Study On The Preparation And Properties Of Epitaxial KNN Thin Film

Ferroelectric films, which possess ferroelectricity and piezoelectricity at the same time, have great potential application in microelectronic device design. Hence, the study on physical properties of ferroelectric thin films has attracted extensive attention during the past decades. Actually, ferroelectric films have already been proposed to fabricate electronic devices, such as ferroelectric capacitors, ferroelectric field-electric transistors and nonvolatile memories, etc. While, the preparation of high quality thin film was the precondition for the study and application of the ferroelectric films. With the development of the film fabricated methods and improvement of the instruments, high quality thin films can be easily prepared by both physical and chemical methods. Among them, pulse laser deposition (PLD) is most widely used in the fabrication of complex oxide films due to the simple and tunable processing parameters. The thickness of the film can also be accurately controlled by the number of laser pulse. What is more, stoichiometric films can be obtained through the optimization of the deposited oxygen partial pressure and substrate temperature.Recently, Potassium sodium niobate (KNN) is becoming a promising candidate of the conventional PZT piezoelectric material. With chemical doping, the performance of KNN ceramics can be remarkably enhanced, like a higher piezoelectric constant, better temperature stability and saturated ferroelectric hysteresis loops. Contrasting with the ceramic study, little attention has been focused on KNN-based films, especially the development of new chemical compositions with fabulous performance. Studies revealed that the major challenges are preserving the light and highly volatile A-site elements, namely sodium and potassium, and avoiding pyrochlore formation. Therefore, PLD was preferentially considered for the preparation of KNN films.In this study, we investigated the effects of deposited oxygen pressure and substrate temperature on the dielectric and ferroelectric properties of the KNN films prepared by PLD. The dominant electric conduction mechanism of the KNN films was revealed through linear fitting with four main models. The KNN capacitors with different oxide electrodes were fabricated, the effect of oxide electrodes on the dielectric, ferroelectric properties and fatigue behavior was described in detail. The whole thesis consists of four chapters.Chapter1:This chapter is divided into3parts:Firstly, the conceptions of the ferroelectricity and piezoelectricity are introduced briefly and the physical meaning of some main constants is described. Secondly, we give out the component diagram of (KxNa1-x)NbO3and introduce the research background of the doped-KNN ceramics. KNN ceramics with chemical addition show remarkable enhancement of piezoelectricity and ferroelectricity, and may be more temperature stable. Finally, we introduce the fabrication methods and the current research states of the KNN films.Chapter2:The films of0.95(K0.49Na0.49Li0.02)(Nb0.8Ta0.2)03-0.05CaZrO3(KNN) with2wt%MnO2addition were prepared at various oxygen partial pressures and substrate temperatures on Lao.7Sro.3Mn03(LSMO) coated SrTiO3(STO)(001). The oxygen pressure of LSMO was optimized. The crystallinity and surface morphology of KNN films were characterized by XRD and SEM. Then, we discussed the effects of deposited oxygen pressure and substrate temperature on the crystallinity. The dielectric constant and ferroelectric hysteresis loops of KNN films were measured in order to analyze the effect of deposition parameters on the physical properties, namely oxygen pressure and substrate temperature. The highest dielectric constant and relatively higher remnant polarization shown up in the film deposited at35Pa and680℃. At last, electric conduction mechanism of the films was studied by fitting of leakage current with four main conduction models. The result indicted that ohmic, space charge-limited current and Pool-Frenkel emission came up as the dominant conduction mechanism in turn with the increase of the applied electric field.Chapter3:LSMO, La0.07Sr0.93Sn03(LSSO) and Nb-doped STO were used as bottom electrodes for the preparation of Pt/KNN/Nb:STO, Pt/KNN/LSSO/STO, Pt/KNN/LSMO/STO and Pt/KNN/LSMO/LAST capacitors. In spite of the high dielectric loss, the measurement result of dielectric constant and ferroelectric hysteresis loop showed that the KNN capacitors with LSMO bottom electrode showed a relative higher polarization. Among them, Pt/KNN/KSMO/STO capacitor showed the highest dielectric constant of1185and a remanent polarization (2Pr) of24.88μC/cm2. Chapter4:The KNN capacitors with various combinations of LSMO and LSSO as thin film electrodes were fabricated by PLD. The LSMO/KNN/LSMO capacitor showed antifatigue characteristic in the fatigue testing. However, KNN capacitors with LSSO electrode tend to get fatigue and breakdown easily.