ELECTRON-STATES AND LUMINESCENCE TRANSITION IN POROUS SILICON

The theoretical analysis of two different Si wires of size 5X4 and 3X4 , simulating porous Si, has been performed through the linear-muffin-t in-orbitals method in the atomic sphere approximation. All the atomic core energies were self-consistently computed and used to directly com pare the energies of the quantum wires and that of the crystalline Si, by aligning the 2p core level of a Si atom located at the center of t he wire to that corresponding to crystalline Si. The optical propertie s of the wires have been computed by evaluating the imaginary part of the dielectric function. The main results are (i) the opening of the g ap is asymmetric; 1/3 of the widening is in the valence band, while 2/ 3 in the conduction band; (ii) the near band-gap states originate main ly from Si atoms located at the center of the wire; (iii) the imaginar y part of the dielectric function shows a low-energy structure, strong ly anisotropic, that follows the blueshift for the gap and is identifi ed as responsible of the luminescence transition; (iv) the spatial loc alization of the valence- and conduction-band states participating in the luminescence transition shows that all the Si atoms of the wire ar e collectively involved.