Growth And Characterization Of BiFeO₃ Thin Films On Si Substrates

As one of multifunctional properties,multiferroicity has great potential application value in novel data storage,energy conversion,photoelectric sensing and so on.At present,the most widely studied multiferroic materials belong to complex transition metal oxides.These complex oxides,with the existence of lattice,charge,orbital and spin degrees of freedom and the interplay among them,are the key to the study of a new generation of low powered,green and environmentally friendly multifunctional photoelectric devices.BiFeO3 is a very rare multiferroic material for its ferroelectricity and antiferromagnetism at room temperature and has been a hot material in the field of ferroelectric and multiferroic research.Researchers have found many novel functional characteristics in BiFeO3,such as the bulk photovoltaic effect and switchable diode effect,showing great application prospect.With the development of the technology of film growth,it is possible to control the growth of thin film at the atomic scale using Laser-molecular beam epitaxy,and therefore the quality of oxide thin films,heterostructures and superlattice structures is greatly improved.The BiFeO₃ thin films grown on Si substrates can integrate semiconductor with ferroelectric oxide to design novel devices for information storage and sensing.The growth of BiFeO₃ thin films with good ferroelectricity on Si substrates plays an important role in the design and development of multifunctional oxide devices.However,it is difficult to grow single-phase BiFeO₃ thin films directly on Si substrate due to the influence of lattice mismatch and mutual diffusion.Up to now,the reported BiFeO₃ thin films grown on Si substrate often use many kinds of oxides as the buffer layer between them in order to reduce the lattice mismatch and mutual diffusion.In this thesis,the growth and physical properties of BiFeO₃ thin films directly on p-type conducting Si substrate are investigated and some results have been achieved as the following.By using Laser-molecular beam epitaxy technique,we have successfully grown the single-phase BiFeO₃ thin films with good ferroelectricity directly on Si substrate by exploring and optimizing the growing conditions of thin film.We found that a predeposition process of about 2 nm BiFeO₃ thin film performed at room temperature and high vacuum conditions in the initial stage of film growth can not only inhibit the growth of amorphous oxides on Si surface,but also effectively avoid the formation of impurity phase and is the key to obtain single-phase BiFeO₃ thin film.Besides,X-ray diffraction measurements illustrate that the obtained BiFeO₃ thin films show pure-phase polycrystalline state.The crystallinity of the BiFeO₃ thin film increases with the increasing film thickness.Transmission electron microcopy indicate that there is an amorphous interface layer around 3 nm in thickness between the BiFeO₃ thin film and Si substrate.Ferroelectric hysteresis loops measurements manifest that BiFeO₃ thin films have good ferroelectricity,and the remnant polarization increases with the increase of the film thickness,whereas the coercive field decreases with the increasing film thickness.In addition,the thin films have small leakage and the amorphous interface layer may contribute to the reduction of leakage current.Spectroscopic ellipsometry measurements demonstrate that the fundamental absorption edge of the BiFeO₃ thin film shows a slight red shift and the band gap energy decreases with the increasing thickness,which are mainly related to effect of grain size.