P. Unsworth

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.

VARIATIONS IN THE LIFETIME OF 3d HOLE STATES IN ULTRATHIN Fe FILMS GROWN ON Cu(100) DEDUCED FROM THE LMM AUGER SPECTRA OF Fe

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L2 ionisation edge, and in coincidence with the Fe 2p3/2 and Fe 2p1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M4,5 holes of L3M4,5 double hole states are localised for longer than the L3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

Influence of the heterojunction on the field emission from tetrahedral amorphous carbon on Si

In order to study the influence of the back barrier on the electron emission properties of tetrahedral amorphous carbon (ta-C), we have deposited identical films on p and n+ Si. The valence and conduction band offsets were measured for ta-C on p and n+ Si using in situ x-ray photoelectron spectroscopy and optical spectroscopy. From the band measurements it is shown that there is a substantial back barrier to emission. We show that for films having very similar properties, the electron emission can be influenced by the ta-C/Si heterojunction.

Charge-Transfer Satellites in K L23 XAS Data for K/Si(111)-(2 × 1): Evidence for Strong Ionic Bonds

We report K L23 edge X-ray absorption spectra (XAS, both normal and grazing incidence) and photoelectron spectra (XPS) for K/Si(111)-(2 × 1) at three different coverages (0.1, 0.3 and 0.5 ML). The data show features which exhibit a very strong variation in both the energy, intensity, and line shape with incidence angle and coverage. Some of these features correspond to charge-transfer satellites and give a measure of the hybridization of the ground state. They indicate very ionic bonding.

Local Lattice Distortion in Ordered and Disordered Cu3Pd Alloys

We have combined extended X-ray absorption fine structure (EXAFS) and electron diffraction to study both ordered and disordered Cu3Pd, and have established that in the first shell of neighbours the Cu-Pd distance is ~ 0.03 A greater than that for Cu-Cu. This result lends support to recent theoretical treatments which assume a distribution of bondlengths in disordered alloys, and draws attention to the fact that Vegards law does not require a single, mean bondlength, as assumed in the application of mean-field theories.

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.

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.

Insight into the local density of states at Si sites at the submonolayer Si/Ge(001)-2 × 1 interface from Si KLV Auger spectroscopy.

An analysis of the differences observed between the Si KLV Auger spectra of the Si/Ge(001)-2 × 1 interface and pure Si indicates that the electronic structure of the interface is characterized by a reduction in the local p DOS at the Si sites and a transfer of p valence charge from Si to Ge. As a result, the screening of core-ionized Si sites at the interface is significantly shifted towards s screening compared with the situation for pure Si. It is possible that there is an increase in the on-site electron correlation energy, UP, for Si sites at the interface as compared with pure Si.

Reflection anisotropy spectroscopy of the oxidized diamond (001) surface

We report reflection anisotropy spectroscopy (RAS) measurements of the oxidized (001) surface of a type IIb natural diamond. These measurements were made possible due to recent developments in diamond surface preparation. We compare RAS results from the hydrogenated, clean and oxidized C(001) surface and demonstrate that RAS is sensitive to the structural transition of the surface from the 2 × 1 reconstruction of the clean surface to the 1 × 1 reconstruction of the oxidized surface.