THERMAL NITRIDATION OF SI(111) SURFACES WITH N-2 MOLECULES STUDIED BY X-RAY PHOTOELECTRON-SPECTROSCOPY

The initial stage of thermal nitridation of Si (111) surfaces using N-2 gas has been studied by X-ray photoelectron spectroscopy. It was found, for the first time, that under N-2 pressure of 1.3 x 10(-3) Pa the nitridation rate increases with temperature from 600 to 700 degrees C, reaches a constant value and then falls off abruptly at the critical temperature (similar to 880 degrees C). Since the critical temperature coincides with that of the transformation of Si (111) from 7 x 7 to 1 x 1, we ascribe the abrupt change to significant differences in reactivity with nitrogen between 7 x 7 and 1 x 1 surfaces. This interpretation is supported by a smoother temperature dependence of nitridation on (100) 2 x 1 surfaces.