Theory of the luminescence of porous silicon

Abstract

The physical origin of the luminescence bands of porous silicon is discussed using results of calculations of electronic and optical properties of silicon nanostructures. Silicon crystallites, wires or molecules are investigated as possible sources of emission of light. We show that the slow visible band in porous silicon is probably due to quantum confinement in silicon crystallites with dimensions lower than ∼4.5 nm. The long radiative lifetime in this band is due to a small 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 range. The physical origin of the fast blue band of porous silicon is examined in light of these results.