Resonantly excited photoluminescence from porous silicon and the question of bulk phonon replicates

High surface area porous silicon displays a light-induced visible photoluminescence (PL) in the 600-800-nm range, which has been associated with a variety of mechanisms, including emission from quantum confined silicon crystallites, surface-based states, surface confined defects, or molecular emitters. Key features used to support the quantum confinement hypothesis include the "phonon replicates," which, while observed only at very low temperatures have been interpreted as proof of a silicon phonon-assisted radiative process within the Si crystallites. For the quantum confinement model to be valid, only 2 or fewer of these "replicates" should be observed. We offer an alternate interpretation of the observed replicate features, which is not only consistent with the data presented by the quantum confinement proponents, but also is commensurate with the observation of additional structure by a number of researchers in the field. It is suggested that the observed "replicates" can be ascribed to transitions involving surface bound fluorophors that are associated with the silanone-based silicon oxyhydrides. Support for this interpretation is provided by quantum chemical calculations of the various silanone species as well as a variety of experimental observations.