Techniques are provided for fabricating a three-dimensional, charge-trapping memory device with improved long term data retention. A corresponding three-dimensional, charge-trapping memory device is also provided which includes a stack of alternating word line layers and dielectric layers. A charge-trapping layer is deposited in a memory hole. The refractive index of portions of the charge-trapping layer which are adjacent to the word line layers is increased relative to the refractive index of portions of the charge-trapping layer which are adjacent to the dielectric layers. This can be achieved by doping the portions of the charge-trapping layer which are adjacent to the word line layers. In one approach, the charge-trapping layer is SiON and is doped with Si or N. In another approach, the charge-trapping layer is HfO and is doped with Hf. In another approach, the charge-trapping layer is HfSiON and is doped with Hf, Si or N.