Effects of lithium doping on solution processed ZnO/IGZO thin film transistors

Abstract

The solution-processed oxide thin-film transistors has attracted attention because of the possibility of its low-cost fabrication and large area application such as display panels. Also oxide semiconductors are expected to be the active layer material of TFTs in next generation flat panel displays because of its superior characteristic to the amorphous silicon, which is used in current flat panel displays. This thesis handle the fabrication of solution-processed oxide TFTs with lithium doping and its electrical characteristics. For the oxide channel layer, spin coated zinc oxide (ZnO) thin films and indium gallium zinc oxide (IGZO) thin films were used as an active layer respectively. The process conditions for each solution-processed oxide TFTs were carefully optimized to obtain reasonable performance and its TFTs characterization was carried out using various measurements tools. First, I investigated about the effects of lithium doped zinc oxide (Li-ZnO) TFTs with various lithium (Li) concentration on the performance and environmental stability of TFTs behavior. It was found that appropriate amount of lithium doping considerably reduced the background conductivity of ZnO films. Moreover, lithium doping improved the orientation of ZnO crystallites along c-axis. In case of 5 at. % lithium doped ZnO TFTs has a higher field-effect mobility of 3.07 cm2/V s than others. However, 15 and 25 at. % Li doped ZnO TFTs showed good environmental stability with reasonable on/off current ratio and saturation mobility under ambient conditions. Second, lithium doped indium gallium zinc oxide (Li-IGZO) TFTs also showed the enhanced performance with appropriate amount of lithium doping. In contrast with lithium doped ZnO films, the lithium doped IGZO thin films showed the amorphous phase. However lithium doping reduced the oxygen vacancies and enhanced metal oxide bonding. The 5 % (mole ratio) lithium doped IGZO film showed the lowest area ratio of peak in oxygen vacancies with 7.16%. In case of the 15 % lithium doped IGZO film showed the highest area ratio of peak in metal oxygen bonding. In addition, electric characteristics of TFTs showed the enhanced saturation mobility and on/off current ration. Especially the 15 % lithium doped IGZO TFTs have four times higher saturation mobility (3.11 cm2/Vs) than un-doped IGZO TFTs (0.77 cm2/Vs) with annealing temperature 400 °C. From both solution-processed lithium doped ZnO and IGZO TFTs characteristics, we concluded that appropriate amount of lithium doping can enhance the saturation mobility on solution processed oxide TFTs with reasonable on/off current ratio.