In a method of forming an aluminum oxide layer, an aluminum source gas and a dilution gas can be supplied into a chamber through a common gas supply nozzle so that the aluminum source gas may be adsorbed on a substrate in the chamber. A first purge gas can be supplied into the chamber to purge the physically adsorbed aluminum source gas from the substrate. An oxygen source gas may be supplied into the chamber to form an aluminum oxide layer on the substrate. A second purge gas may be supplied into the chamber to purge a reaction residue and the physically adsorbed remaining gas from the substrate. The operations can be performed repeatedly to form an aluminum oxide layer having a desired thickness.