R.J. Cole

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.

Extra-atomic relaxation energy calculations using an extended potential model

Abstract We show how the ground state potential model used to analyse XPS shifts can be extended to account for final state relaxation. The extended potential model is found to describe rather well extra-atomic relaxation contributions to the XPS and Auger metal-atom shifts.

Tetrahedral amorphous carbon–silicon heterojunction band energy offsets

Non-hydrogenated tetrahedral amorphous carbon (ta-C) has shown superior field emission characteristics. The understanding of the emission mechanism has been hindered by the lack of any directly measured data on the band offsets between ta-C and Si. In this paper results from direct in situ X-ray photoemission spectroscopy (XPS) measurements of the band-offset between ta-C and Si are reported. The measurements were carried out using a filtered cathodic vacuum arc (FCVA) deposition system attached directly to an ultra-high vacuum (UHV) XPS chamber via a load lock chamber. Repeated XPS measurements were carried out after monolayer depositions on in situ cleaned Si substrates. The total film thickness for each set of measurements was approximately 5 nm. Analysis of the data from undoped ta-C on n and p Si show the unexpected result that the conduction band barrier between Si and ta-C remains around 1.0 eV, but that the valence band barrier changes from 0.7 to 0.0 eV. The band line up derived from these barriers suggests that the Fermi level in the ta-C lies 0.3 eV above the valence band on both p and n+Si. The heterojunction barriers when ta-C is doped with nitrogen are also presented. The implications of the heterojunction energy barrier heights for field emission from ta-C are discussed.

Potential parameters for analysis of chemical shifts for the elements Lithium to Argon

Abstract Chemical shifts in the core level photoelectron spectra and Auger spectra of atoms provide insight into the nature of the chemical bonding and the local electronic structure. Recent progress has been made in the development of a core potential model for chemical shifts which treats implicitly the effects of electron relaxation, charge transfer, screening and polarisation. Here we present calculated potential model parameters for the elements LiAr. A parameter related to electronegativity is introduced which depends on the charge state and orbital character of the valence electrons of these elements. The physical meanings of the parameters and their trends across the period are discussed.

Off-Site Contributions to Electron Correlation; An Extension to the Hubbard Model Studied by Auger Spectroscopy

A new theory of the lineshape of CVV Auger processes has been developed which extends the Hubbard Hamiltonian to include off-site correlation effects. The new theory shows that the neglect of off-site correlations explains the failure of the standard Cini-Sawatzky model of CVV processes to yield simultaneous agreement with the experimental kinetic energy and profile of the N6,7O4,5O4,5 Auger spectra of Au which correspond to the U > W, quasi-atomic, case. The theory predicts that the contribution from off-site correlations will be less important in the U < W, bandlike region, and this is shown to be consistent with the results of high-resolution measurements of the KVV spectrum of Si.

Separating ground state and screening contributions to chemical shifts

A recently developed potential model is used to derive expressions for the chemical shifts measured by electron spectroscopy. The accuracy and reliability with which the initial state charge and core hole screening charge can be determined from experimental chemical shifts are discussed. The model is applied to the free-atom to metal shifts of Na, Mg and Zn. It predicts appropriate values of the valence charges and indicates perfect local screening of core holes in the metallic phase of these elements.

Complementary vibrational and reflectance anisotropy spectroscopic determination of molecular azimuthal orientation

Abstract Azimuthal orientations of 9-anthracene carboxylate (9-AC) on clean and p(2×1)O/Cu(110) surfaces were determined from a reflectance anisotropy spectroscopy (RAS) signal derived from an intramolecular electronic transition. The magnitude of the molecular signal on the p(2×1)O/Cu(110) surface is 4–5 times larger than on the clean surface. We present a complete vibrational assignment of adsorbed 9-AC based on Fourier transform infrared and on- and off-specular high resolution electron energy loss spectroscopy (HREELS) and ab initio calculations. Correlation of the off-specular HREELS on p(2×1)O/Cu(110) with the RAS results demonstrates that the magnitude of the RAS signal depends on the degree of azimuthal orientation.

THE DEPENDENCE OF THE ATOMIC CORE POTENTIAL ON VALENCE CHARGE FOR CU

Abstract We summarize a potential model approach to the calculation of initial and final state contributions to chemical shifts in photoelectron and Auger electron spectra. Model parameters for Cu are presented which relate the core potential to the atomic charge. The effects of valence configuration changes on the model parameters are investigated.

Photoionisation cross-sections as a probe of 4d hybridisation in dilute Ag Alloys

Abstract The photon energy dependence of the photoionisation cross-section of 4d valence levels is observed to be sensitive to the chemical environment. We discuss the modification of the cross-section variation with photon energy in terms of the hybridisation of the 4d orbitals of Ag impurities in Al and Cd hosts.

Charge transfer and electronic activation at an Sb δ-layer in Si(001)

Abstract Thin sheets of dopant atoms (δ-layers) have been shown to give unprecedented microscopic control over interfaces between semiconductors, though there exists very little understanding of the fundamental changes in the electronic structure in the vicinity of the δ-doped layer. Recent work in electron spectroscopy has shown that the combination of environmentally determined photoelectron and Auger electron shifts within the Auger parameter can provide insight into the differences in local electron screening and charge transfer. In this work, we apply the technique to the study of Sb δ-layers in epitaxially grown Si(001) and have been able to determine for the first time using spectroscopic techniques the existence of two different environments for Sb in Si, and that only 17% of the donor atoms are electrically active.