ABSTRACT
Silicon is an indirect band-gap material, light emission is a phonon-mediated process with low probability (spontaneous recombination lifetimes in the ms range). In standard bulk silicon, competitive non-radiative recombination rates are much higher than the radiative ones and most of the excited e-h pairs recombine non-radiatively. This yields very low internal quantum efficiency (^ w lO -6) for bulk silicon luminescence. In addition, fast non-radiative processes such as Auger or free carrier absorption severely prevent population inversion for silicon optical transitions at the high pumping rates needed to achieve optical amplification. Despite of all, during the nineties many different strategies have been employed to overcome these materials limitations [4]. The most successful ones are based on the exploitation of low dimensional silicon where silicon is nanostructured and hence the electronic properties of free carriers are modified by quantum confinement effects. A steady improvement in low dimensional silicon LED performances has been achieved and silicon LEDs are now only a factor of ten out of the severe market requirements [10,11].
