Curvature effects on optical response of Si nanocrystals in SiO2 having interface silicon suboxides

Several models of oxygenated and hydrogenated surfaces of Si quantum wells and Si nanocrystals (NCs) of variable shapes have been constructed in order to assess curvature effects on energy gaps due to the three Si suboxides. Si suboxides in partially oxydized models of nanocrystals of spherical shapes (or quantum dots ) are shown to reduce energy gaps compared to the hydrogenated nanocrystals, consistent with previous results in the literature. This trend is shown to be reversed when planar interfaces are formed in Si NC inside SiO2 thin films, as in Si quantum wells. At planar interfaces (or surfaces ) the electronic charge density is shown to become extended and to distribute among the Si suboxides, thus generating along these planar interfaces extended, or delocalized, states. This delocalization of the electronic states then increases the energy gap compared to equivalent hydrogenated interfaces (or surfaces). Determination of geometric effects of curvature on the band gaps are based on density-functional theory (DFT) using the real-space numerical approach implemented in the "Pseudopotential Algorithm for Real-Space Electronic Structure" (PARSEC) program. A method for measuring interface effects due to Si suboxides in Si NC in SiO2 is also suggested by comparing photoluminescence (PL) between as-grown and annealed Si-NC samples. The DFT calculations suggest that blueshift of more than 0.2 eV of the PL should be observed in as-grown samples having Si suboxides at their planar interfaces, in comparison to annealed samples above 950 degrees C that have fewer or no Si suboxides once annealed, providing that the bulk of Si NC remains unaltered.