Development and Study on the Optical and Electrical Properties of Magnesium Niobate Thin Films

  • 何 毅達

Student thesis: Doctoral Thesis


Binary metal oxides with high optical band gaps (Eg) have been researched and developed for several applications in recent years The high optical transparencies and excellent electrical and mechanical properties lead these materials to have high potentials on transparent electronic devices On the other hand amorphous oxide films offer several advantages such as lower processing temperatures better surface uniformity lower leakage current and superior mechanical flexibility compared to crystalline films These excellent properties and advantages make amorphous transparent oxide films (a-TOFs) powerful candidates for dielectric layers to realize transparent electronics In addition several materials systems have been found to possess stable resistive switching (RS) behavior attracting great attention as candidates for next-generation non-volatile memory applications Niobium-based oxides have been extensively studied for applications such as microwave dielectrics optoelectronics and photoactive materials Among these materials columbite MgNb2O6 (MNO) exhibits a high dielectric constant excellent microwave properties good thermal stability high transparency a wide bandgap and optical activation These excellent properties make MNO a promising material for transparent electronics or multi-functional optoelectronics As mentioned above the main study of this dissertation is divided by three parts: 1 Fabrication and Characteristic of Sol-Gel Derived MgNb2O6 Thin Films Transparent amorphous-MgNb2O6 (MNO) thin films were fabricated on ITO/glass substrates using the sol-gel method The change in the chemical states as well as the optical and dielectric properties of MNO films at various annealing temperatures is investigated The MNO films exhibited the amorphous phase when the annealing temperature was below 600°C From X-ray photoelectron spectroscopy (XPS) the major parts of the films’ chemical states can be indexed as Mg2+ Nb5+ Nb4+ and O2- Further the Nb4+ element can be reduced at higher annealing temperatures The average transmission percentage in the visible range (λ=400–800 nm) is over 80% for all MNO/ITO/glass samples while the optical band gap (Eg) for all samples is estimated at ~5 eV In addition the dielectric constant was calculated to be higher than 20 under a 1 MHz AC electric field with a leakage current density below 2×10-7 A/cm2 at 1 V In addition the effects of annealing atmosphere (N2 air and O2) on the electrical properties of sol-gel derived MNO/ITO heterostructures are discussed All samples also exhibited the amorphous phase and were highly transparent The percentage of Nb4+ content increased when the films were annealed in the oxygen deficient conditions which could lead to semiconducting films In addition the results show that the electrical properties of sol-gel derived MNO thin films could be tuned based on the annealing atmosphere Moreover the conduction mechanisms of MNO /ITO heterostructures are also discussed The results show that MNO thin films have potential for use in multifunctional optoelectronic applications due to their flexible electrical properties and good transparency 2 The Effects of Zinc Substitution on the Electrical Properties of MgNb2O6 Thin Films For lowering the fabrication temperature the Al/Mg1-xZnxNb2O6 (MZxNO)/ITO devices were investigated in this part According to the experimental results the great optical-electrical properties of the devices can be obtained at x=0 2 The dielectric constant average transparency and optical band gap of the devices are 21 2 (@1 MHz) ~80% and 4 86 eV after 400°C annealing respectively Compared with 500°C-annealled MNO samples the dielectric constant has highly enhancement More important the fabrication temperature of MZ0 2NO thin films is 100°C lower than that of MNO From low to high applied electric field the leakage conduction mechanisms of Al/MZxNO/ITO devices are ohmic conduction space-charge-limited conduction (SCLC) and FN tunneling respectively Moreover the critical electric field strength of the conduction mechanisms changed with zinc substitution proportions 3 Resistive Switching Behaviors of MgNb2O6 Thin Films The resistive switching (RS) behavior of the Al/MNO/ITO devices was investigated From the DC voltage sweep test the air-annealed MNO samples exhibited stable and reproducible bipolar resistive switching (BRS) behavior; however the samples annealed in an O2-rich environment showed no RS property These results suggest that the RS behavior of the MNO memory devices is highly related to the oxygen vacancy concentration and distribution within the MNO films In addition forming-free unipolar resistive switching (URS) behavior was observed when the MNO films were annealed under an N2H2 atmosphere In order to determine the origin of the BRS and URS behaviors cross-sectional high-resolution transmission electron microscopy images of the MNO samples were acquired The RS behavior of the MNO films can be ascribed to the release and recombination of electrons and oxygen vacancies
Date of Award2015 Aug 14
Original languageEnglish
SupervisorCheng-Liang Huang (Supervisor)

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