Equivalent Step Structures along Inequivalent Crystallographic Directions on Halogen-Terminated Si(111)-(1×1)Surfaces

Steps in inequivalent crystallographic directions are usually assumed to have different atomic structures. However, this Letter demonstrates that bilayer steps in the two principal crystallographic directions ( $〈\overline{1}\overline{1}2〉$ and $〈11\overline{2}〉$) of the Br-terminated Si(111)- $(1\times 1)$ surface have the same atomic edge structure, due to the introduction of stacking faults along the $〈11\overline{2}〉$ step edges. Similar results are also observed for Cl- and I-terminated Si(111) surfaces. This strong preference for a particular step edge structure determines the surface morphology of steps, islands, and etch pits, and has profound ramifications for dry etching and chemical vapor deposition growth for this system.