Methods of fabricating a semiconductor device are provided. A semiconductor substrate is provided that includes a single crystalline structure within at least a defined region thereof. A thin layer is formed on the semiconductor substrate. The thin layer is patterned to form a plurality of spaced apart field structures and to expose therebetween portions of the semiconductor substrate having the single crystalline structure. A non-crystalline layer is formed on the exposed portions of the semiconductor substrate having the single crystalline structure. The non-crystalline layer is planarized to expose upper surfaces of the field structures and define non-crystalline active structures from the non-crystalline layer between the field structures. A laser beam is generated that heats the non-crystalline active structures to change them into single crystalline active structures having substantially the same single crystalline structure as the defined region of the semiconductor substrate.