Origins of visible-light emissions in porous silicon

The electronic and optical properties of porous silicon (p-Si) have been theoretically investigated using the minimal sp ³-basis set tight-binding method, with inclusion of second nearest neighbors and spin-orbit interactions. A hypothetical model of p-Si was assumed and calculations of band structures, with focus on bandgap energy E _g, oscillator strength (OS) and recombination rate (RR), were carried varying the porosity and the mean distance d between pores. Similar calculations were also performed for other confined silicon nanostructures as hydrogen-passivated silicon nanocrystals (Si:H NCs) and silicon nanowires (Si-NWs). For these two latter systems, the results of E _g versus the size d are found to be in excellent agreement with the available measured photoluminescence (PL) data of experimental p-Si samples. On one hand, the results show that sizes in the range d = 1-3 nm are responsible for emission in the visible-light energy window. On the other hand, our results also suggest that the emission properties of p-Si is strongly affected by the 2D and 3D quantum confinement (QC) characters of the involved band edge states. Furthermore, our theoretical values for the RR are found to be larger and closer to the experimental ones in the case of Si:H NCs with respect to the case of Si-NWs. This fact suggests that the intense measured PL features are mainly due to 3D confined nanostructures.