Room-temperature 1.54 $\mu$m photoluminescence of Er:O${}_x$ centers at extremely low concentration in silicon

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 $E{r}^{3+}:{}^4{I}_{13\mathrm{/}2}\to {}^4{I}_{15\mathrm{/}2}$. 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 $\lambda =792nm$ 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.