Quasi-confinement, localization and optical properties in porous silicon

The quasi-confinement concept, where electrons can find ways out through the necks between the pores, is discussed and its consequences in the localization and optical properties of porous silicon are analyzed. The polarized light absorption is studied by observing the oscillator strength behaviour. The localization is quantified using the inverse participation ratio (IPR), which gives the number of sites occupied by the wave function. The pore structure is simulated by a supercell model, where a tight-binding Hamiltonian with an sp(3)s* basis is used and empty columns of atoms are produced in an otherwise perfect silicon structure. These columns are passivated with hydrogen atoms. The results show that the bandgap broadens and the conduction band minimum shifts towards the gamma point, as the porosity increases. Likewise, the oscillator strength analysis reveals a significant enlargement of the optically active zone in the k-space, due to the localization of the wavefunction,which has been analyzed by looking at the IPR. (C) 1997 Elsevier Science S.A.