The femtosecond optical response of porous, amorphous and crystalline silicon

The carrier dynamics in porous silicon have been investigated using femtosecond time-resolved spectroscopy. The results are compared to those obtained under identical conditions on a-Si:H, crystalline silicon and GaAs. Porous silicon is an indirect bandgap material with a free carrier absorption cross section that is lower than that of a-Si:H and comparable to that of crystalline silicon. Our measurements indicate carrier trapping within 200 fs followed by a slower decay on a subnanosecond time scale that speeds up at elevated injected densities. This effect is interpreted as efficient Anger recombination of carriers generated in, and confined to, nanometer sized crystallites.