W. Schattke

DIFFRACTION AND HOLOGRAPHY WITH PHOTOELECTRONS AND FLUORESCENT X-RAYS

Abstract We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emission of fluorescent x-rays. The current status of photoelectron diffraction studies of surfaces, interfaces, and other nanostructures is first briefly reviewed, and then several recent developments and proposals for future areas of application are discussed. The application of full-solid-angle diffraction data, together with simultaneous characterization by low energy electron diffraction and scanning tunneling microscopy, to the epitaxial growth of oxides and metals is considered. Several new avenues that are being opened up by third-generation synchrotron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possibility of time-resolved measurements of surface reactions with chemical-state resolution, and circular dichroism in photoelectron angular distributions from both non-magnetic and magnetic systems. The addition of spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the measurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or circular-polarized excitation of spin-orbit-split levels. The direct imaging of short-range atomic structure by both photoelectron holography and two distinct types of x-ray holography involving fluorescent emission is also discussed. Both photoelectron and x-ray holography have demonstrated the ability to directly determine at least approximate atomic structures in three dimensions. Photoelectron holography with spin resolution may make it possible also to study short-range magnetic order in a holographic fashion. Although much more recent in its first experimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, including both surface and bulk regions.

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)

A subroutine package for computing Green's functions of relaxed surfaces by the renormalization method

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.

Impact ionization rate and high-field transport in ZnS with nonlocal band structure

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}

Unoccupied band structure ofNbSe2by very low-energy electron diffraction: Experiment and theory

Abstract We present a subroutine package for the computation of Greens functions of relaxed surfaces and the bulk within the framework of a tight-binding basis. The application of a highly convergent renormalization scheme allows treatment of large unit cells and may include spin-orbit interaction in a suitable parametrization. The onedimensional semi-infinite solid is solved analytically and serves as a testing example.

PHOTOEMISSION WITHIN AND BEYOND THE ONE-STEP MODEL

The impact ionization rate in ZnS is calculated using a nonlocal empirical pseudopotential band structure and compared to previous results using a local calculation. The two resulting rates are then compared and simple fit formulas are presented. These are included in an ensemble Monte Carlo simulation of electron transport in bulk ZnS. The calculated impact ionization rate is then compared to experimental impact ionization coefficient data; reasonable agreement between the experimental data and the calculated impact ionization rate is obtained with an appropriate choice of optical deformation potentials.

Hole initiated impact ionization in wide band gap semiconductors

A combined experimental and theoretical study of very low-energy electron diffraction at the (0001) surface of

Electronic band structure of gallium nitride: a comparative angle-resolved photoemission study of single crystals and thin films

{2\mathrm{H}\ensuremath{-}\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_{2}

PHOTOELECTRON DIFFRACTION: SPACE, TIME, AND SPIN DEPENDENCE OF SURFACE STRUCTURES

is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with