S-INDUCED FACETING TRANSFORMATIONS ON FE(111)

We report previously unobserved structural transformation on the Fe(11 1) surface with respect io segregation of S studied by a combination o f STM. LEED and AES. An evolution of different surface topographies an d LEED patterns was observed as a function of S coverage. At low cover ages of S and other impurities of N and O, a complex ''clock'' LEED pa ttern was observed along with a faceted surface consisting of triangul ar terraces sl mounds and sawtooth structures. The terraces display a typical width of similar to 1400 Angstrom separated by multi-height st eps us high as similar to 50 Angstrom. The clock pattern appears only after annealing the crystal to temperatures above 520 K. Annealing of the crystal at higher temperatures (greater than or equal to 700 K) in duced increased segregation of bulk S resulting in the transformation to a (1 x 1)-S LEED pattern. In contrast to the multi-height steps obs erved for the clock surface. STM measurements on the (1 x 1)-S phase r evealed a planar topography consisting of monatomic steps spaced at ap proximately 100 Angstrom from each other, Upon further annealing of th e crystal, additional S was driven to the ( 1 x 1)-S surface resulting in the transformation to a new LEED pattern corresponding to a (2 roo t 3 x 1)R30 degrees structure. STM images for the (2 root 3 x 1)R30 de grees structure revealed that the steps have coalescence to reform the multi-height steps and wide terraces similar to that observed on the dock surface. In addition to he coalescence of the steps. an unusual f aceting behavior was observed for this high-S coverage phase. consisti ng of numerous triangular pits of varying sizes with edges aligned alo ng the (110) directions of the surface. Within large triangular pits a very-periodic ''staircase'' topography was observed. The pits are as deep as 50 Angstrom with edges as long as 800 Angstrom. Our data show that S. at varying coverages. interacts very strongly with the steps o f the Fe(111) surface. (C) 1998 Elsevier Science B.V.