Effects of solvents and Al doping on structure and physical properties of BiFeO3 thin films

In this study, BFO and BFAO thin films were prepared on fluorine-doped tin oxide (FTO) substrates via spin coating with two different acid solvents. One is nitric acid solution (solvent I), the other is a mixture of 2-methoxyethanol and glacial acetic acid (solvent II). The structure, morphology, elemental valence states, and ferroelectric properties of BiFeO3 (BFO) and BiFe0.96Al0.04O3 (BFAO) films were investigated. X-ray diffraction (XRD) results show that all the films are R3c structure and have no impurity phase. The SEM results show that the BFO thin film prepared by solvent II is more compact, uniform, and has low porosity. Raman spectra show that Al doping causes structural distortion. Al doping can solve the problem of porosity and increase the density of BFO thin film. Therefore, the density of BFAO-II sample is better. X-ray photoelectron spectroscopy (XPS) shows that the presence of Al reduces the oxygen vacancy content. This is beneficial to reduce the leakage current density and improve the ferroelectric properties. The leakage current density of BFAO-II sample is 10-3 A/cm2, and the double residual polarization value is 110.2 μC/cm2. Compared with BFO-II sample, it is significantly improved. In addition, Al doping can reduce the band gap of BFO films. This work will be a new idea for the further application of Al doped bismuth ferrite films. P-E curves of BFO (I, II) and BFAO (I, II) films: (a) for BFO-I and BFAO-I and (b) for BFO-II and BFAO-II. P-E curves of BFO (I, II) and BFAO (I, II) films: (a) for BFO-I and BFAO-I and (b) for BFO-II and BFAO-II. High performance BFO, BFAO thin films were prepared by sol–gel method. The BFO thin films have excellent properties with the mixed solution of 2-methoxyethanol and glacial acetic acid as solvent. Al doping can effectively improve the ferroelectric properties and reduce the band gap width of BFO thin films. High performance BFO, BFAO thin films were prepared by sol–gel method. The BFO thin films have excellent properties with the mixed solution of 2-methoxyethanol and glacial acetic acid as solvent. Al doping can effectively improve the ferroelectric properties and reduce the band gap width of BFO thin films.

structural distortion and lead to double hysteresis loops, because of the electric field-induced transition from paraelectric state to ferroelectric state. Synchronously, according to some literature transition elements (Mn, Zn, Ti, Al, etc.) are used to substitute for Fe to solve the problem of large leakage current [21][22][23][24].
Ferroelectric materials essentially have spontaneous electric polarization (P S ) below T C [25]. Theoretical P S values of BFO films are considerable (~150 μC/cm 2 ). Unfortunately, saturated electric polarization loops (P-E loops) of BFO materials are obstructed to observe for poor quality and resistivity. The ferroelectric properties of thin film samples are generally better than those of bulk materials. In particular, for epitaxial BFO films, the electric polarization is directional, P s values of [111] and [001] are observed to be 100 μC/cm 2 and 65 μC/cm 2 , respectively [26]. Most notably, Zhang et al. [27] experimentally estimated P s value of BFO epitaxial thin films with tetragonal phase that exceed 140 μC/cm 2 under a high test electric field (~1000 kV/cm). Perovskite BiAlO 3 (BAO) which has R3c symmetry and is similar to typical BFO. Theoretically, BAO possess an excellent ferroelectric value (P s~9 0 μC/ cm 2 ) and high Curie temperature (T C~5 30°C) [28]. BAO modified Bi 0.5 (Na, K) 0.5 TiO 3 antiferroelectric ceramics possess excellent energy storage characteristics [29]. Hence, in this work, nonmagnetic Al was selected and substituted for Fe in BFO films via the spin coating method. The spin coating method is widely used because it is low cost [16]. In this study, the structure, morphology, elemental valence states and ferroelectric properties of BiFeO 3 (BFO) and BiFe 0.96 Al 0.04 O 3 (BFAO) films were investigated. Optimized Al doping BFO films can be used in the construction of next generation environmentally friendly intensive nanodevices.  2 , EG), and ethanolamine (C 2 H 7 NO) were used as raw materials. Two kinds of precursors were prepared: one was an aqueous solution of nitric acid (solvent I), and the other was a mixture of 2-methoxyethanol and acetic acid in a volume ratio of 3:1 (solvent II). Citric acid was used as a complexing agent in both solvents, and appropriate amounts of glycol and ethanolamine are added to stabilize the solution. Excess Bi (4 at.%) was used to compensate for volatilization. The concentration of cation (Bi 3+ , Fe 3+ , Al 3+ ) is about 0.5 mol/L, fluorine-doped tin oxide conductive glass (FTO, SnO 2 : F) were served as substrates. BiFeO 3 (BFO) and BiFe 0.96 Al 0.04 O 3 (BFAO) thin films were synthesized via spin coating. The precursor solvent was spin coated on FTO substrate with a maximum rotation speed of 4000 rpm for 15 s layer-by-layer, and then it was dried in an electric vacuum drying oven at 85°C for 10 min. Subsequently, the FTO substrates with gel layers were pushed into a 550°C-preheated muffle furnace for 10 min to remove the organic solvent. After 14 cycles of the coating/pyrolysis processes, the BFO or BFAO precursor films were sintered at 550°C for 30 min in air. BFO and BFAO films that were prepared by an aqueous solution of nitric acid were denoted as BFO-I and BFAO-I, respectively, and those that were prepared using 2-methoxyethanol and glacial acetic acid were denoted as BFO-II and BFAO-II, respectively.

Characterization
Crystal structures of the thin films were analyzed from Xray diffraction (XRD, D/max 200) measurements. Ultraviolet-visible spectrophotometry (UV-Vis, GBC Gintra-20) was used to record the absorption spectra. Field emission scanning electron microscopy (FE-SEM, TES-CAN VEGA3) was employed to observe the microstructures (surface morphology and thickness) and energy dispersive spectroscopy (EDS, Thermo) to obtain surface element information of the sample. Raman spectra of the films were collected using a LabRAM HR Evolution (Horiba, France). X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250xi (Thermo, USA) with Al Kα radiation to determine the chemical states of samples, and the adventitious C1s peak at 284.8 eV was used to calibrate all other binding energies. The electrical properties (mainly P-E loops and leakage current density) were characterized using a ferroelectric test system (aix ACT). Before polarization measurement, several circular Pt upper electrodes with a diameter of 0.6 mm were deposited on the surface of films utilized ion sputtering with shadow masks.

Structure and morphology
Typical XRD patterns of BFO thin films are shown in Fig.  1. All of the thin film samples have sharp diffraction peaks. Compared to the standard PDF (BiFeO 3 : PDF#71-2494), it can be concluded that all of the thin film samples remained pure phase, and crystal structures of the samples could be indexed to distorted rhombohedral structure with space group R3c. In addition, some of the diffraction peaks do not correspond to BFO mainly correspond to conductive oxides (SnO 2 : F) on the surface of the FTO substrate, and no secondary phases were observed in any of the samples. The lattice constants of BFO-II and BFAO-II are 3.956 Å and 3.953 Å, respectively. Because the radius of Al 3+ is smaller than that of Fe 3+ , the lattice constant of BFO film decreases.
To investigate the growth quality of Al-doped BFO films using two kinds of solvents, SEM was employed to directly observe the microstructures of the films. Figure 2 shows SEM images of the surface of films. There are some pores on the surface of the BFO films prepared using the two solvents. The average grain size of BFO-I is the largest of all of the samples, and this was about 167.6 nm. Al doping refines the grain size, reduces pores, and increases the density of the film. The average grain size of the BFAO-I sample was about 120.7 nm. On the whole, the grain size of the BFO films that were prepared using the 2methoxyethanol and glacial acetic acid solvent were more refined than those of the BFO films that were prepared by nitric acid solvent. The crystal sizes of BFO-II and BFAO-II films were 148.4 and 111.7 nm, respectively. At the same time, a cross-sectional sample of each film was prepared using liquid nitrogen embrittlement treatment, and SEM was used to observe the composition of the cross-section. There was an obvious interface (BFO/FTO) in the crosssection, as seen in Fig. 3   results in a film that is more compact and has thinner thickness.
To further study the element distribution and content on the surface of the film sample, we carried out surface element scanning of the sample prepared with 2methoxyethanol and glacial acetic acid as the solvent. Scanning results are shown in Fig. 4. Bi, Fe, O, and Al elements with less content are evenly distributed on the surface of the BFAO-II sample. The atomic proportion that measured from the energy spectrum basically conforms to the proportion of metal ions in the preparation of raw materials, according to the atomic ratio of the element contents.
The information of BiFe 0.96 Al 0.04 O samples prepared with 2-methoxyethanol and glacial acetic acid as solvent can be further obtained by XPS test. Figure 5 shows the XPS results of BFAO-II sample. Figure 5a shows the XPS spectra of BFAO-IIsamples in the energy range of 50-1200 eV. It can be clearly seen from the diagram that the corresponding peaks of Bi, Fe, O, and Al are present, and there are no impurity elements. The peak fitting of Bi, Fe and O is shown in Fig. 5b-d. Figure 5b shows that Bi 4f 7/2 and Bi 4f 5/2 peaks are at 158.6 eV and 164.1 eV. As shown in Fig. 5c, the Fe 2p peaks at 710.3 eV and 724.1 eV are divided into two fitting peaks, which are Fe 2+ and Fe 3+ respectively. The concentration ratio of Fe 3+ /Fe 2+ is 42:58.   5d clearly shows that O 1 s peak is composed of two peaks, the ratios between O 2ions and oxygen vacancies is 75:25. The oxygen vacancy concentration is low, which is conducive to the reduction of leakage current and the improvement of ferroelectric properties. After XPS calculation, the atomic ratio of Bi, Fe, O, and Al is about 54.84:20.30:23.68:1.2, which is basically the same as that of EDS, which indicates that Al element is doped into bismuth ferrite film.

Raman and UV-Vis spectra
To further study the influences that Al doping and different solvents have on the structure of the film samples, Raman spectra were recorded of all the samples. BFO has a perovskite structure of the R3c space group. There are 27 kinds of phonon vibration modes that are produced by 10 atoms in the cell, which can be defined as r opt = 4A 1 + 5A 2 + 9E. However, 5A 2 modes are not Raman active, and 9E modes are doubly degenerate. Figure 6 depicts the Raman spectra of the film samples, these 13 Raman active vibration modes (4A 1 + 9E) [30] are seen in Fig. 6a. The ferroelectricity of BFO is usually attributed to E-1, E-2, and 4A 1 . Low frequency modes (A 1 -1 and A 1 -2) and intermediate frequency modes are associated with Bi-O [31,32]. Figure 6b is an enlarged view in the range of 60-250 cm −1 . There are four obvious Raman vibration modes in this low frequency region, E-1 (~70 cm −1 ), A 1 -1 (~140 cm −1 ), A 1 -2 (~170 cm −1 ), and A 1 -3 (~230 cm −1 ). Also, an E-2 mode (~115 cm −1 ) corresponds to Fe. For Al-doped samples BFAO (I, II), A 1 -1 and E-1 are narrower, and the strength of E-2 is weaker. Undoubtedly, Al doping leads to a certain anharmonic vibration of the lattice and weakens the vibration of Fe in the lattice. However, the A 1 -3 mode corresponds to the octahedral structure of FeO 6 , and Al doping enhances its strength. This indicates that the octahedral structure of FeO 6 is distorted; this gives rise to the Jahn-Teller effect, which is a result of Al doping [33]. These observations indicate that film samples that were prepared by two solvents introduce Al enter to BFO lattice.
The spin coating thin film is nanoscale and sensitive to optical signal. The UV-Vis absorption spectra of BFO (I, II) and BFAO (I, II) films were recorded, and the light absorption curves are shown in Fig. 7a. As seen in the absorption curves, the film samples mainly absorb 300-500 nm blue-green light, and the absorption peak of the film is at 450 nm for BFO (I, II). When 0.04-Al was introduced into the films, the absorption range of the BFAO films became wider. The absorption peak underwent a slight red shift, with the peak moving to the vicinity of 500 nm. Al doping improved the absorption and utilization ability of the visible light.
The band gap width (E g ) of each film sample was then calculated using the classical Tauc formula, as written in below equation (1) [34] where α is the absorption coefficient that originates from measuring, h is Planck's constant (h = 6.62607015 × 10 −34 J·s), ν is the photon frequency (test frequency), A is a constant, E g is the bandgap, and n is the exponential coefficient that correlates to the nature of the E g transition (direct E g : n = 2). The (αhν) 2hν cures of the films were plotted in Fig. 7b. The values of the forbidden band width (E g ) of the BFO (I, II) and BFAO (I, II) samples are 2.28, 2.26, 2.17, and 2.21 eV, respectively. This work indicates that the E g of BFO films can be adjusted by changing the preparation of the solvent or by introducing Al. Thus, these approaches improve the absorption ability of visible light, facilitate the application of BFO film in photocatalytic decomposition of methyl orange and other organic matters, and provide a new idea for photovoltaic devices.

P-E loops and leakage current
The ferroelectric properties of BFO (I, II) and BFAO (I, II) samples were tested using an aix ACT system. As discussed in the previous section, SEM was used to test the thickness of BFO (I, II) and BFAO (I, II) samples. The test voltage (V) divided by the thickness of the film (d) can be converted into electric field strength (E = V/d).
The P-E curves of BFO (I, II) and BFAO (I, II) samples are shown in Fig. 8. The residual polarization value (+P r ) of the BFO film that was prepared using nitric acid is about 10 μC/cm 2 , whereas the residual polarization values (+P r ) of BFO and BFAO samples prepared using 2methoxyethanol and glacial acetic acid in positive electric field are 33.2 μC/cm 2 and 57.6 μC/cm 2 , respectively. The hysteresis loop of BFO is not completely symmetrical, 2P r (+P r add -P r ) and coercive field 2E c (+E c add -E c ) were applied to describe the ferroelectricity. Excessive Al doping weakens the ferroelectric properties of BFO films, and this may be a result of the spontaneous polarization of BAO (P s~9 0 μC/cm 2 ), which is lower than BFO (P s~1 50 μC/cm 2 ), theoretically. The leakage current cures of the film samples were then tested. The problem of high leakage always hinders the application of BFO film on commercialized devices. Figure 9a shows leakage current curves of the BFO (I, II) and BFAO (I, II) samples. The values of leakage current of the thin film samples prepared with nitric acid as the solvent were larger, and the test field was lower than that of the thin film sample that were prepared with 2-methoxyethanol and glacial acetic acid as the solvent. The leakage of BFO-I was about 3 × 10 −2 A/cm 2 at 150 kV/cm. However, the leakage of BFAO-II is about 1 × 10 −3 A/cm 2 at 150 kV/cm and 1 × 10 −2 A/cm 2 at 300 kV/cm. These test results are related to the quality of the thin films. The SEM images above also show that there are more pores in the two samples that were prepared using the nitric acid precursor, and the density of the thin film is lower than that of the BFO-II and BFAO-II films.
To study the conduction mechanism of the sample, we calculated the slope to describe the evolution of the leakage current with the field strength. The logJ-logE curves in the positive bias region are displayed in Fig. 9b. According to the slopes (S) obtained with different values of electric field strength (E), all of the logJ-logE curves can be divided into three stages. The slope of the first stage is about 1.5, and this indicates that the Ohmic conduction mechanism is dominant under a low electric field. Under a higher electric field, the value of the slope increases, and the control conductivity mechanism is gradually evolving. In the third stage, the slopes of BFAO film that was prepared with each of two solutions are about 3. At higher electric field the dominant conduction mechanism of Al doped film is Schottky conductivity mechanism. The evolution of the conductivity mechanism indicates that the concentration of defective dipoles changes. The sharp increase in the slope may be a result of the complete filling of injected electrons by an effective trap related to the defects [37]. Al doping may reduce the concentration of oxygen vacancies. The slope of the undoped sample is lower; however, the leakage current value is larger than Al-doped films. The transition period from the Ohmic contact to the Schottky conduction mechanism of undoped BFO is longer, and the test electric field cannot continue to increase because of the poor quality of the film.

Conclusions
BiFeO 3 and BiFe 0.96 Al 0.04 O 3 thin films were successfully synthesized via the sol-gel method and spin coating on FTO substrates. All of the samples have rhombohedral structure with the R3c space group. Al doping gives rise to the Jahn-Teller effect with some structural distortion of the BFO film; it also leads to grain refinement and improved comprehensive properties for high density. Average grain sizes of the samples are 167.6 (BFO-I), 148.4 (BFO-II), 120.7 (BFAO-I) and 111.7 (BFAO-II) nm, and the cross section thickness of these samples are 845.27, 770.73, 818.16, and 714.28 nm, respectively. The leakage cures of the films were dominated by the Ohmic and Schottky conduction mechanisms. BFAO-II possesses excellent ferroelectricity (2P r~1 10 μC/cm 2 ), and its leakage current density (~1 × 10 −2 A/cm 2 at 300 kV/cm) is lower than that of undoped BFO films (BFO (I, II)). The E g values of samples are 2.28 (BFO-I), 2.26 (BFO-II), 2.17 (BFAO-I), and 2.21 (BFAO-II) eV. This work provides a new idea for the application of BFAO films in photocatalysis and photovoltaic devices.