A method for forming an amorphous silicon thin film is disclosed. In some embodiments, a method includes loading a substrate into a reaction chamber; and conducting a plurality of deposition cycles on the substrate. Each of at least two of the cycles includes: supplying a silicon precursor to the reaction chamber during a first time period; applying radio frequency power to the reaction chamber at least partly during the first time period; stopping supplying of the silicon precursor and applying of the radio frequency power during a second time period between the first time period and an immediately subsequent deposition cycle; and supplying hydrogen plasma to the reaction chamber during a third time period between the second time period and the immediately subsequent deposition cycle. The method allows formation of an amorphous silicon film having an excellent step-coverage and a low roughness at a relatively low deposition temperature.