Provided is a method of forming an amorphous silicon layer, the method including supplying a reaction gas and an inert gas onto a substrate having an underlayer thereon, and stabilizing the gases while power for generating plasma is not being applied to a chamber, depositing an amorphous silicon layer on the underlayer by using plasma of the reaction gas, which is generated by applying a high-frequency (HF) power of 500 W to 700 W and, at the same time, applying a low-frequency (LF) power lower than the HF power to the chamber, providing a purge gas into the chamber, and pumping the chamber.