Electronic states and optical properties of Si/SiO₂ superlattices

We study the electronic structure of {Si}_m{SiO₂}_n superlattices (SLs) grown along the [001] direction, using tight-binding methods. Detailed atomic models of the Si/SiO₂ interface are considered. A clear feature of the results is the essentially direct band-gap structure with flat bands along the ZΓ symmetry line of the SL-Brillouin zone which has a blueshifted energy gap due to quantum confinement. The calculated densities of states are enhanced at the valence and conduction band edges, as compared with silicon. The optical properties of the SLs are calculated using a parametrization of the imaginary part of the dielectric function of bulk Si. The strong confinement of the electron-hole pairs in the Si wells and their tendency to localize at the low-dielectric {SiO₂} interfaces due to the mutual Coulomb attraction lead to strong electrostatic effects. These produce an interplay of several length scales in determining possible regimes of high radiative efficiency. Our results have implications for the understanding of the luminescence in porous Si and Si-based nanostructures like the amorphous Si/SiO₂ SLs studied recently.