Tight-binding description of disordered nanostructures: an application to porous silicon

We present the calculations of the coefficient of light (photo) absorption in porous silicon (por-Si) using the supercell tight-binding sp³s^* model, in which the pores are columns digged in crystalline silicon. The disorder in the pore sizes and the undulation of the silicon wires are taken into account by considering nonvertical interband transitions. The results obtained for 8- and 32-atom supercells show a strong dependence on the pore morphology, i.e., the absorption coefficient changes with the shape and size of the silicon wires even at constant porosity. The absorption spectrum of this model for por-Si is defined by the interplay between the decrease in the indirectness of the material (connected to the absorption processes assisted by the scattering on the pores), which effectively reduces the direct gap, and the increase of the gap due to the quantum confinement.