Flexoelectric Control of Ferroelectric Properties and Electronic Functions in Epitaxial BiFeO3 Thin Films

Abstract

Flexoelectricity is the generation of an electric field by a strain gradient via electro-mechanical coupling. Although it was first theoretically reported by Kogan in the 1968, there have been few studies on flexoelectricity in bulk solid materials. It is because the flexoelectric effects are expected to be quite small in the rigid bulk solids. Recently, however it was reported that the strain gradient in epitaxial oxide thin films could be 6 or 7 orders of magnitude larger than the corresponding bulk values. As a result, flexoelectric effect has been emerging as a fascinating means for exploring the physical properties in epitaxial ferroelectric thin films. In this dissertation, I will report the effect of deposition temperature to the defect evolution and ferroelectric properties in BiFeO3 thin films. I could successfully control the strain evolution of the BiFeO3 films by varying the deposition temperature and the film thickness. I will show that the flexoelectric effect can reverse the as-grown polarization direction and associated changes in the electronic functional properties of BiFeO3 thin films. Finally, I will show that the unusual coupling between internal electric field and defect formation in BiFeO3 epitaxial thin films. By tailoring the internal electric field via flexoelectricity, I control the defect formation and achieve a nearly defect-free BiFeO3 film that exhibits perfectly functional performances. Ferroelectric materials promise a broad range of functional electronic properties, which are generally governed by various defects. Critical to practical applications of ferroelectric properties is our ability for understanding and controlling the defect formation. To investigate the effects of deposition temperature to the defect evolution and the ferroelectric properties of the BiFeO3 thin films, I grew BiFeO3 thin films on (001) SrTiO3 substrates using pulsed laser deposition at temperatures in the range of 570–600°C at intervals of 10°C. Interestingly, I found that defects appeared at temperatures greater than 590°C and threshold temperature is between 580°C and 590°C. The defects led to significant changes in the optical absorption and impurity peaks in X-ray diffraction data. Analysis of the X-ray diffraction data indicates that the defects are Fe2O3. Atomic force microscopy measurements showed that the appearance of defects accompanied an abrupt increase in the surface roughness. Furthermore, the presence of the defects significantly affected ferroelectric hysteresis. Our results suggest that the evolution of defects in BiFeO3 thin films depends strongly on the deposition temperature. The flexoelectric effect can play an important role in determining the domain configurations and electronic transport properties in ferroelectric epitaxial thin films, due to its intrinsic and universal existence in every dielectric material. In BiFeO3 epitaxial films with a large strain gradient, the flexoelectric and interfacial effects compete with each other in establishing the self-polarization state. The competing effects in the films were introduced by fabricating BiFeO3 thin films of two different strain states, varying the deposition temperature and/or the film thickness. We found that uniaxially, fully strained BiFeO3 films were self-poled, having a downward polarization

this indicated that the interfacial effect was dominant. In contrast, the relaxed films had upward self-polarization, indicating that the flexoelectric effect was dominant. Interestingly enough, the two kinds of films also exhibited different unidirectional current flows, referred to as the diode effect. By understanding the self-poling mechanisms in BiFeO3 films, such as ferroelectric hysteresis and electronic transport characteristics, the configuration of the as-grown films can be optimized to allow full utilization of the ferroelectric functional device. Finally, internal fields (Eint) can be induced in ferroelectric thin film during the growth. Numerous origins have been proposed for the built-in electric field, namely, interfacial effects, flexoelectricity, defects, and piezoelectricity, and so on. I demonstrate that the defect formation in BiFeO3 thin films critically depends on the internal electric field in the films. The large, systematic control of internal electric field is achieved via flexoelectricity, which can thereby modify the defect formation and associated electronic functions of the films. Such a flexoelectric control can be utilized to achieve a nearly defect-free BiFeO3 film that exhibits perfectly functional performances, such as imprint-free polarization switching and switchable diode effect. This results highlight that flexoelectricity can dramatically modify the defect formation even with a small variation of growth parameters, emphasizing its potential key role in defect engineering. Our study provides novel insight into defect engineering, as well as a foundation for fully utilizing functional materials.