Investigation of Low-temperature Solution-processed Thin-Film Transistors for Flexible Displays

Abstract

This thesis describes the electrical behaviour of solution-processed zinc oxide thin film transistors (ZnO TFTs) fabricated at low temperature. First, the electrical properties of solution-processed ZnO films are reported. Spin-coated ZnO films annealed at 150 °C exhibit significant sensitivity to the ambient environment. However, their stability can be improved by hydrogen plasma treatment. Zinc oxide TFTs (channel width/length = 4000/200 μm) fabricated by chemical spray pyrolysis at the low process temperature of 140 °C are investigated. The resulting transistors exhibit a saturation mobility of 2 cm2/Vs measured in air; this value is reduced to 0.5 cm2/Vs under vacuum. The effect of hydrogen plasma treatment on spin-coated ZnO TFTs is then studied. The electrical characteristics of untreated TFTs exhibit large hysteresis and a positive threshold voltage shift on repeated measurements. These effects are reduced by the hydrogen plasma and an increase in carrier mobility is observed. In a further investigation, a solution-processed silicon dioxide gate insulator for application in the TFTs is used; a perhydropolysilazane (PHPS) precursor is spin-coated with subsequent thermal treatment to form the SiO2 layer. Exposure to oxygen plasma leads to an acceleration of the conversion reaction, resulting in good insulating properties (leakage current density of ~10-7 A/cm2) and TFT performance (channel width/length = 1000/50 μm, carrier mobility of 3.2 cm2/Vs, an on/off ratio of ~107, a threshold voltage of -1.3 V and a subthreshold swing of 0.2 V/decade). Finally, a photolithographic process is introduced for the fabrication of ‘short’ channel solution-processed ZnO TFTs. Optimum processing conditions are established and used for the fabrication of transistors having various channel dimensions. Devices with a minimum channel length of 5 μm possessed a mobility of 1.5 x 10-2 cm2/Vs, on/off ratio of 106 and good contact between the S/D electrodes and the semiconductor. The relatively low mobility could originate from gate insulator roughness caused by the photolithographic processes.