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 electr on diffraction (LEED) investigations of the interaction of low-energy nitrogen ions with an Si(111)-7 x 7 surface in the initial stage of ni tridation. On silicone nitride islands showing a quadruplet LEED patte rn a triangular periodicity of white protrusions with an average separ ation of 10-11 Angstrom was observed in the STM image. Furthermore, th e symmetry directions of the white protrusions were rotated about 10 d egrees with respect to those of Si(111) surface, which was consistent with the LEED observation of dominant diffraction spots in that direct ion. The preferential appearance of dark Si adatoms due to bonding wit h nitrogen atoms on the center adatom sites compared with corner sites is explained in terms of the thermal stability of the product after n itridation. We also found that elevation of the nitridation temperatur e to 950 degrees C dramatically improved the quality of the silicone n itride layer owing to the improved mobilities of reacting species.