The physical limits of downscaling the SiO2 thickness of rare earth implanted metal–oxynitride–oxide–semiconductor-based light emitters are explored by investigating the drop down of the electroluminescence power efficiency with decreasing SiO2 thickness of Tb-implanted devices. It will be experimentally shown that there is a dark zone with an extension of about 20 nm behind the injecting interface in which the hot electrons have not yet gained enough kinetic energy in order to excite the Tb3+ luminescence centers. In addition, replacing the host matrix SiO2 by SiON results in a decrease of power efficiency by two orders of magnitude what is consistent with the experimental data about the hot energy distribution in these media.