BREAKDOWN OF THE K-CONSERVATION RULE IN SI NANOCRYSTALS

We show that light emission from different systems of silicon nanocrys tals does behave as expected for indirect-band-gap quantum dots. Photo luminescence excited on the low energy part of the distribution of Si nanocrystals exhibits a set of narrow peaks associated with Si TA and TO momentum-conserving phonon-assisted optical transitions. These spec tra allow us to determine the ratio of no-phonon transitions to TA and TO phonon-assisted processes over a wide range of confinement energie s. The ratio between these recombination channels changes by 2 orders of magnitude with increasing confinement energy. For confinement energ ies above 0.7 eV the radiative transitions are governed by no-phonon q uasidirect processes.