Effect of (Zn,Mn) co-doping on the structure and ferroelectric properties of BiFeO3 thin films

BiFe_1-2xZn_xMn_xO₃ (BFZMO, with x = 0–0.05) thin films were synthesized via sol–gel method. Effects of (Zn, Mn) co-doping on the structure, ferroelectric, dielectric, and optical properties of BiFeO₃ (BFO) films were investigated. BFZMO thin films exhibit rhombohedral structure. Scanning electron microscopy (SEM) images indicate that co-doping leads to a decrease in grain size and number of defects. Leakage current density (4.60 × 10^?6 A/cm²) of BFZMO film with x = 0.02 was found to be two orders of magnitude lower than that of pristine BFO film. Owing to decreased leakage current density, saturated P–E curves were obtained. Maximum double remnant polarization of 413.2 μC/cm² was observed for BFZMO thin film with x = 0.02, while that for the BFO film was found to be 199.68 μC/cm². The reason for improved ferroelectric properties is partial substitution of Fe ions with Zn and Mn ions, which resulted in a reduction in the effect of oxygen vacancy defects. In addition, co-doping was found to decrease optical bandgap of BFO film, opening several possible routes for novel applications of these (Zn, Mn) co-doped BFO thin films.     

Tunable multiferroic properties of Mn substituted BiFeO3 thin films

The current study explored the influence of Mn substitution on the electrical and magnetic properties of BiFeO₃ (BFO) thin films synthesized using low cost chemical solution deposition technique. X-ray diffraction analysis revealed that pure rhombohedral phase of BiFeO₃ was transformed to the tetragonal structure with P4mm symmetry on Mn substitution. A leakage current density of 5.7×10⁻⁴ A/cm² which is about two orders of magnitude lower than pure BFO was observed in 3% Mn doped BFO thin film at an external electric field >400 kV/cm. A well saturated (p-E) loops with saturation polarization (P_sat) and remanent polarization (2P_r) as high as 60.34 µC/cm² and 25.06 µC/cm² were observed in 10% Mn substituted BFO thin films. An escalation in dielectric tunability (n_r), figure of merit (K) and quality factor (Q) were observed in suitable Mn doped BFO thin films. The magnetic measurement revealed that Mn substituted BFO thin films showed a large saturation magnetization compared to pure BFO thin film. The highest saturation ~31 emu/cc was observed for 3% Mn substituted BFO thin films.     

Influences of Cr Doping on the Electrical Properties in BiFeO3 Thin Films

Cr-doped BiFeO₃ (BFO) thin films were deposited on Pt(200)/TiO₂/SiO₂/Si(100) substrates by a chemical solution deposition method. The dielectric constant and dissipation factor of the BFO thin films decrease from 165 and 0.054 (undoped) to 100 and 0.02 (3 mol% Cr-doped) at a frequency of 10 kHz as the Cr content increases. The leakage current and ferroelectric properties were improved significantly by means of Cr doping. The leakage current density of 4.1×10^-6 A/cm² for the 3 mol% Cr-doped BFO thin film is about four orders of magnitude lower than that of undoped BFO thin film at an external electric field of 100 kV/cm. The 3 mol% Cr-doped BFO thin film exhibited a well-saturated hysteresis loop with a large remanent polarization (P_r) of 61 μC/cm² at room temperature. The reason for the improved leakage current and ferroelectric properties in Cr-doped BFO thin films can be attributed to the reduced oxygen vacancies in the films by Cr doping.

Communicated by Dr. George W. Taylor     

Solution-processed BiFeO3 thin films with low leakage current

Bismuth ferrite (BiFeO₃) is an attractive multiferroic material that shows strong ferroelectric and antiferromagnetic properties. Nevertheless, producing high-quality oriented BiFeO₃ on technology-important platinized silicon substrates by low-cost solution deposition methods is still challenging. In this work, polycrystalline Mn and Ti co-doped BiFeO₃ (BFO) thin films were fabricated on platinized silicon substrates by a solution deposition method. PbTiO₃ nanocrystals were used as a seed layer between the electrode and the BFO thin films to induce a preferential (100) pseudocubic orientation. We show that the introduction of a PbTiO₃ seed layer strongly reduces the leakage current. The films show excellent room-temperature ferroelectric properties at low frequencies (300 Hz), with epitaxial-like remanent polarization as high as 51 μC/cm² and coercive field of 500 kV/cm.     

(001)-oriented BiFeO3 films integrated on Si with enhanced electrical performances

Highly (001)-oriented pure-phase BFO films were prepared on traditional Si substrates via radio frequency magnetron sputtering (RFMS). The crystallinity of the films is found to be increased, that is, higher degree of (001) texture, larger grain size, less grain boundary, denser surface morphology, and better thickness uniformity, with increased film thickness. These factors have significant influences on the electrical properties of BFO films, that is, dielectric response, as well as ferroelectric polarization and leakage current characteristics. The 240-nm-thick film exhibits relatively poor electrical properties compared with other three thicker films, which is mainly due to its small grain, the enhancement of the clamping effect of neighboring grains, and the absence of domain walls. The essential roles of the evolution and distribution of grains/domains and defect charges in leakage mechanism and ferroelectric switching polarization were also investigated systematically. It was found that 600-nm-thick BFO film has the lowest leakage current density (as low as 1.8 × 10⁻⁶ A / cm² @ 90 kV/cm) and followed a mixed SE or SCLC conduction behavior, while the leakage behavior in other films is dominated by SE and P-F currents. All (001)-BFO films have a giant electrical polarization which is solely originated from the contribution of ferroelectric domain switching, and it has lower switching voltage and faster switching rate in thicker films.     

Multiferroic properties of La/Er/Mn/Co multi-doped BiFeO3 thin films

Bi_0.9-xLa_xEr_0.1Fe_0.96Co_0.02Mn_0.02O₃ (BLa_xEFMCO) thin films were prepared by sol-gel method. The grain size, grain boundary resistance, oxygen vacancies and the amount of Fe²⁺ of the films were reduced by multi-ion doping to reduce the built-in electric field of the films. An applied voltage was adopted to regulate the effects of the directional alignment of the oxygen vacancies, defects, and defect pairs on the ferroelectric domains at the grain boundaries to control the ferroelectric polarization of the films. Meanwhile, the capacitance peak also reveals the effects of the ferroelectric domains switching, the migration of oxygen vacancies, and the directional alignment of defect pairs on the ferroelectric properties. In addition, the remnant polarization value of the BLa_0.01EFMCO thin film reaches 152?μC/cm², the squareness of the hysteresis loop (R_sq) is calculated to be 1.03, and the maximum switching current is 1.50?mA. The typical butterfly curves under positive and negative electric fields indicate the films with the enhanced ferroelectric properties. Moreover, the BLa_0.01EFMCO thin film exhibits the enhanced ferromagnetic properties, and its saturation magnetization (M_s) is 2.32 emu/cm³. Therefore, the ferroelectric properties of the BFO film can be enhanced by the multi-ion doped BFO film to reduce the grain boundary resistance (Rgb), the interface Schottky barrier formed by the asymmetric electrode material at the top and bottom of the film, and the built-in electric field formed by the film internal defect or defect pairs.     

Synthesis and properties of multiferroic 0.7BiFeO3−0.3BaTiO3 thin films by Mn doping

Multiferroic BiFeO₃?BaTiO₃ thin films that simultaneously exhibit ferroelectricity and ferromagnetism at room temperature were prepared by chemical solution deposition. Perovskite single-phase 0.7BiFeO₃?0.3BaTiO₃ thin films were successfully fabricated in the temperature range 600–700 °C on Pt/TiO_x/SiO₂/Si substrates. As the crystallization temperature was increased, grain growth proceeded, resulting in higher crystallinity at 700 °C. Although the 0.7BiFeO₃?0.3BaTiO₃ thin films exhibited poor polarization (P)?electric field (E) hysteresis loops owing to their low insulating resistance. The leakage current at high applied fields was effectively reduced by Mn doping at the Fe site of the 0.7BiFeO₃?0.3BaTiO₃ thin films, leading to improved ferroelectric properties. The 5 mol% Mn-doped 0.7BiFeO₃?0.3BaTiO₃ thin films simultaneously exhibited ferroelectric polarization and ferromagnetic magnetization hysteresis loops at room temperature.     

Ferroelectric,dielectric properties and electrical conduction mechanism of epitaxial B1-xDyxFeO3 (x = 0.05, 0.075, 0.1, 0.125) thin films prepared by pulsed laser deposition

We report that Dy doping can be used to enhance ferroelectric properties and reduce leakage current in (001) epitaxial Bi_1-xDy_xFeO₃ (BD_xFO) thin films where x = 0.05, 0.075, 0.1, 0.125. The 120 nm thin BD_xFO films were prepared by pulsed laser deposition (PLD). Saturated ferroelectric hysteresis (P-E) loops were observed at room temperature with an increase in remanent polarization (P_r) with the increase in applied electric field and Dy concentration. The improved P_r in BD_xFO films from x = 0.05 to 0.1 is attributed to reduction in oxygen vacancies with an increase in Dy content that prevents electric domain back-switching. However, BD_xFO (x = 0.125) film exhibit leaky and unsaturated loop which may be due to the appearance of microscopic defects in the film. The leakage current in BD_xFO films decreased by an order of magnitude as Dy content increased from x = 0.05 to 0.1. The detailed leakage current mechanism analysis is also presented. The results of positive up negative down (PUND) method confirmed that intrinsic polarization is due to ferroelectricity and not due to leakage. Relative permittivity (ɛ') of BD_xFO films increased whereas dielectric loss tangent (tan δ) reduced as Dy content increased from x = 0.05 to 0.1.     

Enhanced dielectric and energy-storage properties in BiFeO3-modified Bi0.5(Na0.8K0.2)0.5TiO3 thin films

Lead free Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ thin films doped with BiFeO₃ (abbreviated as BNKT-xBFO) (x = 0, 0.02, 0.04, 0.08, 0.10) were deposited on Pt(111)/Ti/SiO₂/Si substrates by sol-gel/spin coating technique and the effects of BiFeO₃ content on the crystal structure and electrical properties were investigated in detail. The results showed that all the BNKT-xBFO thin films exhibited a single perovskite phase structure and high-dense surface. Reduced leakage current density, enhanced dielectric and ferroelectric properties were achieved at the optimal composition of BNKT-0.10BFO thin films, with a leakage current density, dielectric constant, dielectric loss and maximum polarization of < 2 × 10⁻⁴ A/cm³, ~ 978^, ~ 0.028 and ~ 74.13 μC/cm² at room temperature, respectively. Moreover, the BNKT-0.10BFO thin films possessed superior energy storage properties due to their slim P-E loops and large maximum polarization, with an energy storage density of 22.12 J/cm³ and an energy conversion efficiency of 60.85% under a relatively low electric field of 1200 kV/cm. Furthermore, the first half period of the BNKT-0.10BFO thin film capacitor was about 0.15 μs, during which most charges and energy were released. The large recoverable energy density and the fast discharge process indicated the potential application of the BNKT-0.10BFO thin films in electrostatic capacitors and embedded devices.     

Investigation of Tb-doping on structural transition and multiferroic properties of BiFeO3 thin films

Pure BiFeO₃ (BFO) and Bi_1−xTb_xFeO₃ (BTFO) thin films were successfully prepared on FTO (fluorine doped tin oxide) substrates by the sol–gel spin-coating method. The effects of Tb-doping on the structural transition, leakage current, and dielectric and multiferroic properties of the BTFO thin films have been investigated systematically. XRD, Rietveld refinement and Raman spectroscopy results clearly reveal that a structural transition occurs from the rhombohedral (R3c:H) to the biphasic structure (R3c:H+R-3m:R) with Tb-doping. The leakage current density of BTFO_x=0.10 thin film is two orders lower than that of the pure BFO, i.e. 5.1×10⁻⁷ A/cm² at 100 kV/cm. Furthermore, the electrical conduction mechanism of the BTFO thin films is dominated by space-charge-limited conduction. The two-phase coexistence of BTFO_x=0.10 gives rise to the superior ferroelectric (2P_r=135.1 μC/cm²) and the enhanced ferromagnetic properties (M_s=6.3 emu/cm³). The optimal performance of the BTFO thin films is mainly attributed to the biphasic structure and the distorted deformation of FeO₆ octahedra.     

Temperature dependence of ferroelectric property and leakage mechanism in Mn-doped Pb(Zr0.3Ti0.7)O3 films

High-quality ferroelectric films of Mn-doped Pb(Zr_0.3Ti_0.7)O₃ (PMZT) were prepared using the sol-gel method, and the temperature dependence of ferroelectric, dielectric, and leakage current properties (J–E) were explored in detail using the top electrode/ferroelectric films/bottom electrode capacitor heterostructure. The enhancement of polarization and dielectric properties by element doping is clearly observed by 3% Mn-doping. Such enhancement is beneficial for the application of these films in ferroelectric random-access memory. In addition, the analysis of leakage current reveals symmetric behavior with 3% Mn-doping and the leakage current density gradually increases with increasing temperature, which may be due to the movement of domain wall and oxygen vacancy. The dominant leakage current conduction mechanism is bulk-limited ohmic or interface-limited Schottky emission conduction within a wide temperature range. The results might be meaningful for further work on ferroelectric electrical devices with improved ferroelectric and dielectric properties.     

Ferroelectric properties of PZT/BFO multilayer thin films prepared using the sol-gel method

In this study, Pb(Zr_0.52Ti_0.48)O₃/BiFeO₃ [PZT/BFO] multilayer thin films were fabricated using the spin-coating method on a Pt(200 nm)/Ti(10 nm)/SiO₂(100 nm)/p-Si(100) substrate alternately using BFO and PZT metal alkoxide solutions. The coating-and-heating procedure was repeated several times to form the multilayer thin films. All PZT/BFO multilayer thin films show a void-free, uniform grain structure without the presence of rosette structures. The relative dielectric constant and dielectric loss of the six-coated PZT/BFO [PZT/BFO-6] thin film were approximately 405 and 0.03%, respectively. As the number of coatings increased, the remanent polarization and coercive field increased. The values for the BFO-6 multilayer thin film were 41.3 C/cm² and 15.1 MV/cm, respectively. The leakage current density of the BFO-6 multilayer thin film at 5 V was 2.52 × 10^-7 A/cm².     

Structural,electric and multiferroic properties of Sm-doped BiFeO3 thin films prepared by the sol–gelprocess

Pure polycrystalline Bi_1−xSm_xFeO₃ (BSFO) (x=0–0.12) thin films were successfully prepared on FTO/glass substrates by the sol–gel method. The influence of Sm doping on the structure, dielectric, leakage current, ferroelectric and ferromagnetic properties of the BSFO films was investigated. X-ray diffraction analysis and FE-SEM images both reveal a gradual rhombohedra to pseudo-tetragonal phase transition with the increase of Sm dopant content. On one hand, a proper amount of Sm doping can decrease the leakage current densities of the BSFO thin films. On the other hand, excess Sm substitution for Bi will lead to multiphase coexistence in the film, the lattice inhomogeneity results in more defects in the film, which can increase the leakage current density. The result shows that defects in the complexes lead to electric domain back-switching in the BSFO_x=0.06 thin film, resulting in a decreased dielectric constant, leakage current and remanent polarization. The BSFO_x=0.09 thin film is promising in practical application because of its highest dielectric constant, remanent polarization and remanent magnetization of 203–185, 70 μC/cm² and 1.31 emu/cm³, respectively.     

Tailoring the multiferroic properties of BiFeO3 by co-doping Er at Bi site with aliovalent Nb,Mn and Mo at Fe site

Single phase multiferroic undoped BiFeO₃ notoriously suffers due to the poor spin–charge coupling resulting in limitations to device applications. The present work focuses on the tailoring of its multiferroic and magnetoelectric coupling properties by synthesizing multiferroic Bi_0.95Er_0.05Fe_0.98TM_0.02O₃ (TM = Nb, Mn and Mo) ceramics. The ferroelectric, magnetic, current leakage measurements and magnetoelectric effect were investigated. XRD along with the Reitveld refinement results confirms that all the samples possess perovskite based rhombohedral structure and reveals that doping of (Er, Nb), (Er, Mn) and (Er, Mo) induced the crystallographic distortion in the BFO lattice and hence induced a variation in the bond lengths and bond angle. Dual doping significantly enhanced the electrical, magnetic properties and magnetoelectric coupling as compared to BiFeO₃. Doping has lowered the leakage current by three to four orders compared to BFO. The lattice distortion, reduced leakage current and destruction of spin–cycloidal structure could be the origin for these improved features. The (Er, Nb) doped BiFeO₃ yields enhanced ferroelectric character with the maximum polarization value of 0.46 µC/cm², maximum ME coupling of 0.22 mV/cm at a magnetic field of 130 G, an improved magnetization with a remanance value of 0.0903 emu/g and the lowest leakage current density.     

Significantly improved energy storage properties of sol-gel derived Mn-modified SrTiO3 thin films

In this paper, x mol% Mn-doped SrTiO₃ (STMx, x?=?0, 0.5, 1, 3 and 5) thin films were synthesized by a sol-gel method. The effect of Mn doping on the microstructure and electrical performance was investigated. STMx (x?≤?1) thin films shows a single cubic perovskite phase while impurity phase appears for STM3 and STM5 thin films confirmed by X-ray diffraction. X-ray photoelectron spectra reveals that STM1 thin film has the lowest concentration of oxygen vacancy. The dielectric constant and loss of STMx (x?≤?1) films display good frequency stability, while decrease with the frequency for STM3 and STM5 thin films. And all samples display excellent bias stability of dielectric constant; this is advantageous for applications in a high electric field. The ferroelectric test demonstrates that the electrical breakdown strength increases and leakage current decreases for Mn doped SrTiO₃ films. A great recoverable energy storage density of 23.8?J/cm³ with an efficiency of 69.8% at 2.286?MV/cm is obtained in STM1 thin film. Furthermore, STM1 thin film shows good frequency stability of energy storage properties. It indicates that Mn doping is a simple and effective method to improve the energy storage properties of dielectric capacitors.     

Electrical properties of chemical solution deposited (Bi0.9RE0.1)(Fe0.975Cu0.025)O3−δ (RE=Ho and Tb) thin films

Pure BiFeO₃ (BFO) and (Bi_0.9RE_0.1)(Fe_0.975Cu_0.025)O_3?δ (RE=Ho and Tb, denoted by BHFCu and BTFCu) thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. The BHFCu and BTFCu thin films showed improved electrical and ferroelectric properties compared to pure BFO thin film. Among them, the BTFCu thin film exhibited large remnant polarization (2P_r), low coercive field (2E_c) and reduced leakage current density, which are 89.15 C/cm² and 345 kV/cm at 1000 kV/cm and 5.38×10^?5 A/cm² at 100 kV/cm, respectively.     

Substantial reduction of leakage currents in La/Er/Zn/Ti multielement-doped BiFeO3 multiferroic thin films

Multi-element doping is an effective method to suppress the leakage of BiFeO₃ (BFO). A systematic study on the effect of various elements (La, Er, Zn, Ti) doping on the leakage performance, mechanism and other electrical properties of BFO films was performed As the kinds of doping elements increases, the leakage current density of the BFO film gradually decreases. The leakage current density is gradually reduced from 5.78 × 10^?2 A/cm² doped with one element (La) to 1.25 × 10^?2 A/cm² doped with two elements (La, Ti), 4.13 × 10^?3 A/cm² doped with three elements (La, Ti, Zn), and 4.53 × 10^?4 A/cm² doped with four elements (La, Er, Zn, Ti). Finally, compared with pure BFO films, the leakage current density in doped BFO films is reduced by two orders of magnitude. Moreover, the conduction mechanism in doped BFO films is gradually changed from space charge limited current to ohmic conduction. This work provides an effective method to ameliorate the leakage of ferroelectric materials and lays a foundation for the practical application of BFO-based films.     

Piezoresponse force microscopy characterization of rare-earth doped BiFeO3 thin films grown by the soft chemical method

The multiferroic behavior with ion modification using rare-earth cations on crystal structures, along with the insulating properties of BiFeO₃ (BFO) thin films was investigated using piezoresponse force microscopy. Rare-earth-substituted BFO films with chemical compositions of (Bi_1.00−xRE_xFe_1.00O₃ (x=0; 0.15), RE=La and Nd were fabricated on Pt (111)/Ti/SiO₂/Si substrates using a chemical solution deposition technique. A crystalline phase of tetragonal BFO was obtained by heat treatment in ambient atmosphere at 500 °C for 2 h. Ion modification using La³⁺ and Nd³⁺ cations lowered the leakage current density of the BFO films at room temperature from approximately 10⁻⁶ down to 10⁻⁸ A/cm². The observed improved magnetism of the Nd³⁺ substituted BFO thin films can be related to the plate-like morphology in a nanometer scale. We observed that various types of domain behavior such as 71° and 180° domain switching, and pinned domain formation occurred. The maximum magnetoelectric coefficient in the longitudinal direction was close to 12 V/cm Oe.     

Microstructures,magnetic, and dielectric properties of Ba-doped BiFeO3 nanoparticles synthesized via molten salt route

As the paradigm of magnetoelectric multiferroic materials, BiFeO₃ (BFO) has potential applications in spintronics, memory devices, sensors, and actuators. However, its large leakage current and small magnetism at room temperature restrict its practical applications. It is demonstrated that the substitutions of Bi by alkali earth elements at A-site of BFO can significantly reduce the leakage current and enhance the remanent magnetization of BFO. In this work, Ba-doped BFO nanoparticles Bi_1-xBa_xFeO₃ (x = 0, 0.05, 0.10, 0.15 and 0.20) were synthesized via molten salt route. X-ray diffraction patterns revealed that with increasing the Ba-doped content the formation of the impurity phase was depressed and the rhombohedral distortions of these nanoparticles were suppressed, as confirmed by Raman spectra. X-ray photoelectron spectroscopy measurements reveal that the Fe element in the nanoparticles exists in the dual valence states of Fe³⁺ and Fe²⁺, and two kinds of oxygen atoms (lattice oxygen atoms and the adsorbed oxygen atoms) exist in the nanoparticles. With increasing the Ba-doped content, the content ratios of Fe³⁺ to Fe²⁺ ions were generally increased, whereas the oxygen vacancy concentrations were decreased. The average particle sizes of the Ba-doped BFO nanoparticles were decreased as compared with that of nondoped BFO nanoparticles. In contrast, the room temperature magnetization of the Ba-doped BFO nanoparticles was greatly enhanced by Ba-substitution, as confirmed by the M-H loops. At room temperature, the remanent magnetization and coercive field of the Bi_0.8Ba_0.2FeO₃ nanoparticles were 0.51 emu/g and 1130 Oe, respectively. Furthermore, the leakage current density was reduced by one order of magnitude at x = 0.2 and the dielectric properties are also improved by Ba-substitution. The improvements on the remanent magnetization, leakage current density as well as dielectric properties of the Ba-doped BFO nanoparticles make them promising candidates for spintronics and dielectric energy storages.     

Ferroelectric switching and current characteristics dependence on ferroelectric polarization direction of microwave synthesized BiFeO3 nanocubes

Herein, we studied the ferroelectric switching and current characteristics of BiFeO₃ (BFO) nanocubes dispersed on the surface of a Nb-doped SrTiO₃ (Nb:STO) substrate based on the ferroelectric polarization orientation. The microwave synthesis method afforded BFO nanocubes with an average size of ～50 nm, which were dispersed on the Nb:STO substrate surface and the substrate was subsequently subjected to heat treatment at 500 °C for 1 h. The piezoelectric d₃₃ hysteresis loop, ferroelectric domain structure, and ferroelectric polarization switching characteristics of the 50-nm-sized BFO nanocubes were examined using piezoresponse force microscopy. Finally, atomic force microscopy confirmed the dependency of current characteristics on the ferroelectric polarization orientation of the BFO nanocubes, verifying the applicability of BFO nanocubes as storage media for ferroelectric polarization information.