Absence of band-gap surface states on clean amorphous silicon

Abstract

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 SiOx 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 SiOx emission.