Improvement in the photoluminescence efficiency of porous silicon using high-energy silicon ion irradiation

An order of magnitude enhancement in the intensity of visible room temperature photoluminescence (PL) from porous silicon (PS) was observed when irradiated with 10 MeV (7(+)) silicon ions from a pelletron source. The effect was associated with a blue shift of 70 nm. The stability of PL with respect to ambients was seen to be remarkably improved. The energy band gap determined from photoluminescence and photoreflectance measurements indicated a shift from 1.67 eV to 1.83 eV. Subsequently partial restructuring of Si-O-Si and Si-H type species into Si-OH was confirmed by infrared measurements recorded before and after irradiation. The effects have been correlated to a reduction in the extent of non radiative recombination centres as a consequence of chemical restructuring of the surface. By assuming that the restructured surface is sufficiently thick to reduce the crystallite size, the blue shift can be accounted for. Formation of Si-OH bonds at the surface was also observed when PS was intentionally exposed to low-energy (10-30 eV) OH- ions from an electron cyclotron resonance (ECR) plasma; associated with this, the PL intensity was enhanced.