A method of manufacturing a semiconductor device, the method including providing a metal precursor on a substrate to form a preliminary layer that includes a first metal; providing a reducing agent on the preliminary layer, the reducing agent including a compound that includes a second metal; and providing a reactant on the preliminary layer to form a metal-containing layer, wherein the second metal has multiple oxidation states, the second metal in the reducing agent having a lower oxidation state among the multiple oxidation states prior to providing the reducing agent on the preliminary layer.