Abstract
Despite the widespread use of Si and Ge materials, or their alloys in optoelectronics, these materials present the disadvantage of having an indirect-gap. We first computed the band structure porous Si passivated by H-atoms using the supercell model and Density Functional Theory. The computations were performed on a supercell containing 72 Si-atoms, where the pores were designed by deleting central Si-columns in [001]-direction. The band structure revealed a direct gap when the porosity is 56.9 %. This gap was then modulated by gradual substituting of Si by Ge-atoms, reaching finally porous Ge. The PDOS indicate a rise of the density of states in the conduction band by Ge-substitution. The computations are extended to analyze the spectra of optical and dielectric functions, according to the light polarizations parallel and perpendicular to the pores direction. The refractive indices were fitted showing a weakening of the optical constants compared to those of bulk silicon.

Key words
porous silicon, porous sige alloys, direct band gap, SiGe optical properties, SiGe PDOS

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