Session VC15 - Amorphous, Porous, and Other Phases of Si/C.
ORAL session, Thursday morning, March 25
Room 265W, GWCC
We present the numerical results for the imaginary part of dielectric function (\epsilon_2(ømega )) of porous Si, obtained in an sp^3s tight-binding supercell model. Calculations have been carried out for 8-, 32-, 128-atom supercells, with different \left< 001 \right> columns of Si-atoms removed to simulate a wide range of porosities. The dangling bonds are saturated with hydrogen atoms. The onset of the optical absorption and the shape of \epsilon_2(ømega ) are governed by an interplay between two opposite effects: (i) quantum confinement, which widens the energy gap, and (ii) the decrease in the ``indirectness'', which is due to the absorption processes accompanied by the scattering on pores, and moves the optical gap of the material to lower frequencies. This interplay is illustrated in samples of the same porosity but with different confinement lengths. We have studied also the effects of pore morphology by changing the shape of the removed columns. The results are compared with available experimental data, and with effective medium calculations.