Fabrication And Characterization Of Zno-based Thin Film Transistors

ZnO-based thin film transistors (TFTs) have attracted much attention due to the wide range of application in driving flat panel displays. However, ZnO films tend to display high carrier concentration in the as-prepared state due to the existence of Vo and Zni, even in nominally undoped films, leading large off-current in ZnO-based TFTs and making they operate in depletion mode. Thus, one of the main problems in ZnO-based TFTs is the control of the carrier concentration in the active layers. In this doctoral dissertation, ZnO-based TFTs were fabricated and the electrical properties of the TFTs were improved by optimize the depotion condition. The details are as follows:The ZnO thin films were deposited on quartz and SiO2/p-Si substrates by radio frequency magnetron sputtering and ZnO-TFTs were fabricated on SiO2/p-Si substrates. The effect of ZnO films' thickness on the optical properties of the films and the effect of ZnO films'thickness and annealing temperature on the electrical properties of the TFTs were investigated. The influence of W/L on the mobility of TFTs was also discussed. The results indicated that the transistor with an40nm thick ZnO and annealed at950℃exhibited the best performance, with a threshold voltage of-2.4V, a field effect mobility of27.5cm2/V s, an on/off ratio of7×103, and an off-current of8.33×10-7A. The on current increased and the mobility decreased as the W/L increased.The ZnO:Ga (GZO) thin films were deposited on quartz and SiO2/p-Si substrates by radio frequency magnetron sputtering and GZO-TFTs were fabricated on SiO2/p-Si substrates. The effect of substrate temperature and sputtering Oxygen flow on the optical properties of the films and the effect of substrate temperature, sputtering Oxygen flow, annealing temperature and GZO films'thickness on the electrical properties of the TFTs were investigated. The time-dependent instability of the TFT was studied. The results indicated that when the substrate temperate was950℃, Oxygen flow was1SCCM, annealing temperature was800℃, and the GZO films'thickness was35nm, the transistor exhibited the best performance, with a threshold voltage of14.7V, a field effect mobility of10.13cm2/V s, an on/off ratio of1.07×106, and an off-current of1.34×10-9A. The transistor stored in ambient air without any protection showed little difference after8months, but degenerated after12months, the degeneration was main related with the humidity in air. The MgZnO thin films were deposited on quartz and SiO2/p-Si substrates by rf-MBE and MgZnO-TFTs were fabricated on SiO2/p-Si substrates. We investigated the effect of substrate temperature, Mg temperature and MgO buffer layers' thickness on the optical properties of the films and the effect of MgO buffer layers' thickness on the electrical properties of the TFTs. The results indicated the MgZnO TFT with4nm MgO buffer layer exhibit the best performance, with a threshold voltage of28.6V, a field effect mobility of1.85cm2/V s, an on/off ratio of above106and an off-current lower than10-10A.