Improved ferroelectric properties and band-gap tuning in BiFeO3 films via substitution of Mn

Multiferroic BiFe1−xMnxO3 (x = 0, 0.04, 0.08, 0.12) films have been prepared on Pt/Ti/SiO2/Si and ITO/glass substrates via the solution-gelation technique. The impacts of Mn doping of BFO thin films on the structure, morphology, leakage current, ferroelectric properties and optical band gap have been systematic investigated. From the XRD patterns, all samples match well with the perovskite structure without an impurity phase and the thin films exhibit dense and smooth microstructure. A leakage current density of 1.10 × 10−6 A cm−2 which is about four orders of magnitude lower than that of pure BiFeO3 was observed for the 8% Mn doped BFO thin film at an external electric field <150 kV cm−1. An increase in the remnant polarization with Mn substitution was observed, with a maximum value of ∼19 μC cm−2 for the 8% Mn-substituted film. Moreover, optical absorption spectra indicate that the doping of Mn has an effect on the energy band structure. Compared with pure BiFeO3, Mn doped thin films present an intense red shift as shown in the UV-visible diffuse absorption together with the decreased direct and indirect optical band gaps. In addition, this work gives insight into the relationship between ferroelectric remnant polarization and band-gap and finds that the optical band gap decreases with the increase of residual polarization.


Introduction
Multiferroic materials, which possess simultaneous ferroelectric, ferromagnetic, and ferroelastic ordering, have attracted much attention. Among these materials, BiFeO 3 (BFO) is the only single phase multiferroic material with both a high ferroelectric Curie temperature (T C $ 1103 K) and a high antiferromagnetic Néel temperature (T N $ 643 K). [1][2][3] It is also a known Pb-free and environmentally friendly material. [4][5][6] Additionally, BFO thin lms exhibit remarkable ferroelectric photovoltaic effects. 7 Compared to most classical ferroelectric materials, BFO has a small band gap with reported values in the range of 2.6-3.0 eV, 8 and a very large remnant ferrroelectric polarization, which can offer exciting opportunities for use in both optoelectronics and solar energy devices towards abundant renewable clean energy harvesting. 9 However, the application of BFO is seriously hindered due to its high leakage current. 4 Some reports also showed weak polarization in BFO thin lms, which is attributed to the presence of impurity phases and oxygen vacancies. 10 To overcome these problems signicant efforts have been made, the substitution technology at the A or B position of perovskite crystals is the most widely used. [11][12][13] Meanwhile, further improve the photovoltaic characteristics, it is necessary to narrow band-gap of the ferroelectric materials with larger polarization and smaller leakage. Therefore, in this work, thin lms BiFe 1Àx Mn x O 3 (BFMO, x ¼ 0, 0.04, 0.08, 0.12) were prepared on Pt/Ti/SiO 2 /Si and ITO/glass substrates by the solution-gelation technique. The microstructure, surface morphologies, ferroelectric and photovoltaic band-gap of BFO and Mn-doped BFO were discussed in detail.

Experiment
The BFMO thin lms were prepared on Pt/Ti/SiO 2  Acetic anhydride was also added as dehydrating agent. 10 mol% of excess Bi was added to compensate for bismuth loss during the heat treatment. Then the solution was stirred at the right temperature for several hours to form a homogeneous sol. The resultant solutions with concentration of 0.3 mol L À1 were deposited on substrates by spin-coating with the velocity 4000 rpm for 30 s. The as-deposited wet thin lms were preannealed at 450 C for 180 s and crystallized at 600 C for 300 s in a rapid thermal process furnace. The spin-coating and thermal treatments were repeated several times to obtain the desired lm thickness.
The crystalline structures of the lms were analyzed by Xray diffraction (XRD, D/max-2500 V Rigaku) with Cu-Ka radiation. The surface morphologies were investigated by an atomic force microscope (AFM, Bruker MultiMode 8). The ferroelectric and leakage current properties were measured using a multiferroic tester system (MultiFerroic 200 V, Radiant Technologies). The optical absorption spectrum was measured by an ultraviolet-visible spectrophotometer (Hitachi U-4100).

Results and discussion
3.1. XRD analysis Fig. 1 shows the X-ray diffraction patterns (XRD) of Mn substituted BFO thin lms grown on the Pt/Ti/SiO 2 /Si substrates. All diffraction peaks of the thin lms exhibit a rhombohedral perovskite structure with (111) orientation, which is good agreement with the ICDD (PDF # 72-2112  15)). It indicates that all these thin lms are single-phase BiFeO 3 materials in the same structure and the doping does not affect the lattice symmetry. In fact, their extranuclear structure of the outermost two layers exhibits high similarity. 16 The (111) and ( 111) peaks overlap completely and the intensity remains roughly the same, while the (120) and ( 120) peaks also overlap completely and the intensity is decreasing gradually. Since that the dopants with different ionic radius could cause the distortion of crystal lattice to various degrees and even change their syngony. 17 Futhermore, all lms show the (111) preferred orientation for the (111)-Pt/Ti/SiO 2 /Si substrate.
The average crystallite size of the samples were calculated by using Scherer's formula: 18 where K is the shape factor (0.89), l is the wavelength of Cu (Ka), b is the full width half maxima (in radians), q is the angle of diffraction, and the data is shown in Table 2. It was observed that crystallite size increase with Mn-doping from x ¼ 0 to 0.08 in BiFe 1Àx Mn x O 3 .   impair the properties of the lms severely. By contrast, the BFMO thin lms exhibit very dense, pore free and uniform microstructure. Thus the substitution of Mn is found to be benecial to enhance the microstructure of the lms.  Fig. 2(e)-(g), respectively. The ferroelectric domain structures of the BFO and 12% Mn-BFO exhibit a fractal growth habit, a domain size similar to the grain size and upward or downward polarization within each domain ( Fig. 2(e)). The domain structures of the 8% Mn-BFO show more homogeneous domains and the domains are pinned at the ground boundaries ( Fig. 2(f)). The density of the domain walls in the 8% Mn-BFO is less than in the BFO and 12% Mn-BFO. Therefore, it is expected that there is a lower leakage of current and large residual polarization in the 8% Mn-BFO ( Fig. 4 and 5) since certain domain walls in the BFO and 12% Mn-BFO are much more conductive than the domains themselves. 22 In addition, there exist three types of domain walls, namely those that separate domains 71 , 109 and 180 different in polarization. 23 Domain patterns can develop with either a (100)-type plane for 109 walls or a (101)-type plane for 71 walls, respectively. 24 As shown from the XRD spectra, the polycrystalline lms should be a mixture of all three types of domain wall. 25 The domain size in the BiFe 0.92 Mn 0.08 O 3 lm is larger than that in the BiFeO 3 lm, which could originate from the tensile strain enhanced by the Mn doping, the bigger grain size or even the mist dislocation between the lm and the substrate. 26 The domain structure and the density of the domain walls of the BFMO also suggests that it is easier to polarize than the BFO. Fig. 3(a) presents the typical leakage current behavior (log J-E curves) of BFMO lms. The measured leakage current density of BFMO (x ¼ 0, 0.04, 0.08, 0.12) thin lms is 9.04 Â 10 À2 A cm À2 , 1.02 Â 10 À4 A cm À2 , 1.14 Â 10 À7 A cm À2 and 1.10 Â 10 À6 A cm À2 at an applied electric eld of 146.9 kV cm À1 , repectively. The doped BFMO (x ¼ 0.04, 0.08) thin lms exhibit lower

Leakage current analysis
It can be seen from the formula that Fe 3+ becomes more stable aer Mn doping and limit the movement of oxygen vacancies. 29 (ii) The increase of (Mn Fe 3+ 3+ )* can suppress Fe 3+ turning into Fe 2+ by doping of trivalent ions (Mn 3+ ) in BFO lms. 30 More signicantly, the leakage current density does not present a monotonous decreasing trend with the increase of Mn doping content. As Mn doping content increased from 8% to 12%, the leakage current density began to become larger for 1.10 Â 10 À6 A cm À2 . This is because the increase in Mn content causes a gradual transformation of BFMO thin lms from an insulator to semiconductor and also increase its density of free electrons in same fashion. 31 Noticing that, the BiFe 0.92 Mn 0.08 O 3 thin lm showed the lowest current density in the low electric eld region (E < 150 kV cm À1 ) as shown in the Fig. 3(d). Based on the AFM results, a at and dense surface should be a source of lower leakage current in low electric eld. From internal factors, a at and a dense surface indicate that the grain size is more uniform, so the defect can be reduced. From extrinsic factors, the at thin lms surface could result to a better Pt/BFMO interface leading to the decrement of the leakage current. 29 We also found that in the low electric eld area, the leakage current of Mn-doped BFO lms are lower than the undoped one.
To further study the source of leakage current, we analyzed the leakage mechanism of BFMO lms. Fig. 3(b) shows the J-E plots of BFMO thin lms in logarithmic scales during the positive process. The plots reveals a near linearity in the range of applied electric elds and they obey the power law of J f E a . We can judge the leakage mechanism of the samples by the slope of a. 32,33 Ohmic conduction and space charge limited conduction (SCLC) are the two most common leakage mechanism with the tting slope of 1 and 2, respectively. 34 The slope under the low electric eld is 1.22, 0.98, 1.03, 1.19 for the samples with x ¼ 0%, 4%, 8%, 12% respectively. Which indicates that the ohmic conduction is dominant conduction mechanism in these samples. Similar analyses were conducted to determine the dominant leakage mechanism in the samples during the negative. The tting results for BFMO (x ¼ 0, 0.04, 0.08, 0.12) thin lms are show in Fig. 3(c), in which a linear tting with slope z 1 would suggest the ohmic mechanism during the negative. It is shown that the free carriers in BFMO  Paper lms play an important role in this stage, and there is no space charge effect. 35 Therefore, at low electric eld, the decrease of leakage current is not due to the decrease of BFO defect number, but mainly due to the inhibition the transformation of Fe 3+ into Fe 2+ by Mn doping, and the interface effect plays a major role. Fig. 4(a) shows the hysteresis loops at different applied voltages for same frequency. It can be seen that the hysteresis loop is strongly dependent on the applied voltage. For the applied voltage lower than 6 V ($300 kV cm À1 ), although a hysteresis loop could be observed, the polarization was not saturated around the maximum of the applied voltage, indicating that polarization switching is incomplete. As the applied voltage increased, the reversal proceeded more completely. At 6 V ($300 kV cm À1 ), the relatively saturated loop was observed. Fig. 4(b) shows the P-E hysteresis curves of Mn-doped BFO lms with different Mn concentrations at the same electric eld. The ferroelectric polarization and other parameters have been summarized in Table 3. It is evident that ferroelectricity of BFMO thin lms has been greatly enhanced with the increase in Mn substitution from 0% to 8%, whereas the P r of 12% Mn substituted BFO thin lms is smaller. The lower residual polarization value and non saturated behavior of 12% Mn doped BFO thin lms is associated with the existence of large leakage current component. 21 It also reported by Das and his co-workers that high leakage characteristics of BFO ceramics could produce difficulty in attaining saturated polarization. 36 The remnant polarization (2P r ) and coercive eld (E c ) increase with the elevation of Mn content from 0% to 8% in BFMO thin lms. This increase in 2P r value with the increase in Mn content from 0 to 8% can be attributed to combine defect of (i) lowering of defects and (ii) increase in the grain size.  39 It is well known that grain size dependent domain structure, domain nucleation and domain mobility greatly inuence the ferroelectric properties of ferroelectric thin lms. 40 The grain boundaries act as a pinning center for polarization and produce hindrance of polarization switching. Thus, the small grain size BFMO thin lms experience more suppression of ferroelectric character whereas polarization switching is much easier inside the larger grain. The increase in the tetragonality of a crystal structure with increase in Mn doping could be another reason for the enhancement of ferroelectricity as propounded by Ederer et al. 41 This possibility can be easily ruled out as in our experiment, no other phases appeared, as shown in Fig. 1 XRD. Table 3 summarizes the different parameters obtained from the (P-E) loops of BFMO thin lms. The sudden decrease in the remnent polarization 2P r value of 12% Mn doped BFO thin lm is attributed to decrease in the grain size. A small conveyance in polarization hysteresis loops along the direction of the positive eld in BFMO thin lms can be imputed of actors like crystallographic defects, work function difference and thermal history between top and bottom electrodes. 42,43 The piezoelectric response of the 8% Mn-BFO thin lm has been studied simply by piezoresponse force microscopy. Typical buttery loops were observed for 8% Mn-BFO sample, as presented in Fig. 5(a) and (b). Local piezoelectric phase hysteresis loops and amplitude-voltage loops were also recorded at xed location as a function of AE15 dc bias superimposed on ac modulating voltage. Complete phase reversal of about 180 and piezo-actuation amplitude variation of more than 4.5 mV under the dc bias of AE5 V reects the better ferroelectric properties of 8% Mn-BFO thin lms important for practical applications of interest. 44

Band-gap analysis
The most direct and perhaps the simplest method for probing the band structure of semiconductors is to measure the absorption spectrum. Fundamental absorption, which manifests itself by a rapid rise in absorption, can be used to determine the energy gap of the semiconductor. To investigate the inuence of Mn doping on the optical absorption of BFO, the absorption optical spectra of pure BFO and Mn-doped BFO samples were measured at room temperature, as shown in Fig. 6(a). The absorption band edge of BFO thin lm appeared at 572 nm, which was similar to those previously reported, 45,46 indicating that BFO could respond to visible light for photocatalytic reaction. The fundamental absorption edge is seen to be shied towards higher wavelength with increasing Mndoping concentration of the BFMO thin lms (Fig. 6(a)). Compared to pure BFO, the Mn-doped BFO samples exhibited enhanced absorption capability especially in the visible light region, and the absorption intensity became gradually stronger where a is the absorption coefficient given by Ab/t, where Ab is absorbance and t is thickness of the lm, h is Planck constant (h ¼ 4.14 Â 10 À15 eV s), n is photon frequency, A is a constant, E g is band gap energy, and n, a number equal to 2 for direct band gap semiconductors and 1/2 indirect band gap semiconductors. 48 Here we use absorbance to calculate the band gap. Whether Ab or a is used, the coefficient A is different and has no effect on E g . Fig. 6(c-f) shows the Tauc plots in the form of (ahn) n versus photon energy for direct and indirect band gap semiconductors. By extrapolating the linear part of the absorption curve to the abscissa, the band gap can be determined according to the particular model used. 49 49 It should be noted that at room temperature, the phonon absorption branch usually dominates the (ahn) 1/2 plot. 50 For pure BFO, the linear portions of the direct and indirect Tauc plots almost overlap with each other, implying that this electronic transition contains both direct and indirect features, which is consistent with the atness of the valence band edge of BFO. 51 Mn substitution results in effective separation of the direct and indirect portions around this transition, which is most pronounced at 8% concentration. It is observed that while the direct transition remains almost unchanged, the indirect transition strongly red-shis with increasing Mn concentration ( Fig. 6(b)). All these observations point to the fact that Mn substitution makes the band-edge transitions more indirect, which directs us to look into the photocarrier dynamics in these lms. With the increase of the Mn dopant content, the band gap of doped samples was gradually decreased, and the band gap energy is minimum for the 8% Mn-doped BFO thin lm, in turn leading to a higher absorption capability. Undoubtedly, the enhanced absorption property of Mn-doped BFO would probably improve the photocatalytic activity of BFO, as discussed below. The smaller band gap values of the lms predict a possibility of higher absorption of visible light that may lead to potential photocatalytic application in photovoltaic devices. Combined with the change of residual polarization value of doped bismuth ferrite lms (Table 3), it is found that the optical band gap decreases with the increase of residual polarization value.

Conclusions
In summary pure and Mn doped BFO thin lms were successfully prepared on Pt/Ti/SiO 2 /Si and ITO/glass substrates by the solution-gelation technique. Structural characterization by Xray diffraction revealed that all samples exhibited a rhombohedral structure with (111) preferred orientation and without impurity phase. It is observed that improved ferroelectric property and leakage current density in 8% Mn-doped BFO thin lm due to a decrease in the oxygen vacancy density, a stabilization of the perovskite structure, and increase in the grain size. The absorption spectrum for BFMO (x ¼ 0, 0.04, 0.08) thin lms presents a signicant red shi and moves towards the visible region. Thus, photovoltaic behaviors are attributed to the narrower band-gap. This work gives insight into the relationship between ferroelectric remnant polarization and band-gap and found that the optical band gap decreases with the increase of residual polarization. This also provides an available way to exploring the mechanism of ferroelectric photovoltaic, getting more extensively applied in the new photovoltaic cells and other novel photoelectronic devices.

Conflicts of interest
There are no conicts to declare.