Morphological effects on the electronic band structure of porous silicon.

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Porous silicon presents a fascinating pore morphology, which could be relevant for its efficient luminescent properties in the visible spectrum. This work attempts to give some insight into the understanding of its optical properties, by studying the electronic band structure. The porous structure is modeled as empty columns of different sizes and shapes, produced into an otherwise perfect silicon crystal. The columns are passivated with hydrogen atoms. A tight-binding Hamiltonian on an ${\mathit{sp}}^{3}$${\mathit{s}}^{\mathrm{*}}$ basis set is applied to supercells of 8, 32, 128, and 256 atoms. Due to the simplicity of the model, morphology effects can be analyzed in detail, even in the case where the column diameter oscillates is included. The results show that the band gap broadens and the conduction-band minimum shifts towards the \ensuremath{\Gamma} point, producing an almost direct band gap, as the porosity increases. A strong splitting of the originally degenerate states at the top of the valence band is also observed, for certain morphologies. Finally, the implications of quasiconfinement, where electrons can find ways out through the necks between the pores, are discussed. \textcopyright{} 1996 The American Physical Society.