| Since its introduction in 2003,amorphous indium gallium zinc oxide?a-IGZO?has been widely studied by the research community due to its high mobility,uniformity on large area and high transparency in visible light region.Therefore,it will be one of the most ideal active layers in the next generation of flexible thin film transistors.In this paper,the structure of gradient doping active?GDA?layers with two or three layers is proposed,based on the classical a-IGZO TFT with single active layer.Its research work is as follows:?1?Firstly,the effects of different preparation processes on the morphology of Al2O3and PMMA thin films were analyzed,and then the effects of different insulation processes based on Al2O3 and PMMA on the performance of a-IGZO TFT were studied.The results show that the performance of Al2O3 device is relatively poor because of rough surface and pointed cone on the surface of Al2O3 films.These defects bring in high off current,up to 1.1×10-6 A.The performance of PMMA device is relatively good and the off current which is reduced by 78%compared to the Al2O3 device.?2?After determining the use of PMMA as the insulating layer of a-IGZO TFT,the effect of IGZO fabrication process on the electrical properties of IGZO thin film is investigated.It is found that the conductivity of IGZO thin film reduces exponentially with the increase of oxygen partial pressure.Then the effect of different preparation process of a-IGZO thin film on the performance of single active layer a-IGZO TFT device is studied.It is found that when the oxygen partial pressure is 2%,the device performance is the best.The mobility,the ratio and off current of PMMA device is 1.01 cm2/Vs,1×103and 2.4×10-7 A,respectively.?3?In order to further improve the performance of the device,a bilayer gradient doping structure is proposed.It is found that when the oxygen flow rate of the lower active layer is smaller than the oxygen flow rate of the upper active layer,the device gets higher performance.Its mobility is up to 2.4 cm2/Vs which increases 1.29 cm2/Vs and switching ratio is 1.87×103 increased by 87%compared to single active layer devices.However the off current of the device is not obvious cut back.?4?To solve the problem of large off current,a three-layer gradient doping structure is further proposed.The effects of the thickness of BL layer and ML layer,post-annealing and doping concentration of ML layer on the performance of a-IGZO TFT devices were studied successively.The switch ratio of this three layer gradient doping device is 10-fold higher than other devices.Besides,its off-state current is low to 4.0×10-10 A. |
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