UCLA

In this dissertation, the effects of electrical defect states on the device performance and stability were examined and characterized for wide bandgap metal-oxide semiconductors. While previous research on metal-oxides in electronic device has mostly focused on the effect of light and/or bias stress on the device performance and stability, research presented here demonstrates and focuses on how electrical defect states within bandgap or at the interfacial layer between the channel and dielectric layer can affect the device performance and stability under light or/and bias stress. These results can help improvements to material sputtering growth which in turn can help improve overall device performance.

The first part of the dissertation characterizes the shallow-level defect state distribution under the conduction band minimum in Pd/ZnO Schottky barrier diode by a deep level transient spectroscopy. The second part details the deep-level defect states in sputtered ZnO

thin-film transistors as a function of oxygen ratio during sputtering growth. Photo-induced threshold voltage-shift measurements under monochromatic illumination were used to characterize the deep-level defect distribution. As a result, the deep-level defect states of ZnO

films were formed at the range of 1.8 – 2.1 eV below the conduction band minimum and were gradually decreased with increasing oxygen ratios.

The last part examines and characterizes sputtered a-ITZO TFTs under light and/or bias stress as a function of oxygen ratios. We confirmed that the photo stability and persistent photocurrent in a-IZTO TFTs were improved due to the reduced concentration of deep-level defects associated with oxygen vacancies at high oxygen ratios during sputtering growth. However, the device stability significantly degraded under bias stress with or without light at high oxygen ratios due to increased defect states at the interfacial layer between the channel and dielectric layer. The increased interfacial defects were characterized by XRR measurements and a hysteresis curve for various oxygen ratios. We have shown that good photo-bias device stability of sputtered a-ITZO TFTs can have a strong trade-off relation between photo and bias stress depending on oxygen ratios during growth by sputtering. In order to take advantage of the improved ITZO material growth at a high oxygen ratio, the interface-related problems must be solved.