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.