Undoped BiFeO3 Research Articles

Synthesis and multiferroic properties of Bi0.8A0.2FeO3 (A=Ca,Sr,Pb) ceramics

Bi 1 − x A x Fe O 3 ceramics (A=Ca,Sr,Pb) were sintered by conventional mixed oxide route. The crystallographic structure of all samples is characterized by the rhombohedral symmetry (space group R3c). The existence of switchable ferroelectric polarization is verified by piezoresponse force microscopy that is proven to be a useful technique in semi-insulating ferroelectrics. Magnetic properties of Ca and Sr-doped ceramics are found to reproduce the antiferromagnetic behavior of undoped BiFeO3 without any enhancement of the magnetization. On the contrary, Pb-doped compound demonstrates appearance of a weak ferromagnetism. It is thus shown that Pb doping of BiFeO3 is a promising way for preparing multiferroic materials.

Recent Progress in Ferroelectric Random Access Memory Technology

AbstractIn this paper, ferroelectric materials suitable for realizing high-density 1T1C-type (capacitor-type) FeRAM are first reviewed. It is found in BiFeO3(BFO) films formed by chemical solution deposition that leakage current density at a low electric field increases by substitution of Mn and Cr atoms for Fe atoms. But, it is also found that the breakdown characteristic is much improved by substitution of these atoms. Because of the better breakdown characteristic, the leakage current densities in the 3 and 5% Mn-substituted films are lower than that in an undoped BFO film at an applied electric field of 1MV/cm at room temperature, and thus well saturated hysteresis loops in P-E (polarization vs. electric field) characteristics are observed in these films.Next, recent technological progress in transistor-type FeRAM, in which data are stored in a single ferroelectric-gate FET(field effect transistor), is discussed. It is demonstrated that the data retention time of ferroelectric-gate FETs is much improved by use of HfO2-based buffer layers which are inserted between the ferroelectric film and Si substrate for preventing interdiffusion of constituent elements. Particular attention is paid to FETs with a Pt/SrBi2Ta2O9/HfO2/Si gate structure, in which the data retention time longer than 30 days has been attained. Finally, the cell structure and operation principle of 1T (one transistor)-type FeRAM are discussed.

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

Enhanced ferroelectricity in Ti-doped multiferroic BiFeO3 thin films

Ti 4 + ion-doped BiFeO3 thin films were prepared by sol-gel spin-coating technique on (111)Pt∕Ti∕SiO2∕Si substrates. X-ray diffraction and scanning electron microscope revealed the single phase, and good surface and cross-section morphologies of the films, respectively. Leakage current density measurement indicated that the quality of the BiFeO3 films was improved by Ti4+ doping. By introducing a small amount of Ti ions into the sol-gel solution-processed BiFeO3 films, large enhancement in both remnant and saturation polarizations of the doped-BiFeO3 films in comparisons with the undoped BiFeO3 films was observed, due to the reduced leakage current, stabilization of the ferroelectric distortion by Ti4+, and more homogenous microstructure.

Doping Effect of Rare-Earth Ions on Electrical Properties of BiFeO3 Thin Films Fabricated by Chemical Solution Deposition

Rare-earth-ion (La and Nd)-doped BiFeO3 (BFO) thin films were fabricated by depositing sol–gel solutions on Pt/Ti/SiO2/Si(100) structures. It was found from X-ray diffraction analysis that BFO thin films were polycrystalline and 5 at. % La and Nd ion dopings did not cause structural changes in the BFO thin films. By 5 at. % La doping in BFO thin films, breakdown field increased and leakage current density in the high-electric-field region decreased, compared with those of undoped BFO thin films. Saturated polarization–electric field curves were obtained for La-doped BFO films at room temperature owing to the decreased leakage current density. At 80 K, the remanent polarization of the Nd-doped thin films was almost the same as that of the undoped film (approximately 90 µC/cm2), whereas that of the La-doped thin films was significantly lower (52 µC/cm2).

Enhanced ferroelectric properties of Cr-doped BiFeO3 thin films grown by chemical solution deposition

Multiferroic Cr-doped BiFeO3 (BFO) thin films were prepared on Pt∕TiO2∕SiO2∕Si(100) substrates by a chemical solution deposition method. From the x-ray photoelectron spectroscopy measurements, it was observed that the valence number of Fe ion in undoped and Cr-doped BFO thin films was found to be almost 3+. It was found that Cr-doped BFO thin films exhibited good ferroelectric properties, such as improved leakage-current density and P-E hysteresis characteristics. The 3mol% Cr-doped BFO thin film showed a leakage-current density of 9.2×10−7A∕cm2 at 100kV∕cm and a large remanent polarization (Pr) of 61μC∕cm2 at room temperature.

Excellent Room-Temperature Ferroelectricity in Mn-substituted BiFeO3 Thin Films Formed by Chemical Solution DepositionΨ

AbstractMn-substituted BiFeO3 (BFO) thin films were formed by chemical solutions deposition on Pt/Ti/SiO2/Si(100) structures. Effects of the Mn-substitution on the structure and ferroelectricity of BFO films were examined. We found that the lattice structure of the film is sensitive to the Mn-substitution and the secondary phase is appears in 50% Mn-substituted BFO films. The leakage current were increased with the Mn-substitution. However, the 5% Mn-substituted BFO film shows low leakage current than undoped BFO films in a high electric field than 0.5 MV/cm. Due to the low leakage current in Mn-doped 3, 5 and 7% BFO films, the saturated P-E hysteresis loops with remanent polarization around 100 μC/cm2 were obtained at RT.