Structure and energetics of Si nanocrystals embedded in a-SiO2

We develop realistic models of Si nanocrystals embedded in a-SiO2 using a Monte Carlo approach. The interface structure and its energetics are studied as a function of the nanocrystal size. We find that the low-energy geometries at the interface are Si-O-Si bridge bonds. Remarkably, their fraction strongly declines as the size becomes smaller. Concurrently, the embedding causes substantial deformation in such small nanocrystals. Based on these findings, an alternative explanation is given for the reduced optical gaps in this size regime.