Observation of two-step excitation of photoluminescence in silicon nanostructures

Efficient visible-range photoluminescence with photon energy higher than th e photon energy of the exciting radiation is observed in nanostructures of porous silicon subjected to heat treatment in vacuum. The photoluminescence intensity is found to be virtually identical for cw and femtosecond excita tion by Ti:sapphire laser radiation with the same average power. The result s can be explained by a two-step cascade photoluminescence excitation proce ss in which optical passivation of defects of the dangling silicon bond typ e occurs. (C) 1998 American Institute of Physics. [S0021-3640(98)00322-3].