  The demand for single photon sources at $\lambda~=~1.54~\mu$m, which follows
from the consistent development of quantum networks based on commercial optical
fibers, makes Er:O$_x$ centers in Si still a viable resource thanks to the
optical transition of $Er^{3+}~:~^4I_{13/2}~\rightarrow~^4I_{15/2}$. Yet, to
date, the implementation of such system remains hindered by its extremely low
emission rate. In this Letter, we explore the room-temperature
photoluminescence (PL) at the telecomm wavelength of very low implantation
doses of $Er:O_x$ in $Si$. The emitted photons, excited by a $\lambda~=~792~nm$
laser in both large areas and confined dots of diameter down to $5~\mu$m, are
collected by an inverted confocal microscope. The lower-bound number of
detectable emission centers within our diffraction-limited illumination spot is
estimated to be down to about 10$^4$, corresponding to an emission rate per
individual ion of about $4~\times~10^{3}$ photons/s.
