Effects of simultaneous doping with boron and phosphorous on the structural, electronic and optical properties of silicon nanostructures

We show, by means of ab-initio calculations, that by properly controlling the doping a significant modification of both the absorption and the emission of light of silicon nanocrystals can be achieved. We have considered impurities, boron and phosphorous (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbour sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron–hole pair due to light absorption determines a geometry distortion that in turn results in a Stokes shift between absorption and emission spectra. In order to give a deeper insight in this effect, in one case, we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. Moreover, we also investigate how the properties of the codoped nanoclusters are influenced by the insertion of more impurities (multidoping). Finally, we have studied the effect of B and P codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanostructures.