J.M.C. Thornton

Reconstructions of the GaAs(111)B surface

Abstract The (111)B surface of GaAs has been investigated using scanning tunnelling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1) LT , (√19×√19) and (1 × 1) HT with increasing annealing temperature, corresponding to decreasing surface As concentration. We find the (1 × 1) LT to be a mixture of the local structures of the (2 × 2) and (√19×√19) phases as well as some elements of a (3 × 3) structure. This is behaviour consistent with a system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1) LT transition, atomic resolution images of the (√19×√19) surface reveal only a threefold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding. This is responsible for the disorder found in the (√19×√19) reconstruction, since the structure may form in one of two domain rotations. At lower surface As concentration, the (1 × 1) HT surface adopts a random structure containing small domains of a (√7×√7)R19.1° reconstruction.

Existence of Ga-vacancy and as-trimer induced (2 × 2) phases on the GaAs(111)A surface

Abstract The GaAs(111)A-(2 × 2) surface has been examined using scanning tunnelling microscopy (STM), and found to exhibit two phases with the same periodicity. At high surface As concentrations images consistent with an As-trimer structure are seen, which transforms into a Ga-vacancy structure as the excess As desorbs at higher temperatures. Both filled and empty state images of the vacancy structure are of atomic resolution, and reveal a relaxation in the surface bilayer in agreement with predictions.

The localisation of 3d hole states in Fe and FeAl studied by Auger vacancy satellite spectroscopy

Abstract For Ni, Cu and Zn the occurrence of the L2L3M4,5 Coster-Kronig transitions give rise to L3M4,5-M4,5M4,5 Auger vacancy satellite transitions to low kinetic energy of the main L3M4,5M4,5 transitions. In pure Fe the intensity of the Auger vacancy satellites are negligibly small even though L2L3M4,5 Coster-Kronig occur, a fact which has been attributed to the delocalisation of the M4,5 (3d) hole state within the lifetime of the L3 hole. In this work we present Auger spectra for Fe and FeAl excited using radiation obtained on station 5U.1 of the Daresbury synchrotron. Our spectra support earlier work regarding the relative lifetime of the L3 and M4,5 hole state in Fe though our spectra are different in detail. We also show that the Fe 3d hole is significantly more localised in FeAl than in Fe.

A new approach to the determination of charge transfer in metal alloys

Abstract An account is given of recent progress in studying charge transfer in alloys by focusing attention on the atomic core potential and evaluating environmentally induced changes in this quantity by measuring shifts in the initial and final state Auger parameters. The importance of the information on the local physical structure of alloys that can be obtained from the EXAFS technique will also be discussed. Recent work on AuMg and AuZn alloys and the CuPd alloy system will be described.

Charge transfer and redistribution in the formation of the K/GaP(110) interface: a photoelectron spectroscopy study

We present the results of a photoelectron spectroscopy study of the K/GaP(110) interface grown at T = 120 K. We have investigated the system looking at the Ga 3d and P 2p core level spectra by analysing their lineshape. This analysis shows that the transferred charge in the adatom-substrate bonding formation affects the electronic and chemical environment of both surface Ga and P atoms. The results suggest relevant differences with the intuitive picture of strong ionic localized bonding between the alkali atom and surface Ga. They are instead more consistent with a delocalized redistributed valence charge between the adsorbate and the substrate. We also conclude that the system at ML is metallic, with an increasing disorder and inhomogeneity at the highest coverage studied. The similarities and differences with the Na/GaP(110) and the other alkali metal/GaAs(110) systems are discussed.

Surface reconstructions and phase transitions on the GaAs(111)B surface

The (111)B surface of GaAs has been investigated using scanning tunneling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)LT, (19 × 19) and (1 × 1)HT with increasing annealing temperature, corresponding to decreasing surface As concentration. The (1 × 1)LT is of particular interest, since it only occurs in a narrow temperature window between the two more established reconstructions, the (2 × 2) and the (19 × 19). We find the (1 × 1)LT to take the form of a mixture of the local structures of both the (2 × 2) and (19 × 19) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)LT transition, atomic resolution images of the (19 × 19) surface reveal only a three-fold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding due to the 23.4° rotation of the surface unit cell relative to the substrate. This is responsible for the disorder found in the (19 × 19) reconstruction, since the structure may form in one of two domains. At lower surface As concentration, the (1 × 1)HT surface adopts a structure combining small domains of a (7 × 7)R 19.1° structure and random disorder. There is no apparent similarity between the (1 × 1)LT and (1 × 1)HT structures, which may be due to our measurements being conducted at room temperature and without an As flux to control the surface As concentration.

A photoemission study of the (2 × 2) reconstructions of GaAs{111} surfaces

Abstract The {111} surfaces of GaAs have been examined using soft X-ray photoemission (SXPS) following the thermal desorption of a protective As cap. A change in As and Ga 3d core-level lineshape has been correlated with a known phase transition from an As-trimer (2 × 2) reconstruction to a Ga-vacancy (2 × 2) with increasing temperature. The photoemission from the Ga-vacancy surface is found to be very similar to the cleaved (110) surface, on which a rehybridization occurs resulting in the separation of empty and filled surface orbitals on to Ga and As atoms respectively. The (111)B surface also yields a (2 × 2) reconstruction after de-capping, with the As-rich nature of the As-trimer reconstruction causing the Ga emission to be entirely bulk-like. Two surface components are found in the As 3d emission, which are attributed to trimer and rest-atom features on the surface

A Cooper minimum photoemission study of the Au95Pd5 alloy

We present the results of a valence band (VB) photoemission study of a alloy in a photon energy range covering the Cooper minima of the Pd 4d and Au 5d photoionization cross sections, ( = 90 - 280 eV). The spectra show the presence of the Pd 4d-derived virtual bound state (VBS) which we find to lie between the Au 5d band and the Fermi level with a binding energy of 1.45 eV. The intensity of this peak goes through a minimum at = 140 eV. The results are discussed in terms of the spatial distortion of the Pd 4d wavefunction in the alloy. Subtractions between the alloy and metallic Au spectra at different photon energies reveal marked differences in the Au 5d part of the valence band. These differences are ascribed to modifications in the density of states caused by the presence of the impurity and to the variation in the photon energy dependence of the cross section of different parts of the VB spectrum.

A comparison of observed and simulated scanning tunneling images of the reconstructed GaAs(001) surface

We have generated scanning tunneling microscopy images of the reconstructed GaAs(001) surface using an ab initio pseudopotential method and the Bardeen tansfer Hamiltonian approximation. These images are compared with observed images for different bias voltages. We find that there is a strong correlation between theory and experiment.

Copper L3-M4,5M4,5 Auger and Auger satellite structures in polycrystalline Cu50Pd50 alloy

Abstract The Cu L 3 M 4,5 M 4,5 and Cu L 3 M 4,5 - M 4,5 M 4,5 M 4,5 Auger vacancy satellite spectra of polycrystalline Cu 50 Pd 50 have been measured using tunable monochromatic x-rays from a synchrotron. The photon energy dependence of the Cu Auger satellite lineshape has been recorded for increasing photon energies. The Auger and Auger vacany satellite spectra of Cu in Cu 50 Pd 50 are found to be identical to those of pure Cu. This result is discussed in terms of the electronic structure of CuPd alloys.