Absorption of Si, Ge, and SiGe alloy nanocrystals embedded in SiO2 matrix

Abstract

Using the atomistic sp3d5s∗ tight-binding method, we calculate the optical absorption spectra due to phononless optical transitions in Si, Ge, and SiGe alloy nanocrystals embedded in an amorphous dielectric SiO2 matrix. For the SiO2 matrix, we use a virtual crystal approximation assuming the cubic crystalline structure, similar to β crystobalite, in the proximity of the nanocrystal surface. The optical absorption is analyzed in detail: its dependency on the Ge content and nanocrystal size and the role of the SiO2 matrix are revealed. Our recent experimental measurements of the optical absorption in co-sputtered thin SiO2 films with arrays of Si, Ge, and SiGe alloy nanocrystals are discussed and compared with the simulations.