P. Czuba

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.

Sputtering of alkali halides studied by a mass selected time of flight spectroscopy

Abstract Energy distributions of alkali and halogen atoms sputtered with a low-energy electron beam have been measured for NaCl, CsI and KBr single crystals. Energy spectra for alkali atoms can be described by the Maxwell-Boltzmann distribution. In case of halogen atoms, for some crystals, an additional, non-thermal component appears. The behaviour of this component was investigated as a function of the target temperature, the beam energy, its current density and the ejection angle. The results can be described by creation of focused replacement sequences and H-centre migration model. The ejection of non-thermal halogen atoms from the (100) surface of KBr single crystal is strongly forward peaked along the normal. This effect is caused by a thin damage layer appearing on the surface due to a strong nonstoichiometry of the erosion process itself.

Atomic excitation in electron- and ion-induced sputtering of alkali halides*

In this work we present a systematic study of the atomic excitation processes stimulated either by ion or electron bombardment of the NaCl- or CsCl-type halides. Spectra and intensities of the discrete lines emitted by sputtered alkali atoms have been measured as a function of the beam mass, its energy, the target temperature and the distance from the surface. In particular, we have found that the radiation emitted by sputtered alkali atoms extends to surprisingly large distances from the electron bombarded surface of alkali halides. The results are discussed in terms of the relevant sputtering mechanisms responsible for alkali halide erosion. It is suggested that the atomic excitation of ion sputtered particles takes place at the surface in the final collision of the cascade, while most of the observed radiation in the case of electron bombardment originates in gas-phase collisions above 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.

Thermally assisted desorption processes in electron bombarded alkali halides

Abstract The desorption of alkali and halogen atoms induced by the interaction of energetic electrons with surfaces of alkali halide crystals has been studied by means of an angular-resolved and mass-selected time-of-flight spectroscopy. It has been found that a considerable fraction of halogen atoms was ejected with hyperthermal energies of the order of 0.1 eV. However, alkali atoms and the remaining part of halogen emission had thermal (Maxwellian) spectra of kinetic energies. In this paper we will report on systematic investigations of these thermal desorption processes for single crystal (100) NaCl, KCl, KBr, RbBr, and Kl surfaces. The relative yield of the thermal component has been measured as a function of electron beam energy and beam current density at various sample temperatures. It will be shown that thermal halogen emission can be explained by thermally assisted diffusion of interstitial halogen atoms produced in the bulk of the crystal from decaying self-trapped excitons. The origin of the alkali atom component will be described as due to neutralization and subsequent thermal evaporation of excess alkali atoms from the halogen deficient surface.

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...