Effect of Nd and Mn Co-Doping on Dielectric, Ferroelectric and Photovoltaic Properties of BiFeO3

Bi1−xNdxFe0.99Mn0.01O3 (BNFMO, x = 0.00~0.20) films were epitaxially grown on Nb:SrTiO3 (001) substrates using pulsed laser deposition. It was found that the Nd-doping concentration has a great impact on the surface morphology, crystal structure, and electrical properties. BNFMO thin film with low Nd-doping concentration (≤16%) crystallizes into a rhombohedral structure, while the high Nd-doping (>16%) will lead to the formation of an orthogonal structure. Furthermore, to eliminate the resistive switching (RS) effect, a positive-up–negative-down (PUND) measurement was applied on two devices in series. The remnant polarization experiences an increase with the Nd-doping concentration increasing to 16%, then drops down with the further increased concentration of Nd. Finally, the ferroelectric photovoltaic effect is also regulated by the ferroelectric polarization, and the maximum photocurrent of 1758 μA/cm2 was obtained in Bi0.84Nd0.16Fe0.99Mn0.01O3 thin film. BNFMO films show great potential for ferroelectric and photovoltaic applications.


Introduction
BiFeO 3 (BFO), one of the few multiferroic materials with antiferromagnetic, ferroelectricity, and ferroelectric photovoltaic responses at room temperature, has captured much attention [1][2][3]. Moreover, the low cost and simple fabrication approach of flexible BFO films can also be utilized in wearable hybrid piezoelectric-triboelectric nanogenerators H-P/TENG [4][5][6][7]. To grow BFO thin films, high substrate temperature during growth is needed for physical or chemical methods. During annealing, the volatilization of Bi 3+ will produce oxygen vacancy content and transform Fe 3+ to Fe 2+ due to the charge compensation, which will lead to the weak ferroelectric properties of the sample [8]. Thus, it is generally difficult to obtain relatively regular ferroelectric polarization hysteresis (polarization voltage, P-V) loops in BFO films. In recent years, in order to enhance the properties of BFO, doping is widely adopted [9][10][11][12][13][14]. Additionally, research on BFO thin films, co-doped by Nd and transition metal elements, has been frequently reported [15][16][17][18][19][20]. Despite this, there has been little research completed on eliminating the effects of leakage current during ferroelectric polarization hysteresis measurements of BFO films. For example, the Nd and Mn co-doped BFO epitaxial film prepared by Kawae et al. [19,21] and the Nd-doped BFO films manufactured by Xue et al. [16] with large ferroelectric polarization cannot obtain the real properties of ferroelectric polarization due to the severe leakage of the current. Thereby, not only does the technology of thin film preparation need to be enhanced, but more advanced measurement methods to eliminate the effect of environment and the leakage of current are also required when attempting to promote the ferroelectric properties of BFO films by using the approach of ion-doping substitution. Moreover, the photovoltaic effect of BFO also attracted the attention of researchers [22][23][24] and doping is often adopted to increase the photovoltaic effect. Mn-doping of BFO prepared by Nakashima et al. [25] enhanced the photovoltage. In a Pt/Mn 1 at%-doped BFO/Pt coplanar capacitor with an inter-electrode distance of 260 µm, V OC reached 280 V at room temperature (RT) (852 V at 80 K). An approach, 'gap-state' engineering, has been developed to produce photoferroelectrics of Mn-BFO film reported by Matsuo et al. [26], in which defect states within the bandgap act as a scaffold for photogeneration. The tunable and low bandgap Bi 2 FeCrO 6 (BFCO) thin films were engineered by Nechache et al. [27] with an unprecedented power conversion efficiency of 8.1%. Gupta et al. [28] reported the superior photovoltaic property in BiFe 0.95 Si 0.05 O 3 films with high photosensitivity V OC~0 .45 V and I SC~0 .75 mA.
The photocurrent is one of the important factors for judging the merits of the photovoltaic effect. Qin et al. [29][30][31] reported that PLZT thin films generally have an optimal thickness peak with the maximum photocurrent, and the range of the peak is around 30-40 nm. In addition, the crystal structure of polycrystalline film has more defects, reducing the lifetime of photo-generated carriers. However, the sample preparations of ion-doped BFO films reported in previous work are relatively thick and mostly polycrystalline films. For example, the thickness of Nd-doped and Nd-Cr co-doped BFO polycrystalline films prepared by Shi et al. [32] and Zhang et al. [33] are thick and the dielectric permittivity of the selected Ag electrode is small. According to previous reports [29], the selection of electrodes is also an important factor that affects the photocurrent of ferroelectric films. Additionally, the thickness of Nd-doped BFO single crystal films prepared by Tan et al. [34] is too thick, while the La-doped BFO polycrystalline films prepared by Biswas et al. [35] have poor quality. Thus, achieving a reliable ferroelectric polarization hysteresis loop and enhancing the photovoltaic response of BFO films becomes important.
We summarize related studies on ion-doping of BFO films and identify the strengths and weaknesses of each. In addition, previous reports have not proposed the influence of Nd and Mn co-doping on ferroelectric photovoltaic properties of BFO thin films. Thus, in this paper, five ratios were selected to study the effect of Nd ion-doping on the morphology, structure, dielectric, ferroelectric, and photovoltaic properties of BiFe 0.99 Mn 0.01 O 3 film in detail. Additionally, 1 at% Mn 4+ -doping on the Fe-site is adopted to optimize the ferroelectric properties. For BFO materials, the ion substitutions with a high valence (4+) for the Fe site usually result in the decrease in oxygen vacancy content and the increase in Fe 2+ [36,37] (Bi 1−x Nd x Fe 0.99 Mn 0.01 O 3 , x = 0.00, 0.05, 0.10, 0.16, 0.20, BNFMO). The 40-nm-thick epitaxial BNFMO films with high quality and no impurity phase were prepared on NSTO. The structure of two devices in series was utilized for P-V loop measurements of BNFMO films during the positive-up-negative-down (PUND) measurement, and successfully deducting the influence of leakage current, resistive switching (RS) effect, and environmental factors. In addition, we obtained the maximum photocurrent of 1758 µA/cm 2 in Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 thin film, which is much larger than that in previous reports in Nd-doped BFO under the same light intensity.

Materials and Methods
The BNFMO films were grown on 0.5-mm-thick single-crystal Nb (0.7 wt%)-doped SrTiO 3 (001) (NSTO) substrates via pulsed laser deposition (PLD). The substrates were first etched with a buffered hydrogen fluoride (BHF) solution before preparing the films, and then annealed in a furnace with flowing air at 950 • C for 2 h. A KrF excimer laser (λ = 248 nm) operating at 2 Hz was applied for target ablation, the laser fluence was 1.5 J/cm 2 with a spot area of 7.5 mm 2 on the target, and the target-substrate distance was set at 6.5 cm. The deposition was performed under an oxygen partial pressure of 0.1 Torr at a substrate temperature of 630 • C for 30 min.
The X-ray diffraction (XRD, λ = 1.5406 Å, Bruker, D8-ADVANCE, Karlsruhe, Germany) patterns of the BFO films were measured at room temperature. All XRD data were collected at 2θ ranges from 10 • to 60 • with a Cu X-ray source. The scan rate and step size of the collecting diffraction data were set as 0.02 • /s and 0.02 • /step, respectively. The morphologies Crystals 2022, 12, 500 3 of 14 of NSTO substrates after annealing and BNFMO films were observed by scanning probe microscope (Multimode 8, Oxford Instruments, Oxford, UK). The ferroelectric domain switching and local piezoelectric responses were analyzed by the piezoresponse force microscopy (PFM) mode using a commercial atomic force microscope (AFM) (Asylum Research, MFP-3D, Oxford Instruments, Oxford, UK). The Au/BNFMO/NSTO/In heterojunction was prepared to study the characteristics of the leakage current density versus the applied electric field (J-E) and the dielectric properties of the BNFMO thin films. The top Au contacts with the thickness of 100 nm were deposited via sputtering as the top electrode, and square size of 200 × 200 µm was patterned in the liftoff process. Thicknesses of the films were obtained by the scanning electron microscope (SEM) (JSM-7001F, JEOL, Tokyo, Japan), and the amplification factor and acceleration voltage were set as 200,000 and 10 kV, respectively. Leakage currents were measured by a current source meter (2400, Keithley, Cleveland, OR, USA). All current data were collected from −2.5 to 2.5 V, and the voltage variation rate was set as 0.2 V. The electrical parameters of the materials were measured as a function of frequency (1 kHz -1 MHz) using a semiconductor electrical performance comprehensive tester (4200A-SCS, Cleveland, OR, USA). Additionally, the oscillating voltage amplitude was set as 0.5 V. The measurements of polarization current (I-V) and P-V loops were performed via a semiconductor device analyzer (WGFMU, B1500A, Agilent Technologies, Santa Clara, CA, USA) with the frequency of 10 KHz. The current density-voltage (J-V) curves were measured by a current source meter (2400, Keithley, Cleveland, OR, USA) when the films were illuminated with a 405 nm monochromatic laser. The light intensity was calibrated to 160 mW/cm 2 by an irradiation meter (FZ-A, photoelectric instrument factory of Beijing Normal University, Beijing, China). Figure 1a demonstrates the surface topography of the etched substrate. Flat atomiclevel steps separated were observed in the inset of Figure 1a. Figure 1b shows the crosssectional SEM image of the BNFMO (40 nm) thin film grown on a NSTO substrate. The X-ray diffraction and Raman spectra were recorded to determine the crystal structure of the BNFMO (x = 0.00~0.20) thin films. Figure 1c shows XRD patterns of the BNFMO film on the Nb-doped SrTiO 3 (001) substrates grown at 630 • C. All films present the perovskite structure with the (001) orientation. In all the BNFMO films, there is no impurity and second phase observed, which indicates that Nd 3+ and Mn 4+ ions have entirely entered into the BFO lattice. The diffraction peaks of the thin films shift to the higher angle region with the increase in neodymium content. It is because Nd 3+ ions with smaller ionic radius (0.995 Å) are substitutes for Bi 3+ ions with a relatively smaller ionic radius (1.03 Å) on A sites, which leads to the lattice contraction. Additionally, there may be a tetragonal or orthorhombic structure transition induced by the Nd-doping elements according to the previous reports [16,38], which can be further confirmed by Raman spectra. Figure 1d shows the Raman spectra of the BNFMO (x = 0.00~0.20) films at room temperature. Three A 1 vibration modes and seven E vibration modes are observed in the rhombohedral BiFeO 3 (the E vibration mode with low wavenumbers below 100 cm −1 was not detected), and the crystal structure is determined by comparing the relative positions of the characteristic vibration modes [39]. The modes of A 1 -1, A 1 -2, and A 1 -3 are related to the vibration of the Bi-O bond, while the modes of A 1 -4 and E are closely related to the vibration of the Fe-O bond. In the range of x = 0.00~0.16, the peak strength of the A 1 -1 and A 1 -2 modes enhances drastically and reaches the maximum at x = 0.16, thereby indicating that the electron cloud of the Bi-O bond and the bond angle of the Fe-O bond have changed [16]. The Raman peaks at 139, 172, 227, 262, 279, 373, 474, and 529 cm −1 of the thin BNFO (x = 0.00~0.16) films are assigned to the vibration modes of A 1 -1, A 1 -2, A 1 -3, E-3, E-4, E-5, A 1 -4, and E-9, as shown in Figure 1d. The result indicates that the BNFMO (x = 0.00~0.16) films are still rhombohedral bismuth ferrite. The positions of the characteristic peaks are exceedingly close to that of the trigonal phase block and thin BiFeO 3 film reported in the literature. Therefore, the original rhombohedral structure is not changed when the Nd ion doping is below 16%. In addition, the E-8 mode gradually strengthens and widens with the increase in Nd-doping amount. This change is consistent with the tendency of transition from R3c to Pbnm phase structure [40]. When the doping amount reached 20%, the intensity of A 1 -1, A 1 -2, and A 1 -3 modes decreases greatly, the vibration modes of E-3, E-4, and E-8 are shifted, respectively, to 268, 288, 548 cm −1 , and the E-5 vibration mode disappeared. This behavior indicates a strong variation in Bi-O and Fe-O-Fe bonds and a transformation from rhombohedral to orthorhombic structure for the Bi 0.80 Nd 0.20 Fe 0.99 Mn 0.01 O 3 film [41].  Figure 1d shows the Raman spectra of the BNFMO (x = 0.00~0.20) films at room temperature. Three A1 vibration modes and seven E vibration modes are observed in the rhombohedral BiFeO3 (the E vibration mode with low wavenumbers below 100 cm −1 was not detected), and the crystal structure is determined by comparing the relative positions of the characteristic vibration modes [39]. The modes of A1-1, A1-2, and A1-3 are related to the vibration of the Bi-O bond, while the modes of A1-4 and E are closely related to the vibration of the Fe-O bond. In the range of x = 0.00~0.16, the peak strength of the A1-1 and A1-2 modes enhances drastically and reaches the maximum at x = 0.16, thereby indicating that the electron cloud of the Bi-O bond and the bond angle of the Fe-O bond have changed [16]. The Raman peaks at 139, 172, 227, 262, 279, 373, 474, and 529 cm −1 of the thin BNFO (x = 0.00~0.16) films are assigned to the vibration modes of A1-1, A1-2, A1-3, E-3, E-4, E-5, A1-4, and E-9, as shown in Figure 1d. The result indicates that the BNFMO (x = 0.00~0.16) films are still rhombohedral bismuth ferrite. The positions of the characteristic peaks are exceedingly close to that of the trigonal phase block and thin BiFeO3 film reported in the literature. Therefore, the original rhombohedral structure is not changed when the Nd ion doping is below 16%. In addition, the E-8 mode gradually strengthens and widens with the increase in Nd-doping amount. This change is consistent with the tendency of transition from R3c to Pbnm phase structure [40]. When the doping amount reached 20%, the intensity of A1-1, A1-2, and A1-3 modes decreases greatly, the vibration   Figure 3, the grain size reduces slightly with the increase in Nd concentration, which is caused by more and more Nd ions stacked at the grain boundaries and preventing the grain from growing up with the increase in Nd concentration. Additionally, it may also be due to the substitution of Bi 3+ by Nd 3+ , protecting the Bi 3+ from volatilizing during the sintering process; thus, reducing the generation of oxygen vacancy content. The reduction will relatively reduce the oxygen content of films during the preparation of the films, which will also decrease the growth speed of grains [42,43]. stacked at the grain boundaries and preventing the grain from growing up with the increase in Nd concentration. Additionally, it may also be due to the substitution of Bi 3+ by Nd 3+ , protecting the Bi 3+ from volatilizing during the sintering process; thus, reducing the generation of oxygen vacancy content. The reduction will relatively reduce the oxygen content of films during the preparation of the films, which will also decrease the growth speed of grains [42,43]. The average grain sizes of BNFMO (x = 0.00, 0.05, 0.10, 0. 16, 0.20) films are 98 (±5), 87 (±5), 73 (±3), 68 (±3), and 47 (±4) nm, respectively.   As shown in Figure 2b,c, we adopted piezoresponse force microscopy (PFM) to obtain the local ferroelectric properties of the BNFMO films. The acquired PFM out-of-plane phase and amplitude images were acquired after applying −8 and +8 V biases on the surface of the BNFMO films. Based on the PFM results, the polarization reversal becomes more apparent when the doping amount of Nd ion increases from 0 to 16%, which is ascribed to the enhanced ion or electron displacement polarization caused by Nd ions [16]. When the doping amount of Nd ion reached 20%, the contrast of the phase and amplitude out of the plane became lower which is due to the formation of the orthorhombic structure of the film. The results are consistent with the Raman spectrum. Figure 4 shows the characteristics of the leakage current versus the applied voltage (I-V) and dielectric properties of the BNFMO films. The leakage current of the BNFMO (x = 0.00~0.16) films decreased significantly with the increase in the Nd ion concentration, as shown in Figure 4a. The inset of Figure 4a shows the structure of the device used for the measurement. The reduction in oxygen vacancy content and other impurity defects leads As shown in Figure 2b,c, we adopted piezoresponse force microscopy (PFM) to obtain the local ferroelectric properties of the BNFMO films. The acquired PFM out-of-plane phase and amplitude images were acquired after applying −8 and +8 V biases on the surface of the BNFMO films. Based on the PFM results, the polarization reversal becomes more apparent when the doping amount of Nd ion increases from 0 to 16%, which is ascribed to the enhanced ion or electron displacement polarization caused by Nd ions [16]. When the doping amount of Nd ion reached 20%, the contrast of the phase and amplitude out of the plane became lower which is due to the formation of the orthorhombic structure of the film. The results are consistent with the Raman spectrum. Figure 4 shows the characteristics of the leakage current versus the applied voltage (I-V) and dielectric properties of the BNFMO films. The leakage current of the BNFMO (x = 0.00~0.16) films decreased significantly with the increase in the Nd ion concentration, as shown in Figure 4a. The inset of Figure 4a shows the structure of the device used for the measurement. The reduction in oxygen vacancy content and other impurity defects leads to poor local conductivity, which is consistent with other reports reducing oxygen vacancy concentration by replacing a volatile Bi ion with a nonvolatile Nd ion. However, due to the excessive doping, there are more defect charges with the Bi 0.80 Nd 0.20 Fe 0.99 Mn 0.01 O 3 thin film; thus generating an increase in leakage current compared to Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 film [44]. Figure 4b illustrates a systematic reduction in dielectric permittivity (E r ) of BNFMO (x = 0.00~0.20) films with Nd-doping [42] at high frequencies, which is attributed to the polarization relaxation phenomenon. It can be ascribed to fact that the contribution of interfacial, dipolar, ionic, and atomic polarizations decreases with increasing frequency and finally vanishes (except electronic polarization) at extremely high frequencies [45,46]. The Positive-up-negative-down (PUND) was generally adopted to measure the ferroelectric properties, in which five triangular pulse signals were implemented, and the PUND measurement effectively removed the effects of the leakage current, dielectric current, and experimental setup (R, L). The measured currents included polarization current (I F ), leakage current (I L ), and dielectric current (I ε ). The ferroelectric (FE) polarization currents (I PUND ) in the negative and positive directions were defined as I PUND− = I P − I U = I F− and I PUND+ = I N − I D = I F+ , respectively [49]. In this paper, the ferroelectric hysteresis loop of the BNFMO (x = 0.00~0.20) films was measured using the PUND method to eliminate the influence of leakage current. However, the resistive switching (RS) effect caused by the movement of oxygen vacancy content also causes erroneous polarization current values [50,51]. As shown in Figure 5a, the I D current increases in the positive direction due to the occurrence of resistance switching (RS) current (I S ) from a high resistance state (HRS) to a low resistance state (LRS) in the fifth pulse (I D ). The current measured in the positive direction (I PUND+ = I N − I D = I F+ − I S+ ) is lower than the actual ferroelectric polarization current (I F+ ). On the contrary, I PUND− current exceeds the actual I F− (I PUND− = I P − I U = I F− + I S− ). Therefore, the measured current I PUND− exceeds the actual ferroelectric polarization switching current (I F− ) [49]. Thus, the existence of the resistive switching effect still leads to the incorrect P-V loop of BNFMO films whose polarization (−) (P r− ) value observably exceeded the polarization (+) (P r+ ) value.

Results and Discussion
In this study, in order to deduct the influence of the RS effect during the PUND measurement, the PUND measurement was carried out for BNFMO (x = 0.00~0.20) connected in series with single and back-to-back connected two identical 40-nm-thick BNFMO ferroelectric capacitors, respectively. Firstly, we selected Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 film to study the two different measurement approaches. Figure 5a shows the PUND measurement on Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 film of a single device and two devices in series at 10 kHz. The inset shows the structure for two different measurement methods. The corresponding I-V and P-V loops of two different measurement methods were calculated from the blue and red lines of (a), respectively, as shown in Figure 5c-f. Obviously, as can be seen in Figure 4c,d, the abnormal I-V and P-V loops were observed, but Figure 5e,f demonstrates the symmetrical loops. Furthermore, we measured the DC I-V curves of two different approaches to verify the PUND results. The voltage was applied in the order of 0 V → +5 V (2.5 V) → 0 V → −5 V (−2.5 V) → 0 V, as shown in Figure 5b. The observed results of PUND and DC I-V curves indicate that the structure of two devices in series can reduce the RS effect effectively. When we measure the ferroelectric hysteresis loop, the maximum range of the test instrument is −5 to +5 V. Thereby, if the two devices in series technique is used, the measurement range is only −2.5−2.5 V, which will lead to the P-V loop we measured being unable to reach saturation polarization [52,53]. Hence, further measurements were undertaken to verify the ferroelectric properties of the Bi 0.84 Nd 0. 16  shown in Figure 4c. The increase in the tan δ and decrease in the Ɛr of Bi0.8Nd0.20Fe0.99Mn0.01O3 film indicates the strong dielectric relaxation behavior in film [16]. Positive-up-negative-down (PUND) was generally adopted to measure the ferroelectric properties, in which five triangular pulse signals were implemented, and the PUND measurement effectively removed the effects of the leakage current, dielectric current, and experimental setup (R, L). The measured currents included polarization current (IF), leakage current (IL), and dielectric current (Iε). The ferroelectric (FE) polarization currents (IPUND) in the negative and positive directions were defined as IPUND− = IP − IU = IF− and IPUND+ = IN − ID = IF+, respectively [49]. In this paper, the ferroelectric hysteresis loop of the BNFMO (x = 0.00~0.20) films was measured using the PUND method to eliminate the influence of leakage current. However, the resistive switching (RS) effect caused by the  The approach of two devices in series was further extended to all BNFMO (x = 0.00~0.20) thin films. Two 40-nm-thick BNFO films with the same doping amount were connected in series. Figure 6a presents the P-V and I-V loops of five BNFO films measured by the PUND measurement, in which two devices were in series at 2.5 V with a driving frequency of 10 kHz. Figure 6b shows the schematic diagram of the remnant polarization (P r ) and coercive voltages of BNFMO films vary with increasing Nd-doping concentration. It can be seen that the P r enhances and coercive voltage reduces, respectively, with the increase in Nd ion concentration in the BNFMO (x = 0.00~0.16) films. Additionally, the hysteresis loops of the films, with P r of approximately 2.9, 4.3, 5.5, and 10.7 µC/cm 2 , respectively, were successfully obtained. The P r and coercive voltage values reached the maximum and minimum at x = 0.16, which can be attributed to the enhanced ion or electron displacement polarization caused by Nd ion and the reduction in the leakage current [56]. However, when the doping amount of Nd ions reaches 20%, the P r of the film greatly decreases from 10.7 to 4.9 µC/cm 2 and the coercive voltage increases from 1.70 to 1.75 V, which are attributed to transformation from rhombohedral to the orthogonal structure and paraelectricity of the film. During the variation of x = 0.16-0.20, the crystal structure parameters only change slowly, so the stereochemical activity should have the predominant effect on the observed ferroelectric-paraelectric transition [56,57].  The photovoltaic characteristics of Au/BNFMO/NSTO/In ferroelectric photovoltaic devices were measured with a 405 nm monochromatic laser as the excitation light source. Figure 7a,b presents I-V photovoltaic characteristic diagrams of the ferroelectric photovoltaic device under light and dark environments, and the circuit connection diagram of the ferroelectric photovoltaic devices is shown in the inset of Figure 7b. Compared with the dark environment, the open-circuit voltage (V oc ) and short-circuit (I sc ) current of films increase to a certain extent under illumination, implying that the photo-generated carriers under illumination results in the formation of photo-generated voltage and current, which verifies the photovoltaic effect of films. Generally speaking, there is no signal output in the dark environment, and the experimental results show that there is open circuit voltage and short circuit current, which is because there is an interface in the ferroelectric photovoltaic device, and there would be many defects on the interface, in which the defect charge and vacancy charge will form an internal electric field [34]. As a result, it causes the BNFMO films to also exhibit V oc and I sc in a dark environment.  The Nd-doping not only reduces the dark current density but also raises the photocurrent density, and the Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 thin film exhibits a more than 35-fold enhancement compared with the BFMO film, indicating that the ferroelectric photovoltaic effect is regulated by the ferroelectric polarization. The photocurrent densities of BNFMO (x = 0.00~0.20) films are −52, −108, −1067, −1758, and −964 µA/cm 2 , while the photovoltages are 0.020, 0.050, 0.090, 0.135, and 0.065 V, respectively. Figure 7c demonstrates the change in the J SC and V OC with Nd-doping amount. The power conversion efficiencies (η = P out /P in = JV/P in ) at the point (J, V) of the BNFMO films are shown in Figure 7d. To verify the reliability of the ferroelectric photovoltaic phenomenon, we measured the time dependence of the photocurrent density and photovoltage of Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 film by controlling the light source to turn on and off alternately every 12.5 s, as shown in Figure 7e,f. Obviously, there is no attenuation of the photocurrent density and photovoltage during several cycles. In addition, it should be noted that our experimental results are numerically much larger in short-circuit current compared to reported literature results of Nd-doped BFO. Table 1 [29] found that the ferroelectric film with a thickness of approximately 30-40 nm demonstrated the strongest photocurrent. Secondly, the crystal structure of the epitaxial thin film has minimal defects, enhancing the lifetime of photogenerated carriers. Thirdly, we utilized a relatively high dielectric constant NSTO as the bottom electrode. The high-dielectric-constant electrode tends to distribute the screening charges more widely away from the interface; thus, weakening the polarization screening effect. Fourthly, the electrons and holes can be separated effectively by the large depolarization field which derived from a larger remnant polarization. Finally, Shi et al. [32] reported that the band gap of the BFO film was reduced by Nd-doping, which allows to collect solar spectrum energy effectively for BFO film. Thus, a larger short circuit photocurrent density (1758 µA/cm 2 ) was generated by Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 film.

Conclusions
In summary, Bi 1−x Nd x Fe 0.99 Mn 0.01 O 3 (x = 0.00~0.20) films with a thickness of 40 nm were prepared on Nb-doped SrTiO 3 substrates by PLD to form Au/BNFMO/NSTO heterojunctions. The original rhombohedral structure of the BiFeO 3 thin films remained when the doping amount of Nd ion was less than 16%, while the BNFO thin film structure became orthogonal when the doping amount of Nd 3+ ion reached 20%. To avoid the resistive switching effect and decrease the leakage current, the structure of two de-vices in series was adopted for ferroelectric hysteresis loop measurement. The maximum remnant polarization (P r ) and minimum coercive voltage (V c ) were obtained under the condition of 16% Nd-doping. In addition, we obtained the maximum photocurrent density of 1758 µA/cm 2 in Bi 0.84 Nd 0.16 Fe 0.99 Mn 0.01 O 3 thin film. This is larger than that in previous reports in Nd-doped BFO films under the same light intensity.
Thus, the BNFMO films show great potential for ferroelectric and photovoltaic applications.

Data Availability Statement:
All data that support the findings of this study are included within the article.

Conflicts of Interest:
The authors declare no competing financial interest.