Structural, mechanical and optical properties of Ge nanocrystals embedded in superlattices fabricated by in situ low temperature annealing

Ge nanocrystals (Ge-ncs) embedded in SiO2 superlattices were prepared by in situ low temperature annealing. The formation of size controlled nanocrystals was demonstrated by Raman scattering, X-ray diffraction and transmission electron microscopy measurements. A core–shell structure was observed in the in situ grown Ge-ncs as a result of the lower annealing temperatures. The in situ annealing process also produced significant compressive stress in the superlattices, and the calculation result shows that the hydrostatic pressure induced by the compressive stress was about 2.55 GPa for the nanocrystals with average size of 7.1 nm. The optical absorption edges of the in situ grown superlattices moved to lower wavelength as crystallite size decreased. The presence of Ge-ncs enhanced the film absorption at short wavelength and the absorption coefficient was found to increase in superlattices containing larger nanocrystals. The superlattices exhibited optical band gap energies larger than that of bulk Ge and a blueshift of energies was observed with the decrease of crystallite size. These features are tentatively explained by quantum confinement effect in the Ge-ncs. Our results indicate that in situ low temperature grown Ge-ncs and superlattices are promising candidates for the fabrication of nanoscale devices on low cost substrates.