The localization and crystallographic dependence of Si suboxide species at the SiO2/Si interface

X-ray photoemission spectroscopy has been used to examine the localization and crystallographic dependence of Si+1, Si+2, and Si+3 suboxide states at the SiO2/Si interface for (100)- and (111)-oriented substrates with gate oxide quality thermal oxides. The Si+1 and Si+2 states are localized within 6–10 Å of the interface while the Si+3 state extends ∼30 Å into the bulk SiO2. The distribution of Si+1 and Si+2 states shows a strong crystallographic dependence with Si+2 dominating on (100) substrates and Si+1 dominating on (111) substrates. This crystallographic dependence is anticipated from consideration of ideal unreconstructed (100) and (111) Si surfaces, suggesting that (1) the Si+1 and Si+2 states are localized immediately within the first monolayer at the interface and (2) the first few monolayers of substrate Si atoms are not significantly displaced from the bulk. The total number of suboxide states observed at the SiO2/Si interface corresponds to 94% and 83% of a monolayer for these (100) and (111) substrates, respectively. We speculate that the remaining interfacial substrate Si atoms that are not associated with bonding to oxygen are bonded to impurity species.