T.E. Gallon

A model for the Auger electron spectroscopy of systems exhibiting layer growth, and its application to the deposition of silver on nickel

The dependence of Auger intensities on deposition time for deposits which grow in layer-by-layer fashion is analysed in terms of a simple model. It is shown that it is possible in principle to calibrate the Auger signals as a function of thickness absolutely with no prior assumption of values for sticking coefficients. The extent to which theory and experimental data for the deposition of silver on nickel are in agreement is examined, and values deduced for sticking coefficients and escape depths of Auger electrons in silver. It is also found that silver grows epitaxially on nickel at room temperature, and that silver/nickel bicrystals do not alloy after heat treatment.

The (100) surfaces of alkali halides: II. Electron stimulated dissociation

Abstract This is the second in a set of three papers reporting observations by low energy electron diffraction, Auger emission spectroscopy and quadrupole mass spectrometry of the structure of the (100) surfaces of the alkali halides LiF, KCl, NaCl, NaF and also of MgO. This paper reports observations of the electron stimulated dissociation of these materials as a function of primary electron beam energy and intensity. After removal from the UHV system optical absorption measurements reveal the presence of M and F bands in the damaged surfaces. The amount of absorption in these bands depends upon the material and the temperature during electron bombardment.

The (100) surfaces of alkali halides: I. The air and vacuum cleaved surfaces

Abstract This paper is the first of a series of three which report the results of studies on the (100) surfaces of LiF, NaF and KCl. The air cleaved and vacuum cleaved surfaces were examined by LEED and Auger emission spectroscopy and the partial pressures of the constituent elements were measured on cleavage of the specimen. Auger analysis of the air cleaved surfaces of LiF and NaF showed the presence of carbon and oxygen, and oxygen was detected on the air cleaved surface of KCl. These impurities could not be detected on the vacuum cleaved surfaces of NaF and KCl but were still observed on the vacuum cleaved surface of LiF. Mass spectrometer studies showed dissociation occurred on cleavage for all three alkali halides although different in character in each case. The form of the partial pressure curves suggests that this dissociation may be non-stoichiometric. LEED patterns obtained from the vacuum cleaved surfaces were of much better quality than those from the air cleaved surfaces except for the case of KCl which gave a poor LEED pattern directly after cleavage but which improved with time.

The electron energy loss spectra of some alkali halides in the band gap region

The EELS of a number of alkali metal fluorides, chlorides and bromides have been measured with primary electron beams in the range 50 eV to 1.0 keV. Both vacuum cleaved single crystal and vacuum evaporated thin films were examined and no significant differences in the loss spectra were observed. Data were recorded in the N(E) form using a conventional thermionic CRT gun to excite the spectra. In all cases several peaks were observed in the band gap region of the loss spectra. Measurement of these peaks as functions of primary beam energy and specimen temperature enabled two different tyupes of peak to be identified. One group of peaks was induced by the electron beam irradiation. Some of these beam induced peaks could be identified as arising from surface and volume plasmon excitation in free alkali metal but other beam induced peaks were observed which could not be correlated either with electron energy loss processes in free metal or with the excitation energies of the common colour centres (F, R and M) in the alkali halides. All the beam induced peaks vanished at high temperatures. Each of the alkali halide loss spectra recorded showed a loss peak towards the upper end of the banrd gap region which was independent of irradiation time and specimen temperature. The energy of this peak varied from 3.1 eV below the bulk excitation at the band edge for LiF to a value so close to the bulk exciton that the surface peak could not be clearly resolved in the case of NaBr and KBr. This peak has tentatively been identified as arising from a surface exciton and it may provide a measure of the reduction of the band gap at the surface of the alkali halides.

Quasi-atomic fine structure in the Auger spectra of solid silver and indium

Abstract High resolution M 4 , 5 N 4,5 N 4,5 Auger spectra of Ag and In reveal fine structure, which may be interpreted in terms of spin-orbit splitting of the initial state and multiplet structure in the final state. Interaction between d holes in Ag is an important factor in determining the shape of the Auger spectrum, and so the Auger profile is not related in any simple way to the one-particle density of states.

Auger emission and colour centres in lithium fluoride

Abstract Low energy Auger transitions from lithium fluoride, below 50 eV, have been associated by Gallon and Matthew [Phys. Status Solidi 41 (1970) 343] with transition processes between an ionised lithium ion and neighbouring fluorine ions. Two of these transitions, giving rise to lines at 36 eV and 43 eV were thought to involve de-excitation via colour centre defects. The growth behaviour of these lines has been investigated as a function of temperature, between 200°K and 600°K, and correlations have been established between this behaviour and optical absorption data obtained in situ. The effects of deposited nickel films on the Auger spectrum have also been investigated, and it is concluded that the 43 eV transition arises from lithium metal produced by electron beam dissociation of the lithium fluoride while the 36 eV line does indeed arise from de-excitation via a defect centre.

The (100) surfaces of alkali halides: III. Electron stimulated desorption and overall discussion

Abstract The electron stimulated desorption of common gaseous materials from the (100) surfaces of LiF, KCl, NaF and MgO is reported. Measurements are given of the desorption rate as a function of primary electron beam energy and intensity. Most of the results are interpreted in terms of a simple model in which damage of the surface by the incident electrons allows gaseous impurities trapped in the bulk of the crystal to diffuse to the surface. The results of this set of three papers on the surfaces of alkali halides are discussed in relation to the mechanisms of epitaxial overgrowth. It is concluded that simple experiments on alkali halides are often unreliable in that the surface structure and stoichiometry depend upon a number of generally uncontrolled parameters. M and F centres are suggested as a pair of potential nucleation sites for condensing metal vapour.

Solid state broadening in Auger spectra

Abstract Some Auger spectra in the gas phase show extremely sharp lines and a great deal of fine structure while the Auger spectra of solids, even at high resolution, lack this detailed fine structure and generally have much broader peaks. In an effort to understand the broadening processes which occur on condensation of a gas or vapour to the solid phase, several solids for which high resolution gas phase data exists have been studied. The result of measurements of the M 45 N 45 N 45 Auger spectrum of cadmium are reported and comparison is made with the vapour phase data. Although the solid data contains considerable quasi-atomic fine structure the lines are broader than the vapour phase and by matching computer broadened vapour phase data to the experimental data an estimate of the individual line widths may be made. The relative intensities of the lines in the simulation agree closely with the solid data if loss processes are considered. Measurements have also been made on the same Cd transition in CdS and in this material the Auger lines are much broader than in Cd metal and the fine structure has almost disappeared. Results are also reported of measurements of the Auger spectra of solid rare gases made with the aid of a UHV cryostat capable of operating at temperatures down to 10 K. The gases studied were Xe, Ar and Kr and again comparison with computer broadened gas data enable the broadening of individual Auger lines to be estimated. Measurements of the line breadths have been made at various temperatures and the results of these experiments will be discussed in terms of lifetime and lattice vibrational broadening.

The electron energy loss spectra of some alkaline earth oxides

The electron energy loss spectra of CaO, SrO and BaO have been measured in the range 0–50 eV for primary energies between 100–1300 eV. For CaO comparison with −Im(lϵ) calculated from optical data shows good agreement except in the region 11–15 eV. The electron energy loss spectra of SrO and BaO show great similarity to the spectrum of CaO. Two surface loss peaks are present in the bulk band gap of CaO and SrO, with a third present at the band edge having a surface contribution. The lowest energy loss peak has been shown to be induced by the electron beam. Barium oxide under electron bombardment differs in the respect that an additional loss peak is produced above the band gap possibly due to the liberation of free barium metal.

The electron energy loss spectrum of tin at low primary beam energies

Abstract The electron energy loss spectra of clean and oxidised tin have been measured for primary energies in the range 100–1000 eV. The structure found for clean tin is similar to published spectra recorded with low primary energies except for the presence of a small peak at 4.7 eV. Differences between the reported volume plasmon energy recorded with low and high primary beam energies were noted and a model is presented to explain these differences in terms of plasmon dispersion.