Excitonic transitions in silicon nanostructures probed by time-resolved photoluminescence spectroscopy

Time-resolved photoluminescence spectroscopy has been utilized to reveal excitonic transitions in silicon nanowires and silicon nanocrystals. While previous works on porous silicon and silicon nanocrystals have shown a two-level splitting, e.g., singlet-triplet states, our measurements reveal the fine structure of the excitons including three semi-bright states and a ground dark excitonic state. Surprisingly, for silicon nanowires we have found the slowest semi-dark exciton to fall above the faster semi-bright excitonic state as opposed to silicon nanocrystals. The results are analyzed in terms of spin and orbital selection rules showing that the interchange in the level's hierarchy corresponds to a swap between spin-forbidden and orbitally-allowed states. We assign this surprising phenomenon to the relationship between the exchange interaction and the direct Coulomb interaction, which is affected by the dimensionality of the exciton. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim