ELECTRONIC-STRUCTURES AND OPTICAL-ABSORPTION SPECTRA OF HYDROGEN-TERMINATED SI QUANTUM SLABS

The electronic band structures and the imaginary part of the dielectri c function of hydrogen-terminated Si quantum slabs of (001), (110), an d (111) directions are calculated from first principles. it is shown t hat only the (001) slab actually possesses a direct band gap. The thic kness dependence of the direct band gap is investigated, and the resul t is in good agreement with that of other calculations. The results of the epsilon(2)(omega) 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) sla bs the direct (or quasidirect) absorption edges at the onset energy ar e separated by a shoulder from the rest of the spectrum. This behavior does not appear in the case of the (111) slabs.