We present a study on erbium (Er)-doped silicon (Si)-rich silicon oxide thin films grown by the magnetron cosputtering of three confocal cathodes according to the deposition temperature and the annealing treatment. It is shown that, through a careful tuning of both deposition and annealing temperatures, it is possible to engineer the fraction of agglomerated Si that may play the role of sensitizer toward Er ions. To investigate the different emitting centers present within the films according to the fraction of agglomerated Si, a cathodoluminescence experiment was made. We observe in all samples contributions from point-defect centers due to some oxygen vacancies and generally known as silicon-oxygen deficient centers (SiODC), at around 450–500 nm. The behavior of such contributions suggests the possible occurrence of an energy transfer from the SiODCs toward Er3+ ions. Photoluminescence experiments were carried out to characterize the energy transfer from Si nanoclusters toward Er3+ ions with a nonresonant wavelength (476 nm) that is unable to excite SiODCs and then exclude any role of these centers in the energy transfer process for the PL experiments. Accordingly, it is shown that structural differences have some effects on the optical properties that lead to better performance for high-temperature deposited material. This aspect is illustrated by the Er-PL efficiency that is found higher for 500°C-deposited, when compared to that for RT-deposited sample. Finally, it is shown that the Er-PL efficiency is gradually increasing as a function of the fraction of agglomerated silicon.