Role of surface passivation and doping in silicon nanocrystals

The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and different surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the structure and spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-C-Si bridge bond originates significative excitonic luminescence features in the near-visible range. The structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have been investigated as a function of the nanocrystal size and the impurity positions. We have found that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbours sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental results.