NITRIC-OXIDE ADSORPTION ON THE SI(111)7X7 SURFACE - EFFECT OF POTASSIUM OVERLAYERS

Nitric oxide (NO) adsorption on clean and potassium modified Si(111)7 x 7 surfaces is studied by means of core-level photoemission spectroscopy using Al K(alpha) (1486.6 eV) and Zr M(zeta) (151.4 eV) X-ray lines. The initial sticking probability of NO is enhanced upon K pre-deposition. The NO uptake is, however, self-limited, and does not result into an increased saturation coverage compared to NO adsorption on bare Si. The presence of potassium significantly changes the stoichiometry and reduces the thickness of the saturated silicon oxynitride layer grown on the Si(111)7 x 7 surface after NO exposures. The number of adsorbed nitrogen atoms decreases linearly with increasing K coverage (theta(K)), whereas the amount of adsorbed oxygen, after a decrease in the K submonolayer coverage range, regains its initial value at theta(K) = 1. The relative concentration of adsorbed N and O atoms varies with the K dosage, leading to an oxygen-enriched SiON layer at high K coverages. The reactivity of the Si(111)7 x 7 surface passivated by NO adsorption can be restored by subsequent K deposition at coverages above 1 ML such that further NO or O2 exposures increase considerably the amount of oxygen transferred to the substrate. For the NO exposure case, only a minor change in the number of adsorbed N atoms occurs. These observations are explained in terms of secondary surface reactions involving N2O formation from K-stabilized NO molecules and N adatoms. N2O either desorbs directly or decomposes into O and N2, the latter desorbing into the gas phase. The results reveal the possibility to grow thicker, oxygen-rich silicon oxynitride layers beyond the saturation coverage with the formation of SiON/Si interfaces after thermal annealing at 700-degrees-C, which removes the N atoms by thermal desorption.