BFO Thin Films Research Articles

Effects of zirconium substitution on the electrical and physical properties of metal-ferroelectric (BiFeO3)-insulator (HfO2)-silicon structures for non-volatile memories

Graphical abstractDisplay Omitted We investigate the origin of Zr substitution in BiFeO3 films.The depth profile of XPS analysis is performed.Conduction mechanisms at gate metal/Zr-doped BFO and Zr-doped BFO/Si interfaces are investigated.The theoretical and experimental memory windows have been discussed. Metal-ferroelectric (Zr-doped BiFeO3)-insulator (HfO2)-semiconductor (MFIS) structures have been fabricated by magnetron cosputtering technique. The C-V memory windows of MFIS capacitors as functions of insulator film thickness, DC power for Zr, post-annealing temperature were compared. The leakage current that reduces with increasing the DC power results in larger memory window. The memory window increases with increasing HfO2 insulator thickness due to the effect of charge injection. In addition, the memory window increases with increasing DC power for Zr. A maximum memory window of 1.6V is obtained at the swept voltage of 8V. The temperature-dependent conduction currents of MFIS capacitors with Zr-doped BFO thin films were studied.

Crystal structure and highly enhanced ferroelectric properties of (Tb, Cr) co-doped BiFeO3 thin films fabricated by a sol–gel method

Abstract BiFeO 3 (BFO), Tb-doped BiFeO 3 (BTFO) and Tb–Cr co-doped BiFeO 3 (BTFCO) thin films were successfully prepared on SnO 2 : F (FTO)/glass substrates by a sol–gel method. The influence of Tb and Cr co-doping on the structure, leakage current, charge defects, dielectric and ferroelectric properties of the BTFCO thin film was investigated systematically. X-ray diffraction (XRD) and Raman spectroscopy results clearly reveal the structure transition and (110) preferentially oriented film texture for the co-doped thin film. BTFCO thin film becomes more compact structure and uniform than that of the other two films. The improved electrical properties of BTFCO thin film are observed in comparison with the BFO thin film. Furthermore, the highly enhanced ferroelectric property with a giant remanent polarization (2 P r =161.60 μC/cm 2 ) and the decreased leakage current density (3.76×10 −5 A/cm 2 at 200 kV/cm) via co-doping Tb and Cr are obtained. The X-ray photoelectron spectroscopy (XPS) analyses clarify that the ratios of Fe 2+ /Fe 3+ in the BFO and BTFCO thin films are calculated as 45:55 and 26:74, respectively, which indicate that the presence of Fe 2+ ions is suppressed with co-doping Tb and Cr.

Hydrothermal epitaxial multiferroic BiFeO3 thick film by addition of the PVA

Abstract Hydrothermal method is a much simpler method of epitaxial multiferroic BiFeO 3 (BFO) film than physical/chemical vapor deposition methods. In this work, epitaxial BFO thin films were fabricated on (0 0 1)-oriented single-crystal SrRuO 3 (SRO)/SrTiO 3 (STO) structures by PVA-assistant hydrothermal method. The results of θ –2 θ scan, ω scan and reciprocal space mappings (RSMs) measured by synchrotron high resolution X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) suggest that the BFO films have relaxed to a single-phase monoclinic structure. The electrical properties observed in BFO (0 0 1) thin films are superior to those of hydrothermal epitaxy reported previously, which indicates PVA plays an important role in fabricating BFO films by using hydrothermal method.

Raman spectroscopy of nanocrystalline Mn-doped BiFeO3 thin films

Enhancement of magnetic and electric properties in multiferroic Mn-substituted BiFeO3 thin films (BiFe1− x Mn x O3 with x = 0.00–0.05) grown via the low-temperature chemical solution deposition process on low-cost substrates like indium tin oxide (ITO)/glass and silicon is reported in this work. A detailed structural analysis by high-resolution X-ray diffraction and Raman spectroscopy indicates that the grown films exhibit rhombohedral distorted perovskite structure without any impure phase. Raman spectroscopic analysis shows all 13 Raman active (4A1 + 9E) phonon modes corresponding to distorted rhombohedral BFO R3c structure. BFO thin film with 5% Mn substitution showed maximum saturation and remnant magnetisation (M s = 16.70 emu/g and M r = 2.58 emu/g) and remarkably reduced leakage current due to distortion in the spin cycloid structure and suppression of oxygen vacancies due to partial replacement of Fe3+ ion by Mn3+ ion, respectively. These enhanced multifunctional properties highlight the potential applications of BFMO system in thin film devices which can couple magnetic and ferroelectric order parameters.

The Influence of Bi Content on the Properties of Bismuth Ferrite Thin Films Fabricated by Magnetron Sputtering

In the process of BiFeO3 film preparation by magnetron sputtering, Bi element is volatile, leading to the films which often appear impurity phases. Therefore, Both Bi excessive 5% (B1.05FO) and 8% (B1.08FO) BFO film in Si substrate were prepared by magnetron sputtering. X-Ray Diffraction (XRD) results showed that the BFO thin films fabricated in the Si substrate are perovskite structure, that the B1.08FO film appeared less impurity phases than B1.05FO film, and that stress due to substrate lattice mismatch caused the shift of XRD patterns. In Raman study, it was concluded that both B1.08FO film and B1.05FO film appeared ten Raman peaks in the range from 50cm-1 to 800cm-1, and that B1.08FO Raman peaks intensity was stronger in 137.1cm-1.168.5cm-1 and 215.3cm-1. Spectroscopic ellipsometry test showed that the refractive index and the extinction coefficient of B1.05FO film were 2.25 and 0.07 respectively in 600 nm with 2.67eV of energy gap; the refractive index and the extinction coefficient of B1.08FO film were 2.14 and 0.05 in 600 nm respectively with 2.71eV of energy gap. Atomic Force Microscope (AFM) was used to characterize the film surface morphology, finding that the B1.08FO film prepared in Si substrate was denser while grain size and surface roughness were smaller.

Leakage Current Behaviors of SrTiO3 Capped Mn-doped Polycrystalline BiFeO3 Thin Film

The polycrystalline Mn 1% doped BFO (BFMO) thin film was deposited on Pt(111)/Ti/SiO2/Si(100) substrate by pulsed laser deposition. A thin SrTiO3 capping layer was grown on top of deposited Bi(Fe0.99Mn0.01)O3 thin film. The structural and electrical properties of STO/BFMO thin film were investigated. By capping a thin STO on top of BFMO, leakage current density of capacitor was improved greatly compare to that of the pure BFMO without degrading ferroelectricity.

Unipolar Resistive Switching and Associated Photoresponse in Sm doped BiFeO3 Thin Film Grown by RF Sputtering

ABSTRACTNonvolatile unipolar resistive switching has been observed in Sm doped BFO thin films in Pt/Sm: BFO/SRO stack geometry. The initial forming voltage was found to be ∼ 11 V. After the forming process repeatable switching of the resistance of Sm:BFO film was obtained between low and high resistance states with nearly constant resistance ratio ∼ 105 and non overlapping switching voltages in the range of 0.7-1 V and 4-6 V respectively. The temperature dependent measurements of the resistance of the device indicated metallic and semiconducting conduction behavior in low and high resistance states respectively. The current conduction mechanism of the Pt/Sm:BFO/SRO device in low resistance states was found to be dominated by the Ohmic behavior while in case of high resistance state and at high voltages it deviated significantly from normal Ohmic behavior and was found to correspond the Pool-Frankel (PF) emission. The Pt/Sm:BFO/SRO structure also showed efficient photo-response in high and low resistance states with increase in photocurrent which was significantly higher in low resistance state when illuminated with white light.

Study of strain effect on in-plane polarization in epitaxial BiFeO3thin films using planar electrodes

Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films because of the inherent coupling between the strain and the polarization. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films, using traditional sandwich capacitor devices, because of high leakage current. Hence, a planar electrode device with different crystallographical orientations between electrodes, which is able to measure the polarization response with different electric field orientation, is used successfully in this work to directly measure the in-plane polarization--electric-field ($P$-$E$) hysteresis loops in fully strained thin films. We used BiFeO${}_{3}$ (BFO) as a model system and measured in-plane $P$-$E$ loops not only in the rhombohedral-like (R-like) BFO thin films but also in largely strained BFO films exhibiting the pure tetragonal-like (T-like) phase. The exact magnitude and direction of the spontaneous polarization vector of the T-like phase is deduced thanks to the collection of in-plane polarization components along different orientations. It is also shown that the polarization vector in the R-like phase of BiFeO${}_{3}$ is constrained to lie within the (1$\overline{\mathbf{1}}$0) plane and rotates from the [111] towards the [001] pseudocubic direction when the compressive strain is increased from zero. At high misfit strains such as $\ensuremath{-}4.4%$, the pure T-like phase is obtained and its polarization vector is constrained to lie in the (010) plane with a significantly large in-plane component, \ensuremath{\sim}44 \ensuremath{\mu}C/cm${}^{2}$. First-principles calculations are carried out in parallel, and provide a good agreement with the experimental results.

Study on the Structural, Electrical, and Magnetic Properties of Pure and (Pr3+,Co2+)-Doped BiFeO3 Powders and Thin Films

The present work addresses the systematic evaluation of the influence of the incorporation of praseodymium (Pr) and cobalt (Co) ions in the bismuth ferrite (BFO) structure on its corresponding structural, ferroelectric and magnetic properties. The level of the doping ions varied from 0 to 4 at. %. Pure and doped BFO powders and thin films were synthesized by a modified sol–gel method. In the case of thin films, glycol was added to the main solvent to increase the viscosity of the precursor solutions and promote their adhesion onto platinum substrates. The development of the host BFO structure was confirmed by XRD analyses of samples annealed in air at 700 °C for 1 h (powders) and 500 °C for Pr and Co-doped BFO thin films. The incorporation of specific dopant ions played an important role in the ferromagnetic and ferroelectric behavior in the material.

Study on the Structural, Electrical, and Magnetic Properties of Pure and (Pr3+,Co2+)-Doped BiFeO3Powders and Thin Films

The present work addresses the systematic evaluation of the influence of the incorporation of praseodymium (Pr) and cobalt (Co) ions in the bismuth ferrite (BFO) structure on its corresponding structural, ferroelectric and magnetic properties. The level of the doping ions varied from 0 to 4 at. %. Pure and doped BFO powders and thin films were synthesized by a modified sol–gel method. In the case of thin films, glycol was added to the main solvent to increase the viscosity of the precursor solutions and promote their adhesion onto platinum substrates. The development of the host BFO structure was confirmed by XRD analyses of samples annealed in air at 700 °C for 1 h (powders) and 500 °C for Pr and Co-doped BFO thin films. The incorporation of specific dopant ions played an important role in the ferromagnetic and ferroelectric behavior in the material.

Enhanced electrical properties of rare-earth-substituted (Bi0.9RE0.1)(Fe0.975Cr0.025)O3 (RE = Nd, Gd, Eu) thin films

BiFeO3 and (Bi0.9 RE 0.1)(Fe0.975Cr0.025)O3 (RE = Nd, Gd and Eu, BNFCr, BGFCr, and BEFCr, respectively) thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by using a chemical solution deposition method. All the thin films were annealed at 550 °C for 30 min under a nitrogen atmosphere for crystallization. Compared to the pure BFO, the co-doped thin films exhibited good electrical properties, such as improved leakage current density and P-E hysteresis characteristics. The leakage current densities of the BFO, BNFCr, BGFCr, and BEFCr thin films were 2.58 × 10−3 A/cm2, 6.76 × 10−6 A/cm2, 5.14 × 10−6 A/cm2, and 9.60 × 10−5 A/cm2 at an electric field of 100 kV/cm, respectively. The remnant polarization (2P r ) and the coercive electric field (2E c ) values of the BFO, BGFNi, BGFCo, and BGFCr thin film were 35.2 µC/cm2 and 1376 kV/cm, 101.9 µC/cm2 and 1131 kV/cm, 81.2 µC/cm2 and 962 kV/cm, 67.1 µC/cm2 and 938 kV/cm, respectively, at an applied electric field of 1066 kV/cm. The improved leakage current and ferroelectric properties of the co-doped thin films are related to the change in the microstructure and the decrease in the number of oxygen vacancies.

Electrical properties in lanthanides substituted (Bi0.9 A 0.1)(Fe0.975Co0.025)O3−δ (A = La, Eu, Gd) thin films

Multiferroic BFO and (Bi0.9A0.1)(Fe0.975Co0.025)O3−δ (A = La, Eu, Gd) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by using a chemical solution deposition method. The thin films were annealed at 550 °C for 30 min by using a conventional annealing process under a nitrogen atmosphere. Reduced leakage current density and improved ferroelectric properties were observed in the co-doped thin films compared to the BFO thin film. The enhanced properties observed in the co-doped thin films could be considered as being the result of the suppression of oxygen vacancies and of the modified microstructure.

Electrical properties of thin films deposited with MnO- and MnO2-modified BiFeO3 oxide targets

Mn-modified Bi1.05FeO3 thin films were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by using a pulsed laser deposition method with BFO oxide targets modified with MnO or MnO2, and the effects of the Mn were investigated. The 3% MnO-modified BFO thin film capacitor exhibited a large remnant polarization (2P r = 179 µC/cm2) and low leakage currents over a wide electric-field range. However, different leakage current behaviors were observed in the 10% MnO-modified and 10% MnO2-modified BFO thin films. The leakage current mechanism of the 10% MnO-modified BFO thin film was Ohmic conduction while that of the 10% MnO2-modified BFO thin film was related to the oxygen vacancy-associated electrical conductivity.

Influence of the ion energy on the structure of Bi and Fe2O3 thin films

Compounds containing bismuth, iron and oxygen (BFO) can result in materials with important magnetic and electrical properties for high-technology applications. We plan to prepare such compounds using the simultaneous ablation of bismuth and iron oxide targets. For that reason in the first part of this work we study the plasmas and the materials produced by ablation of bismuth or Fe2O3 targets, and then the two plasmas are combined in order to deposit the BFO compounds. The individual plasmas were characterized using a Langmuir probe, in order to measure the mean kinetic ion energy (E p) and plasma density (N p). Bismuth and magnetite-Fe3O4 thin films were obtained in high vacuum (2.7×10−4 Pa). Meanwhile for the deposition of α-Fe2O3 (hematite) or amorphous bismuth oxide thin films a reactive atmosphere (Ar/O2=80/20) was used. All depositions were made at room temperature. The bismuth thin films crystallized in the rhombohedral metallic system with preferential orientations that depended on the Bi-ion energy used. Bismuth oxide phases were only obtained after annealing of the Bi thin films at different temperatures. Iron oxide thin films reproducing the target stoichiometry were obtained at a certain value of iron-ion energy. Preliminary structural results of the BFO thin films obtained by the combination of the individual plasmas are presented.

Nanoscale piezoresponse and magnetic studies of multiferroic Co and Pr co-substituted BFO thin films

Piezoresponse Force Microscopy (PFM) technique has been employed to acquire out-of-plane (OPP) piezoresponse images and local piezoelectric hysteresis loop in rhombohedrally distorted Bi1−xPrxFe1−yCoyO3 [x=0, 0.05; y=0.05] polycrystalline thin films fabricated via chemical solution deposition method. PFM images revealed that piezoelectric contrast is dependent upon the film composition. Furthermore, negative self-polarization effect was observed in the cobalt substituted BFO film. Well saturated local piezo-hysteresis loops were monitored and an increase was noticed in the piezoelectric coefficient (d33) value with cobalt doping (25.1pm/V) whereas with Pr co-substitution in BFCO film, the piezoelectric behavior was almost suppressed. Pr and cobalt co-substituted film exhibited the lowest leakage current density. Magnetic behavior (M–H curves) exhibited nearly eight times enhancement in the saturation magnetization values in the Co- and Co–Pr substituted films. The present study provides the different elements’ substitution effect on the local piezoelectric and magnetic properties of BiFeO3 multiferroic thin film.

Preparation and properties of rare earth (Eu, Tb, Ho) and transition metal (Co) co-doped BiFeO3 thin films

Ferroelectric (Bi0.9 RE 0.1)(Fe0.975Co0.025)O3-δ (RE = Eu, Tb and Ho) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates via a chemical solution deposition method. All thin films were crystallized in a distorted rhombohedral perovskite structure confirmed by using an X-ray diffraction and a Raman scattering analyses. Compared to the pure BiFeO3 thin film, improved electrical and ferroelectric properties were observed for the co-doped thin films. Among the thin films, the lowest leakage current density of 4.28 × 10−5 A/cm2 was measured at an applied electric field of 100 kV/cm for the (Bi0.9Ho0.1)(Fe0.975Co0.025)O3-δ thin film. This value is approximately three orders lower than that of the pure BFO thin film. Furthermore, a large remnant polarization (2P r) of 60.2 μC/cm2 and a low coercive field (2E c ) of 561 kV/cm at 980 kV/cm were observed from the (Bi0.9Ho0.1)(Fe0.975Co0.025)O3-δ thin film.

Weak ferromagnetism in La-doped BiFeO3 multiferroic thin films

Bi1−xLaxFeO3 thin films (x = 0.0, 0.3, 0.5) were grown on glass substrates by thermal physical vapor deposition. The monoclinically distorted crystal structure of the films was revealed by x-ray diffraction at room temperature. Field and temperature (up to 1000 K) dependences of magnetization were studied. Saturation of the room temperature magnetic hysteresis loop has been observed at magnetic field above 0.15 T, demonstrating the weak ferromagnetic nature of the thin films. Our magnetic force microscopy results show clearly the presence of magnetic domains in BFO thin films. These structural and magnetic properties suggest the absence of magnetic spiral spin structure in monoclinically distorted BFO-based thin films.

Structural and pyroelectric properties of sol–gel derived multiferroic BFO thin films

Multiferroic BFO/PZT multilayer films were fabricated by spin-coating method on the (1 1 1)Pt/Ti/SiO 2/Si substrate alternately using PZT(30/70), PZT(70/30) and BFO alkoxide solutions. The structural and ferroelectric properties were investigated for uncooled infrared detector applications. The coating and heating procedure was repeated six times to form BFO/PZT multilayer films. All films showed the typical XRD patterns of the perovskite polycrystalline structure without presence of the second phase such as Bi 2Fe 4O 3. The thickness of BFO/PZT multilayer film was about 200–220 nm. The ferroelectric properties such as dielectric constant, remnant polarization and pyroelectric coefficient were superior to those of single composition BFO film, and those values for BFO/PZT(70/30) multilayer film were 288, 15.7 μC/cm 2 and 9.1 × 10 −9 C/cm 2 K at room temperature, respectively. Leakage current density of the BFO/PZT(30/70) multilayer film was 3.3 × 10 −9 A/cm 2 at 150 kV/cm. The figures of merit, F V for the voltage responsivity and F D for the specific detectivity, of the BFO/PZT(70/30) multilayer film were 6.17 × 10 −11 Ccm/J and 6.45 × 10 −9 Ccm/J, respectively.

Evidence of bulk photovoltaic effect and large tensor coefficient in ferroelectric BiFeO3thin films

We present a study on the bulk photovoltaic effect (BPVE) in ferroelectric thin films, in which the photocurrent is measured with symmetric electrodes in the plane perpendicular to the ferroelectric polarization to eliminate the effect of the depolarization field and interfacial energy barriers. We show evidence of a strong BPVE in epitaxial BiFeO${}_{3}$ (BFO) films as thin as 40 nm. By taking into account the nonuniform absorption of incident photons, we obtain the bulk photovoltaic tensor coefficient ${\ensuremath{\beta}}_{22}$ for BFO thin films. Remarkably, it is about five orders of magnitude larger than that of other typical ferroelectric materials for visible wavelengths. The results indicate that the BPVE in ferroelectric BFO thin films could be further explored for solar light photovoltaic applications.

Magnetoelectric properties of Mn-substituted BiFeO3 thin films with a TiO2 barrier

Multiferroic thin films with the general formula TiO2/BiFe1−xMnxO3 (x=0.00, 0.05, 0.10 and 0.15) (TiO2/BFMO) were synthesized on Au/Ti/SiO2/Si substrates using a chemical solution deposition (CSD) method assisted with magnetron sputtering. X-ray diffraction analysis shows the thin films contained perovskite structures with random orientations. Compared with BFMO films, the leakage current density of the TiO2/BFMO thin films was found to be lower by nearly two orders of magnitude, and the remnant polarizations were increased by nearly ten times. The enhanced ferroelectric properties may be attributed to the lower leakage current caused by the introduction of the TiO2 layer. The J–E characteristics indicated that the main conduction mechanism for the TiO2/BFMO thin film was trap-free Ohmic conduction over a wide range of electric fields (0–500kV/cm). In addition, ferromagnetism was observed in the Mn doped BFO thin films at room temperature. The origin of ferromagnetism is related to the competition between distortion of structure and decrease of grain size and decreasing net magnetic moment in films due to Mn doping.