Interaction of low-energy nitrogen ions with an Si(111)-7x7 surface: STM and LEED investigations

We report on scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) investigations of the interaction of low-energy nitrogen ions with an Si(111)-7 x 7 surface in the initial stage of nitridation. On silicone nitride islands showing a quadruplet LEED pattern a triangular periodicity of white protrusions with an average separation of 10-11 Angstrom was observed in the STM image. Furthermore, the symmetry directions of the white protrusions were rotated about 10 degrees with respect to those of Si(111) surface, which was consistent with the LEED observation of dominant diffraction spots in that direction. The preferential appearance of dark Si adatoms due to bonding with nitrogen atoms on the center adatom sites compared with corner sites is explained in terms of the thermal stability of the product after nitridation. We also found that elevation of the nitridation temperature to 950 degrees C dramatically improved the quality of the silicone nitride layer owing to the improved mobilities of reacting species.