A method of manufacturing a semiconductor device includes forming a plurality of recess regions on an upper surface of a substrate, forming a first oxide layer in the recess regions, forming a polysilicon layer on the first oxide layer, forming a second oxide layer by oxidizing the polysilicon layer, and forming a gap-fill layer on the second oxide layer to fill the recess regions, wherein at least a portion of the polysilicon layer remains between the first oxide layer and the second oxide layer after forming the second oxide layer.