The inverse energy transfer between Ge nanocrystals and erbium in SiO2 and its dependence on microstructure

The electroluminescence (EL) of Er-implanted SiO₂ layers containing Ge nanocrystals (NCs) was investigated and correlated with microstructural results obtained by transmission electron microscopy, Raman spectroscopy and X-ray diffraction. In case of EL, and in contrast to the behaviour of Er-doped Si-rich SiO₂ known from the literature it appears that there is an inverse energy transfer from Er to Ge-related oxygen deficiency centres which are located at the surface of the Ge NCs or in the transition region between the NC and the SiO₂ matrix. This is indicated by the increase of the blue-violet, Ge-related EL in presence of Er, although the Ge-related photoluminescence, which was excited by UV wavelengths non-resonant to Er, decreases at the same time. The microstructural results reveal that the maximum increase of the Ge-related EL occurs when the Ge NCs are not amorphized and/or fragmented by the Er implantation but surrounded by an Er shell. Finally a qualitative model is given explaining the different behaviour of Er-implanted SiO₂ containing Ge NCs compared to the case of Si NCs by the different quality of the NC interface to the SiO₂ matrix.