P. Piatkowski

Electron stimulated desorption of neutral species from (100) KCl surfaces

Abstract Composition changes of a (100) KCl surface bombarded by 1 keV electrons have been studied by Auger electron spectroscopy. Intensity ratios of characteristic alkali and halogen Auger lines were monitored as a function of target temperature and beam current density. In addition, for the first time angle-resolved energy distributions of electron desorbed K and Cl atoms were measured using mass-analyzed time of flight techniques. For temperatures higher than about 100°C, a near-stoichiometric surface composition was obtained and a significant non-thermal component was observed in the kinetic energy distributions of Cl atoms emitted normal to the (100) surface. These results can be interpreted in terms of new concepts regarding the excitonic mechanism of electron stimulated desorption (ESD).

Electron-stimulated desorption from ionic crystal surfaces

Abstract Inelastic interactions of electrons with surfaces of ionic crystals result in emission of various particles such as ions, atoms and molecules. We will review such electron-stimulated desorption processes for the particular class of ionic crystals, namely for alkali halides. In this case, a dominant fraction of the emission is in the form of halogen and alkali atoms characterized by a thermal (Maxwellian) spectrum of translational energies. For several alkali halides (potassium and rubidium chlorides, bromides, and iodides), however, a significant part of the halogen atoms is ejected with nonthermal energies, i.e. energies of the order of 0.1 eV. The results of recent systematic studies of angular-resolved kinetic energy distributions of the emitted particles will be reported and current views on the electronic mechanisms of desorption will be described. In particular, it will be shown that the ESD mechanism can be understood in terms of the model involving a surface localisation of the so called “hot-holes” created by electron bombardment of alkali halides. A role of hot holes in ESD processes will further be discussed in relation to very recent experimental results obtained for the KBr crystals doped with In impurities which act as efficient hole traps.

Frenkel defect interactions at surfaces of irradiated alkali halides studied by non-contact atomic-force microscopy

Irradiation of alkali halide crystal leads to production of Frenkel defects in the crystal bulk. Subsequent diffusion and interactions of these defects with the surface results in desorption processes at the surface. We have studied surface topography of electron bombarded alkali halide crystals (KBr, NaCI) and the desorption fluxes. It is found that the desorption proceeds in a layer-by-layer mode by growth and linking of pits of monoatomic depth, which results in modulation of surface step density. Electron-stimulated desorption fluxes are correlated with the surface step density. Based on these results it is concluded that H-centers decay at the (100) surface plane with the emission of halogen atoms, F * -centers recombine with the terrace edge and initiate emission of alkali atoms, and F-centers accumulate in the sub-surface region. Above certain temperature (∼450 K for KBr, 370 K for NaCI) the desorption proceeds in more complex, multilayer mode as a result of combination of the above described Frenkel defect mediated mechanism with simultaneous thermal restructuring of the surface.

Secondary ion production by electron and ion bombardment of alkali halides

Abstract We have investigated desorption of positive and negative ions for electron and ion bombarded NaCl single crystals. Our experiments include the first systematic measurements of electron-stimulated desorption of negative ions from an electron irradiated (100) surface of NaCl. Measurements of the mass distributions and relative yields of emitted particles indicate that during electron bombardment, positive ions are formed by gas-phase ionization of neutral atoms and molecules by primary and secondary electrons. In contrast, electron desorbed negative ions are created by direct emission of Cl− from the surface due to electronic processes. A mechanism of ion formation during keV, heavy ion bombardment is discussed in terms of an Auger process and/or direct emission of lattice ions from a collision cascade.

Temperature dependent dynamic ESD processes in alkali halides

Abstract The effect of the sample temperature on angular-resolved kinetic-energy distributions of alkali and halogen atoms, electronically desorbed from single crystal alkali halides, has been measured. It was found that while the emission of particles with thermal energies increased by about a factor of 40 in the temperature range 90–300°C, the nonthermal halogen atom intensity decreased by about a factor of 3. From these temperature dependent measurements the activation energies for thermally assisted defect migration processes have been estimated. The results will be compared with the data available in the literature and the predictions of a recently proposed model for electron-stimulated desorption (ESD) of alkali halides.

Structure and electronic properties of ionic nano-layers MBE-grown on III–V semiconductors

Thin epitaxial NaCl and KBr layers of various thickness (from 3 to 100 monolayers) have been deposited on (100) surfaces of GaAs and InSb semiconductors by means of molecular beam epitaxy (MBE). Electronic and structural properties of the freshly prepared films were subsequently investigated in the attached UHV analytical chamber by means of low energy electron diffraction, low electron energy loss spectroscopy, Auger electron spectroscopy and an electron holography. Alkali halide growth mode was found to be a two-dimensional layer-by-layer type (Franck van der Merve growth mode). It was possible to demonstrate that the first monolayer of alkali halide on the AIIIBV semiconductor is arranged by the strong bond formed between the halogen ion and the AIII metallic element. In case of NaCl/GaAs(100) system a local atomic configuration was found for an early stage of epitaxy (three to five monolayers) by means of the electron holography. A detailed analysis of the reconstructed diffraction patterns revealed that an initial formation of the Cl-Ga bond occurred in the system, and after deposition of five monolayers NaCl, the substrate was uniformly covered by a layer at least three monolayers thick. q 2000 Elsevier Science S.A. All rights reserved.

A comparison of electron-stimulated desorption of halogen atoms from different alkali-halide single-crystals

Abstract Kinetic energy distributions of halogen atoms emitted due to electron-stimulated desorption (ESD) were measured for a (100) surface of NaCl-type alkali-halide crystals (NaCl, NaI, NaF, KCl, KBr, KI, RbCI, RbBr, RbI) under the same experimental conditions. It was found that energy spectra of halogen atoms emitted from potassium and rubidium chlorides, bromides, and iodides, kept at temperatures below 250°C, consist of two components: the thermal (Maxwellian) peak and the higher energy peak (at about 0.25 eV) whose energy is temperature-independent. From the energy spectra and complementary total yield measurements relative desorption rates for both thermal and nonthermal components were determined for all investigated samples. The results are compared with predictions of the Rabin-Klick criterion and the self-trapped exciton relaxation energy calculations by Song et al. Finally, it is shown that the hyperthermal halogen emission could be well correlated with a measure of the excess space between t...

Electron-stimulated desorption of thin epitaxial films of KBr grown on (100)InSb

Abstract We have measured time-of-flight (TOF) distributions of Br atoms desorbed from thin (less than 1000 A) epitaxial films of KBr on (100) InSb with a 2 keV electron beam. Although the general structure of the TOF spectra was similar to that obtained previously for the thick crystals, both the fast and the slow (thermal) components of the distribution were strongly dependent on the film thickness. We argue that this dependence is due to two different diffusion processes involved in the transport of the primary excitation products from the bulk to the surface. By measuring the velocity resolved ESD yield for films of various thicknesses, we determined that a diffusion length of the carriers responsible for the thermal ESD component varied from 30 to 700 A with temperature in the range 20–300°C. In contrast, for the non-thermal desorption we found the carrier diffusion length of about 140 A which did not depend significantly on the temperature.

ELECTRON-STIMULATED POSITIVE ION EMISSION FROM NACL CRYSTAL SURFACE

The energy and the mass distributions of positive ions emitted from the surface of ultrathin epitaxial NaCl films under 1–5 keV electron beam have been measured. Two peaks in the energy spectra have been found. The low energy one, composed mostly of Na+, Cl+ and Cl2+ ions, is temperature dependent, and scales quadratically with the electron current. This peak clearly corresponds to the gas-phase ionisation of neutral atoms. The second, with a maximum around 1 eV, is composed exclusively of Na+ ions, does not depend significantly on the crystal temperature, and scales linearly with the electron current. This last peak seems to be due to ions ejected from the crystal surface as a result of the Coulomb instability. Depending on the crystal temperature and the electron current density either the first or the second component may become dominating, explaining the discrepancies in results obtained so far by different authors.

Photon stimulated desorption from alkali halide surfaces at near threshold energies

Abstract Desorption processes induced in alkali halides by photons of synchrotron radiation (of energy 5–20 eV) have been investigated by means of quadrupole mass spectroscopy. The relative yields and angular characteristics of alkali and halogen atoms emitted from (100) surfaces of KI and RbI crystals were measured as a function of the photon energy and the sample temperature. For comparison, analogous measurements were performed with a 1 keV electron beam. In the latter case the relevant desorption processes are already better understood. We have found that both the fundamental exciton and the fundamental gap excitations result in alkali and halogen desorption with the same angular distributions. This leads to the conclusion that dynamic desorption processes, pronounced for electron excitation, are inefficient during desorption stimulated by photons with near threshold energies. It is argued that such a difference can be understood by taking into account different selection rules, and differences in excitation depth and excitation density distribution for photon- and electron-induced processes.