Preparation Of Multiferroic BiFeO₃Films By Rf Magnetron Sputtering And Investigation Of Their Properties

Multiferroics are defined as materials that contain at least two of ferroic orders, including ferroelectric, ferromagnetic and ferroelastic ones. Multiferroics not only have promising research prospects in the field of materials science and condensed matter physics, but also will stimulate the creation of a new generation of functional devices, such as new memory cells, novel transducers and spintronic devices, etc. Magnetoelectric multiferroics is the mostly studied multiferroic system, which possesses both ferroelectric and ferromagnetic orders, as well as a coupling effect between the two.Among all magnetoelectric multiferroics, BiFeO3(BFO) is one of the few that possesses both high ferroelectric Curie temperature (Tc=1143K) and high anti-ferromagnetic Neel temperature (Tn=643K), so it attracts much more attention than its peers. In this thesis, BiFeO3films and thin films-based capacitors structures were prepared successfully via radio-frequency (RF) magnetron sputtering. The microstructure, composition and properties of the films were analyzed or measured via a variety of instruments, including XRD, XPS, TEM, SEM, AFM, PFM, ferroelectric tester and AGM. The main contents and results of this thesis are:(1) The effect of parasitic phases on the microstructure and properties of BiFeO3films were analyzed. By using a stoichiometric target of bismuth ferrite, the existence of Bi2O3and y-Fe2O3parasitic phases were revealed in films deposited at different substrate temperatures. Magnetic hysteresis tests via AGM indicated that, the inclusion of crystalline γ-Fe2O3significantly improved the magnetic properties of the films.(2) The effect of sputtering atmosphere on the microstructure and properties of BiFeO3films were studied. It’s well known that the glow discharge of RF magnetron sputtering mainly depends on the ionization of inert gases (such as Ar). The ratio of Ar/O2affects the distribution of the glow discharge, the species in the discharge and the energy of the sputtering species, which could have a profound influence on the deposition rate, and hence a significant impact on the microstruture and properties of the films. On one hand, inclusion of oxygen in the sputtering atmosphere helps to maintain the stoicheometry of BiFeO3films, leading to an improved electrical performance. On the other hand, inclusion of too much oxygen results in a sharp decrease of films deposition rate, leading to a porous film with increased surface and interface roughnesses, deteriorating the film’s electrical property. Our study revealed that the films prepared at Ar/O2=4:1show a dense surface, microstructure and a smooth film surface, together with the best overall ferroelectric property.(3) The effect of film thickness on the microstructure and properties of BiFeO3films were investigated. The thickness of a film affects its the surface roughness and preferred orientation, leading to a thickness dependent electrical property. Our investigation suggested that, in a thickness range between150nm and500nm, sputtered BiFeO3films show improved electrical properties with the increase of thickness..(4) The effect of substrate orientation on pure phase BiFeO3thin films were explored. SrRuO3_-coated SrTiO3substrates with different crystalline orientations were used to deposit BiFeO3films with the same thickness. We found that the preferred orientations of BiFeO3films are dominated by the substrate orientation. In addition, surface morphology and electrical properties of BiFeO3films exhibit a strong crystalline anisotropy. The smallest grain size and smoothest interface were revealed for the(111)-oriented BiFeO3, while the best ferroelectric performance was found in the (001)-BiFeO3film. Moreover, the dielectric constants are different for films with different orientations, while the dielectric loss tangent is only weakly dependent on film orientation.