A direct measurement of the electronic structure of Si nanocrystals and its effect on optoelectronic properties

Abstract

Since reports that silicon nanocrystals (Si-NCs) can exhibit direct transition emission, the silicon laser field is at a juncture where the importance of this discovery needs to be evaluated. Most theoretical models predicted a monotonic increase in the bandgap and experimental information currently available on the electronic structure at the Γ valley of these promising materials is circumstantial as it is obtained from emission measurements where competing non-radiative relaxation and recombination processes only provide an incomplete picture of the electronic structure of Si-NCs. Optical absorption, the most immediate probe of the electronic structure beyond the band-edges, showing the evolution of the Γ valley states with nanocrystal size has not been measured. Here, we show such measurements, performed with high dynamic range, allowing us to observe directly the effect of crystal size on the Γ valley splitting far above the band-edges. We show that the splitting is 100 s of meV more pronounced than predicted by pseudo potential calculations and Luttinger-Kohn model. We also show that ultrafast red-shifting emission can be observed in plasma enhanced chemical vapor deposition prepared Si-NCs.