We present the microstructural evolution in Ge-rich SiO2 doped with Er under different fabrication conditions. At sufficiently high Er contents and annealing temperatures, the Er2 O3 and Er2 Ge2 O7 phases are eventually formed, leading to an electroluminescence quenching of Ge-related oxygen-deficiency centers. The correlation between the microstructure and electroluminescence is discussed based on: (i) an Er doping dependent fragmentation/amorphization of Ge nanocrystals, (ii) a temperature dependent Ge diffusion toward the Si/ SiO2 interface, and (iii) the formation of different Er phases. © 2009 The American Physical Society.