ABSTRACT
For a silicon-based active material, the two major approaches were adopted to overcome the difficulty caused by the indirect bandgap of silicon in order to obtain optical gain, microstructuring the silicon materials or introducing light-emitting impurities into silicon-based materials. For the microstructured silicon, such as porous silicon and silicon nanocrystals,5 the light emission mechanism is complicated, and these materials are subjected to strong light-scattering and absorption, thus limiting the optical gain. For silicon containing light-emitting impurities such as erbium, it is usually difficult to reach high doping concentrations because erbium has a very low solid solubility in silicon. The most severe limitation was an energy back-transfer from the excited erbium ion to the silicon lattice at room temperature.6 Later it was discovered that silicon nanocrystals excite erbium ions efficiently, significantly increasing the light emission from erbium in silicon oxide or silicon nitrides.7 The study of erbium-doped silicon nanocrystalline (ncSi)-rich silicon oxide and nitride is currently a very active research field. Compared with the resonant absorption of a photon, erbium ions can have about two orders of magnitude higher excitation cross section with the sensitization of silicon nanocrystals, thus these material systems emit light efficiently. Waveguides and lightemitting devices based on these materials have been fabricated. However, the optical gain is limited by the high carrier absorption of silicon nanocrystals.
