Atomic and electronic structure of silicon nanocrystals embedded in a silica matrix

The atomic structures and the optical and electronic properties of silicon nanocrystals (nc-Si) in a beta cristobalite matrix are studied using DFT calculations provided by the AIMPRO code. Five atomic models are considered (two nanocrystal diameters of 5.6 and 11 angstrom with and without interface defects). After total relaxation, the mean Si-Si distances in nc-Si are found to be 6% higher than those in perfect bulk silicon. The optical and electronic properties are influenced by many parameters, among which are the nanograin density and size. The quantum confinement effect is demonstrated by the increase of energy gap when decreasing nanograin size. The energy gap of nc-Si is adjusted by using B3LYP functional calculations; the energy gap of 5.6 angstrom nc-Si is found to be equal to 3.4 eV while that of 11 angstrom nc-Si is equal to 3.1 eV. In the band structure, the levels due to nc-Si appear in the forbidden band of SiO2. The electronic density of these levels is presented in 3D. A redshift is observed in the optical absorption spectrum as the nc-Si size increases, and the absorbance of nc-Si/SiO2 is proportional to the nanograin density. The system is more stable as the distance between nanograins increases. We have also studied two kinds of nc-Si/SiO2 interface defects (Si-O-Si and Si=O bonds). It is found that the Si-O-Si bridge bond leads to the most stable configuration. The presence of Si=O double bonds reduces the nc-Si energy gap and leads to a redshift in the absorption spectrum. The Si-O-Si bonds produce the inverse effect, i.e. an energy gap increase associated with a blueshift in the absorption spectrum.