In one aspect, a method for forming a 3D memory device is disclosed. A selective epitaxial sacrificial layer is formed above a substrate, and a dielectric stack is formed above the selective epitaxial sacrificial layer. A first opening extending vertically through the dielectric stack and the selective epitaxial sacrificial layer is formed. A portion of the first opening extending vertically through the selective epitaxial sacrificial layer is enlarged. A memory film and a semiconductor channel are subsequently formed in this order along sidewalls and a bottom surface of the first opening. The selective epitaxial sacrificial layer is removed to form a cavity exposing a portion of the memory film. The portion of the memory film exposed in the cavity is removed to expose a portion of the semiconductor channel. A selective epitaxial layer is epitaxially grown from the substrate to fill the cavity and be in contact with the portion of the semiconductor channel.