Surfacelike band-edge states on H-covered and oxidized ultrasmall Si quantum boxes: a theoretical study

We present new surfacelike electronic states induced by oxidation of H-covered ultrasmall Si 5×5×Nz (Nz=7, 11, and 15) quantum boxes (QBs) with (100) planes (where a 5×5×Nz QB is a structure composed of 5, 5, and Nz Si monolayers along the [100], [010], and [001] directions, respectively). Electronic state calculations are done for a Si dihydride backbonded to oxygen on the (001) surface of the QBs using the extended Hückel-type nonorthogonal tight-binding method. It is shown that the backbond oxidation creates new surfacelike states localized on the (001) surface not only at the conduction-band edge, but also at the valence-band edge, while preserving the original surfacelike states due to interhydride interactions. The occurrence of these surfacelike states explains the experimentally observed redshifts in the band gap of Si nanocrystals by oxidation. The characteristics of dipole-allowed optical transitions between the oxidation-induced surfacelike band-edge states are also discussed.