Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a method for forming a 3D memory device is disclosed. A channel opening extending vertically is formed above a substrate. A semiconductor plug is formed in a lower portion of the channel opening. A memory film and a channel sacrificial layer are subsequently formed above the semiconductor plug and along a sidewall of the channel opening. A semiconductor plug protrusion protruding above the semiconductor plug and through a bottom of the memory film and the channel sacrificial layer is formed. A cap layer is formed in the channel opening and over the channel sacrificial layer. The cap layer covers the semiconductor plug protrusion. A semiconductor channel is formed between the memory film and the cap layer by replacing the channel sacrificial layer with a semiconductor material epitaxially grown from the semiconductor plug protrusion.