Study of Ferromagnetic and Field Effect Properties of Zinc Oxide Thin Films.

摘要

Spintronics (spin transport electronics), in which both spm and charge of carriers are utilized for information processing, is perceived to be a candidate to extend and possibly to become the next-generation electronics. Its advantages include nonvolatility (data retention without electrical power), lower energy consumption, faster processing speed, and higher integration densities in comparison with the current semiconductor devices relying solely on electron charge. To realize a spin-field effect transistor, two respects are prerequisite. On the one hand, the mechanism of ferromagnetism should be addressed before one could prepare magnetic semiconductor films in a controllable way. On the other hand, excellent field effect properties should be sought through a convenient and low-cost strategy for manufacturing future nano-scale spintronic devices. This thesis is comprised of two parts. Firstly, it deals with the synthesis, characterization, and magnetism of transition-metal-doped or un-doped zinc oxide (ZnO) thin films. Secondly, it focuses on the field effect properties of solution processable ZnO thin films, which are not only of great interest for current charge-carrier based thin film transistors, but also of fundamental importance in future spin-based transistors.;A facile spin-coating technique has been developed to fabricate ZnO thin films. Even without magnetic element doping, the film is found to show room temperature ferromagnetism. A broad series of advanced microscopic and spectroscopic techniques are utilized to characterize the thin films properties. Oxygen vacancy defects are tentatively attributed to the observed ferromagnetism. Following the similar method, Ga doped or Ga, Co co-doped ZnO thin films are prepared. The ferromagnetism is enhanced with Ga doping, providing more carriers. It is discovered that room temperature ferromagnetism can exist in both highly conductive regime and the less conductive or near insulating regime. Transition metal doping is not necessary to realize ferromagnetic coupling. By annealing single crystalline ZnO in high vacuum, similar hysteresis loops indicative of ferromagnetism have been observed, which excludes the interfacial and grain boundary effects in polycrystalline ZnO as the origin of ferromagnetism.;High performance thin film transistors are featured by high carrier mobility, low power consumption, facile and low cost fabrication process, low temperature preparation, and high throughput to meet the demands in next generation large area and flexible display technology. By developing a novel process to fabricate alumina gate dielectric and proper doping into ZnO thin films, the high performance devices are attained with larger mobility than most ever reported devices fabricated by low cost solution process without high temperature annealing. By employing this technique, the device is endowed with almost all features mentioned above. Spin-coated alumina provides unique dielectric properties, excellent thermal stability, visible transparency, low cost, and seamless integration with oxide channel layers, which will probably pave the way for wide applications in transparent flexible electronics in future.;This thesis research contributes a deeper insight into the microscopic origin of ferromagnetism in wide-band-gap oxide based DMSs. Meanwhile, a novel process of alumina gate dielectric dedicated to low operating voltage application has been unveiled, with which the field-effect performance of ZnO TFTs has been significantly improved. This technique may not only make significant impact on conventional charge based transparent electronics, but also might provide clues and simple platforms in future to investigate the spin-related field effect properties in ZnO.