In a method of forming a silicon-rich nanocrystalline structure by an ALD process, a first gas including a first silicon compound is provided onto an object to form a silicon-rich chemisorption layer on the object. A second gas including oxygen is provided onto the silicon-rich chemisorption layer to form a silicon-rich insulation layer on the object. A third gas including a second silicon compound is provided onto the silicon-rich insulation layer to form a silicon nanocrystalline layer on the silicon-rich insulation layer. The first gas, the second gas and the third gas may be repeatedly provided to alternately form the silicon-rich nanocrystalline structure having a plurality of silicon-rich insulation layers and a plurality of silicon nanocrystalline layers on the object.