We have investigated phosphorus ion ͑P + ͒ implantation in Si nanocrystals ͑SiNCs͒ embedded in SiO 2 , in order to clarify the P donor doping effects for photoluminescence ͑PL͒ of SiNCs in wide P concentrations ranging in three orders. Some types of defects such as P b centers were found to remain significantly at the interfaces between SiNCs and the surrounding SiO 2 even by high-temperature ͑1000°C͒ annealing of all the samples. Hydrogen atom treatment ͑HAT͒ method can efficiently passivate remaining interface defects, leading to significant increase in the intensity of PL arising from the recombination of electron-hole pairs confined in SiNCs, in addition to significant decrease in interface defects with dangling bonds detected by electron spin resonance. From both the results of the P dose dependence before and after HAT, it is found that the amount of remaining defects is higher for samples with SiNCs damaged by implantation with relatively lower P + doses and then annealed, and that through HAT the observed PL intensity increases surely as the P concentration increases up to a critical concentration. Then it begins to decrease due to Auger nonradiative recombination above the critical concentration which depends on the size of SiNCs. These results suggest an effect of relatively low concentration of P atoms for the enhancement of PL intensity of SiNCs and we present an unconventional idea for explaining it.