The Atomic Geometries of GaP(110) and ZnS(110) Revisited: A Structural Ambiguity and its Resolution

The atomic geometries of GaP(110) and ZnS(110) are reexamined using our R-factor minimization procedure, developed for GaAs(110) and previously applied to GaSb(110) ZnTe(110), InAs(110) and AlP(110), to analyze experimental elastic low-energy electron diffraction intensities. Unlike most of the earlier cases, both GaP(110) and ZnS(110) exhibit two distinct minimum-R_x structures which cannot be distinguished by analysis of the shapes of the intensity profiles alone. One region of best-fit structures exhibits top-layer displacements normal to the surface characterized by a small bond-length-conserving, top-layer rotation (ω~ 2-3°), a small relaxation of the top layer away from the surface, and a 10% expansion of the top-layer bond length. The other region of best-fit structures is the conventional one: nearly bond-length-conserving rotations of ω = 26-28° in the top layer and a small (~ 0.1 A) contraction of the uppermost layer spacing. This ambiguity may be removed, however, by consideration of the integrated beam intensities. The conventional region of structural parameters provides a decisively better description of the relative magnitudes of the integrated beam intensities and hence is the preferred structure.