Abstract
Thin film transistors are employed as the pixel-driving elements of liquid crystal displays (LCDs). As the development of LCD toward large size and high quality panels, to enhance the TFT switching characteristic is necessary. Particularly, the large photo-induce leakage current is an issue that need to be solved imminently. In this thesis, we use several ways to realize a low photo leakage current TFT. First, we introduce a deep level trap center, copper, into Si film to act as recombination centers. The a-Si TFT was dipped in CuCl2 solution to let copper diffuse into bulk channel layer. The experimental results have shown the photo leakage current of a-Si TFT after dipping is lowered. Second, because the lower absorption coefficient of uc-Si, we utilize the conventional PECVD to direct deposit uc-Si as TFT active layer. We have successfully fabricated the uc-Si TFT which have photo leakage current lower than conventional a-Si TFT with island-out structure. In addition, we also study the photo leakage current of a-Si TFT before and after UV treatment by hysteresis measurement. We concluded that trap states created by back UV treatment was suitable for suppress TFT photo leakage current, since the on-state of TFT was not affected by back UV treatment, and the hole accumulation rate was lowered after back UV treatment. Besides, in order to increase the on-state current of TFT, we deposited the uc-Si film as TFT active layer and the CuMg alloy was used as source/drain metal. We proposed a novel fabrication process by annealing to let Mg out-diffuse from CuMg alloy to the interface between CuMg alloy and uc-Si to form Ohmic contact layer, replaced the conventional n+ doped a-Si as Ohmic contact layer. We have successfully fabricated this novel uc-Si TFT. Compared with conventional TFT which use n+ doped a-Si as Ohmic contact layer, there is no n+ layer deposition in our process. Hence, there would be no back channel etching process. Therefore, this simpler process is suitable for mass production.