Matrix role in Ge nanoclusters embedded in Si3N4 or SiO2

Ge nanoclusters (NCs), synthesized by ion implantation and annealing up to 900 degrees C, result small (similar to 2 nm) and amorphous in Si3N4, crystalline and much larger in SiO2. The NCs ripening and crystallization kinetics in Si3N4 is retarded by larger interfacial energy and lower diffusivity of Ge in comparison to SiO2. Ge NCs absorb light more efficiently when embedded in Si3N4 than in SiO2. A significant effect of the barrier height on absorption was evidenced, in agreement with effective mass theory predictions. The smaller bandgap of Ge NCs embedded in Si3N4 and their closeness is promising features for light harvesting applications.