The Reliability of Zinc Oxide Based Thin Film Transistors Under Extreme Conditions
Type of DegreePhD Dissertation
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In this Ph.D. dissertation, I report the device instability of ZnO thin film transistors (TFTs) under extreme environmental conditions. It is extremely important to identify the cause of device instability under different environmental conditions. The types of defects and its influence in ZnO TFT performance have been analyzed in this work. The device instability under electrical stressing and subsequent relaxation have been investigated for ZnO TFTs. A systemic comparison between ambient and vacuum conditions was carried out to investigate the effect of adsorption of oxygen and water molecules, which leads to the creation of defects in the channel layer. The observed subthreshold swing and change in field effect mobility under gate bias stressing have supported the fact that oxygen and moisture directly affect the threshold voltage (VTH) shift. We have presented the comprehensive analysis of device relaxation under both ambient and vacuum conditions to further confirm the defect creation and charge trapping/de-trapping process since it has not been reported before. It was hypothesized that chemisorbed molecules form acceptor-like traps and can diffuse into the ZnO thin film through the void on the grain boundary, being relocated even near the semiconductor/dielectric interface. The stretched exponential and power-law model fitting reinforces the conclusion of defect creation by oxygen and moisture adsorption on the active layer. I have investigated the displacement damage (DD) effect on the electrical characteristics of ZnO thin film transistors (TFTs) based on its location of origin in the device structure. The area subjected to maximum proton dose induces maximum DD effect in that particular location. ZnO TFTs with two different passivation layer thicknesses were prepared to obtain maximum proton dose distribution in either the ZnO channel layer or ZnO/SiO2 interface. The devices were irradiated by a proton beam with an energy 200 keV and 1×1014 protons/cm2 fluence. Transport of Ions in Matter (TRIM) simulation, followed by calculation of depth distribution of the non-ionizing energy loss (NIEL), illustrated different proton dose distribution profiles and NIEL profiles along the depth of the device for these two types of samples. The sample with maximum proton dose peak at the ZnO/SiO2 interface exhibited a significant degradation in device electrical characteristics after the irradiation compared to negligible degradation of the sample where the maximum proton dose was absorbed in the ZnO layer. Therefore, one must be cautious when studying the radiation hardness of proton-irradiated ZnO TFTs since the displacement damage induces drastic changes on the device characteristics based on the damage location.