THEORY OF THE LUMINESCENCE OF POROUS SILICON

The physical origin of the luminescence bands of porous silicon is dis cussed using results of calculations of electronic and optical propert ies of silicon nanostructures. Silicon crystallites, wires or molecule s are investigated as possible sources of emission of light. We show t hat the slow visible band in porous silicon is probably due to quantum confinement in silicon crystallites with dimensions lower than simila r to 4.5 nm. The long radiative lifetime in this band is due to a smal l oscillator strength typical of an indirect bandgap semiconductor. We calculate that the presence of a dangling bond in a crystallite must kill its luminescence in the visible range but can produce an emission in the infrared region, in agreement with recent experimental results showing a correlation between the presence of dangling bonds and the infrared emission of light. We show that polysilanes can emit visible light in some cases but with radiative lifetimes in the nanosecond ran ge. The physical origin of the fast blue band of porous silicon is exa mined in light of these results.