Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si substrate by a (SiO2 + Ge)/(SiO2 + GeO2) superlattice approach, using an rf magnetron sputtering technique with a Ge + SiO2 composite target and subsequent thermal annealing in N-2 ambient at 750 degrees C for 5 min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8 cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge-0 in the (SiO2 + Ge) layer and Ge4+ in the (SiO2 + GeO2) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO2 + Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO2 matrix film.