Abstract

The discovery in 1990 [1] that porous silicon is a strong light emitter in the visible at room temperature has changed our assessment of the usefulness of silicon as an optoelectronic material and raises the possibility of practical silicon light emitters [2]. The luminescence from light emitting porous silicon (LEPSi) usually occurs in the deep red to orange region with a quantum efficiency of a few percents at 300 K and >10% at low temperature. The luminescence decays with a lifetime that ranges from ~1-10 μsec at 300 K to >1 msec at low temperature. More than 15 different models have been proposed to explain the strong luminescence from LEPSi [3] but only two models have received strong experimental support. In the first model, light emission results from the recombination of a free electron-hole pair or exciton across the bandgap of silicon nanoparticles [1,4]. The emission is in the visible because the effective bandgap is increased by quantum confinement and the quantum efficiency is high because the bandgap of these silicon nanoparticles has become quasi-direct and because nonradiative recombination is suppressed thanks to the good passivation of the nanoparticle surface. In the second model, light emission results from radiative recombination involving carriers trapped in states localized on the nanoparticle surface [5]. These surface states are in the forbidden gap of the silicon nanocrystallites which is increased by quantumconfinement. The long lifetime of the luminescence is explained by the small overlap between the wavefunctions of carriers localized at physically separated states. To date, no experiment has been able to decide which model is correct.

© 1994 Optical Society of America