H. Khemliche

Grazing incidence fast atom diffraction: An innovative approach to surface structure analysis

An alternative diffraction technique, based on grazing incidence scattering of high energy atoms, is applied to surface structure determination of crystalline surfaces. This technique, named GIFAD for grazing incidence fast atom diffraction, uses the same geometry as reflection high energy electron diffraction but is less invasive, more surface sensitive, and readily interpretable quantitatively. We present here a demonstration of this approach on a prototypical II–VI compound, ZnSe(001). Besides providing lattice parameter with high accuracy, we show that GIFAD gives straightforward access to the surface valence electron density profile, allowing clear identification of an electron transfer from Zn to Se.

Determination of the geometric corrugation of graphene on SiC(0001) by grazing incidence fast atom diffraction

We present a grazing incidence fast atom diffraction (GIFAD) study of monolayer graphene on 6H-SiC(0001). This system shows a Moire-like 13 × 13 superlattice above the reconstructed carbon buffer layer. The averaging property of GIFAD results in electronic and geometric corrugations that are well decoupled; the graphene honeycomb corrugation is only observed with the incident beam parallel to the zigzag direction while the geometric corrugation arising from the superlattice is revealed along the armchair direction. Full-quantum calculations of the diffraction patterns show the very high GIFAD sensitivity to the amplitude of the surface corrugation. The best agreement between the calculated and measured diffraction intensities yields a corrugation height of 0.27 ± 0.03 A.

Dynamic grazing incidence fast atom diffraction during molecular beam epitaxial growth of GaAs

A Grazing Incidence Fast Atom Diffraction (GIFAD) system has been mounted on a commercial molecular beam epitaxy chamber and used to monitor GaAs growth in real-time. In contrast to the conventionally used Reflection High Energy Electron Diffraction, all the GIFAD diffraction orders oscillate in phase, with the change in intensity related to diffuse scattering at step edges. We show that the scattered intensity integrated over the Laue circle is a robust method to monitor the periodic change in surface roughness during layer-by-layer growth, with oscillation phase and amplitude independent of incidence angle and crystal orientation. When there is a change in surface reconstruction at the start of growth, GIFAD intensity oscillations show that there is a corresponding delay in the onset of layer-by-layer growth. In addition, changes in the relative intensity of different diffraction orders have been observed during growth showing that GIFAD has the potential to provide insight into the preferential adatom attachment sites on the surface reconstruction during growth.

High resolution imaging of superficial mosaicity in single crystals using grazing incidence fast atom diffraction.

A new table top technique is used to simultaneously analyze the local morphology of crystalline surfaces as well as the misalignment of large scale domains at the topmost surface layer. The approach is based on fast atom diffraction at grazing incidence (GIFAD); the diffraction pattern yields the structural characteristics and the topology of the surface electronic density with atomic resolution. If superficial mosaicity is present, diffraction patterns arising from each mosaic domain can be distinguished, providing high sensitivity to the properties of each of the domains. Taking NaCl(001) as an example, we observe a discrete tilt angle distribution of the mosaic domains following an arithmetic progression with a 0.025° ± 0.005° difference; a twist mosaic angle of 0.09° ± 0.01° is also observed.

Surface exciton population in proton impact with LiF(1 0 0)

Abstract We have recently shown that the large energy loss experienced by slow protons interacting with a LiF surface is primarily caused by the population of surface excitons [11]. These states are most probably populated by electron transfer from H − ions formed at halogen sites, whereas secondary electron emission results from direct detachment of the negative ions in collisions with halogen sites. We report here on the impact energy dependence of the charge-state of the scattered projectile, its energy loss and the associated electron yield. From these complete measurements, the transfer probability to the surface excitons and its dependence upon impact energy is derived in the range 0.6–10 keV.

Grazing collision of keV protons on LiF correlation between energy loss and electron emission

AbstractGrazing interaction of low energy protons with LiF(1 0 0) is studied using a new coincidence technique combiningenergy loss, charge state analysis and electron spectroscopy. Correlation between the scattered projectile energy loss andthe number of emitted electrons points to an energy loss mechanism not leading to electron emission. Detailed analysisof the energy loss spectra, which show well-resolved structures, suggests that this mechanism corresponds to thepopulation of surface excitons. Moreover the correlation between the projectile final charge state and the number ofemitted electrons sheds new light on the major role played by the negative ion at the surface. Electron removal from thevalence band proceeds mainly through formation of H y at halogen sites, whereas electron emission results from thedetachment of these negative ions at subsequent F y sites. O 1999 Elsevier Science B.V. All rights reserved. 1. IntroductionIn collisions of ions on metals, the processesresponsible for projectile energy loss and electronemission are now well understood. Excitation ofconduction electrons from the Fermi level to theneighboring empty levels of the conduction bandexplains the large stopping power of all metals. Onthe other hand, the excitation of conduction elec-tron to the vacuum requires few eV and explainsthe appearance of a threshold in the ion velocitythat depends on the metal work function. Re-cently, investigations of energy loss and electronemission have been extended to large band gapinsulators. Both energy loss and electron emissionshould exhibit a threshold behavior at relativelyhigh collision energy since the band gap has to beovercome for every electronic excitation. In thatrespect, it is di†cult to understand the sustainedsecondary electron emission [1] in H

Discharging dynamics of insulator surfaces irradiated by highly charged ions

Guiding and focusing of keV to MeV ions by insulator micro-capillaries offer exciting perspectives for the production of low divergence micro-sized beams and the spatial control over the irradiated zone. These effects result from the local charging of the capillary inner wall and depend strongly on the charging/discharging dynamics of the insulating material. This dynamics has been studied on various glass insulator surfaces (borosilicate, fused silica and quartz) by grazing incidence highly charged ion beams. We propose simple experimental methods to derive the relevant time constant and study the influence of temperature and different material properties to the charging process.

Transmission of ions through a tapered-glass capillary: Imaging the dynamics of the charging-up process

Transmission of 230 keV Xe23+ through a single macroscopic glass capillary with micrometric outlet is observed with a position sensitive detector. During the charging-up process of the capillary, we notice oscillations of the transmitted beam position. We ascribed this phenomenon to a sequential creation of charge patches along the capillary.