In order to get shallow high doping of Si with optically active complexes ErOn, Er followed by O recoils implantation was realized by means of subsequent Ar+ 250–290keV implantation with doses 2×1015–1×1016cm–2 through 50-nm deposited films of Er and then SiO2, accordingly. High Er concentration up to 5×1020cm−3 to the depth of 10nm was obtained after implantation. However, about a half of the Er implanted atoms become part of surface SiO2 during post-implantation annealing at 950°С for 1h in the N2 ambient under a SiO2 cap. The mechanism of Er segregation into the cap oxide following the moving amorphous–crystalline interface during recrystallization was rejected by the transmission electron microscopy (TEM) analysis. Instead, the other mechanism of immobile Er atoms and redistribution of recoil-implanted O atoms toward cap oxide was proposed. It explains the observed formation of two Er containing phases: Er–Si–O phase with a high O content adjacent to the cap oxide and deeper O depleted Er–Si phase. The correction of heat treatments is proposed in order to avoid the above-mentioned problems.