Adsorption and co-adsorption of boron and oxygen on ordered α-SiC surfaces

Boron layers (grown by thermal decomposition of B10H14) on the (0001) Si- and (0001) C-terminated surfaces of α-SiC have been studied using Auger and electron energy-loss spectroscopies and low-energy electron diffraction. Adsorption of O2 on the clean and B-adsorbed surfaces was also studied. Cleaning the (0001) surface by annealing in a flux of Si vapor gives a (3 × 3) structure which converts to 3 × 3 upon further annealing in vacuum. The (3 × 3) consists of an ordered layer of Si chemisorbed on the Si termination layer, while the 3 × 3 involves an ordered arrangement of Si vacancies. A (1 × 1) structure is observed for the clean C-face. Adsorption of B on the (3 × 3) Si-face eliminates the reconstruction, and further annealing produces a complicated superstructure. Stronger interaction occurs on the 3 × 3 Si-face leading to an incommensurately ordered (1 × 1) layer loosely termed “Si boride” due to the substantial changes in SiL2,3VV and B KLL Auger lineshapes. On the C-face, a disordered B layer forms. On B-free surfaces, room-temperature chemisorption of O2 is slower on the (1 × 1) C-face than on the (3 × 3) Si-face, but the rates of O uptake become comparable with increasing coverage as chemisorption gives way to oxidation. For the 3 × 3 Si-face, chemisorption is rapid, but oxidation very slow. For all three surfaces, adsorbed B suppresses chemisorption of O2 but has little or no effect on oxidation.