Measurement And Characterization Of Piezoelectric Properties Of ZnO Nanobelts And BDT Ferroelectric Thin Films

Because of prominent ferroelectric properties such as high polarization value, negligible polarization fatigue, high Curie temperature and low leakage current, bismuth layered structure materials (BLSF) are proper to apply at high temperature and high frequency conditions and have attractive application in ferroelectic memory fields. With the development of micro-electric technology and the tendency of high integration, the preparation and properties of BLSF ferroelectric thin films have attracted great interests worldwide.In chapter 1, we reviewed the general situation, potential application, preparation methods and research status for ferroelectric thin films and II-VI nanobelts. On the basis of the review, the aim of this thesis is focused on the follows: 1 the preparation of Bi_4-xDy_xTi₃O₁₂ (BDT) thin films by metal-organic decomposition (MOD); 2 the effect of annealing temperature on surface morphology, ferroelectric properties and piezoelectric coefficient of BDT thin films; 3 the electromechanical coupling coefficient of II-VI nanobelts.In chapter 2, BDT lead-free ferroelectric thin films were prepared by metal-organic decomposition (MOD), and the effects of annealing temperature on microstructure, ferroelectric, and piezoelectric properties were investigated by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectrometer, ferroelectric tester, and scanning probe microscope (SPM). The ferroelectric and piezoelectric behaviors were established from polarization-electric field (P-E) hysteresis loops and displacement-electric field (D-E)"butterfly"curves, respectively. The remnant polarization 2Pr, spontaneous polarization 2Ps (16.2μC/cm2 and 23.3μC/cm2 under 690 kV/cm) and effective piezoelectric coefficient d33 (63 pm/V under the bipolar driving field of 310 kV/cm) of BDT thin film annealed at 700°C are better than those of the other temperatures. The improved d33 may make BDT thin films a promising candidate for piezoelectric thin film devices.In chapter 3, ZnO nanobelts were prepared by thermal evaporation method, and different metal thin film electrodes were prepared by electron diffraction, vapor plating and sputtering method. Microstructure was investigated by FE-SEM, XRD, transmission electron microscopy (TEM). The work function and the photovoltaic properties of ZnO nanobelts spreading on the metal electrode were investigated by Kelvin probe system. The best rectifying properties are obtained for metals with completed-shell (Pt, Ti), while the metals with few electrons on the d-shell (Ag) form ohmic-like contacts. The work functions of Pt, Ti and Ag are obtained to be 5.25 eV, 4.65 eV and 4.33 eV, repectively. The contact between Pt, Ti and ZnO nanobelt is Schottky contact, while the contact between Ag and ZnO is ohmic contact.In chapter 4, the test program of electromechanical coupling coefficient of II-VI nanobelts has been constructed. We used structure mode of nanobelts freely supported beam to deduce theoretical formula of electromechanical coupling coefficient of nanobelts, and validated the feasibility of using electrical contact resistance (ECR) system. By analyzing force factor of test, we designed reasonable poroid structure electrode and placed nanobelts on the poroid structure electrode. ECR test system used to operate on the nanobelts can exert force on the nanobelts, while outputs electrical energy comes out through the electrodes. Meanwhile, the mechanism of output currents was also explained.