Absorption and emission of light in nanoscale silicon structures

A unified model of phonon-assisted and zero-phonon radiative transitions in nanoscale silicon structures is presented. For characteristic sizes above 15--20 {}, phonon-assisted transitions dominate, while zero-phonon transitions, allowed due to the finite-size effect, are more important for smaller length scales. Light emission from porous silicon is analyzed on the basis of these results showing that phonon-assisted transitions should dominate the emission in the observed red band.