First-principles investigation of crystal geometry, band gap modulation, and optical responsein porous Si1 − mGem alloys

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.