Three-dimensional semiconductor memory devices and methods of fabricating the same

A method of fabricating a three-dimensional semiconductor memory device, comprising:

• forming a lower insulating layer and a lower conductive structure on a substrate;
• forming a mold structure comprising insulating layers and sacrificial layers alternatingly stacked on the lower conductive structure;
• forming an opening to penetrate the mold structure and the lower conductive structure and expose the lower insulating layer;
• sequentially forming a vertical insulating layer and a first semiconductor layer to cover an inner wall of the opening;
• forming a through hole to expose the substrate below the opening, the through hole penetrating the first semiconductor layer, the vertical insulating layer and the lower insulating layer;
• and
• forming a second semiconductor layer in the through hole to the substrate with the first semiconductor layer.