Characterization of Li-Doped ZnO Thin Films and Their Application to Resistive Random Access Memory Devices and Acoustic-Based Sensors

摘要

Zinc oxide (ZnO) is a direct wide-bandgap (Eg ~ 3 37 eV) n-type multifunctional semiconductor material with many unique electrical optical acoustic and mechanical properties The literature contains many proposals for enhancing the structural and electrical properties of ZnO thin films and extending their applications The present thesis focuses on improving the microstructure of Li-doped ZnO (LZO) thin films with particular regard to their potential application to resistive random access memory (ReRAM) devices and thin-film bulk acoustic-wave resonator (TFBAR) sensors The thesis commences by investigating the use of an in-situ post-sputtering annealing process performed under vacuum conditions to improve the piezoelectric and ferroelectric properties of 3 at% LZO thin films grown by radio frequency (RF) magnetron sputtering The results confirm that the piezoelectric and ferroelectric properties of the LZO thin film are both improved through a careful control of the annealing conditions An enhanced RF magnetron sputtering process is then proposed in which 3 at% LZO thin films are prepared with a synchronous DC-bias voltage applied to the substrate in order to improve the structural piezoelectric and dielectric properties of the films In the second stage of the thesis an investigation is performed into the potential of LZO films for non-volatile memory (NVM) applications The investigations focus particularly on the effects of the Li dopant level on the formation and distribution of defects in the LixZn1-xO structure and the subsequent impact of these defects on the resistive random access memory (ReRAM) device performance It is shown that the Li dopant concentration has a direct influence on the distribution of the defects in the LixZn1-xO film and thus has a critical effect on the resistance ratio of the device The multi-step (i e normal 2- 3- and 4-step) RESET behavior of a 6 at% LZO ReRAM device is systematically explored under various compliance currents (CC) in terms of current-voltage (I-V) curves (RS cycles and linear fitting results) in-situ transmission electron microscopy (TEM) observations (evolution of the conductive filaments (CFs) within the ReRAM device) electrochemical impedance spectroscopy (EIS) measurements (Nyquist plot and equivalent circuit) and chemical bonds (Li+ ions) Finally longitudinal-mode and contour-mode TFBARs are synthesized incorporating LZO films treated by UV-ozone illumination for 30 ~ 120 min The microstructural piezoelectric and energy binding properties of the various LZO films are examined and compared The results show that the UV illumination process improves both the crystallization properties of the LZO films and their piezoelectric performance In addition it is shown that given an appropriate UV illumination time the TFBARs exhibit a high resolution for atmospheric sensing and an approximately linear response for relative humilities in the range of 30 ~ 90%

獎項日期	2016 11月 9
原文	English
監督員	Sheng-Yuan Chu (Supervisor)