THEORETICAL ASPECTS OF THE LUMINESCENCE OF POROUS SILICON

The luminescence in the visible range of porous silicon is analyzed in the hypothesis of quantum confinement. We calculate the electronic an d optical properties of silicon crystallites and wires with sizes betw een 0 and 4.5 nm. The band-gap energies of such confined systems are i n agreement with the photon energies observed in luminescence. We calc ulate the radiative recombination times of the confined excitons. We c onclude that experimental nonradiative processes in porous silicon are more efficient than calculated radiative ones at T=300 K. The high ph otoluminescence efficiency of porous silicon is due to the small proba bility of finding a nonradiative recombination center in silicon nanoc rystallites. Recently, it has been proposed that the low-temperature d ependence of the experimental radiative decay time of the luminescence of porous silicon could be explained by the exchange splitting in the fundamental exciton. We show that the influence of the valley-orbit s plitting cannot be excluded. The sharp optical-absorption edge above 3 .0 eV is not proof of the molecular origin of the properties of porous silicon because silicon nanostructures present a similar absorption s pectrum. We calculate the nonradiative capture of electrons or holes o n silicon dangling bonds and show that it is very dependent on the con finement. We find that the presence of one dangling bond at the surfac e of a crystallite in porous silicon must destroy its luminescent prop erties above 1.1 eV but can produce a luminescence below 1.1 eV due to a radiative capture on the dangling bond.