A memory device, including a first memory region including a first substrate, a plurality of first semiconductor devices on the first substrate, and a first interlayer insulating layer covering the plurality of first semiconductor devices; and a second memory region including a second substrate on the first interlayer insulating layer and a plurality of second semiconductor devices on the second substrate, the second substrate including a first region in a plurality of grooves in the first interlayer insulating layer and a second region including grains extending from the first region, the second region being on an upper surface of the first interlayer insulating layer.