W. K. Ford

Low-energy electron diffraction studies of Si(111)-(√3 × √3)R30°-Bi system: Observation and structural determination of two phases

Abstract The initial epitaxial growth of Bi on a Si(111)-7 × 7 surface has been studied as a function of overlayer coverage and deposition conditions using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Experiments show for the first time that two equilibrium phases are formed in the growth process, each displaying a (√3 × √3)R30° LEED symmetry: an α-phase occurring at 1 3 ML Bi coverage and substrate temperature of 360° C, and a β-phase at 1 ML and 300°C. Quantitative structural information for each phase was determined by multiple scattering analysis of I - V curves. The analysis is facilitated by comparing LEED intensity data measured for each phase with calculated values using appropriate structural models. The T 4 geometry for the α-phase and a trimer geometry for the β-phase are revealed to be the “best” models; detailed atomic coordinates have been determined for each phase.

Ionicity dependence of surface bond lengths on the (110) cleavage faces of isoelectronic zincblende structure compound semiconductors: GaP, ZnS, and CuCl

Previously measured low‐energy electron diffraction (LEED) intensity data for the (110) surfaces of the isoelectronic series GaP, ZnS, and CuCl, are reanalyzed using dynamical LEED calculations, a new selection of independent structural variables, and an automated optimization procedure. The bond lengths of the atoms in the top layer and perpendicular displacements of the cation‐anion sublattices in the top two layers are selected as the independent structural variables. All other bond lengths are taken to be equal to their bulk values. An automated gradient method search procedure is utilized to obtain these structural variables by optimizing the fit of the measured intensities by the calculated ones as determined by the x‐ray R factor. This procedure permits a precise comparison of the bond lengths of species in the top layer from one material to another. Our major finding is that the bond lengths, d, between the anion and cation in the top layer depend on the spectroscopic ionicity, f, via [(d−dB)/dB]=...

Observation and LEED I–V analysis of the Ge(111)-1×1-Sb system

Abstract When antimony is adsorbed at submonolayer coverage on a cleaved Ge(111) surface, the clean (2 × 1) surface transforms to a (1 × 1) structure. A dynamical low energy electron diffraction (LEED) intensity anlysis was performed to study this system. The results suggest that Sb adatoms replace the outermost Ge atoms, resulting in a well-ordered bulk-like (1 × 1) surface. This system, like that of a Sb overlayer on Si(111) surfaces, demonstrates that adatoms can remove a surface reconstruction.

Physical processing effects on polycrystalline YBa 2 Cu 3 O x

The effect of heating YBa 2 Cu 3 O x in vacuum to 600 °C has been studied using photoelectron spectroscopy and diamagnetic susceptibility measurements. Evidence of two chemically distinct copper and barium species is found in single phase samples at room temperature cleaned by gentle heating at 450 °C. Such annealing also increases the volume diamagnetic susceptibility of the samples which suggests that the preferred stoichiometry of growth does not lead to an optimum superconducting phase. Samples cleaned by vacuum scraping or ion bombardment reveal more amorphous XPS structure and are less indicative of bulk properties.

Low magnetic field superconducting phase diagram of the high-Tc YBa2Cu3O9-δ

Abstract The low magnetic field phase diagram of the high-Tc superconductor YBa2Cu3O9-δ with δ∼2.1 has been determined in fields to 5000 0e. The lower critical field is about 300 0e near T=0 and less than 100 0e at liquid nitrogen temperature. The mixed phase shows significant relaxation and field cooling effects.

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.