Electronic structures and optical-absorption spectra of hydrogen-terminated Si quantum slabs

The electronic band structures and the imaginary part of the dielectric function of hydrogen-terminated Si quantum slabs of (001), (110), and (111) directions are calculated from first principles. It is shown that only the (001) slab actually possesses a direct band gap. The thickness dependence of the direct band gap is investigated, and the result is in good agreement with that of other calculations. The results of the ${\mathrm{\epsilon }}_2$(ω) spectra show that there is a major peak located at about 4–5 eV for all the slabs. In the case of (001) and (110) slabs the direct (or quasidirect) absorption edges at the onset energy are separated by a shoulder from the rest of the spectrum. This behavior does not appear in the case of the (111) slabs.