Optical dielectric function of intrinsic amorphous silicon

The imaginary part of the optical dielectric function ε₂(ω) has been calculated using a continuous-random-tetrahedral network as the structural model for the atomic positions. Here the electronic energies and wave functions are determined by first-principles calculations with the method of linear combinations of atomic orbitals (LCAO), and the momentum matrix elements are evaluated directly from the LCAO wave functions. The calculated dielectric function is in good overall agreement with experiment. At energies within 1 eV above the threshold, the ε₂ curve shows some structures that are due to interband transitions between the localized states near the band gap.