OXYGEN-ADSORPTION ON HYDROGEN-PREADSORBED SI(111)7X7 OBSERVED BY 2ND-HARMONIC GENERATION (SHG)

Oxygen adsorption was in situ observed by second harmonic generation ( SHG) on both clean and hydrogen-preadsorbed Si(111)7 x 7. The sticking probability of molecular oxygen was decreased by monohydride formatio n, although it was not a completely passivating surface. This is a dif ferent effect of monohydride from that on Si(111)1 x 1 for passivation . Hydrogen adsorption at room temperature leading to both monohydride and polyhydride formation showed two different stages of sticking prob ability: with increasing exposure for hydrogen preadsorption, sticking probability was furthermore decreased with oxygen adsorption. This is expected due to the formation of polyhydride species on Si(111)7 x 7 which was found to be necessary for passivating reconstructed Si(111)7 x 7.