Three-dimensional (3D) NAND memory devices and methods are provided. In one aspect, a fabrication method includes depositing a cover layer over a substrate, depositing a sacrificial layer over the cover layer, depositing a layer stack over the sacrificial layer, forming a channel layer extending through the layer stack and the sacrificial layer, performing a first epitaxial growth to deposit a first epitaxial layer on a side portion of the channel layer that is close to the substrate, removing the cover layer, and performing a second epitaxial growth to simultaneously thicken the first epitaxial layer and deposit a second epitaxial layer on the substrate. The layer stack includes first stack layers and second stack layers that are alternately stacked.