Study Of Fabrication And Stability Of ZnO Thin Film Transistors Based On ALD Technology

Thin film transistors (TFTs) have been widely used in liquid crystal displays (LCDs) and also organic light-emitting diode (OLED) panels. Traditional amorphous silicon based TFTs have several drawbacks making them difficult to meet the market demands, such as the low mobility and high photosensitivity. Recently, transparent oxide semiconductors based thin film transistors have gained a lot of attention and zinc oxide (ZnO) has been a promising candidate to replace Si-based TFTs because of its simplest composition and a relatively low cost. However, the instabilities of ZnO TFT limit its pratical application. In this study, ZnO films were deposited by means of ALD using diethylzinc (DEZ) and deionized water as Zn and oxygen precursors respectively. We mainly aimed at the process optimization and the elimination of the instabilities of ZnO TFTs.We fabricated our ZnO TFTs on glass substrates using a double layer Al2O3 gate dielectric with the first thin gate dielectric for the protection of ZnO channel in the next photolithographic process. Prior to the fabrication of ZnO TFTs, the deposition conditions of ZnO films were investigated to get the optimized temperature in the growth or annealing of ZnO.We found various instabilities in our ZnO TFTs during the further electrical measurements, including hysteresis, hump effect, and the shift of threshold voltage both in positive and negative direction. The mechanisms behind these instabilities were studied. We believe that the main orgins of these instabilities were the water molecular diffusion in ZnO and the charge trapping mechanism at the channel/dielectric interface.Furthermore, we developed an effective process methodology to greatly enhance the stability of ZnO TFTs by using RTA treatments three times during the fabrication process. The first RTA treatment on ZnO films in O2 ambient is adopted to control the carrier density in ZnO channel. After the semiconductor channel was patterned, a second RTA process was carried out to the ZnO channel with the first thin Al2O3 dielectric to minimize the defects at the channel/dielectric interface and eliminate the water diffusion. Then a third RTA process was applied on the second gate dielectric layer to make the Al2O3 dielectric more resistant in the next photolithographic process. We found that our ZnO TFTs with extremely high stability by the optimized fabrication method we developed. Our ZnO TFTs obtained excellent defect-controlled channel and a positive VON of 2.2V. The ?VONn of our ZnO TFTs could be less than 0.2V after measuring the transfer characteristics five consecutive times which indicated a high electrical stability.