Absence of band-gap surface states on clean amorphous silicon

Abstract

Photoemission studies have been made of the clean and oxidized amorphous Si surface at hv = 10·2, 21·2 and 40·8 eV. For the clean surface, no emission from surface states in the band gap is found and exposure to small amounts of oxygen results in no preferential decreases in emission near the valence band edge. We place an upper limit of 4 × 10¹³ surface states/cm² in the gap and conclude that any residual states in the gap are not intrinsic to the amorphous Si vacuum interface. A small amount of oxygen induces a broad valence band peak 2 to 5 eV below the valence band maximum; this we associated with a SiO_x structure. Furthermore, we find that exposure to 10⁸ L oxygen produces valence band structure similar but not identical to SiO₂ and a depletion of the SiO_x emission.