A method of fabricating a vertical NAND semiconductor device can include changing a phase of a first preliminary semiconductor layer in an opening from solid to liquid to form a first single crystalline semiconductor layer in the opening and then forming a second preliminary semiconductor layer on the first single crystalline semiconductor layer. The phase of the second preliminary semiconductor layer is changed from solid to liquid to form a second single crystalline semiconductor layer that combines with the first single crystalline semiconductor layers to form a single crystalline semiconductor layer in the opening.