Study On The Preparation And Properties Of Epitaxial BiFeO₃ Thin Film Capacitors And Heterostructures

BiFeO3 (BFO)with a perovskite-type structure is now perhaps in the future the only single-phase material that is both magnetic and a strong ferroelectric at room temperature, TN=673K, TC=1103K. Great prospect captured the imagination worldwide.The residual polarization of BFO(Pr) reach up to 90-100μC/cm2, which provides an alternative choice of a lead-free ferroelectric material. How because the pure phase BFO bulk and film preparation conditions are relatively harsh, and there is a large leakage current, it is difficult to obtain saturated hysteresis loop. Chemical-doping can improve the leakage performance. The optical band-gap of BFO is 2.8eV, that means at wavelength of 500nm or more in the visible region BFO is transparent. So it is useful in invisible electronic circuits.In this thesis Pulsed Laser Deposition method was used to deposit thin films. A mature technology of obtaining pure BFO thin film is at our disposal. Epitaxial and transparent ferroelectric capacitor was successfully fabricated by employing a new transparent conductive oxide Lao.o7Sro.93Sn03 as electrode. Leakage was greatly suppressed with Mn-doping as a result the ferroelectric properties were improved. The junctions using hetero-epitaxial n-LSSO\p-BFO (BFMO) were proposed and examined. We demonstrated that they show good rectifying characteristics and resistive hysteresis. TheⅠ-Ⅴresults of adjusting the thickness of BFO thin films and Mn-doping reveal rectifying characteristics and resistive hysteresis related to oxygen vacancies of BFO; The stress relief mechanisms in epitaxial La0.7Sr0.3Mn03(LSMO, a common electrode materials) grown on LAST(001), NGO(001) and NGO(110) substrates were investigated, also.The dissertation consists of five chapters, the arrangement of the chapters and the main contents in each chapter are presented as follows:In chapter 1, first we make a brief introduction to the perovskite structure including crystal structure, lattice distortion and the d orbital splitting. Then from the research background, development and basic application we introduced the concept of ferroelectric, multiferroic, transparent conductive oxide, perovskite manganese oxides and ferroelectric-based devices mainly about the formula and concepts involved in Subsequent chapters.In chapter 2, the film preparation methods and samples measurements are introduced, especially the details of the pulsed laser deposition method, x-ray diffraction reciprocal space mapping.In chapter 3, take advantage of the High-definition XRD RSM, We investigated the stress relief mechanisms in epitaxial La0.7Sr0.3MnO3 (LSMO), a common electrode materials) grown on LAST(001), NGO(001) and NGO(110) substrates, and demonstrate that for LSMO\NGO(110) biaxial mismatch stress is relieved differently along in-plane directions perpendicular to each other:one is accommodated by decrease of the out-plane-angle of the orthorhombic LSMO unit cell, while another is partially by periodic lattice modulations.In chapter 4, new conductive and transparent LSSO film was employed as electrodes for fabrication of epitaxial BFMO ferroelectric capacitors. All the films show single-crystalline quality as revealed by HRDXRD. The LSSO\PZT heterostructures grown on the STO are transparent with transmittance comparable to that of the pure STO substrates at wavelength of 500-2500nm. The capacitors show square polarization-electric field hysteresis loops, but poor fatigue resistance. By transmittance measurement we got that the optical band gap of BFMO is 2.8eV. The effects of substrate orientation on Pr show the easy axis of spontaneous polarization of BFMO lies close to [111].In chapter 5, Systematic studies on theⅠ-Ⅴcharacteristics of the LSSO\BFO (BFMO) heterostructures were executed. A resistive hysteresis and a diode-like behavior were observed. TheⅠ-Ⅴresults of adjusting the thickness of BFO thin films and Mn-doping reveal rectifying characteristics and resistive hysteresis related to oxygen vacancies of BFO.In chapter 6, Epitaxial (001)-, (118)-, and (104)-oriented Bi3.15Nd0.85Ti3O12 (BNT) films were grown on LSMO buffered LAST substrates of (100), (110) and (111) orientations, respectively. All the films show single-crystalline quality as revealed by HRDXRD. The capacitors show saturated polarization-electric field hysteresis loops and poor fatigue resistance.