1 Introduction

ABO3-type perovskite structure BiFeO3 (BFO) has attracted much attention because it is a known Pb-free and environmentally friendly material that simultaneously shows electric and magnetic ordering in the same phase [1–3]. Based on the specific characteristics, including magnetoelectric coupling effects [1], high Curie temperature (T C ~ 1,103 K) and Néel temperature (T N ~ 643 K) [4], and small optical bandgap (E g ~ 2.5 eV) [5], BFO thin films have potential applications in magnetic/ferroelectric data storage media, spintronics, nonvolatile memories, photocatalytic compound and ultrafast optoelectronic devices [1, 3, 5, 6]. Though as promising as BiFeO3 films are, there are still drawbacks, such as large dielectric loss, small remnant polarization, high coercive field, high leakage current and inhomogeneous magnetic spin structure [4, 7, 8], to be overcome before practical applications. One way of doing so is by doping BFO films with other elements. In order to enhance ferroelectric properties, reduce leakage current, modify its spatially inhomogeneous spin-modulated incommensurate structure, and intensify magnetoelectric interaction, several research groups have tried A-site, B-site or (A, B)-sites of BFO films substituting by La [2, 7], Tb [9], Gd [10], Sc [4], Ti [11], Cr [12], La and Ti [13], La and Mn [14] as well as La and Nb [15]. More recently, Shannigrahi et al. reported that impure phase appeared in 0.5 mol% Sc-doped BFO film, and leakage current, ferroelectric and magnetic properties were improved for 0.3 mol% Sc-doped BFO [4]. Wang et al. investigated that Ti4+ ions were used to substitute some of Fe cations to form \( {\text{Ti}}_{\text{Fe}}^{ \bullet \bullet } \) as substituting Fe2+ or \( {\text{Ti}}_{\text{Fe}}^{ \bullet } \) as substituting Fe3+, which could eliminate oxygen vacancies in the Ti-doped BFO films because of the requirements of charge compensation [11]. Cheng et al. found that the ferroelectric properties were significantly improved, and the ferromagnetic moments were kept at a high level for the (La, Nb)-codoped BFO films [15]. Being a lanthanide element, however, Ce has remained relatively unexplored in doping BFO films.

In this study, crystal structures, surface chemical compositions and bonding states of pure and Ce-doped BFO films were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Chemical shifts of Bi 4f7/2, Bi 4f5/2, Fe 2p3/2, Fe 2p1/2, O 1s, Ce 3d5/2 and Ce 3d3/2 peaks for pure BFO and Bi0.8Ce0.2FeO3 films were analyzed by fitting the corresponding XPS narrow-scan spectra. Dielectric and ferroelectric properties of the pure and Ce-doped BFO film capacitors were also discussed.

2 Experimental details

Bismuth nitrate [Bi(NO3)3 · 5H2O], iron nitrate [Fe(NO3)3 · 9H2O] and cerium nitrate [Ce(NO3)3 · 6H2O] were suitably dissolved in the mixtures of 2-methoxyethanol (CH3OCH2CH2OH) and glacial acetic acid (CH3COOH), while Ce contents were determined by composition formulae Bi1−x Ce x FeO3 (abbreviated as BCFO), where x is equal to 0 (for pure BFO), 0.05, 0.10, 0.15 and 0.20, respectively. Meanwhile, acetylacetone (CH3COCH2COCH3) and N-N-dimethyl formamide [(CH3)2NOCH] in 1 to 1 molar ratio with respect to Fe(NO3)3 · 9H2O were used to stabilize BCFO solutions. The stock solutions were stirred at 318 K for 240 min to obtain homogeneous BCFO precursors. BCFO sols were spin-coated onto Pt (111)/TiN/Si3N4/Si (100) substrates at 3,000 rpm for 20 s. After each spin-coating, the green BCFO films were dried at 393 K for 10 min, and then prebaked at 693 K for 30 min in the air to remove organic contaminations. This procedure was repeated several times to obtain appropriate thickness. Finally, BCFO films were annealed in a quartz tube furnace at 873 K for 60 min under a N2 ambience, and then cooled down slowly to room temperature.

Crystal structures of BCFO films were analyzed by XRD (D8 Advance, Bruker Corp.) with Cu K α radiation. Thicknesses of BCFO films were measured by scanning electron microscope (SEM) (Sirion FEG, FEI Co.). Surface chemical compositions and binding states of the corresponding elements for BFO and Bi0.8Ce0.2FeO3 films were investigated by XPS (VG MultiLab 2000, Thermo Electron Corp.). The XPS Al K α source operating at 300 W provided nonmonochromatic X-rays at 1,486.6 eV. Photoelectric peak of C 1s located at 284.6 eV was assigned to carbon from adventitious contaminations, and it was used as the criterion to rectify binding energies of XPS spectra. Software of XPSPEAK Version 4.1 was used to fit the narrow-scan spectra of Bi 4f, Fe 2p, O 1s and Ce 3d after Shirley-type background subtraction [16, 17]. For comparison, 120-nm-thick Pt top electrodes with diameter of 0.3 mm were sputtered on BCFO films through a metal shadow mask. To understand Ce-doped influence on dielectric and ferroelectric properties of BFO film, capacitance-frequency and dielectric loss-frequency curves for the BCFO (x = 0, 0.1 and 0.2) film capacitors were measured by a precision impedance analyzer (Agilent 4294A, Agilent Technologies Ltd.). Polarization–electric field (P–E) hysteresis loops and leakage current density–electric field (J–E) curves for the same BCFO capacitors were measured by a precision materials analyzer (RT6000, Radiant Technology Inc.). All measurements were carried out at room temperature.

3 Results and discussion

Figure 1 shows XRD patterns of BCFO films annealed in 873 K for 60 min under a N2 ambience. It is found that all the BCFO films are polycrystalline and exhibit a single-phase perovskite structure. Non-perovskite phase such as Bi2Fe4O9, Bi36Fe2O57 and Bi12(Bi0.5Fe0.5)O19.5 [6, 18] are not detected in the XRD patterns. The obvious peak-splitting XRD pattern for the BCFO film with x = 0 indicate that the BFO film is rhombohedral phase (space group R3c) [10]. As shown in the broken square of Fig. 1, (104) and (110) peaks of BCFO films move closer together along with broadening, indicating a structural change [19, 20] with increasing Ce content. Since ionic radii of Ce3+ (1.18 Å) and Ce4+ (1.02 Å) are smaller than that of Bi3+ (1.20 Å) [21], Ce cations are partially substituted for Bi3+ with increasing Ce dopant, resulting in smaller grain sizes of the BCFO films compared to those of the BFO counterparts.

Fig. 1
figure 1

XRD patterns of Bi1−x Ce x FeO3 (x = 0, 0.05, 0.1, 0.15 and 0.2) films deposited on Pt/TiN/Si3N4/Si substrates. As shown in the broken square of Fig. 1, (104) and (110) peaks merge when x increases from 0 to 0.2

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Figure 2 shows XPS survey spectra of BFO and Bi0.8Ce0.2TiO3 films. There are XPS photoelectron peaks and the corresponding Auger lines of Bi, Fe, O and C elements on the BFO film surface. Herein, C 1s peak located at 284.6 eV is used as the criterion to rectify the binding energy of XPS spectra. Characteristic peaks for Bi 4f7/2 (158.56 eV), Bi 4f5/2 (163.90 eV), Fe 2p3/2 (710.33 eV), Fe 2p1/2 (723.69 eV), O 1s (529.52 eV) and Auger peaks for Fe L3M45M45 (783.86 eV), Fe L3M23M45 (838.24 eV) and Fe LM23M23 (888.29 eV) [17] are identified in Fig. 2. Undoubtedly, there is no signal of Ce element in the pure BFO film. As for Bi0.8Ce0.2FeO3 film, there are XPS signals of Bi, Ce, Fe, O and C elements, where Ce 4d5/2 (108.71 eV), Ce 4d3/2 (112.43 eV), Ce 3d5/2 (882.32 eV) and Ce 3d3/2 (900.64 eV) and Auger peak for Ce M45N45N45 (832.87 eV) [17] are assigned in Fig. 2. It is concluded that some Ce–O bonds are present in Bi0.8Ce0.2FeO3 matrix.

Fig. 2
figure 2

XPS survey spectra of BFO and Bi0.8Ce0.2FeO3 films annealed at 873 K for 60 min under a N2 ambience

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Figure 3 shows the fitted narrow-scan spectra of Bi 4f, Fe 2p, O 1s and Ce 3d peaks for the BFO and Bi0.8Ce0.2FeO3 films. Figure 3a(1) shows that pure Bi 4f doublet consists of two peaks at 158.56 and 163.90 eV, which are mainly identified as a signal from Bi–O bonds [17]. Spin-orbit splitting energy (Δ) of the pure Bi 4f doublet is 5.34 eV, which is comparable to theoretical value (ΔBi 4f) of 5.31 eV [16]. Two fitted subpeaks located at 158.28 and 163.61 eV are ascribed to Bi (4f7/2)–O and Bi (4f5/2)–O bonds, while other subpeaks located at 158.84 and 164.18 eV may be related to Bi–O–Fe bonds in oxygen octahedron and/or relaxed Bi phase, probably caused by oxygen vacancies and cation defects [17, 22]. Figure 3a(2) shows that compared to the pure Bi 4f doublet, Bi 4f7/2 (158.89 eV) and Bi 4f5/2 (164.19 eV) peaks of the Bi0.8Ce0.2FeO3 film shift towards higher binding energy regions by amounts of 0.33 and 0.29 eV, respectively. Herein, two subpeaks located at 158.56 and 163.86 eV are ascribed to Bi (4f7/2)–O and Bi (4f5/2)–O bonds, while other subpeaks located at 159.22 and 164.52 eV may be related to Bi–O–Fe and Bi–O–Ce bonds in oxygen octahedron and/or relaxed Bi phase. It is conceivable that Ce cations are substituted for Bi3+ at A-site in the Bi0.8Ce0.2FeO3 matrix because of the decreased Bi 4f peak area in comparison with the pure counterpart. As reported by Gao et al., the N-doped Ba 3d5/2 and Ba 3d3/2 peaks shift towards the lower binding energy regions because the bonding energies of Ba–N bonds are lower than that of Ba–O bonds considering the slightly lower electronegativity of N element (3.04) than that of O element (3.44) [21]. So the chemical shifts of Bi 4f, Fe 2p and O 1 s peaks for the Bi0.8Ce0.2FeO3 film may be related to the various electronegativity values for Bi (2.02), Ce (1.12), Fe (1.83) and O (3.44) elements [16, 21, 23]. To estimate ionicity of Bi–O, Ce–O and Fe–O bonds for the Bi0.8Ce0.2FeO3 film, fraction of ionicity (F i) [24] is given by:

$$ F_{i} = 1 - \exp \left[ { - \frac{{\left( {\Updelta EN} \right)^{2} }}{4}} \right] $$
(1)

where ΔEN is difference in the anion and cation electronegativities. According to Eq. (1), the fraction of ionicity [F i(Ce–O)] (0.74) is much larger than F i(Bi–O) (0.40), and bonding energy of (Bi,Ce)–O bond in oxygen octahedron may be larger than that of single Bi–O bond, suggesting that the Bi 4f doublet of Bi0.8Ce0.2FeO3 film shifts to the higher binding energy region.

Fig. 3
figure 3

The fitted XPS narrow-scan spectra of (a) Bi 4f, (b) Fe 2p, (c) O 1 s and (d) Ce 3d peaks for (1) BFO and (2) Bi0.8Ce0.2FeO3 films. The black solid lines represent experimental result, red short dots represent the fitting results after Shirley-type background subtraction, the dashed peaks with colors of blue, magenta, violet and green are fitted subpeaks. Iterations are performed until a minimum for square of the difference (χ2) between the experimental curve and the fitted curve is reached for each fitted sum curve

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Figure 3b shows the fitted Fe 2p narrow-scan spectra for the BFO and Bi0.8Ce0.2FeO3 films. In Fig. 3b(1), the pure Fe 2p doublet consists of two wide peaks of Fe 2p3/2 (710.33 eV) and Fe 2p1/2 (723.69 eV), which are mainly ascribed to Fe–O bonds [17]. Spin-orbit splitting energy of the pure Fe 2p doublet is equal to 13.36 eV, which is compared to theoretical value (ΔFe 2p) of 13.6 eV for Fe2O3 [17]. Figure 3b(2) shows that compared to the pure Fe 2p doublet, Fe 2p3/2 (710.76 eV) and Fe 2p1/2 (724.27 eV) peaks of the Bi0.8Ce0.2FeO3 film shift towards higher binding energy regions by amounts of 0.43 and 0.58 eV, respectively. Herein, two subpeaks located at 711.51 and 725.15 eV are ascribed to (Fe 2p3/2)2–O3 and (Fe 2p1/2)2–O3 bonds, while the subpeaks located at 710.00 and 723.38 eV may be related to Fe–O bonds for Fe2+, Fe–O–Bi bonds, Fe–O–Ce bonds in oxygen octahedra and/or other relaxed Fe phase [16, 25]. It is the reason why the Fe 2p3/2 and Fe 2p1/2 peaks of the Bi0.8Ce0.2FeO3 film shift towards the higher binding energy regions in comparison with the Fe 2p doublet of the BFO film. According to ratio of the fitted peak areas for Fe3+ and Fe2+, concentration ratios of Fe3+ and Fe2+ in the BFO and Bi0.8Ce0.2FeO3 films are 67:33 and 70:30, respectively, indicating that the presence of Fe2+ ions are less in the Bi0.8Ce0.2FeO3 film compared to that of the BFO film. This could be an evidence for the decreased oxygen vacancies in the Bi0.8Ce0.2FeO3 film.

Figure 3c shows the fitted O 1s narrow-scan spectra for the BFO and Bi0.8Ce0.2FeO3 films. Figure 3c(1) shows that a broad O 1s peak (529.52 eV) for the BFO film consists of three subpeaks located at 528.77, 529.34 and 530.45 eV. Since the BFO film consists of three components (Bi2O3, Fe2O3 and FeO) in BFO solid solution, the subpeaks are mainly ascribed to Fe2–(O 1s)3 (529.6 eV), Fe–(O 1s) (529.8 eV), Bi2–(O 1s)3 (530.0 eV) bonds [17] and relaxed O phase, especially for the fitted subpeak located at the higher binding energy region. It may be mainly attributed to the absorbed oxygen (531.0 eV) [17] associated with oxygen vacancies and surface species, such as H2O and CO2 absorbed from the air during the sol–gel process [16, 17]. Figure 3c(2) shows that compared to the counterpart of the BFO film, O 1s peak (530.01 eV) for the Bi0.8Ce0.2FeO3 film shifts towards higher binding energy region by amount of 0.49 eV. These subpeaks are mainly ascribed to Fe2–(O 1s)3 (529.6 eV for the O 1s peak), Fe–(O 1s) (529.8 eV), Bi2–(O 1s)3 (530.0 eV) and Ce2–(O 1s)3 (530.3 eV) bonds [17]. Considering that Ce3+ ions are partially substituted for Bi3+ ions, and binding energy of O 1s in Ce2–(O 1s)3 bond is larger than that of Bi2–(O 1s)3 bond, suggesting that the O 1s peak of the Bi0.8Ce0.2FeO3 film shifts toward the higher binding energy regions in comparison with the BFO counterpart.

Figure 3d shows the fitted Ce 3d5/2 peak located at 882.24 eV and Ce 3d3/2 peak located at 900.61 eV. Both of them are mainly assigned to Ce–O bonds. Spin-orbit splitting energy of the Ce 3d doublet is equal to 18.37 eV, which is compared to theoretical value (ΔCe 3d) of 18.10 eV for Ce or 18.30 eV for CeO2 [17]. The fitted area ratio for Ce 3d5/2 and Ce 3d3/2 peaks is about 1.47, which is comparative to theoretical value of 1.50 for Ce 3d doublet [17]. Figure 3d also shows that each of the Ce 3d peak is accompanied by two satellite lines, called 3d94f0 and 3d94f2 [17, 26]. Furthermore, each of the Ce 3d5/2 and Ce 3d3/2 peak could be fitted by two subpeaks separated by 1.33 ± 0.01 eV. The subpeaks located at 882.91 and 901.27 eV are assigned to Ce 3d5/2–O and Ce 3d3/2–O bonds for Ce3+. Meanwhile, the subpeaks located at 881.57 and 899.95 eV may be associated with Ce 3d5/2–O and Ce 3d3/2–O bonds for Ce4+ [17], (Bi, Ce)–O bonds and/or relaxed Ce phase, probably caused by oxygen vacancies and other defects.

Figure 4 shows typical dielectric properties of the BCFO (x = 0, 0.1 and 0.2) film capacitors at the measured frequency ranging from 40 Hz to 1 MHz. The corresponding dielectric constant (ε r) and loss tangent (tanδ) for the BFO, Bi0.9Ce0.1FeO3 and Bi0.8Ce0.2FeO3 capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. It means that compared to the BFO counterparts, both ε r and tanδ of the Bi0.8Ce0.2FeO3 capacitor decrease when 20 mol% Ce doped into the BFO matrix. Moreover, this result could be comparable to the values of PLD-derived Bi0.8La0.2FeO3 (ε r = 86, tanδ = 0.69) and Bi0.8La0.2Nb0.01Fe0.99O3 (ε r = 184, tanδ = 0.038) films [15] deposited on Pt/Ti/SiO2/Si substrates at 100 kHz as reported by Cheng et al., respectively.

Fig. 4
figure 4

Dielectric constants and loss tangents of Bi1−x Ce x FeO3 (x = 0, 0.1 and 0.2) film capacitors as a function of frequency ranging from 40 Hz to 1 MHz

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Figure 5 shows P–E hysteresis loops for the BFO, Bi0.9Ce0.1FeO3 and Bi0.8Ce0.2FeO3 film capacitors. These capacitors have typical ferroelectric characteristics, and the corresponding remnant polarization [P r = (P r + − P −r )/2] of 1.08, 1.43 and 2.04 μC/cm2 and coercive field [E c = (E c + − E −c )/2] of 120, 112 and 123 kV/cm, respectively, are obtained under the applied electric field of 388 kV/cm. P r value of the Bi0.8Ce0.2FeO3 capacitor is larger than that of the BFO film, or those of the BiFe1−x Sc x O3 (x = 0.1 and 0.3 mol%) films [4] or the Ti4+ and Ni2+ doped BFO films prepared by PLD process [27], but it is much small than that of the Gd-doped BFO film prepared by metal organic decomposition process [10]. According to Wang et al., ferroelectricity of BFO film originated from the displacements of Bi with respect to the FeO6 cages along (111) plane [22]. Compared to the BFO counterpart, the increased P r of the Bi0.8Ce0.2FeO3 film might be attributed to the structural change in the oxygen octahedron, since Ce cations are feasibly substituted for Bi3+ as evidenced by the XPS spectra.

Fig. 5
figure 5

Polarization-electric field hysteresis loops of Bi1−x Ce x FeO3 (x = 0, 0.1 and 0.2) capacitors

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Figure 6 shows J–E curves of the BCFO (x = 0, 0.1 and 0.2) capacitors. Leakage current densities of BFO, Bi0.9Ce0.1FeO3 and Bi0.8Ce0.2FeO3 capacitors are 1.38 × 10−2, 1.06 × 10−3 and 1.47 × 10−4 A/cm2 at 388 kV/cm, respectively. It means that the leakage current density of the Bi0.8Ce0.2FeO3 capacitor is about two orders of magnitude lower than that of the BFO counterpart. One reason for this is that an appropriate amount of Ce doping reduces grain sizes of the BCFO film as evidenced by XRD result, leading to an increase in the density of grain boundaries, which makes contribution to the decreased leakage current density [9]. Another reason is that the decreased Bi volatilization and the restrained reduction of Fe3+ to Fe2+ are realized by virtue of the reducible oxygen vacancies and cation defects after Ce-doping, as proofed by XPS data.

Fig. 6
figure 6

Leakage current density-electric field curves of Bi1−x Ce x FeO3 (x = 0, 0.1 and 0.2) capacitors when electric filed ranges from −388 to 388 kV/cm

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4 Conclusions

BCFO films were deposited on Pt/TiN/Si3N4/Si substrates by sol–gel technique. Compared to the BFO counterparts, the fitted Bi 4f7/2, Bi 4f5/2, Fe 2p3/2, Fe 2p1/2 and O 1s peaks for the Bi0.8Ce0.2FeO3 film shift towards the higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. The corresponding ε r and tanδ of the BFO, Bi0.9Ce0.1FeO3 and Bi0.8Ce0.2FeO3 capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. The corresponding P r of 1.08, 1.43 and 2.04 μC/cm2 and E c of 120, 112 and 123 kV/cm are obtained under the electric field of 388 kV/cm, respectively. The leakage current density of the Bi0.8Ce0.2FeO3 capacitor is about two orders of magnitude lower than that of the BFO counterpart. Compared to the BFO counterparts, the increased P r and the decreased leakage current density of the Bi0.8Ce0.2FeO3 film might be attributed to the decreased Bi volatilization and the restrained reduction of Fe3+ to Fe2+ by virtue of the reducible oxygen vacancies and cation defects after Ce-doping, as proved by the XPS spectra.