Doped BiFeO3 Films Research Articles

Highly stable photovoltaic effects in A2+–Zr4+ (A = Ca, Sr, Ba) co-doped BiFeO3 films with self-polarization

The photovoltaic responses and temperature stabilities of A2+–Zr4+ (A = Ca, Sr, Ba) composite doped BiFeO3 films with self-polarization were enhanced in the R to MA phase transition.

Coercive field modified via partial ion substitution, mechanical load and charge injection in (Ba, Ta, Cr) doped BiFeO3 films

Changes in the coercive field of ferroelectric materials translate into variations in the operating power consumption of ferroelectric devices. Here, we have used partial ion substitution to reduce the coercive field in BiFeO3 films deposited by the pulsed laser technique. We have also used mechanical loads without interruption during switching events at the nanoscale to follow the changes in the coercive field. Furthermore, we have analyzed the dependence of the coercive field concerning the injected charges during the writing process. The partial ion substitution of Ba, Ta, and Cr in the BiFeO3 structure causes a reduction of ∼50% in the coercive field due to a weakening in the Asites-O bonds. The mechanical load experiments show that linearly increasing mechanical loading from ∼50 nN to ∼600 nN tends to increase the signal amplitude of the piezoresponse and move the coercive field locations. The linear unloading reduces the amplitude and returns the coercive field to values close to before the force experiment, pointing out a reversible process. Such behavior can be explained by considering that the piezoelectric response is supported and enhanced by the electric field created by the flexoelectric effect during the mechanical loading process due to the small tip radius. Finally, the application of negative (positive) voltage during the writing process caused the loops to shift to the left (right), changing the locations of the coercive field.

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.

Effects of Mn doping on ferroelectric, ferromagnetic and optical properties of BiFeO3 thin films

Mn-doped BiFeO3 films were prepared by chemical solution deposition method on fluorine doped tin oxide conductive glass via spin-coating technique. XRD pattern reveals the presence of the impurity phase of Bi2Fe4O9 in BiFeO3 films, while no impurity phase is found in the Mn-doped thin films. XPS spectrum demonstrates the purity of prepared thin films by exhibiting the desired elements alone and it also shows that Mn-doping restrains the change of Fe3+ valency. The leakage current result shows that Mn-doping causes the change of conduction mechanism led by the change of microstructure and the restraint of Fe ions valence. BiFe0.95Mn0.05O3 thin film has a good ferroelectric performance with high remnant polarization (Pr) value of 27 μC/cm2. Mn doped BiFeO3 films have stronger saturation magnetization than the BiFeO3 thin films. UV vis NIR transmittance spectra depicts the optical property, which shows the optical band gap decreases with the concentration of Mn increases.

Hydrogen sensing characteristics of perovskite based calcium doped BiFeO3 thin films

Perovskite oxide based thin film gas sensors have long been considered as potential alternatives to commonly investigated binary metal oxides based sensors. BiFeO3, which is a prototype of p-type perovskite based semiconducting oxides, has recently drawn significant attention for its promising gas sensing characteristics. In the present work, the hydrogen sensing characteristics of calcium doped BiFeO3 has been reported by varying the film thickness, doping concentration, operating temperature, and test gas concentration. The films were deposited on glass substrates by sol-gel route using spin coating. X-ray diffraction analyses confirmed formation of phase pure films and scanning electron microscopy confirmed their uniform and dense microstructure. The Ca-doped BiFeO3 sensors exhibit higher sensitivity compared to pure BiFeO3 sensors. It is reported that the film thickness and Ca doping concentration play major role to control hydrogen sensing characteristics of the deposited films. The sensor based on 15% Ca-doped BiFeO3 sensor exhibited very high sensitivity (∼212% at 500 ppm H2), and selectivity towards hydrogen at a moderate operating temperature (∼250 °C). The enhanced gas sensing response of the doped BiFeO3 films has been attributed to the higher oxygen vacancy concentration induced by incorporation of aliovalent Ca2+.

Electrical properties of calcium doped BiFeO3 films on LaNiO3 coated Pt substrates

Pure and calcium-modified (CaxBi1-xFeO3, x = 0.0, 0.1, 0.2, 0.30) thin films were fabricated on Pt(111)/Ti/SiO2/Si substrates by the soft chemical method using LaNiO3 as the bottom electrode. Highly (200)- oriented BFO film was coherently grown on LNO at 500?C. Ca-doped BiFeO3 films have a dense microstructure and rounded grains. The conventional problem of the leakage current for the highest doped film was reduced from 10-5 to 10-10 with remarkable improvement in the film/electrode interface, chemical homogeneity, crystallinity, and morphology of the BFO film. Enhanced ferroelectricity was observed at room temperature due to the bottom electrode. Fatigue-free films were grown on LaNiO3 bottom electrodes with no degradation after 1?1010 switching cycles at an applied voltage of 5 V with a frequency of 1MHz. After several tests the capacitors retained 77% of its polarization upon a retention time of 104 s. Room temperature magnetic coercive field measurements indicate that the magnetic behaviour is influenced by the nature of the bottom electrode.

MOCVD Growth of Perovskite Multiferroic BiFeO3 Films: The Effect of Doping at the A and/or B Sites on the Structural, Morphological and Ferroelectric Properties

Bismuth ferrite (BiFeO3) materials have been the subject of intense research activity in the last two decades. The great interest arises from the BiFeO3 being one of the rare multiferroic compounds in which ferroelectricity and magnetism coexist at room temperature. To improve these properties several studies have been reported on the doping at the A and/or B sites of the BiFeO3 perovskite structure. In this short review, the attention is focused to the synthesis of BiFeO3 and BiFeO3 doped with Ba or Dy at the A site and Ti at the B site through Metal Organic Chemical Vapor Deposition (MOCVD). The applied MOCVD process consists of an in situ one step approach using a multi‐metal source precursor mixture containing the Bi(phenyl)3 and Fe(tmhd)3 (phenyl = ‐C6H5; H‐tmhd = 2,2,6,6‐tetramethyl‐3,5‐heptandione) as source of Bi and Fe ions. This study evidences the effect of doping on the structural, morphological and piezo/ferroelectric properties of BiFeO3 and doped systems. In summary, this mini‐review illustrates the possibility to apply a simple MOCVD approach to produce good quality pure and doped BiFeO3 films.

Leakage current characteristics and Sm/Ti doping effect in BiFeO3 thin films on silicon wafers

The leakage current of BiFeO3 thin films can be reduced by Sm or Ti doping, and their codoping effect is significant. X-ray photoelectron spectroscopy revealed that the reduction in the leakage current can be ascribed to decreased Fe2+ ions and oxygen vacancies. The leakage mechanisms of pristine and doped BiFeO3 films were studied by analyzing their leakage current characteristics through curve fitting. It was found that Ti could stabilize the trapped carriers, while Sm might decrease the trap energy and contrarily benefit the excitation of the trapped carriers, which explains the greater effect of Ti in reducing the leakage current than that of Sm. The piezoelectric properties of Sm/Ti-codoped BiFe0.9Ti0.1O3 (BFO) films were comparable to those of pristine BFO owing to the enhanced electric breakdown despite their reduced remanent polarization.

Large Piezoelectric Response Induced by the Coexistence of Low-Symmetry and Self-Polarization in Li+-Nb5+-Doped BiFeO3 Polycrystalline Films

Pure BiFeO3 (S1) and Li+-Nb5+-pair-doped BiFeO3 (S2, S3, and S4) films were prepared by the radio-frequency magnetron sputtering onto the LaNiO3-buffered Pt/Ti/SiO2/Si substrate. The phase structure of the doped BiFeO3 films is low-symmetry T-like phase owing to the chemical pressure induced by the substitution of larger Li+ and Nb5+ for smaller Fe3+, which is different from the pure BiFeO3 film (high-symmetry R phase). Compared with the pure BiFeO3 film, the dielectric, ferroelectric, and piezoelectric performances are greatly enhanced, especially for the piezoelectric response of 9 wt % Li+-Nb5+-doped BiFeO3 (S3) film. The converse piezoelectric coefficient d33 of S3 film is 114.1 pm/V, which is comparable to that of lead-based piezoelectric films. The excellent piezoelectricity of S3 film is considered as the coupling results of low-symmetry T-like phase BiFeO3 and upward self-polarization because the polarization vectors of the T-like phase and nanodomains rotate easily under small external stimuli. In addition, the reduced leakage currents in the doped BiFeO3 films also contribute to the enhanced dielectric, ferroelectric, and piezoelectric performances.

Comparative study of Mn3+ and Mn2+ doping effects on structure and electrical properties of BiFeO3 thin films

BiFeO3 (BFO) thin films doped differently with 5mol% Mn3+ and 5mol% Mn2+ were prepared on (111)Pt/Ti/SiO2/Si substrates by a sol–gel method with rapid thermal annealing. Systematic analyses about the two different doping effects and their underlying mechanisms were performed. Pure and doped BFO films were well crystallized in perovskite structure, whilst doping of Mn3+ and Mn2+ resulted in slight distortion in the lattice structure. After doping, smaller grain size, diminished structural defects and denser microstructure were confirmed. Both Mn3+ and Mn2+ dopants could reduce the concentration of oxygen vacancies in BFO films, whereas Mn3+doping exhibited better ability. As a result, the Mn3+ doped film showed the highest dielectric permittivity among those three samples, εr=105 at 100kHz; meanwhile, the Mn3+ doped film showed a superior 2Pr value of 79.5µC/cm2, compared to 63.2µC/cm2 for the Mn2+ doped film and unachievable intrinsic ferroelectric polarizations in the undoped BFO film because of high leakage contribution. The leakage current density in medium/high electric field region was dramatically decreased by Mn doping, accompanied with a change of conduction mechanism from the space charge limited conduction to the Schottky mechanism.

Resistive switching in doped BiFeO3 films

Polycrystalline Bi0.9Sr0.1FeO3 and Bi0.9Ca0.1Fe0.9Co0.1O3 films (BXFO) were synthesized by sol-gel spin coating method. Local resistive switching behavior was investigated. Bipolar resistive switching characteristics were clearly observed in both two samples. However, for the Bi0.9Sr0.1FeO3 film, the switching from the low resistance (LRS) to the high resistance (HRS) occurred at positive bias and the switching from the HRS to the LRS at negative bias. On the contrary, the resistance of Bi0.9Ca0.1Fe0.9Co0.1O3 film was switched from the LRS to the HRS when a negative bias was applied. Possible mechanisms for both resistive switching forms were discussed on the basis of the Schottky-like barrier.

Spin cycloid destruction in Pr doped BiFeO3 films studied by conversion-electron Mossbauer spectroscopy

Crystalline Bi1−xPrxFeO3 films were fabricated on Si (100) substrates by sol-gel technique. X-ray diffraction revealed a decrease of lattice constants of the doped samples evidencing substitution of Bi by Pr ions. Ferromagnetism with low saturation magnetization Ms = 1–3 emu/cm3 of the films were observed by Vibrating sample magnetometry. Systematic change of Mossbauer parameters of the films with Pr doped concentration increase were obtained by Conversion-electron Mossbauer spectroscopy (CEMS). The results of both one- and two-sextet fittings of the CEMS spectra provide evidence for destruction of the spin cycloid in Pr-doped BiFeO3.

Synthesis and characterization of Bi1 − xNdxFeO3 thin films deposited using a high throughput physical vapour deposition technique

The high throughput synthesis of BiFeO3 and rare earth doped BiFeO3 films using a modified molecular beam epitaxy technique is reported. Optimum conditions for deposition have been established and compositionally graded Bi(1−x)NdxFeO3 (x=0.08 to 0.24) thin films have been fabricated on platinised silicon substrate (Si/SiO2/TiO2/Pt) with the aim of finding the optimum Nd dopant concentrations for enhanced piezoelectric properties. For x<0.12, the structure and symmetry were identical to that of the R3c BiFeO3 end member. For x>0.20, the structure and symmetry were consistent with the NdFeO3 end member (Pnma). For compositions 0.12<x<0.2, a gradual transition from R3c to Pnma was observed via a mixed phase region but no compositional interval could be unambiguously identified in which the intermediate PbZrO3-like structure, reported by Karimi et al. (2009) [6], existed as a single phase. Piezoresponse force microscopy remanent hysteresis measurements of the film revealed a statistical increase in the piezoelectric response at x≈0.11 within the R3c region adjacent to the mixed phase field.

BiFeO&lt;SUB&gt;3&lt;/SUB&gt; Films Doped in the A or B Sites: Effects on the Structural and Morphological Properties

Metal-Organic Chemical Vapor Deposition (MOCVD) has been applied to the fabrication of BiFeO3 films undoped and doped with Ba or Ti on SrTiO3 (100) and YSZ (100) substrates. The films have been deposited using a multi-metal source, consisting of the Bi(phenyl)3, Fe(tmhd)3 and Ba(hfa)2 tetraglyme or Ti(tmhd)2(O-iPr)2 (phenyl = -C6H5, H-tmhd = 2,2,6,6-tetramethyl-3,5-heptandione; O-iPr = iso-propoxide; H-hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; tetraglyme = CH3O(CH2CH2O)4CH3) precursor mixture. The structural and morphological characterization of films has been carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Chemical compositional studies have been performed by energy dispersive X-ray (EDX) analysis. Structural and morphological characterizations point to the formation of homogeneous and flat surfaces for both undoped and doped BiFeO3 films.

Site modification in BiFeO3 thin films studied by Raman spectroscopy and piezoelectric force microscopy

Site modification in BiFeO3 films was investigated using Raman spectroscopy and piezoelectric force microscopy to explore effect of A- and B-site modifications on structural evolution and ferroelectric behavior of doped BiFeO3 films. Our Raman results revealed the subtle structural changes in the BiFeO3 films caused by the changes of the short-range force in the crystal lattice induced by ionic radii mismatch. The observation of ferroelectric domain and the control of domain switching through an electric field in these doped BiFeO3 films were realized by piezoelectric force microscopy. Piezoelectric response of these doped BiFeO3 films illustrated dependence of their properties upon the lattice symmetry and film microstructure.

Substantially enhanced ferroelectricity in Ti doped BiFeO3 films

We have grown BiFeO3 (BFO) and 5% Ti doped BiFeO3 (BFTO) thin films on LaNiO3/Si structures by simple sol–gel deposition. X-ray diffraction analysis confirmed that BFO has an intense (1 1 0) peak and the BFTO film is randomly oriented and adopts a rhombohedrally distorted perovskite structure. No impure phase was identified in the two films. A cross section scanning image revealed that the BFMO film has homogeneous thickness. Surface scanning electron microscopy images indicated that the BFTO film has a more compact structure. The BFTO film showed larger remanent polarization (Pr) and a small coercive field. The Pr values are 16.0 µC cm−2 and 8.0 µC cm−2 and coercive fields are 189 kV cm−1 and 416 kV cm−1 for the BFTO and BFO films at the same applied electric field, respectively. Through the Ti substitution, the dielectric property is also enhanced and leakage conduction is reduced.

Reduced leakage current in La and Ni codoped BiFeO3 thin films

La (2.5%) and Ni (2.5%) codoped BiFeO3 (BLFNO) thin films were formed by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. BiFeO3 (BFO), La(5%)-doped BFO, and Ni(5%)-doped BFO films were studied for comparison. X-ray diffraction analysis showed that the crystal structure of all films was single perovskite phase, however, structural modifications were observed in La-doped films. The leakage current density at 500kV∕cm was reduced by approximately three orders of magnitude by codoping La and Ni atoms, compared with BFO films. In BLFNO films, well saturated remanent polarization of 70μC∕cm2 was obtained at room temperature at 10kHz with the reduced coercive field by approximately 30%.