The Investigation On The Fabrication,Performance And Stability Of A-InZnO-TFTs

Oxide thin-film transistors (TFTs) have attracted ever-increasing attention due to high carrier mobility as well as high visual transparency. As a key element in TFTs, qualified high permittivity (high-k) dielectrics determine the performance of the device. High-k Ta2O5has been considered as a promising candidate due to its important role in increasing the capacitive coupling between the gate and the active channel layer, thereby improving the subthreshold swing and reducing the operation voltage. For practical application considerations, the stability and reliability issues of TFTs are most critical. This thesis has carried out the research on the preparation and electrical stability study of InZnO thin-film transistors using Ta2O5gate dielectric. The main conclusions are listed as follows:Firstly, photolithography technology combined with wet etching method was explored and optimized to fabricate InZnO/SiO2TFTs. The electrical stability of InZnO/SiO2TFTs had also been investigated. When subjected to positive bias stress, the InZnO/SiO2TFTs displayed monotonously positive threshold voltage (VTH) shift, which was stemmed from the mechanism of charge trapping in the channel/dielectric interface or in the dielectric. The InZnO/SiO2TFTs annealed at250℃for1hour in atmosphere displayed excellent electrical stability, which was attributed to the reduction in trap-state density at the interface as well as the curing of the defect states in the bandgap of InZnO.Secondly, Tantalum oxide (Ta2O5) films were deposited on ITO glass substrates by dc reactive magnetron sputtering at different substrate bias voltages in the range from0to-145V. Our results showed that a moderate substrate bias voltage could densify and smoothen the films, resulting in the improvement of their electrical properties. Under an optimal substrate biasing condition (Vs=-135V), Ta2O5films exhibit attractive physical properties, i.e., relatively large refractive index and a flat surface, a permittivity as high as-23, a small dielectric loss (-0.01at1kHz) and a low leakage current density of1.45×10-1A/cm2at1MV/cm. The evolution of the films’properties was believed to be a result of the beneficial’soft-hammering’ effect of ion bombardment, which can contribute to activate the mobility of the deposited atoms arriving onto the growing film surface. That is to say, the deposited atoms had high kinetic energy enough to diffuse more freely on the film surface and to occupy or approach the equilibrium lattice position.Finally, on the basis of optimizing the Ta2O5films, InZnO/Ta2O5TFTs were fabricated by photolithography technology. The InZnO/Ta2O5TFTs annealed at200℃ for1hour in atmosphere showed good electrical properties, such as a large saturation mobility (μsat) of18cm V-1s-1, a small subthreshold voltage swing (S) of141mV/decade, a low VTH of1.7V, and a high drain current on/off ratio (1on/Ioff) of8×10/. The bias stress stability of InZnO/Ta2O5TFTs was also evaluated. However, an anomalous negative VTH shift under positive bias stress was observed. It was suggested that electron detrapping from the high-k Ta2O5dielectric to the gate electrode was responsible for this VTH shift, which was supported by the logarithmical dependence of the VTH shift on the stress time.