Initial stages of nitridation of Si(111) surfaces: X-ray photoelectron spectroscopy and scanning tunneling microscopy studies

Abstract

Submonolayer regime of thermal nitridation of clean Si(111) using N2 gas has been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) in ultrahigh vacuum. Nitrogen coverage after a certain exposure time gradually increases with increasing nitridation temperature above ∼ 500°C and abruptly decreases at around the (7 × 7) ↔ (1 × 1) transition temperature of the Si substrate. This result suggests that the sudden change of coverage for (111) is due to higher reactivity of nitrogen with the (7 × 7) surface than with the unreconstructed (1 × 1) surface. Furthermore, chemical shifts of Si 2p and N 1s moving toward higher binding energy with increasing nitridation temperature indicates that nitrided regions tend to be condensed, forming completely-nitrided (Si3N4) islands. These XPS results are evidenced by STM observations; the formation of dark regions, which is assigned to silicon nitride, is strongly suppressed on terraces in the “1 × 1” temperature range and the dark regions become larger, having a characteristic triangular shape with increasing temperature in the “7 × 7” temperature range. The temperature-induced condensation of those islands are confirmed by an annealing experiment.