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Title: Thin film engineering for transparent thin film transistors
Author: Abusabee, K. M.
ISNI:       0000 0004 5348 3261
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2014
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Zinc oxide (ZnO) and Indium Gallium Zinc Oxide (IGZO) thin films are of interest as oxide semiconductors in thin film transistor (TFT) applications, due to visible light transparency, and low deposition temperature. There is particular interest in ZnO and IGZO based transparent TFT devices fabricated at low temperature on low cost flexible substrates. However, thermal annealing processes are typically required to ensure a good performance, suitable long term stability, and to control the point defects which affect the electrical characteristics. Hence there is interest in post deposition processing techniques, particularly where alternatives to high temperature thermal treatments can be utilised in combination with low temperature substrates. This thesis presents the results of a series of experimental studies as an investigation into photonic (excimer laser) processing of low temperature ZnO and IGZO thin films deposited by RF magnetron sputtering and/or by high target utilisation sputtering (HiTUS), to optimise the microstructure and electrical properties for potential use in thin film electronic applications. ZnO thin films were grown at various deposition parameters by varying oxygen flow rates, RF power, oxygen concentration, and growth temperatures. Subsequently, the films were subjected to three different annealing processes: (i) Thermal Annealing (furnace): samples were thermally annealed in air at temperatures ranging from 300 °C to 880 °C for 1 hour. (ii) Rapid Thermal Annealing: samples were annealed in nitrogen and oxygen environment at temperatures of 600 °C, 740 °C, 880 °C, and 1000 °C, and dwell times of 1-16 s. (iii) Excimer laser annealing: samples were annealed at ambient conditions using a Lambda Physik 305i 284 nm, 20 ns pulse KrF excimer laser with a beam delivery system providing a homogenised 10 mm x 10 mm uniform irradiation at the sample plane. Processing was undertaken at fluences in the range of 0 to 350 mJ/cm2 at single and multiple pulses. IGZO thin films were also investigated following RF magnetron deposition without intentional substrate heating and at various other deposition conditions, followed by laser processing in air at laser energy densities in the range of 0 to 175 mJ/cm2 with single pulse. Processed ZnO films were characterised by room temperature photoluminescence excitation which exhibited that laser annealing at high fluences resulted in suppression of the observed visible deep level emission (DLE) with evolution of a strong UV near band emission (NBE) peak, indicating a reduction of intrinsic defects without film degradation or materials loss that occurred by thermal and rapid thermal annealing. Also the intensity of the NBE peak was strongly influenced by the films growth temperature, with the results showing that as the growth temperature increased beyond ambient; the intensity of the resultant NBE peak decreased as a function of laser energy. TEM studies demonstrate that laser processing provides a controlled in-depth crystallisation and modification of ZnO films. Therefore, laser processing is shown to be a suitable technique to control the crystal microstructure and defect properties as a function of two lasers processing parameters (fluence, number of pulses) - realising optimised film properties as a localised region isolated from the substrate or sensitive underlying layers. In terms of electrical properties, the results indicated a significant drop in sheet resistance as a function of laser anneal from highly resistive (>5 MΩ/sq.) to about 860 Ω/sq. To produce IGZO thin films without intentional substrate heating with lowest sheet resistance as a function of laser processing, low deposition pressure, low oxygen concentration, and high RF power are required. Room temperature Hall effect mobility of 50 nm thick IGZO increased significantly as the laser energy density increased from 75 mJ/cm2 to 100 mJ/cm2 at single pulse reaching values of 11.1 cm2/Vs and 13.9 cm2/Vs respectively.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available