| Bismuth ferric oxide, which as a species of multiferroic materials have been widely studied in recent years, because of its abundant physics and potential applications in information storage, spintronic devices, optical devices and sensors. Bismuth ferric oxide is one of the well-known single-phase multiferroic materials with ABO3structure and G-type antiferromagnetic behavior, which has comparatively high Curie temperature (TC～1103K) and high Neel temperature (TN～643K). Recently, researchers have detected photovoltaic effect in BiFeO3diode structure, which has inaugurated much attention in its potential photovoltaic application. Thus, the study on the optical properties of BiFeO3thin films is helpful to better understand the effect. Although some reports on the optical band gap of BiFeO3with different method have been reported, such as the optical band gap of BiFeO3film was analyzed about2.74eV by molecular-beam epitaxy,2.82eV by wet chemical method,2.80eV by magnetron sputtering, and2.70eV by Sol-Gel process, most of those investigations were mainly focused on BiFeO3. As we known, proper ion substitution in BiFeO3is an efficient way to improve the optical, electric and magnetic properties of the BiFeO3films. The major work includes the manufacture of pure and ions doping BiFeO3films on different substrate, and the studies of microstructure, optical, electric and magnetic properties of BiFeO3films. The major work can be summarized as follows:(1) Transparent multi-component solutions were prepared by dissolving iron nitrate, bismuth nitrate (5%mol excess to compensate the Bi loss), lanthanum nitrate, magnesium nitrate and cobalt nitrate in ethylene glycol and acetic acid under constant stirring velocity. The final concentration for the stock solutions was0.15mol/L. The resultant solutions were deposited on substrates by spin-coating sol-gel method. The as-deposited wet thin films were crystallized at600℃for300s in a rapid thermal process (RTP) furnace. These processes were repeated for several times to have desired film thickness. In order to investigate electrical properties, the top Pt electrodes with a diameter of0.03mm were sputtered on the surface of thin films which fabricated on the LNO/Si substrate through a mask.(2) The crystal structures of BiFeO3and ions doping BiFeO3films were investigated by X-ray diffraction patterns. We can judge the ions doping impacting the crystal structures of BiFeO3by XRD pattern. No impurity phases of doping oxides can be observed in XRD patterns, confirms that Co atoms have been successfully incorporated into the host lattice. The surface morphologies and cross-section micrographs of films were observed by scanning electron microscopy (SEM) images. The Atomic Force Microscope was used to investigate the surface of films. These films are well crystallized since favorable grains can be identified clearly. The grain size ranges of Bi1-xLaxFe1-yMgyO3(x=y=0.0.0.03、0.08and0.12) thin films from80to120nm. This result suggests that proper Co doping in BiFeO3thin film could restrain the formation of voids. The average thickness of BiFe1-xCoxO3(x=0,0.03,0.05,0.1) thin films from cross-section micrographs are about635,632,647and652 nm, respectively.(3) The study of crystalline structure evolution with ion substitution on BiFeO3can also be analyzed through Raman spectra. By fitting the measured spectra and decomposing the fitted curves into individual Lorentzian components, the natural frequency of each Raman active mode was obtained in all samples. The shift to lower wave number of E-4modes in BiFe1-xCoxO3(x=0,0.03,0.05,0.1) thin films clearly shows that the substitution of Co on Fe sites induces both the compressive structural distortion on Bi sites. The refractive index n, extinction coefficient k and optical band gap of samples were obtained by Spectroscopic ellipsometry and transmittance spectra. The optical band gaps of BiFe1-xCoxO3(x=0,0.03,0.05,0.1) films are2.66,2.63,2.61and2.53eV, respectively. The optical band gap of Bi1-xLaxFe1-yMgyO3(x=y=0.0,x=0.08,y=0.08and x=y=0.8) thin films are about2.64、2.73.2.75and2.78eV.(4) The variation of dielectric properties and dielectric loss factor of La and Mg doping of BiFeO3films measured at room temperature as a function of frequency in the range of100Hz-1MHz. The magnetization versus magnetic field hysteresis loops for the BiFe1-xCoxO3(x=0,0.03,0.05,0.1) thin films at300K. The saturated magnetization with very weak hysteresis loop was observed in the case of BiFeO3film, suggestive of thin ferromagnetic behavior. The remanent magnetization Mr of BiFe1-xCoxO3(x=0.03,0.05,0.1) thin films are about1.8,4.1and11.6emu/cm3, respectively. The saturation magnetizations Ms and coercivity Hc of BiFe1-xCoxO3(x= 0.0,0.03,0.05,0.1) thin films increases with increasing Co composition. |
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