The luminescence transition in porous silicon: The nature of the electronic states

The theoretical analysis of two different wires of size 5x4 and 3x4, simulating porous Si has been performed through the linear muffin tin orbitals method in the atomic sphere approximation. We show that the opening of the local density approximation gap is asymmetric: 1/3 of the widening is in the valence band, while 2/3 is in the conduction band; the near-bandgap states originate from Si atoms located at the centre of the wire; the imaginary part of the dielectric function shows a strongly anisotropic low-energy side structure, identified as responsible for the luminescence transition; the spatial localization of the valence and conduction band states involved in the luminescence transition shows that all the Si atoms of the wire are collectively involved.