Formation of Ge nanocrystals embedded in a SiO2 matrix: Transmission electron microscopy, x-ray absorption, and optical studies

The structure of Ge nanocrystals embedded into a SiO2 matrix on either Si(100) or quartz-glass substrates has been studied by transmission electron microscopy, x-ray absorption fine structure, and Raman scattering. The size of the nanocrystals (5-20 nm) is found to strongly depend on the substrate material, while their shape--either spherical nanocrystals with multiple twinning defects or faceted single crystals--depends on the location of the nanocrystals in the matrix. We found that Ge nanocrystals are predominantly formed from the amorphous Ge phase, preexisting in the samples with higher Ge concentration, the formed nanocrystals are randomly oriented and have a Ge-Ge bond length of 2.45±0.01 Aa, i.e., are relaxed