Experimental Evidence of Si Nanocluster delta-Layer Formation in the Vicinity of Ion-Irradiated SiO₂-Si Interfaces

Predictions on formation and self-alignment of Si nanocrystals (NCs) at ion-irradiated SiO₂-Si interfaces have been proven by cross-section transmission electron microscopy (TEM). The model is based on ion-mixing of SiO₂-Si interfaces resulting in a region of SiO_x (x<2). During annealing, the interface restores and in the tail of the mixing profile Si excess precipitates and forms a self-aligned layer
of Si NCs in SiO₂. This Si NC delta-layer structure can hardly be observed by conventional TEM techniques due to the very low mass contrast of tiny Si NCs embedded in SiO₂. Here, a contrast enhancing method is presented based on alloying of these Si NCs with Ge. For this purpose, a thin Ge layer is embedded into the oxide sufficiently far from the interface mixing range, thus, preventing interference with the Si precipitate formation. During annealing, diffusing Ge monomers attach to the Si NCs, mainly due to the energetically favored Si-Ge bond, resulting in Si_1-xGe_x NCs with an enhanced mass contrast in TEM.