Supercell approach to the optical properties of porous silicon

We calculate the optical constants of porous silicon (por-Si) from the elec tronic band structure obtained by means of an sp(3)s* tight-binding Hamilto nian and a supercell model, in which the pores are columns dug in crystalli ne Si. The position of the absorption edge of the material is defined by tw o competing effects: (i) transitions assisted by the scattering of carriers on the lattice of pores, which effectively decrease the "indirectness" of per-Si and result in a redshift of the absorption edge, and (ii) quantum co nfinement, which increases the band gap. The interplay between these effect s is illustrated by calculating the imaginary part of the dielectric functi on for 8-, 32-, and 128-atom supercells with different porosities. We also show how the supercell model can be extended to take into account weak diso rder, which produces nonvertical optical transitions in k space and smoothe ns the absorption spectra. Our results, obtained without any adjustable par ameters, are compared with experimental data.