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High Performance Metal Oxide Thin Film Transistors via Cluster Control and Interface Engineering


Around 100 years has passed since the first cathode ray tube has been fabricated. Fast and free transition of graphs provided much convenience for human communication. Generations of display were developed and flat panel display (FPD) techniques are developing tremendously recently. Various demands are raised including high definition, large area, flexibility, etc. Backplane need improving to meet these, especially the thin film transistor (TFT) units. High mobility, easy process and good interfaces are desired. Solution processed amorphous InGaZnO proves a competitive candidate for TFT semiconductor materials. Its electronic performance, uniformity and switching properties turned out among the best. However, problems remain to be solved including mechanism interpretation, precursor control, morphology and interface. Chapter 1 will introduce the history and state of art of TFT in more details.

In the following parts of this dissertation, I’ll discuss the electronic behavior, morphology and interface of IGZO TFT. In Chapter 2, we performed gated four-probe measurements to extract the intrinsic mobility and contact resistance as functions of gate voltage and temperature. Contact resistance was proved to play a major role in mobility degradation at high gate bias, whereas, band-like transport dominates. We proposed UV-O3 which modified the contact regions and mobility was boosted from 23 to 30 cm2/Vs.

In Chapter 3, clusters in precursor solution, which has critical effects on morphology, are discussed. Cluster size distribution was narrowed and size was brought down by acac. Small roughness of metal oxide was achieved and saturated mobility increased from 4.0 to 5.5 cm2/Vs. In a positive bias stress test, turn on voltage shift decreased from 1.6 to 0.3 V/10000s. Cluster size control is a promising way to tune the morphology of solution processed metal oxide film.

Small sized high definition display is placing more challenge on backplane TFTs. IGZO is one of the candidates but the unsatisfactory performance of small sized IGZO TFTs is limiting their applicability. Hence, a novel weak acid modification (WAM) strategy was introduced to generate more oxygen vacancies for higher mobility, and to lower the surface roughness. Electrode-IGZO contact was enhanced. Contact resistance was reduced from 9.1 kΩmm to 2.3 kΩmm, as measured by the gated four probe (GFP) method. Field effect mobility for small sized devices was boosted from 1.5 cm2/Vs to 4.0 cm2/Vs. Additionally, a 12�12 transistor and organic light emission diode array built from the modified IGZO TFT devices has been demonstrated.

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