The electroluminescence (EL) at 1.54 um of metal-oxide-semiconductor (MOS) capacitors with Er3+ ions embedded in the silicon-rich silicon oxide (SRSO) layer has been investigated under different polarization conditions and compared with that of erbium doped SiO2 layers. EL time-resolved measurements allowed us to distinguish between two different excitation mechanisms responsible for the Er3+ emission. Energy transfer from silicon nanoclusters (Si-ncs) to Er3+ is clearly observed at low-field AC excitation and increases with driving frequency. We demonstrate that sequential electron and hole injection at the edges of the pulses create excited states in Si-ncs which upon recombination, transfer their energy to Er3+ ions. On the contrary, direct impact excitation of Er3+ by hot injected carriers starts at Fowler-Nordheim injection threshold (above 5 MV/cm) and dominates for larger fields.