Si and Ge nanocluster formation in Silica matrix

High resolution transmission electron microscopy, scanning transmission electron microscopy and cathodoluminescence have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers. Commonly, cathodoluminescence emission spectra of pure SiO2 are identified with particular defect centers within the atomic network of silica including the nonbridging oxygen hole center associated with the red luminescence at 650nm (1.9 eV) and the oxygen deficient centers with the blue (460 nm; 2.7 eV) and ultraviolet band (295 nm; 4.2 eV). In Ge+ ion implanted SiO2 an additional violet emission band appears at 410 nm (3.1 eV). The strong increase of this violet luminescence after thermal annealing is associated with formation of low-dimension Ge aggregates like dimers, trimers and higher formations, further growing to Ge nanoclusters. On the other hand, pure silica layers were modified by heavy electron beam irradiation (5 keV; 2.7 A/cm2) leading to electronic as well as thermal dissociation of oxygen and appearance of under-stoichiometric SiOx. This SiOx will undergo a phase separation and we observe Si cluster formation with a most probable cluster diameter of 4 nm. Such largely extended Si clusters will diminish the SiO2 related luminescence and Si crystal related luminescence in the near IR appears.