C. Solterbeck

Valence-band photoemission from the GaN(0001) surface

A detailed investigation by one-step photoemission calculations of the

ENERGETIC AND SPATIAL BONDING PROPERTIES FROM ANGULAR DISTRIBUTIONS OF ULTRAVIOLET PHOTOELECTRONS : APPLICATION TO THE GAAS(110) SURFACE

\mathrm{GaN}(0001)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)

How to Determine Fermi Vectors by Angle-Resolved Photoemission

surface in comparison with recent experiments is presented in order to clarify its structural properties and electronic structure. The discussion of normal and off-normal spectra, reveals through the identified surface states, clear fingerprints for the applicability of a surface model proposed by Smith et al. [Phys. Rev. Lett. 79, 3934 (1997)]. Especially the predicted metallic bonds are confirmed. In the context of direct transitions, the calculated spectra allow us to determine the valence-band width and to argue in favor of one of two theoretical bulk band structures. Furthermore, a commonly used experimental method to fix the valence-band maximum is critically tested.

OPTICAL POTENTIAL AND ESCAPE DEPTH FOR ELECTRON SCATTERING AT VERY LOW ENERGIES

Angle-resolved ultraviolet photoemission spectra are interpreted by combining the energetics and spatial properties of the contributing states. One-step calculations are in excellent agreement with new azimuthal experimental data for GaAs(110). Strong variations caused by the dispersion of the surface bands permit an accurate mapping of the electronic structure. The delocalization of the valence states is discussed analogous to photoelectron diffraction. The spatial origin of the electrons is determined, and found to be strongly energy dependent, with uv excitation probing the bonding region. {copyright} {ital 1997} {ital The American Physical Society}

Observation of circular dichroism in angular distribution of the valence band photoelectron from Si(001) in UPS region

Angle resolved photoemission spectroscopy (ARPES) has been commonly applied to evaluate the shape of Fermi surfaces by employing simple criteria for the determination of the Fermi vector k_F parallel to the surface such as maximum photoemission intensity at the Fermi level or discontinuity in the momentum distribution function. Here we show that these criteria may lead to large uncertainties in particular for narrow band systems. We develop a reliable method for the determination of Fermi vectors employing high resolution ARPES at different temperatures. The relevance and accuracy of the method is discussed on data of the quasi two dimensional system TiTe_2.

Estimation of photoemission intensities and the surface photoelectric effect

Abstract Electron spectroscopy at low kinetic energies, e.g., valence band photoemission with vacuum ultraviolet light, is sensitive to the fine structure of the electron damping, i.e., to the magnitude and energy dependence of the optical potential. The basis of this quantity is usually given by a semiquantitative analytical derivation together with empirical findings from the escape depth. Only recently the optical potential has been determined ab-initio via calculating the self-energy. Here, this approach is used to calculate the self-energy and from this the wave functions in the conduction band regime with scattering boundary conditions for the surface system. For the former, the GW approximation is applied. For the latter, algebraic solvers in the Laue representation have been used to solve the Schrodinger equation for arbitrary potentials. The wave function is investigated to extract physical quantities, like the angle and energy dependent escape depth, which are significant in discussing electron scattering.

Electron-scattering states at solid surfaces calculated with realistic potentials

Abstract Circular dichroism in angular distribution was observed for the valence band photoelectron from nonmagnetic and nonchiral material Si(001) with the incident photon energy in UPS region. The obtained two-dimensional angular distributions are to a certain extent consistent with an analysis of simple electronic dipole transition. Calculated angular distributions based on highly accurate one-step model showed better agreement considering photoelectron diffraction effect.

Full-hemisphere valence band photoemission spectra calculated for the ideal Si(001) surface

Abstract Going beyond a simple Fresnel like correction, surface-state peaks in normal emission spectra offer a sensitive probe to detect the screening of the light wave by the surface of the solid below the plasma frequency. The background resulting from distant bulk peaks should vary rather slowly with photon energy as compared with the intensity of the surface state. Above the plasma frequency the surface sensitivity of screening vanishes and bulk-plasmon excitation dominates the dependence on photon energy. The photocurrent is estiamted here by approximations to the matrix elements between the initial and final photoemission states. In the case of surface states the two-density-step hydrodynamic model is applied for the electrodynamic response. It has proven to yield similar results as the selfconsistent calculation on one hand and it is well suited by geometrical aspects on the other. The electronic density distribution of a surface state may be regarded as an overlayer sheet representing the lower density step of that model. The behavior of the emission from dangling bond states of semiconductors dependent on photon frequency is discussed for frequencies below and above the plasma frequency.

A theoretical investigation of photoemission spectra from (GaAs)2(AlAs)2 superlattices

Scattering states with LEED asymptotics are calculated for a general non-muffin tin potential, as e.g. for a pseudopotential with a suitable barrier and image potential part. The latter applies especially to the case of low lying conduction bands. The wave function is described with a reciprocal lattice representation parallel to the surface and a discretization of the real space perpendicular to the surface. The Schroedinger equation leads to a system of linear one-dimensional equations. The asymptotic boundary value problem is confined via the quantum transmitting boundary method to a finite interval. The solutions are obtained basing on a multigrid technique which yields a fast and reliable algorithm. The influence of the boundary conditions, the accuracy and the rate of convergence with several solvers are discussed. The resulting charge densities are investigated.

Scattering states for very low energy electron spectroscopy

Angle resolved photoemission spectra from the Si(001) surface have been calculated within the highly accurate one-step model. These spin integrated constant final state spectra for the valence bands have been investigated as a function of the emission direction of electrons. Especially, differences between right and left circularly polarized light in the spectra have been considered. A strong circular dichroism is found depending on the orbital composition of the contributing initial states. From various photoelectron patterns over the full 2π hemisphere an example for a p orbital dominated binding energy is presented and discussed.