S. Mróz

Directional Auger electron spectroscopy (DAES) and directional elastic peak electron spectroscopy (DEPES) in the investigation of the crystalline structure of surface layers: the Ag/Cu(111) interface

Dependence of the Auger signal (directional Auger electron spectroscopy — DAES) and the elastically scattered electron intensity (directional elastic peak electron spectroscopy — DEPES) on the direction of the primary electron beam (E = 600–1500 eV) was measured using a retarding field analyser (LEED optics) for the Cu(111) face, both clean and covered with silver up to 12 ML. Well-developed maxima of DAES and DEPES signals appear when the primary beam is parallel to one of the close-packed rows of atoms in the sample surface layer, while the angular distribution of the emitted electrons is averaged over the large acceptance angle of the RFA and does not influence appreciably the DAES and DEPES profiles. From the positions of the maxima mentioned above the atomic structure of a few surface layers can be determined. The silver layer was found to be rotated for 60° with respect to the Cu(111) substrate.

High-energy electron diffraction on Cu(111) measured with low-energy Auger electrons: theory and experiment

Abstract Electrons with kinetic energies between 600 eV and 1.5 keV have been diffracted at the surface of a Cu(111) crystal. The intensity of the electron wavefield within the crystal has been measured using low-energy Cu M 2,3 VV Auger electrons integrated over a large solid angle. The results are well reproduced by single-scattering calculations. We show that this technique is chemically selective as well as sensitive to the crystal structure and that it is essentially the time reversal of X-ray photoelectron diffraction but in general more surface-sensitive than the latter. It is therefore well suited to study epitaxial growth and interdiffusion.

Determination of the inelastic mean free path of electrons in silver and copper by measurement and calculation of the elastic scattering coefficient

Abstract The inelastic mean free paths, λ , of electrons in polycrystalline Ag and Cu samples were determined for electron energies in the range 250–1500 eV by means of elastic peak electron spectroscopy (EPES). In this method the coefficient of elastic scattering of electrons, η e , is measured. η e is also calculated theoretically for different assumed values of λ . The value of λ giving the best agreement between the measured and calculated η e is taken as the experimental one. the η e measurements were performed for Ag and Cu samples cleaned by mechanical scraping only and for samples cleaned by ion bombardment in order to estimate the eventual influence of surface roughness on the η e values. Additionally, different methods to calculate η e theoretically are compared.

The interaction of gold with the surface of a cylindrical tungsten single crystal

Abstract Gold layers on the surface of a cylindrical tungsten single crystal were investigated by LEED and work function change measurements. Work function changes during gold adsorption on (110), (111), (112), (115) and (001) faces, and structures deduced from LEED patterns for (110), (112), (332) and (001) faces are presented with a common coverage scale. Upon heating the gold-covered crystal up to 1000 K, faceting occurs with faceted regions as large as 20°, 13° and 4° around the [112], [110] and [332] directions, respectively. The diffusion of the gold layer was studied by the movement of the sharp boundary across the cylindrical crystal surface at a temperature of 1000 K. The diffusion rate was found to be much higher in the (112) region than in the (001) region.

LEED studies of thermal vibrations of atoms on the (111) nickel surface by the alternating current method

Abstract Results of measuring the effective Debye temperature for the (111) nickel surface by LEED method are presented. The measurements of the thermal dependence of the diffracted beam intensity were performed using the alternating current method developed for this purpose in which the energy of the primary electron beam is modulated with a frequency ω around the maximal value of the diffracted beam intensity and the part of the spot microphotometer current with the frequency 2ω is measured. The values θ S = 275 K and θ B = 385 K were obtained for the surface and bulk Debye temperature, respectively. The latter is in good agreement with other experimental work in this field.

Properties of ultrathin silver films on the Ni(001) face

Adsorption and isothermal desorption of silver on the Ni(001) face were investigated using AES, LEED, directional elastic peak (DEPES), and directional Auger (DAES) electron spectroscopies. At room temperature the first silver layer was found to grow up to the coverage θ≈0.74−0.78 where silver islands start to grow on it. At higher temperatures (620 K, for instance) the first layer grows alone up to θ≈0.86−0.89. Next, the first and second layers grow simultaneously up to θ≈1.7 where the first layer is completed and a 3D growth begins. Silver layer consists of close packed Ag(111) islands. Among two possible epitaxial orientations and two stacking sequences possible for each orientation, only one orientation and one sequence are preferable because the silver islands start to grow from the steps at the substrate. The dependence of silver Auger peak height on time, obtained during isothermal desorption consist of a linear part and an exponential one corresponding to the desorption of the second and the first silver layer, respectively. From such dependences for different desorption temperatures binding energies of silver atoms in the second and the first silver layer (3.04 and 3.59 eV/atom, respectively) are calculated.

Temperature effects in directional elastic peak electron spectroscopy for the Cu(011) face: observation of the surface roughening transition

Abstract Polar profiles of directional elastic peak electron spectroscopy (DEPES) and directional Auger electron spectroscopy (DAES) were measured for the Cu(011) face in the vicinity of particular close-packed directions in the sample and for sample temperatures of 400 T C versus T curve, where C is the contrast: C = 2(I max −I max ) (I max + I max ) for a particular maximum in the polar profile, is composed of three linear parts. The slope of this particular part is different for the different maxima investigated but the breaks between the linear parts occur always at the temperatures T 1 ≈ 700 K and T 2 ≈ 1000 K. The former break is ascribed to the appearance of anharmonic thermal vibrations while the latter one indicates the surface roughening transition. DEPES was applied for the first time in the investigation of the surface roughening transition and it was found to be a very convenient and effective method. The surface roughening temperature T R ≈ 1000 K was found for the Cu(011) face, in an excellent agreement with theoretical predictions.

Determination of backscattering factor for quantitative Auger analysis

Abstract The backscattering factor R is necessary in quantitative Auger analysis with the matrix effects corrections. The most popular method of determination of this factor is based on the Monte Carlo simulation of electron trajectories in a solid sample with the use of theoretically calculated cross-sections for the scattering of those electrons on the sample atoms [1]. The cross-sections mentioned above are still not known firmly especially in the cases of low energies and high atomic numbers. In the present work we used the modified Wang method [2]. The energy distribution of the secondary electrons for particular energies of primary electrons is used for the backscattering factor determination, instead of the Monte Carlo calculations. Results for copper, silver, gold and three alloys of these metals are presented for primary electron energies of 1000–2000 eV and compared with the results known from the literature. The best agreement is obtained between our results and those presented by Love [3] and Shimizu [4] for higher ionization energies and by Jabtohski [5] for lower energies.

On the mechanism of contrast creation in directional Auger electron spectroscopy (DAES)

Abstract In (DAES), maxima in the dependence of the Auger signal from a crystalline sample on the primary electron beam incidence angle are used for determination of a sample crystalline structure. To determine a mechanism responsible for creation of those maxima, azimuth profiles in the vicinity of the ( 1 1 1 ) plane for the elastic peak height, current of backscattered (with different ΔE) electrons, and Auger Cu M2,3VV peak height were measured (using an RFA) at different incidence angles for the Cu(011) face. The highest contrasts of the elastic peak were found for incidence angles corresponding to the [011] and [121] directions. However, about two times smaller contrast was still found for other incidence angles when the primary beam pass only the ( 1 1 1 ) plane, and no close-packed direction. This indicates that channeling of primary electrons on close-packed planes plays an important role in the creation of the elastic peak contrast. For backscattered (ΔE > 300 eV) electrons and for the Auger peak, the contrast corresponding to the cross of the ( 1 1 1 ) plane was 2–3 times smaller than that for the elastic peak. This indicates that the Auger electrons excited by backscattered electrons are mainly responsible for the contrast of the low-energy Auger peak. Thus, information on the crystalline structure given by low-energy DAES concerns in an appreciable part the sample layer at the dept comparable with the extinction distance of primary beam electrons. It was shown for Ag MNN DAES profiles obtained for silver monolayer on the Cu(001) face that maxima concerned with the substrate structure vanished if the primary beam energy is as low as 600 eV. Thus, information given by DAES concerns only the first atomic layers in this case.

AES investigations of growth of very thin silver films deposited on the copper and nickel (110) faces

Growth of very thin silver films deposited under UHV conditions on the Ni(110) and Cu(110) faces was studied with the Auger Electron Spectroscopy. Silver 360eV and nickel 61eV Auger peaks kinetics show the Stranski-Krastanov growth mechanism. Two stages of the first ML growth at room temperature was observed. During annealing of 2 ML layer (the rate of temperature increase of 4 K/s) the silver Auger peak increases by about 10% atT ≈ 620 K. This indicates that reconstruction of silver islands takes place. The first monolayer desorbs atT=840 K. The Cu-Ag system was studied with silver 360eV and copper 62eV Auger peaks. The adsorption kinetics, up to 440 K, show the Stranski-Krastanov growth mechanism. After 1 ML deposition, the increase of the substrate temperature leads to the slope decrease of the adsorption kinetics. Adsorption kinetics forT=560–620 K show that after 1.5 ML deposition the silver dissolution occurs and the surface alloy is formed. Above 770 K the shape of kinetics drastically changes due to the decrease of a sticking coefficient.