The present infra-red light-emitting device includes a substrate with a first window layer, a silicon dioxide layer positioned on the first window layer, silicon nanocrystals distributed in the silicon dioxide layer, a second window layer, a transparent conductive layer and a first ohmic contact electrode positioned in sequence on the silicon dioxide layer, and a second ohmic contact electrode positioned on the bottom surface of the substrate. The present method forms a sub-stoichiometric silica (SiO.sub.x) layer on a substrate, wherein the numerical ratio (x) of oxygen atoms to silicon atoms is smaller than 2. A thermal treating process is then performed in a nitrogen or argon atmosphere to transform the SiO.sub.x layer into a silicon dioxide layer with a plurality of silicon nanocrystals distributed therein. The thickness of the silicon dioxide layer is between 1 and 10,000 nanometers, and the diameter of the silicon nanocrystal is between 4 and 8 nanometers.