Methods of forming a material layer according to some embodiments of the inventive concept may include a deposition cycle including providing an adsorption inhibitor on a substrate, purging an excess amount of the adsorption inhibitor, providing a metal precursor on the substrate, purging an excess amount of the metal precursor, and supplying a reactant to form a material layer on the substrate. The adsorption inhibitor may include a group 15 element or a group 16 element.