Fabrication and Characterization of Thin Film Transistors based on Sol-Gel Derived Zinc Oxide Channel Layers
Type of DegreePhD Dissertation
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This dissertation aims to explore film and device characteristics of zinc oxide (ZnO) based channel layers fabricated via the sol-gel spin-coating growth technique. ZnO is wide band gap semiconductor with a wide range of industrial applications and numerous electronic applications such as optoelectronics, flexible transparent electronics and sensors. ZnO films were synthesized with different number of layers by repeating the spin-coating process and then annealed at different temperatures. Micro-Raman, Photoluminescence (PL), Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) were used for film characterization and to investigate crystal quality of the films. The results indicated an increase in the magnitude of E2High Raman shift as well as the near-band-edge PL peak as the annealing temperature increased. This intensification was related to the enlargement of grain size as a result of higher annealing temperature. The area under the curve ratio for UV PL over visible PL, in addition to the intensification of E2High Raman peak, suggested higher film quality as the annealing temperature increases. However, SEM images revealed that annealing temperatures higher than 800 ℃ result in the formation of gaps between grains, causing non-continuity in the film. Bottom gate thin film transistors (TFTs) were fabricated and Current-Voltage (I-V) and Capacitance-Voltage (C-V) measurements were implemented to investigate a possible “kink-effect” (also known as the ”hump-effect”) in the multilayer-channel samples, due to the renowned surface oxygen chemisorption in oxide semiconductors. The measurements hint to the possibility of depletion layer formation at the ZnO-ZnO interlayer-interfaces. The kink effect disappeared after high doses of radiation, suggesting the likelihood of the distortion of the depletion layers due to the displacement damage. After 18 months, the kink effect in unirradiated samples remained existent and in the irradiated samples, it remained absent. Multilayer ZnO TFTs were exposed to a 60Co radiation source and the robustness of ZnO TFTs against gamma irradiation was verified by comparing the transfer and output characteristics of TFTs and the PL of the channel layers before and after irradiation. By applying High Resolution Digital Optical Microscopy and Atomic Force Microscopy (AFM), it was found that the thickness of the ZnO was drastically influenced by the irradiation. A model was provided to explain the possible etching mechanism. Three effective medium approximations were applied to theoretically investigate the modification of the refractive index of the flexible conductive polymer matrix Poly(3,4-ethylenedioxythiophene) and Poly(styrene sulfonic acid) (PEDOT:PSS) by gallium doped ZnO nanoparticle inclusions, as a function of volume fraction of nanoparticle inclusions. Calculations determined the desired volume fraction of the gallium doped nanoparticles to be between 45% and 71% when gallium dopant concentration varies between 0 and 4 percent.