Embodiments of source structure of a three-dimensional (3D) memory device and method for forming the source structure of the 3D memory device are disclosed. In an example, a NAND memory device includes a substrate, an alternating conductor/dielectric stack, a NAND string, a source conductor layer, and a source contact. The alternating conductor/dielectric stack includes a plurality of conductor/dielectric pairs above the substrate. The NAND string extends vertically through the alternating conductor/dielectric stack. The source conductor layer is above the alternating conductor/dielectric stack and is in contact with an end of the NAND string. The source contact includes an end in contact with the source conductor layer. The NAND string is electrically connected to the source contact by the source conductor layer. In some embodiments, the source conductor layer includes one or more conduction regions each including one or more of a metal, a metal alloy, and a metal silicide.