J.D. Riley

Al2O3 prepared by atomic layer deposition as gate dielectric on 6H-SiC(0001)

Al2O3 films were deposited as alternative gate dielectric on hydrogen-terminated 6H-SiC(0001) by atomic layer chemical vapor deposition and characterized by photoelectron spectroscopy (PES) and admittance measurements. The PES results indicate an abrupt interface free of significant Si–suboxide contributions where the Al2O3 layer is connected to SiC by bridging oxygen atoms. The admittance measurements yield an interface state density which is lower than that of the thermally formed oxide and show in particular no increase toward the conduction band edge. Furthermore, a nearly symmetrical band alignment of Al2O3 on 6H-SiC with offsets of 2.2 and 1.8 eV is determined for the valence and conduction bands, respectively. This makes Al2O3 a serious competitor to thermal oxides as gate insulator in SiC devices.

Electronegativity as a unifying concept in the determination of Fermi energies and photoelectric thresholds

Abstract The method proposed by Nethercot for the determination of the Fermi energies and photoelectric thresholds of simple binary compounds has been successfully applied to some AB 2 compounds and metals.

The electronic structure of the valence bands of solid NH3 and H2O studied by ultraviolet photoelectron spectroscopy

Abstract The results of a photoelectron study using ultraviolet 40.81 eV photons (UPS) of the outermost bands of the molecular solids NH 3 and H 2 O are reported. The binding energies, the energy separation, the band widths and the branching ratio of the two outermost bands of solid NH 3 are found not to be significantly different from the 3a l and 1e molecular orbital states of the gaseous NH 3 UPS spectrum. This implies that hydrogen bonding has not produced any significant change in the electronic structure of the valence bands of solid NH 3 . Because of a much smaller intermolecular hydrogen bond length in solid H 2 O compared to that in solid NH 3 , the hydrogen bond does, however, produce a significant change in the valence bands of H 2 O on solidification, and because of the orbital geometry it predominantly affects the 3a l molecular orbital state.

Initial Stages of the Graphite-SiC(0001) Interface Formation Studied by Photoelectron Spectroscopy

Graphitization of the 6H-SiC(0001) surface as a function of annealing temperature has been studied by ARPES, high resolution XPS, and LEED. For the initial stage of graphitization – the 6√3 reconstructed surface – we observe σ-bands characteristic of graphitic sp2-bonded carbon. The π-bands are modified by the interaction with the substrate. C1s core level spectra indicate that this layer consists of two inequivalent types of carbon atoms. The next layer of graphite (graphene) formed on top of the 6√3 surface at TA=1250°C-1300°C has an unperturbed electronic structure. Annealing at higher temperatures results in the formation of a multilayer graphite film. It is shown that the atomic arrangement of the interface between graphite and the SiC(0001) surface is practically identical to that of the 6√3 reconstructed layer.

Design criteria for an angle resolved electron spectrometer of novel toroidal geometry

Abstract A charged particle analyser based on the use of polar trajectories in a toroidal sector is described in which a large number of particle emission angles are detected simultaneously. An analysis of the trajectories of particles in this class of analyser is presented and we develop transfer matrices which enable the imaging properties of such devices to be determined analytically. Expressions for the axial and radial magnification and for the energy resolving power are also derived. The analysis and the derived properties are compared with a numerical simulation and with all available published data.

Surface chemical shifts and photoelectron diffraction in CoSi2

An angle resolved study of the Si(2p) photoelectron line from bulk‐grown CoSi2(111), (100), and (110) surfaces has been performed using synchrotron radiation. A surface shifted component has been observed from annealed (111) and (100) faces, the surface/bulk intensity ratio being a strongly modulated function of polar emission angle and of photoelectron kinetic energy. Surface emission has been found to be significantly stronger from (111) faces than from (100) faces. We adopt a model in which the (100) face is terminated in a single Si layer, whereas we suggest that in the (111) surface, two Si layers exist above the first layer of Co atoms, thereby forming a Si (111)‐like template for the further growth of epitaxial Si. We consider that the interfacial Co atoms are sevenfold coordinated. Fractured CoSi2(111) surfaces are found to exhibit no surface Si.

Tuning the charge carriers in epitaxial graphene on SiC(0001) from electron to hole via molecular doping with C60F48

We demonstrate that the intrinsic electron doping of monolayer epitaxial graphene on SiC(0001) can be tuned in a controlled fashion to holes via molecular doping with the fluorinated fullerene C60F48. In situ angle-resolved photoemission is used to measure an upward shift of (0.6 ± 0.05) eV in the Dirac point from −0.43 eV to +0.17 eV relative to the Fermi level. The carrier density is observed to change from n ∼ (1 × 1013 ± 0.1 × 1013) cm−2 to p ∼ (2 × 1012 ± 1 × 1012) cm−2. We introduce a doping model employing Fermi-Dirac statistics which explicitly takes temperature and the highly correlated nature of molecular orbitals into account. The model describes the observed doping behaviour in our experiment and readily explains why net p-type doping was not achieved in a previous study [Coletti et al., Phys. Rev. B 81, 8 (2010)] which used tetrafluorotetra-cyanoquinodimethane (F4-TCNQ).

Studies in the growth of ZnSe on GaAs(001)

This paper reports a study of the molecular beam epitaxial (MBE) growth of ZnSe on GaAs substrates using elemental sources. Growth rates of ZnSe as a function of Se:Zn flux ratio for constant Zn flux were determined over a wider range of values than previously reported. Careful measurements of atomic fluxes and sample thickness lead to a determination of the sticking coefficients of Zn and Se which are at variance with many previously reported values. The temperature dependence of the sticking coefficients of Zn and Se has been measured carefully and provides evidence for a greater desorption of Zn from the growing surface than previously thought, an effect which persists at low growth temperatures. Measurements at high flux ratios support the use of a precursor model to describe MBE growth of ZnSe on GaAs substrates.

Photoemission studies of graphene on SiC: growth, interface, and electronic structure

The possibility to grow well ordered graphitic films on SiC(0001) surfaces with thicknesses down to a single graphene layer is promising for future applications. Photoelectron spectroscopy (PES) is a versatile technique for investigating a variety of fundamentals and technologically relevant properties of this system. We survey results from recent PES studies with a focus on the growth of graphene and few layer graphene, the electronic and structural properties of the interface to the SiC substrate, and the electronic structure of the films.

Ultraviolet photoelectron spectroscopy of the valence bands of solid NH3, H2O, CO2, SO2 and N2O4

A photoelectron study is presented for the outermost bands of the molecular solid phase of: NH3, H2O, CO2, SO2 and N2O4. Both gaseous and solid phase spectra were acquired with 40.81 eV ultraviolet photoelectron spectroscopy (UPS) techniques (except for gaseous N2O4). For the solids, charging effects were measured systematically and appropriate corrections made. Some valence band shifts were observed, as well as significant changes in valence band widths, between the gas and solid phase spectra, but the shifts were in general much less than those measured by previous workers. These results are interpreted in terms of molecular solid bonding and relaxation effects.