| Ferroelectric materials is an important member of the family of materials, while ferrite bismuth (BiFeO3, BFO) is the star of the ferroelectric materials. Since the BFO first appeared in 1957, the magnetoelectric coupling effect, photostrictive effect and ferroelectric photovoltaic effect in BFO have been found and reported, which indicates the unique advantages and huge potential in the device design field. In this work, the rare-earth element Eu and transition metal element Zn are used to further improve the properties of BFO, The innovative results are showed here:I. The Bi1-xEuxFeO3 (BEFOx, x=0,0.03,0.05,0.07 and 0.10) films were deposited on quartz and LaNiO3/Si (LNO/Si) substrates using Laser molecular beam epitaxy (Laser MBE) technique, the band gap decreases with the increasing Eu concentration, and the A1-1 mode (137 cm-1) of the BEFOx films shifts to higher wavenumber with increasing the amount of Eu.XRD patterns indicate that the films exhibit a pseudocubic perovskite crystal structure, with the increasing x, the peak (100) of BEFOx films suggest a shift in the peak position toward higher 2θ value, which indicates the lattice constant decreases with Eu doping, this may be resulted from a structural distortion of the BFO lattice due to the substitution of Eu, because the radius of Eu3+(-0.95 A) is smaller than that of Bi3+(～1.03 A). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicates all the BEFOx samples show compact and smooth morphologies. As shown by the transmittance spectra and the spectroscopic ellipsometry (SE) spectra, the band gap decreases with the increasing Eu concentration, this may be caused by the impurity levels induced by the Eu doping. These results suggest the Eu doping could increase the photon absorption efficiency in BFO, enhancing the photoelectric property of BFO. As the Raman spectra shown, the peak position of the A1-1 mode (137 cm-1) of the BEFOx films shifts to higher wavenumber with increasing the amount of Eu, which could be expressed by (146.8+1.59x) cm-1.Ⅱ. The interface and strain of BEFOx epitaxial films on SrTiO3 (100) substrates were studied. For the sake of brevity, the SrTiO3 (100) was substituted by STO (100). Scanning Transmission Electron Microscope (STEM) indicates the SrO/FeO2/BiO stacking configuration was formed at the BFO/STO interface, and the BEFOx epitaxial films shows the layer by layer growth mode. The BFO layer is under compressive stress applied by STO (100) substrate.The BEFOx films were grown on STO (100) substrates by Laser MBE technique. XRD analysis indicates the strong (h00) peaks, and AFM shows the root-mean-square roughness (RMS) is up to nanometers (～0.28 nm), which indicates the BEFOx films were epitaxially grown on STO (100) substrates. The transmittance spectra indicaes the band gap is decreased with the increasing Eu dopant, this result is in agreement with the results obtained from the quartz and LNO/Si substrates. According to the STEM, the Fe columns in BFO align with Sr columns in STO, with a SrO/FeO2/BiO stacking configuration at the BFO/STO interface, and the BFO layer is under compressive stress applied by STO (100) substrate. In addition, the RHEED oscillation intensity curve and the AFM images indicates the BEFOx epitaxial films are layer by layer growth mode, and the deposition rate is 0.43 A/s。III. The influences of transition metal element Zn doping on structure and optical properties of BiFeO3 thin films were studied. The transmittance spectra suggests that the band gap increases with the increasing Eu dopant, indicating the Zn doping against increasing the photon absorption efficiency in BFO. M-H images shows that the saturation magnetization (Ms) of BFZOx films is significantly enhanced with increasing Zn dopant.The BFZOx films were grown on quartz substrates by Sol-Gel technique (x=0, 0.03,0.06 and 0.09). XRD and SEM indicate that BFZOx films were well crystallized, the surface was smooth and compact. The band gap of the BFZOx (x=0.0.03,0.06 and 0.09) films is 2.60 eV,2.63 eV,2.65 eV and 2.68 eV, respectively. The increased band gap was caused by the Moss-Burstein effect. M-H loops indicate that the saturation magnetization (Ms) of BFZOx (x=0.03,0.06,0.09) films is significantly enhanced, which increases from 0.25 emu/cm3 (x= 0) to 2.87 emu/cm3 (x=0.09).Ⅳ. The electrical property of BEFOx films were tested. Ferroelectric hysteresis loop (P-E loop)shows the typical ferroelectric properties, and the diode effect was observed.The BEFO* films were grown on electric fluorine-doped tin dioxide (FTO) substrates by Sol-Gel technique, and the Pt electrodes for ferroelectric measurements were deposited by sputtering through a shadow mask (Pt/BEFOx/FTO). XRD and AFM indicate that BEFOx films were well crystallized, the surface is smooth and compact. The P-E loops of the Pt/BEFOx/FTO device indicate favorable ferroelectric properties. For the Pt/Bio.9iEuo.o9Fe03/FTO device, the dielectric constant decreases with the increasing frequency. While the dielectric loss shows moderate increases with the frequency below 10 kHz and abrupt increase when the frequency is beyond 100 kHz. Evec if the Ⅰ-Ⅴ characteristics curve shows no obvious difference between bight field and dark field, the obvious diode effect is observed in the Pt/BEFOx/FTO device. |
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