A method of forming a three-dimensional (3D) memory device includes: forming, over a substrate, a layer stack having alternating layers of a first conductive material and a first dielectric material; forming trenches extending vertically through the layer stack from an upper surface of the layer stack distal from the substrate to a lower surface of the layer stack facing the substrate; lining sidewalls and bottoms of the trenches with a memory film; forming a channel material over the memory film, the channel material including an amorphous material; filling the trenches with a second dielectric material after forming the channel material; forming memory cell isolation regions in the second dielectric material; forming source lines (SLs) and bit lines (BLs) that extend vertically in the second dielectric material on opposing sides of the memory cell isolation regions; and crystallizing first portions of the channel material after forming the SLs and BLs.