Preparation And Characterization Of Bi_xFeO₃(0.80≤x≤1.20) Thin Films

During the recent decades, there has been a spurring trend of research about multiferroic materials. These materials are specialized for possessing ferroelectricity, ferromagnetism or even ferroelasticity spontaneously at the same time. As one of the multiferroic materials, lead-free bismuth ferrite (BiFeO3BFO) has attracted numerous attentions for its relatively large remnant polarization (Pr～100μC/cm2), high Curie temperature (TC～850℃) and high Neel temperature (TN～370℃). These properties make BFO a very promising candidate in the development and application of new-generation multi-function devices.To prepare BFO thin film samples, there are several experimental methods such as sol-gel technique, pulsed laser deposition, magnetron sputtering and molecular beam epitaxy, etc. Each method has its own advantages, but there are still bottlenecks to obtain high-quality pure BFO samples. One of the bottlenecks is the volatility of bismuth which usually leads to off-stoichiometry effects on BFO thin films and so there will be unbearable high conductivity in BFO samples. As a result, even though BFO has been discovered since1960s, there's still little notable progress in its application in devices.A long used step to avoid the influences brought by the volatility of bismuth is to use overdosed bismuth to prepare BFO samples. However, the effect is limited since the impurities cannot be eliminated completely. Hence, to examine the influences of the variation of bismuth composition on crystallization, microstructure, optical and electrical properties of BFO thin films, we started an investigation on BixFeO3thin films prepared by sol-gel technique with x changing from0.80to1.20. Main results are as follows:(1) We have successfully prepared the precursors solution of BixFeO3with x varying from0.80to1.20using sol-gel technique. The BixFeO3thin films were deposited on both Si(100) and LNO/Si(100) substrates. In addition, we also prepared5%La doped BixLa0.05FeO3samples on both substrates to observe how can stoichiometry variation effect doped BFO thin films.(2) The XPS test was adopted to examine that if the ratios of Bi atoms and Fe atoms in the samples were in accordance with our anticipation. The results indicated an excess of bismuth on the surfaces but a lack of bismuth in the inside. Using X-ray diffraction (XRD) technique and atomic force microscope (AFM), we observed the microstructures and surface morphologies of our BixFeO3thin films. We only obtained pure BFO structure without any impurity phases when x was1.00according to our XRD patterns. Meanwhile, there appeared to be a change in the rhombohedral distortion with bismuth content reaching towards both extremes. AFM pictures clearly showed the surfaces of our samples changing with the increasing x value.(3) Raman spectra and spectroscopic ellipsmetry measurements were used to study the influences of variation of bismuth composition on the optical properties of BixFeO3thin films. In Raman spectra, we have identified most of the Raman active mode of BFO. The observed variation of active modes can be generally attributed to the changing of both Bi-O and Fe-O covalent bonds. In addition, by fitting the spectroscopic ellipsmetry data we obtained the refractive index n and extinction coefficient k. The shifting of n indicated a change of strain in BixFeO3thin films with x varying from0.80to1.20. Optical band gaps (Eg) were obtained by Tauc plot. With x increase from0.80to1.20, the optical band gap of BixFeO3thin films increase at first, and then fall down. Possible cause is the changed lattice parameter brought by appearances of impurities.(4) We also performed some electrical tests to examine the ferroelectricity and dielectricity of our BixFeO3samples as well as BixLaFeO3samples. We used LNO thin films as the bottom electrodes and sputtered Pt as the top electrodes. The results showed an increasing Pr with the higher x value in BixFeO3samples. While in BixLaFeO3samples Pr stopped increase when x reached0.95and started to fall down with the further increase of x.