Electronic states and optical properties of silicon nanocrystals terminated by dimers

An electronic state calculation for Si nanocrystals terminated by Si–Si dimers is reported by using the extended Hückel-type nonorthogonal tight-binding method. It is shown that dimer-related surface states appear in the range up to 1 eV below the conduction band edge in all the Si nanocrystals studied, in good agreement with experimentally observed electron trap states in porous Si, and that these surface states in the dimer configuration have stronger optical coupling with the valence band edge in Si nanocrystals than the conduction band edge in the dihydride-terminated configuration, explaining a significant part of the mechanisms governing the luminescence properties of porous Si.