In a method of forming a thin layer having a desired composition, a source gas is provided onto a substrate loaded in a chamber for a first time, and the source gas is chemisorbed onto the substrate. While the source gas is provided, a plasma is generated in the chamber for a second time to change the chemisorbed source gas into the thin layer having the desired composition. The thin layer may have a stoichiometrical composition or a non-stoichiometrical composition.