A method for forming a three-dimensional, 3D, memory device, comprising: forming an alternating dielectric stack on a substrate; forming a temporary top selective gate cut in an upper portion of the alternating dielectric stack and extending along a lateral direction; forming a plurality of channel holes penetrating the alternating dielectric stack; removing the temporary top selective gate cut to form a trench; and forming, simultaneously, a plurality of channel structures in the plurality of channel holes and a top selective gate cut structure in the trench, and wherein forming the plurality of channel structures in the plurality of channel holes comprises: before forming a functional layer, forming an epitaxial layer on a surface of the substrate that is exposed by the plurality of channel holes; forming the functional layer covering the sidewall of each channel hole, the top surface of the epitaxial layer in each channel hole, as well as the sidewalls and the bottom surface of the trench; characterized by forming a protecting layer to cover the functional layer; and removing portions of the functional layer and the protecting layer on the bottom of each channel hole to form a first recess that exposes or extends into the epitaxial layer, wherein the protecting layer is completely removed. forming an alternating dielectric stack on a substrate; forming a temporary top selective gate cut in an upper portion of the alternating dielectric stack and extending along a lateral direction; forming a plurality of channel holes penetrating the alternating dielectric stack; removing the temporary top selective gate cut to form a trench; and forming, simultaneously, a plurality of channel structures in the plurality of channel holes and a top selective gate cut structure in the trench, and wherein forming the plurality of channel structures in the plurality of channel holes comprises: before forming a functional layer, forming an epitaxial layer on a surface of the substrate that is exposed by the plurality of channel holes; forming the functional layer covering the sidewall of each channel hole, the top surface of the epitaxial layer in each channel hole, as well as the sidewalls and the bottom surface of the trench; characterized by forming a protecting layer to cover the functional layer; and removing portions of the functional layer and the protecting layer on the bottom of each channel hole to form a first recess that exposes or extends into the epitaxial layer, wherein the protecting layer is completely removed. before forming a functional layer, forming an epitaxial layer on a surface of the substrate that is exposed by the plurality of channel holes; forming the functional layer covering the sidewall of each channel hole, the top surface of the epitaxial layer in each channel hole, as well as the sidewalls and the bottom surface of the trench; characterized by forming a protecting layer to cover the functional layer; and removing portions of the functional layer and the protecting layer on the bottom of each channel hole to form a first recess that exposes or extends into the epitaxial layer, wherein the protecting layer is completely removed.