Three-dimensional memory devices with improved charge confinement and fabrication methods thereof

A method for forming a three-dimensional (3D) memory device, comprising:

• forming an initial channel structure in a stack structure that comprises interleaved a plurality of first layers and a plurality of second layers over a substrate, the initial channel structure including an initial memory layer and being without a blocking layer;
• forming an initial slit opening in the stack structure;
• removing remaining portions of the second layers to expose portions of the initial memory layer and to form a plurality of gate structures each surrounded by a blocking portion of a blocking layer;
• forming an initial blocking layer to cover remaining portions of the first layers and the exposed portions of the initial memory layer, the initial blocking layer comprising the blocking portion of the blocking layer;
• and
• after forming the initial blocking layer, removing the remaining portions of the first layers and portions of the initial channel structure, comprising removing portions of the initial memory layer covered by the remaining portions of the first layers, to form a sealing structure in contact with each of the gate structures and exposed portions of the initial channel structure.