UV Raman spectroscopy of group IV nanocrystals embedded in a SiO2 matrix

Nanostructures of both Ge nanocrystals formed by thermal oxidation of SiGe layers, and SiGe nanocrystals formed by crystallization of amorphous SiGe nanoparticles deposited by LPCVD have been analyzed by Raman spectroscopy. The nanostructures are formed on a silicon substrate. Raman spectra have been acquired with visible (514.5 nm) and UV (325 nm) excitation lines. When the amount of material is very small, as it has happens in these nanostructures, the visible line is not able to excite the characteristic peaks of the Ge or SiGe in the Raman spectrum; instead the Si second order spectrum of the substrate appears and it can be misinterpreted by attributing it to the Ge-Ge band associated with the nanocrystals. In this work, the use of UV excitation has been demonstrated to enhance the sensitivity respect to the conventional visible excitation, allowing the characteristic peaks of the Ge or SiGe nanocrystals to appear in the spectrum. We attributed this effect to the resonance effects.