A process compatible with very large scale integrated technology for preparing high densities (up to500 μm−2) of well-passivated sub-5 nm diam silicon nanowires is presented. The nanowire formation consists of the deposition of a fine-grained natural mask and the subsequent transfer of the pattern into the silicon substrate by reactive ion etching. To isolate the nanowires from the bulk and from each other, this process was applied on silicon-on-insulator material, where the buried oxide layer acts as an etch stop. The nanowire diameter distribution was then shifted below 5 nm by self-limiting oxidation. Finally, hydrogen-rich amorphous silicon nitride(a-SiNx:H),prepared by plasma enhanced chemical vapor deposition, was deposited onto the structures to act as a source of hydrogen. After annealing in forming gas, visible photoluminescence (PL) is detected under 325 nm excitation. Thea-SiNx:Hwas found to emit broad PL, where peak position and intensity depend on the layer composition. Buried silicon oxide, covered bya-SiNx:Hand annealed in forming gas emits yellow (480 nm), blue (560 nm), and red (650 nm) PL. After subtraction of the PL originating from both silicon oxide and silicon nitride, no light could be attributed to the nanowires, possibly due to insufficient passivation of the wires.