The presence of indirect Er3+ excitation in Si-rich SiO2 is demonstrated for Si-excess concentrations in the range of 2.5–37 at. %. The Si excess concentration providing the highest density of sensitized Er3+ ions is demonstrated to be relatively insensitive to the presence of Si nanocrystals and is found to be ∼14.5 at. % for samples without Si nanocrystals (annealed at 600 °C) and ∼11.5 at. % for samples with Si nanocrystals (annealed at 1100 °C). The observed optimum is attributed to an increase in the density of Si-related sensitizers as the Si concentration is increased, with subsequent deactivation and removal of these sensitizers at high Si concentrations. The optimized Si excess concentration is predicted to generate maximum Er-related gain at 1.54 μm in devices based on Er-doped Si-rich SiO2.