the respective polarity and polarization stability of BiFeO 3 thin films.However, most of these studies were focused on ceramics and bulk materials [11,12], which are not fit for the rapidly developing micro-and nanoelectromechanical system (MEMS&NEMS).Therefore, in this work, Ce doped BiFeO 3 thin films were prepared using solution-gelation method as this technique can well control the stoichiometric ratio.Ce will replace Bi of A sites in perovskite structure of ABO 3 .The choice of the dopant ion was based on the fact that Ce 3+ has a more stable electronic configuration than Bi 3+ , which minimizes the leakage current and further improves ferroelectric properties in BiFeO 3 thin films.The origins of the improved ferroelectric and leakage properties are discussed in detail in this paper.The present work can provide an available way to improve the ferroelectric properties for single phase multiferroic BiFeO 3 based thin films.

Experiment
The pure and Ce doped BiFeO 3 thin films were prepared using the solution-gelation method.All the reactions were carried out at room temperature under ambient conditions.High-purity bismuth nitrate [Bi(NO 3 were obtained from commercial sources (Alfa Aesar); they were dissolved in solvent ethylene glycol monomethyl ether in proper proportions of 0.99 : 1 : 0.1 and stirred until the solutions turn into transparent for about three to four hours.[Bi(NO 3 ) 3 ⋅5H 2 O] should be added by excessive 10% for volatilizing Bi in process of dissolving and annealing.The process of preparing pure BiFeO 3 solution is similar to the above one, but [Bi(NO 3 ) 3 ⋅5H 2 O] and [Fe(NO 3 ) 3 ⋅9H 2 O] had the proportion of 1.1 : 1.Further, the prepared solutions were aged for several days.Then, the thin films were coated by spin coater.Here are conditions of rotation speed; low velocity is 400 r/min and high velocity is 3500 r/min.Next thin films were baked on the heating stage for five minutes at 280 ∘ C and annealed with rapid thermal processor system for five minutes at 550 ∘ C.This process was repeated twelve times and final annealing was for twenty minutes to obtain the BiFeO 3 thin films with the thickness of 300 nm.The orientation, crystal structure, and phase purity of the thin films were analyzed by X-ray diffraction (XRD, Panalytical-Empyrean) with Cu-K radiation ( = 0.15406 nm).The surface morphologies of the thin films were investigated by a scanning electron microscope (SEM, Hitachi S-4800).Raman spectroscopy measurements were performed at room temperature using a confocal Raman spectroscope (NT-MDT NTEGRA Spectra) with a 633 nm excitation laser with an initial power of 10 mW.For the fabrication of BiFeO 3 based multiferroic thin film capacitors, Au top electrodes with a diameter of 0.2 mm were deposited on the surface of the thin films using an ion sputtering method.The ferroelectric and leakage properties of the thin films capacitors were measured using a multiferroic tester system (MultiFerroic100V, Radient Technology, USA) at room temperature.The binding energies of atoms and orbits for the samples were characterized by Xray photoelectron spectroscopy (XPS, KRATOS-AMICUS).The cleaning treatments of thin films were carried out using Ar ion bombardment.And all binding energies were calibrated with respect to C 1s spectral line at 284.8 eV.

Results and Discussion
The typical SEM images of the pure and Ce doped BiFeO 3 thin films were shown in Figure 1.One can clearly observe that both thin films are assembled with uniformly distributed grains.Some small voids can be observed from the surface of the pure BiFeO 3 thin films in Figure 1(a), while for Ce doped BiFeO 3 thin films, it exhibits more dense morphology and less small holes than the pure BiFeO 3 thin films as shown in Figure 1(b).That is, the microstructure of thin films becomes more compact, which leads to the reduced voids for thin films.Thus result is attributed to the fact that Ce is more    stable than Bi element, which reduces oxygen vacancy and increases the combining of elements.This compact surface morphology is available to improve the properties such as ferroelectricity and the leakage current of thin films.Figure 2 presents the XRD patterns of the pure and Ce doped BiFeO 3 thin films.It can be seen that the thin films are polycrystalline perovskite structure without any prominent impurity peaks (e.g., Bi 2 Fe 4 O 9 , Bi 25 FeO 40 , etc.) observed.The absence of the diffraction peaks of Ce and its oxides implies that Ce is incorporated with the BiFeO 3 by the means of substitution for Bi. Figure 1(b) further gives an expanded view on the location of diffraction peaks in the range of 30-33 and 47-57 degree (2).It shows clearly that the pure BiFeO 3 thin film has a perovskite rhombohedral structure belonging to the space group R3c as the (104) and (110) diffraction peaks are almost completely separated, while the two diffraction peaks were greatly overlapped to form one peak with the Ce substitution doping at Bi site, which indicates that the rhombohedral distortion is reduced toward the coexistence of tetragonal and orthorhombic symmetry structure after doping Ce.This structural transformation is consistent with the other reports on rare earth doped BiFeO 3 ceramics [13,14].Besides, it can be seen that the position of all the diffraction peaks does not Figure 3 shows the Raman scattering spectra of the pure and Ce doped BiFeO 3 thin films.Table 1 reveals the Raman scattering modes of pure BiFeO 3 and Ce doped BiFeO 3 thin films including A 1 -1, A 1 -2, A 1 -3, A 1 -4, E-2, E-5, 5E-7, E-8, and E-9 modes, respectively.The observed scattering modes were in close agreement with the irreducible representation: Γ = 4A 1 + 9E [15,16].Combining Table 1 and Figure 3, we found that A 1 -1 A 1 -2, E-2, and A 1 -3 modes have got red shift and the scattering peak at 428.25 cm −1 has disappeared after doping Ce.The reason for this phenomenon can be stated as the following two aspects: one is that the change of the interaction between atoms due to the heavier atom of Bi replaced by the lighter atom of Ce causes the change of the phonon frequencies after doping Ce; another reason is structural phase transformation from rhombohedral symmetry structure to the coexistence of tetragonal and orthorhombic symmetry structure after doping Ce, which leads to the change in Bi-O covalent bonds as the bond controls the E-1, A 1 -1, A 1 -2, A 1 -3, and E-2 modes.
Figure 4(a) illustrates ferroelectric hysteresis and leakage current of the pure and Ce doped BiFeO 3 thin films, respectively.As shown in Figure 4(a), the ferroelectric properties change after doping Ce.For the pure BiFeO 3 thin films, the ferroelectric properties are poor and unsaturated ferroelectric hysteresis is observed, while the polarization has been enhanced for the film of Ce doped BiFeO 3 thin films, especially under large electric field.After doping Ce, the remanent and saturation polarization have been increased from 6.22 c/cm 2 to 25.10 c/cm 2 and from 27.07 c/cm 2 to 49.22 c/cm 2 , respectively.It is caused by the structural transformation from the R3c space group of rhombohedral structure of the pure BiFeO 3 thin films to the Pna2 1 space group of orthorhombic structure of Ce doped BiFeO 3 thin films.Moreover, the improvement of ferroelectricity is also closely related to the leakage current behavior as discussed below.
Figure 4(b) further shows the leakage current versus electric field () characteristics plots recorded with a voltage step width of 0.1 V of the thin films capacitors.It shows that the leakage currents of Ce doped BiFeO 3 thin films are decreased by one to two orders of magnitude in comparison with the pure BiFeO 3 thin films.For the pure BiFeO 3 thin films, a substantial number of oxygen vacancies and Fe 2+ ion are created to compensate the positive charge deficiency caused by the vaporization of Bi [17].They serve as donorlike trapping centers for electrons, which can be activated to be free for current conduction at the applied electric field, further leading to the higher leakage current in the pure BiFeO 3 thin films [18], while the leakage current of BiFeO 3 thin films was decreased after doping Ce, which is attributed to two factors.On one hand, the reduction of leakage current results from the structural transformation as discussed above.The leakage current of the pure BiFeO 3 thin films is larger than that of Ce doped BiFeO 3 thin films, which indicates the rhombohedral structure is closely related to the higher leakage current density.The coexistence structure of tetragonal and orthorhombic symmetry structure in BiFeO 3 thin films is an important season to decrease the leakage current.On the other hand, the creation of oxygen vacancies and the presence of Fe 2+ ion were suppressed as it is effective for Ce substitution doping to control the volatility of Bi atoms.Thus XPS investigations further confirm those points.
In order to intensify the oxidation of Fe, oxygen vacancies, and the elements for the pure and Ce doped BiFeO 3 thin films, XPS survey spectra are presented in Figure 5.It reveals the presence of Bi, Fe, O, and Ce without any other trace impurities except for a small amount of carbon for Ce doped BiFeO 3 thin films.This confirms the chemical compositions of the pure and Ce doped BiFeO 3 thin films.
In order to further intensify the oxidation of Fe, Figure 6(a) presents the representative Fe 2p XPS spectra of the pure and Ce doped BiFeO 3 thin films.Two main XPS peaks for Fe 2p 1/2 and Fe 2p 3/2 were observed in both the pure and the Ce doped BiFeO 3 thin films.Moreover, a satellite peak was also identified, which was considered to be characteristic of the oxidation state of Fe.Due to different d orbital electron configuration, during the relaxation of metal cations, Fe 2+ and Fe 3+ cations always show the satellite peaks with the gap of 6 eV and 8 eV above the Fe 2p 3/2 peaks, respectively [19].
We further analyze the peak of the Fe 2p3/2 by Lorentzian-Gaussian fitting, as shown in Figure 6(b).It suggests that Fe 3+ and Fe 2+ cations are coexistence in both the thin films and the binding energy changes after doping Ce.The Fe 2p3/2 peak position for the pure BiFeO 3 thin films is confirmed at 710.30 eV for Fe 2+ cations and 712.49eV for Fe 3+ ions, respectively, while the Fe 2p3/2 peak position for Ce doped BiFeO 3 films is confirmed at 709.79 eV for Fe 2+ ions and 711.28 eV for Fe 3+ cations, respectively.It is easy to conclude that the binding energy between Fe and O decreases after doping rare earth metal of Ce.The change in Bi-O covalent bonds is attributed to the structural phase transformation from rhombohedral symmetry structure to the coexistence of tetragonal and orthorhombic symmetry structure, which is accordant with the results of the Raman shift spectra discussed above.At the same time, the binding energy between Bi and O does not change after doping Ce, as shown in Figure 6(c).The peak position is confirmed at 164.02 eV for Bi 4f 5/2 and 158.68 eV for Bi 4f 7/2 .Figure 6(d) further presents the XPS peak of C for the pure and Ce doped BiFeO 3 thin films, which suggests that the peak shifts above would come from the change of binding energy rather than the measure errors.
As we know, electron hops between Fe 3+ cations and Fe 2+ ions are accordant with charged compensated oxygen vacancies.More Fe 2+ ions imply more oxygen vacancies, which would cause large leakage current and further decrease ferroelectric properties.Fortunately, the ratio of Fe 2+ to Fe 3+ cations changes from about 1 : 1 to 1 : 2 by calculating the area of peak.It obviously means that the concentration of the Fe 2+ is decreased after doping Ce.Therefore, the Ce doping was helpful to reduce the Fe 2+ ions concentration in BiFeO 3 based thin films, which decrease their leakage currents and further improve ferroelectric properties.

Conclusions
In summary, the pure and Ce doped BiFeO 3 thin films were prepared by solution-gelation process and their phase structures, ferroelectric properties, and leakage were systematically investigated.The enhanced ferroelectric properties have been obtained after doping Ce.Thus enhancement is attributed to the structural transformation from rhombohedral symmetry structure to coexistence of orthorhombic symmetry structure revealed by XRD measurements.At the same time, the leakage current decreases obviously, which originates from the change of the binding energy between Fe-O increases and the decrease of the concentration of the Fe 2+ after doping Ce.The present work provides an available way on enhancing ferroelectric properties and possible multifunctional applications for BiFeO 3 based thin films.

Table 1 :
Raman scattering modes of the pure and Ce doped BiFeO 3 thin films.