Due to its indirect electronic band gap, crystalline silicon is rarely used as the active emitter in semiconductor optics. Interband light emission is predominantly a phonon-assisted process and Si has therefore very poor quantum efficiency. However the development of efficient silicon based light emitters would pave the way toward CMOS compatible monolithic optical interconnects and, therefore, signal processing speeds much higher than currently provided by silicon microelectronics. Enhanced light emission from crystalline Si can be achieved by using two-dimensional photonic crystals with photonic point defect nanocavities. Such defect photonic crystal nanocavities modify the spatial emission profile of light and due to the in-plane photonic band gap a much larger fraction of light is emitted perpendicular to the slab.
Hauke, N.: "Correlation between emission intensity of self-assembled germanium islands and quality factor of silicon photonic crystal nanocavities", 2011.