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 sp(3)s* 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 a ccount by considering nonvertical interband transitions. The results obtain ed for 8- and 32-atom supercells show a strong dependence on the pore morph ology, i.e., the absorption coefficient changes with the shape and size of the silicon wires even at constant porosity. The absorption spectrum of thi s model for per-Si is defined by the interplay between the decrease in the indirectness of the material (connected to the absorption processes assiste d by the scattering on the pores), which effectively reduces the direct gap , and the increase of the gap due to the quantum confinement. (C) 1999 Else vier Science B,V. All rights reserved.