Low-energy (3-100 eV) electrons are shown to be efficient promoters of the superficial nitridation of thin SiO2 films grown on boron-doped Si(100) using low pure NH3 pressures (less-than-or-equal-to 2 x 10(-4) mbar) at ambient temperature. The initial concentration (almost-equal-to 10(15) at. CM-3) of ionized acceptors in silicon near the SiO2/Si interface is first lowered by the nitridation process and then restored by hydrogen annealing at 450-degrees-C. The ultrahigh vacuum experimental set-up permits us to uncouple two important effects from a thermally-assisted (700-950-degrees-C) r.f. NH3 plasma nitridation process : the role played by low-energy (3-5 eV) electrons and the temperature. It is found that the existence of charged particles near the sample surface must be taken into account in the plasma reaction process and that the main effect of thermal activation is to enhance the reaction and the diffusion of nitrogen species through the SiO2 layer up to the SiO2/Si interface.