A. Paton

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‐Rx 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 integrate...

Surface structure determination of the cleavage faces of CdSe via low‐energy positron diffraction

Low‐energy positron diffraction intensities were measured for 10 beams from CdSe(1010) and 14 beams from CdSe(1120). These were compared with calculated intensities for bond‐length‐conserving relaxed surface structures for both surfaces. Using an R‐factor methodology the best‐fit structures were obtained for these surfaces corresponding to local planar tilt angles of ω=15°±5° for CdSe(1010) and ω=27±7° for CdSe(1120). Both results are in excellent correspondence with structures predicted by tight‐binding total‐energy minimization calculations.

Dynamical analysis of low-energy electron diffraction intensities from CdSe(1010)

Abstract The measurement of the intensities of 14 diffracted beams of low-energy (40≤E≤230 eV) electrons normally incident on CdSe( 10 1 0 ) is reported. The temperature of the CdSe surface during the measurements was T≅125 K. The surface were prepared by in situ cleavage within the ultrahigh vacuum system used to perform the intensity measurements. The measured intensities were analyzed using a relativistic, Hara-exchange electron-ion-core potential and an X-ray R-factor structure analysis methodology. This analysis leads to a best-fit structure characterized by a bond-length-conserving rotation of the dimers in the top layer by ω = 23°, Se outward and Cd inward. The X-ray R-factor for this structure is Rx = 0.23 identical to the value obtained for the best-fit surface-structure of the InAs(110) which is the zincblende-structure isoelectronic counterpart of wurtzite-structure CdSe.

Atomic and electronic structure of the (311) surfaces of GaAs

The polar (311) surfaces of GaAs exhibit four inequivalent bulk (1×1) terminations each of which exhibits a metallic electronic character associated with partly occupied surface states. Experimentally prepared surfaces also display a (1×1) symmetry, but the tendency for surfaces to reconstruct towards an insulating ground state suggests adatom surface structure models which yield the saturation of the surface dangling bonds. We examine two Ga–adatom models on an otherwise clean As‐terminated (311) surface. Empirical total‐energy minimization calculations reveal that the As‐terminated surfaces exhibit two possible minimum‐energy adatom structures with the top‐layer (adatom) Ga in a bridge or hollow site. In the bridge‐site configuration, the Ga species occupy twofold sites on top of a relaxed As‐terminated surface. In the hollow‐site configuration, the Ga occupy threefold sites slightly above a nearly unrelaxed surface. Both correspond to simple saturated‐bond models of the surface chemical bonding but onl...

Surface relaxation of PbTe(100)

An analysis of surface Pb core‐level shifts from rock salt structure PbS(100) suggests large top layer contractions for group IVA element chalcogenides, in contradiction to expectations from other binary cubic semiconductors like MgO. To test this hypothesis, we have performed a structure analysis on PbTe(100) using eight beams of diffracted low‐energy electrons from PbTe(100) at 50 K. An intensity analysis, based on relativistic potentials shown to be accurate for fifth row elements, leads to the conclusion that the Pb sublattice in the top layer is contracted by 0.23 A (7%), whereas the Te species is unrelaxed, and the spacing between the second and third layers is 0.07 A (2%) larger than that of the bulk. Our analysis gives results analogous to those found for the (100) surfaces of other cubic materials rather than the large uniform top‐layer contraction suggested by the analysis of the core‐level shifts.

Comparison of the atomic geometries of GaSb(110) and ZnTe(110): Failure of ionicity‐structure correlations

The atomic geometries of the (110) surfaces of GaSb and ZnTe are determined by comparing calculated elastic low energy electron diffraction (ELEED) intensities with those measured at T=125 K. The quality of the model description of the observed intensities is measured by the x ray R factor Rx. Using the minimization of Rx as the structure determination criterion, the atomic geometries of these two surfaces are shown to be very similar, in fact, nearly identical to within the uncertainties inherent in the analysis. GaSb(110) is reconstructed via a bond‐length conserving rotation in its uppermost atomic layer characterized by ω1=30±2°, and the Sb relaxed outward from the substrate. ZnTe(110) exhibits an analogous bond‐length conserving rotation of ω1=28±2° in its uppermost layer. In addition, however, this layer is relaxed by 0.05±0.05 A toward the substrate, and the second layer exhibits a counter relaxation of the Zn outward by 0.025±0.05 A and the Te inward by 0.025±0.05 A. The spectroscopic ionicity of ...

Elastic low‐energy electron diffraction from GaAs(110)‐p(1×1)‐Sb(1 ML)

Auger electron spectroscopy and low‐energy electron diffraction are used to study the interface formed by the evaporation of Sb on room temperature GaAs(110). In contrast to Al, the Sb overlayer is ordered and produces a (1×1) diffraction pattern with intensity profiles very different from those measured from the clean substrate surface. The interface is sharp and stable under heat treatment, the Sb film is continuous and thermal desorption experiments reveal the strong bonding that exists between the first Sb monolayer and the substrate.The atomic structure of the GaAs(110)‐p(1×1)‐Sb(1 ML) system is analyzed with multiple scattering computations. Three types of structures have been examined: chains of Sb atoms parallel and antiparallel to the top layer Ga–As chains, Sb2 dimers attached to the surface Ga species, and a ’’jellium’’ type structure in which one Sb is bonded to the surface Ga atom and the other Sb is randomly distributed above the surface. Only single scattering computation, however, has been used for the later model. Qualitative description of the measured intensities are achieved by structures of the first (antiparallel chains) and third models.