Study of the adsorption of hydrogen and oxygen on Si(100) using positron-annihilation induced Auger electron spectroscopy (PAES)

Abstract

The adsorption of hydrogen and oxygen on a Si(100) surface is investigated by positron-annihilation induced Auger electron spectroscopy (PAES), as well as conventional electron-induced Auger electron spectroscopy (EAES) and low-energy electron diffraction (LEED). The Si L 2,3VV Auger intensities were observed to decrease exponentially with increasing H 2 (O 2) exposure. The sticking coefficients of H 2 and O 2 were found from fits to plots of the attenuation of the Si Auger signal versus gas exposure to have values of 4.4 ± 1.8 × 10 −5 and 2.6 ± 0.6 × 10 −4 for hydrogen and oxygen, respectively. After 9000 L O 2 (4500 L H 2) exposure, the Si LVV signal measured using PAES was found to decrease by 71% (60%) as compared to a 25% (< 10%) decrease of the Si signal found using EAES. This is consistent with the higher degree of surface selectivity expected in PAES. This attenuation of the Si PAES intensity is consistent with previous studies of H 2S adsorption on Cu, which indicated that the positron surface-state wave function is displaced away from substrate atoms by an adsorbed overlayer, resulting in a decrease in the annihilation rate of positrons with core electrons of the substrate. Our results indicate that PAES could be useful in other H adsorption measurements due to its high sensitivity to the presence of a H adlayer.