OXIDATION STAGES OF CLEAN AND H-TERMINATED SI(001) SURFACES AT ROOM-TEMPERATURE

The oxygen uptake on clean Si(001)-2 x 1 and H-terminated Si(001)-1 x 1 surfaces at room temperature was investigated by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Surfaces were cleaned by Ar+-ion sputtering and annealing at 1200 K. H-terminated surfaces were prepared by etching of thermally oxidized samples in hydrofluoric acid. The samples were then exposed to research grade oxygen in the range from 10(14) to 10(30) O2-molecules/cm2. During exposures any excitations of the surface or the gas were avoided. The uptake of oxygen on clean surfaces proceeds in two subsequent steps. The first process saturates at about 1 monolayer and may be attributed to dissociative chemisorption. The second process sets in at a dose of 10(19) O2-molecules/cm2 and follows an inverse-logarithmic growth law. It may be described by field-assisted oxidation (Mott-Cabrera mechanism). The results are compared with similar data for Si(111) surfaces. Irrespective of surface orientation and reconstruction, the oxidation process starts always at the same exposure whereas the initial sticking coefficient and the rate of oxidation depend on the orientation of the investigated surfaces. On HF-treated surfaces, the oxygen adsorption is strongly inhibited: the sticking coefficient amounts only to approximately 10(-12).