Thermal Nitridation of Clean Si Surfaces by N2.

Abstract

Thermal nitridation of clean Si by N2 has been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). It was found that the degree of nitrogen coverage on (111) gradually increases with increasing nitridation temperature and decreases abruptly at around the 7×7 ↔ 1×1 transition temperature, while such a sudden change of coverage was not observed with Si (100) 2×1 surface. This result suggests higher reactivity of N2 with the 7×7 surface than with the unreconstructed 1×1 surface. Furthermore, chemical shifts of Si-2p and N-1s indicates that nitrided regions tend to be condensed forming completely-nitrided (Si3N4) islands with increasing nitridation temperature. These XPS results are evidenced by the behavior of dark and bright regions observed by STM; formation of these regions is strongly suppressed on terraces in the “1×1” temperature range and their dimensions become larger with increasing temperature. The dark regions are assigned from atom resolution images to “8×8” LEED pattern nitrides and the bright regions tentatively to “quadruplet” pattern nitrides.