Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a memory stack including interleaved stack conductive layers and stack dielectric layers, a semiconductor layer, a plurality of channel structures each extending vertically through the memory stack into the semiconductor layer, and an insulating structure extending vertically through the memory stack and including a dielectric layer doped with at least one of hydrogen or an isotope of hydrogen.