Co-doped BiFeO3 Thin Films Research Articles

Significantly improved ferroelectric properties of (Zn,Co) co-doped BiFeO3 thin films prepared by sol-gel method

BiFe1-2xZnxCoxO3 (BFZCO) thin films (x = 0 %, 1 %, 2 %, 3 %, 4 %, 5 %) were deposited on FTO/glass substrates by sol-gel method. We systematically investigated the effects of equal doping of (Zn, Co) on the crystal microstructure, leakage current, leakage mechanism, and ferroelectric properties of BFZCO thin films. The XRD patterns indicate that all samples matched the chalcogenide structure without any impurity phases. The BiFe0.96Zn0.02Co0.02O3 (BFZCO-2) film exhibited a large residual polarization (2Pr = 339.9 μC/cm2) at an electric field strength of approximately 2000 kV/cm. The curve of leakage current density shows that equal doping of (Zn, Co) can considerably decrease the leakage current of BFO thin films at low electric fields. Furthermore, the mechanism of leakage conduction has shifted from Ohmic conduction at low electric fields to SCLC conduction at high electric fields.

Photovoltaic Effect of La and Mn Co-Doped BiFeO3 Heterostructure with Charge Transport Layers.

Bismuth ferrite BiFeO3 (BFO)-based ferroelectrics have great potential as inorganic perovskite-like oxides for future solar cells applications due to their unique physical properties. In this work, La and Mn co-doped BFO thin films with compositions Bi0.9La0.1(Fe1-xMnx)O3 (x = 0, 0.05, 0.1, 0.15) (denoted as BLF, BLFM5, BLFM10, BLFM15, respectively) were prepared via a sol-gel technique on indium tin oxide (ITO) glass. All the films are monophasic, showing good crystallinity. The optical bandgap Eg was found to decrease monotonously with an increase in the Mn doping amount. Compared with other compositions, the BLFM5 sample exhibits a better crystallinity and less oxygen vacancies as indicated by XRD and XPS measurements, thereby achieving a better J-V performance. Based on BLFM5 as the light absorbing layer, the ITO/ZnO/BLFM5/Pt and ITO/ZnO/BLFM5/NiO/Pt heterostructure devices were designed and characterized. It was found that the introduction of the ZnO layer increases both the open circuit voltage (Voc) and the short circuit current density (Jsc) with Voc = 90.2 mV and Jsc = 6.90 μA/cm2 for the Pt/ BLFM5/ZnO/ITO device. However, the insertion of the NiO layer reduces both Voc and Jsc, which is attributed to the weakened built-in electric field at the NiO/BLFM5 interface.

Bandgap modulation and phase boundary region of multiferroic Gd, Co co-doped BiFeO3 thin film

Ferroelectric polarization is a crucial factor to induce photovoltaic effect in ferroelectric materials. Here, a novel modulation of bandgap by Gd and Co co-doped BFO is found for a polycrystalline Bi0.9Gd0.1Fe0.85Co0.15O3 thin film prepared by the sol–gel process. The ferroelectric properties, magnetic properties, and bandgap of the BFO films were altered by doping Gd and Co. This work has led to a greater understanding of bismuth ferrate, and it proposes the Bi0.9Gd0.1Fe0.85Co0.15O3 thin film for the possibility of better preparation of high conversion efficiency ferroelectric photovoltaic devices.

Enhancement of ferromagnetism in a multiferroic La–Co co-doped BiFeO3 thin films

BiFeO3-based materials have attracted considerable attention owing to their room-temperature multiferroic properties and ultrahigh ferroelectric polarization. In this study, La and Co co-doped BiFeO3 (BLFCO) thin films were fabricated on Pt (111)/Ti/SiO2/Si substrate buffered by Nb-doped (0.7 wt.%) SrTiO3 via pulsed laser deposition. Experiments revealed that the film deposited at 0.2 Pa comprised the predominant orientation of BiFeO3 (BFO) (111), and its surface root-mean-square roughness was 0.96 nm. The saturation magnetization of the film reached 25.3 emu cm3, which was an order of magnitude higher than that of the pure BFO film, as well as the remanent magnetization reached 1.8 emu cm−3. The result was attributed to the predominant orientation and small grain size of BLFCO films. The piezo-response force microscopy measurements revealed the co-doped film possesses well repeatable performance of polarization reversal, and the presence of ferroelectric orders with an asymmetric ‘butterfly’ structure. These results are helpful for further improving the performance of BFO multifunctional devices.

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

BiFe1-2xZnxMnxO3 (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.

Spin-casted (Gd–Zn) co-doped BiFeO3 thin films for sustainable oxide-electronics

The emerging paradigm of oxide-based devices are reshaping the frontiers of sustainable electronics and enabling new functionalities like ferroelectric-photovoltaics, photo-catalytic activity, ferroelectric logics, and terahertz-scale actuation. The intriguing advantages of BiFeO3 (BFO) based oxide electronics offers the possibility of combining electric and magnetic degrees of freedom and is of interest in applications ranging from non-volatile random-access memories, multiple state storage media, tunnelling barriers, actuation, and sensors. Here, we investigate structural evolution and device performance of a cost-effective and unexplored BFO doped derivative i.e. (Gd, Zn) co-doped BFO thin films synthesized using commercially viable spin-casting technique. Owing to the competing change in dopant cation sizes w.r.t. Host lattice, ABO3 rhombohedral perovskite structure of the BFO did not transform while the optical band gap sequentially reduced from 2.65 to 2.53 eV with increasing Gd–Zn co-dopant concentration. Further, the effects of co-doping and asymmetric electrodes on the IV-characteristics of capacitors and the underlying conduction mechanism of these devices are investigated. Photoconductivity studies show three orders larger photocurrent arising from grain-boundary contributions in 2% (Gd, Zn) – BFO film exhibiting rapid, stable and repeatable photo-response which makes them useful for photosensitive capacitor applications.

Enhanced Magnetic Properties of Co-Doped BiFeO3 Thin Films via Structural Progression.

Co3+ doping in BiFeO3 is expected to be an effective method for improving its magnetic properties. In this work, pristine BiFeO3 (BFO) and doped BiFe1-xCoxO3 (BFCxO, x = 0.01, 0.03, 0.05, 0.07 and 0.10) composite thin films were successfully synthesized by a sol–gel technique. XRD and Raman spectra indicate that the Co3+ ions are substituted for the Fe3+ ion sites in the BFO rhombohedral lattice. Raman vibration of oxygen octahedron is obviously weakened due to the lattice distortion induced by the size mismatch between two B-site cations (Fe3+ and Co3+ ions), which has an impact on the magnetic properties of BFCxO. SEM images reveal a denser agglomeration in Co-doped samples. TEM results indicate that the average size of grains is reduced due to the Co3+ substitution. XPS measurements illustrate that the replacement of Fe3+ with Co3+ effectively suppresses the generation of oxygen defects and increases the concentration of Fe3+ ions at the B-site of perovskite lattice. Vibrating sample magnetometer (VSM) measurements show that the remanent magnetization (Mr) of BFC0.07O (3.6 emu/cm3) and the saturation magnetization (Ms) of BFC0.10O (48.84 emu/cm3) thin film both increase by approximately two times at room temperature, compared with that of the pure BFO counterpart.

Nd-Cr co-doped BiFeO3 thin films for photovoltaic devices with enhanced photovoltaic performance

BiFeO3 films and Nd-Cr co-doped BiFeO3 films were prepared by sol-gel method followed by spinning process on fluorine-doped tin oxide glass substrates. By testing the ultraviolet-visible absorption spectra, it was found that Nd-Cr co-doping will increase the light absorption rate of the film and reduce the optical band gap. The reduced bandgap can facilitate the transport of carriers. After Nd-Cr co-doping, the leakage current of the film is effectively reduced, which is near four orders of magnitude lower than the leakage current density of the pristine BiFeO3 film. The reduction of leakage current will enhance the ferroelectric polarization. The enhancement of ferroelectric polarization is more favorable for the separation of photogenerated carriers. Compared with the pristine BiFeO3 film, the short circuit photocurrent density, open circuit photovoltage and power conversion efficiency of Nd-Cr co-doped BiFeO3 film are all clearly improved. The Nd-Cr co-doped BiFeO3 films exhibited largely enhanced photovoltaic property, which favored the practical application of BiFeO3-based films in photovoltaic devices.

Expansion of the spin cycloid in multiferroic BiFeO3 thin films

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO3, a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11bar 2] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to TN. These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Modulation of oxygen vacancies assisted ferroelectric and photovoltaic properties of (Nd, V) co-doped BiFeO3 thin films

We are reporting on the improved ferroelectric and photovoltaic properties of (Nd3+, V5+) co-doped (Bi0.95Nd0.05)(Fe1−xVx)O3 (BNFVO) (x = 0.01, 0.03) thin films grown by PLD. BNFVO thin films showed reduced leakage current, lower optical bandgap, and improved ferroelectricity compared to BFO films, which can be explained by valance change and extinguishing oxygen vacancies due to the doping effect. Piezoresponse force microscopy measurements showed an improved domain back switching in doped thin films indicating that the suppression of oxygen vacancies offset the effect of polarization flipping caused by doping. Further, we found a relatively stable and enhanced photovoltaic effect in BNFVO films with an order of magnitude higher photocurrent and almost doubled photovoltage in comparison to BFO films, which can be explained by less recombination between hopping electrons and oxygen vacancies. Our results demonstrate the significance of dopant selection to suppress the oxygen vacancies for improved ferroelectric and photovoltaic properties of BFO films.

The microstructure, leakage current and dielectric behaviors of (Nd,Ti)-codoped BiFeO3 thin films: effect of deposited substrate

(Nd,Ti)-codoped BiFeO3 (BNFTO) thin films have been prepared on LaNiO3 (LNO)/Si, indium tin oxide (ITO)-coated glass, and monocrystalline silicon (Si) substrates by metal organic decomposition combined with sequential layer annealing. The influences of the various substrates on crystal structure, surface and cross-sectional images, insulating and dielectric properties were mainly investigated. All the samples crystallize into the pure rhombohedral perovskite structure with different relative intensities of (l00) peaks. Compared with other two films, BNFTO on ITO/glass shows lower leakage current due to the dense microstructure with relatively small grains. The voltage shift degree of capacitance–voltage curve is not same for BNFTO on ITO/glass and LNO/Si, which can be ascribed to the different content of defect complexes in both samples. The memory window of BNFTO on Si is 1.4 V at the applied voltage of ±5 V. The values of the figure of merit are 7.3 and 9.5 for BNFTO on ITO/glass and LNO/Si substrates, respectively. These present findings suggest that the microstructure and dielectric property depend strongly on the using substrate.

Effect of substrate temperature on the structural and electrical properties of La and Mn co-doped BiFeO3 thin films

The effect of substrate temperature on the structural and electrical properties of multiferroic Bi0.90La0.10Fe0.95Mn0.05O3 (BLFMO) thin films deposited on Pt (111)/Ti/SiO2/Si substrate using pulsed laser deposition (PLD) has been investigated. Films with substrate temperature ranging from 450°C to 650°C have been deposited. The grain size and roughness are found to increase with substrate temperature. The film deposited at 575°C exhibits maximum remnant polarization around 39μC/cm2 and a coercive field of 400kV/cm. The cyclic fatigue study of the sample shows only 4% loss after 108cycles. Complex impedance study of the BLFMO thin films demonstrates electrical homogeneity of the sample. AC conductivity data has been fitted using Jonscher single power law, and value of n found to be <1 indicating translational hopping conduction mechanism. The lowest leakage current found is 4.37×10−6A/cm2 at 575°C. The leakage current mechanism is found to be dominated by space charge limited current and Fowler-Nordheim conduction mechanism.

Growth of epitaxial Mn and Zn codoped BiFeO3 thin films and an enhancement of photovoltage generated by a bulk photovoltaic effect

Recently, the bulk photovoltaic effect of BiFeO3 (BFO) thin films has attracted much attention because of its above bandgap photovoltage for realizing novel photovoltaic devices. In this study, the epitaxial growth of 1-µm-thick Mn and Zn codoped BFO thin films has been demonstrated, and the effects of Mn and Zn codoping on the ferroelectric and bulk photovoltaic properties of the BFO thin films have been investigated. A 0.5% Mn and 0.5% Zn codoped BFO (BFMZO050) thin film on a SrRuO3-buffered vicinal-SrTiO3(001) substrate showed an atomically flat surface with a step-and-terrace structure, a low leakage current of 1.5 × 10−6 A/cm2 at 100 kV/cm, and well-saturated ferroelectric electric displacement–electric field (D–E) hysteresis loops. In addition, a Pt/BFMZO/Pt coplanar capacitor with an interelectrode distance of 260 µm illuminated by a violet laser (λ = 405 nm) showed an enhanced photovoltage of 145 V owing to the reduction in photoconductance by Mn and Zn codoping.

Microstructure, ferroelectric and dielectric properties in Nd and Ti co-doped BiFeO3 thin film

Nd (3 %) and Ti (2 %) co-doped BiFeO3 (BNFTO) thin film was deposited on indium tin oxide-coated glass substrate by a metal organic decomposition process. The BNFTO film exhibits a single perovskite phase with random orientation and good insulating property with low leakage current. Asymmetrical polarization–electric field loops are observed in the BNFTO film: This is mainly attributed to the role of oxygen vacancy-related defect complexes in the film. Additionally, the capacitance of the sample is strongly dependent on both the applied voltage and measuring frequency. Furthermore, with the increase in applied voltages or decrease in frequencies, the dielectric tunability is gradually enhanced. The related physics mechanism for the enhanced performance is also discussed. High dielectric constant of 165, dielectric tunability of 27 % and figure of merit of 13.5 are achieved when a measuring frequency of 1 kHz is applied. These present results can be regarded as a base for further development of high-performance BiFeO3 films. Polarization–electric field (P–E) and capacitance–voltage (C–V) curves of the BNFTO film measured under various electric fields and applied bias voltages.

The influence of Er, Ti co-doping on the multiferroic properties of BiFeO3 thin films

The pure and Er, Ti co-doped BiFeO3 thin films were prepared by chemistry solution deposition. Enhanced ferroelectric and ferromagnetic properties were obtained, which is mainly attributed to that the effect of co-doping Er and Ti leads to the drastic crystal structural transformation from rhombohedral phase to orthorhombic phase. Thus crystal structural transformation not only changes the switching behavior of the polarization path to improve the ferroelectric polarization, but also suppresses the original spiral spin structure to release the locked magnetization. At the same time, the leakage current density is decreased after doping Er3+ and Ti4+, which results from that the crystal structural transformation changes the leakage current mechanism. The present work provides an available way on improving the multiferroic properties of BiFeO3 thin films.

Local polarization switching in Ba–Ni co-doped BiFeO3 thin films with low rhombohedral-symmetry distortion

Low rhombohedral-symmetry distortion of (Ba, Ni) co-doped BiFeO3 multiferroic thin films has been achieved on Pt/TiO2/SiO2/Si substrates using RF magnetron sputtering. X-ray diffraction and Rietveld fitting show that the Bi0.75Ba0.25Fe0.975Ni0.025O3 films have an R3c low rhombohedral-symmetry distortion of hexagonal perovskite close to a pseudocubic-type structure and that their crystallographic volume decreases as the sputtering pO2 increases. Ferroelectric characterization and piezoresponse force microscopy demonstrate that the thin films possess ferroelectric domain structure by polarization switching. In addition, the ferroelectric character is connected to FeO6 octahedral distortion because of the incorporation of Ba2+ and Ni2+ in the A and B sites, respectively, changing the stereochemical activity of co-doped BiFeO3 films. Our findings indicate that there is a direct relationship between the sputtering pO2 and the rate of nucleation and growth; as the pO2 is increased, the deposition rate is lower. The maximum magnetic saturation in these thin films was 40 emu/cm3.

Effects of Ho, Mn co-doping on ferroelectric fatigue of BiFeO3 thin films

Ho, Mn, and Ho, Mn co-doped BiFeO3 thin films were deposited on Pt (100)/Ti/SiO2/Si substrates using solution gelation. The ferroelectric fatigue properties were clearly improved after doping with Ho and Mn. The doped BiFeO3 thin film displayed more strongly increased ferroelectric, leakage current, and fatigue properties compared with the pure BiFeO3 thin film. Thus, the improved performance of the BiFeO3 thin films depends on the structural transformation and reduced oxygen vacancy concentration. The present work provides a feasible approach to enhance the ferroelectric and fatigue properties of BiFeO3-based thin films. Open image in new window

Conduction mechanisms and enhanced multiferroic properties of Sr doped Bi0.85−xPr0.15SrxFe0.97Mn0.03O3 thin films

Pure BiFeO3 (BFO) and Bi0.85−xPr0.15SrxFe0.97Mn0.03O3 (BPSxFMO, x = 0.02–0.05) thin films were deposited on fluorine-doped SnO2 (FTO/glass) substrates by a sol–gel method. The systematic studies of crystalline structure, surface morphologies, leakage conduction mechanisms, ferroelectric and magnetic properties of pure BiFeO3 and Bi0.85−xPr0.15SrxFe0.97Mn0.03O3 thin films were investigated. X-ray diffraction patterns, Rietveld analysis and Raman scattering spectra reveal that the (Pr, Sr and Mn) co-doped BiFeO3 thin films give rise to a trigonal (R3m: R) structure transition with respect to the original rhombohedral structure of pure BiFeO3 film. Fowler–Nordheim tunneling conduction mechanism dominates the leakage current of all the films in the relatively high electric field. The significantly enhanced ferroelectric properties of the Bi0.85−xPr0.15SrxFe0.97Mn0.03O3 thin films were observed in comparison with pure BiFeO3 film. In addition, the BPS2FMO film shows the enhanced ferromagnetism with the saturation magnetization (Ms ~ 1.97 emu cm−3), which is almost three times larger than that of pure BFO film (Ms ~ 0.67 emu cm−3), as a result of the small grain size effect and the collapse of space modulated spin structure by the structure transition.

Photovoltaic Effect in Ferroelectric Pt/(Bi0.9La0.1)(Fe0.97Ta0.03)O3/Graphene Hetrostructures

Recently, observation of switchable polarization-induced ferroelectric photovoltaic effect (FEPV) in BiFeO3 (BFO) has attracted great interest. In order to improve its FEPV properties, co-substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition (PLD). The phase formation of the films was confirmed by X-ray diffraction, Raman spectroscopy, and XPS studies. The FEPV properties in La and Ta co-doped BiFeO3 (BFO) thin films were evaluated under illumination using sandwich configuration with a transparent conducting electrode (TCE) graphene as top electrode. The TCE electrode was transferred by chemical method on BLFTO thin films. The band gap of the films was determined to be 2.66 eV from spectrophotometric measurements, which is very close to that of pure BFO (2.7 eV). The ferroelectric nature of the films was characterized by P-E loop measurements, which indicated that leakage was still the prominent factor for not getting good polarization hysteresis. The piezo-force microscopy (PFM) studies however confirmed the ferroelectric nature of the films. The photovoltaic effect of the films was studied in both geometries, namely top-bottom and planar electrodes configurations. We will present aforementioned investigations and discuss possible photovoltaic applications of the above structure.

Structural, electrical, and multiferroic properties of (Nd, Zn) co-doped BiFeO3 thin films prepared by a chemical solution deposition method

The effects of Nd and Zn co-doping on the structural, electrical, and multiferroic properties of the BiFeO3 thin film were investigated. Pure BiFeO3 (BFO) and (Nd, Zn) co-doped Bi0.9Nd0.1Fe0.975Zn0.025O3−δ (BNFZO) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by using a chemical solution deposition method. X-ray diffraction and Raman scattering analyses revealed the formation of polycrystalline distorted rhombohedral perovskite structures for both of the thin films. As compared to the pure BFO, a low leakage current density of 6.68 × 10−5 A/cm2 (at 100 kV/cm), large remnant polarization (2P r ) of 60 μC/cm2, and low coercive field (2E c ) of 773 kV/cm (at 1,000 kV/cm) were observed for the co-doped BNFZO thin film. Furthermore, the BNFZO thin film showed enhanced magnetization when compared to the BFO thin film. These results indicate that the randomly oriented BNFZO thin film would be a useful nontoxic alternative for lead-containing multiferroic applications.