AN STM Study of the Oxygenation of Silicon

We have made a detailed atomic study of the early stages of oxidation of silicon at room temperature. On the reconstructed 111 surface the reaction occurs rapidly with a sticking coefficient of ~0.02. Three types of defect or reacted site are found on the surface; the number density of the two commonest defects increasing linearly with oxygen dose. The commoner of the two defect types exhibits image contrast consistent with a depletion in electron density from empty (anti-bonding) to filled (bonding) states which we believe is due to the reaction of the oxygen in the layer immediately below the adatoms. Tunnelling spectroscopy at such sites on the surface shows the quenching of the adatom states and the appearance of a peak in the density of states at -0.8 eV. The other common defect appears in the filled state image exhibiting weaker contrast than the first type and indicating an enhancement in the filled state density. Both defect types preferentially react in the faulted half of the unit cell with an 88% probability. The third type of defect, with only a weak dependence on oxygen dose, occurs at the corner of the unit cell and produces a large change in the electron barrier height consistent with uncompensated charge or an electron trap.