Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N⁺(E) has been separated into an ionization coefficient R⁺(E) and a total energy distribution, N(E), i.e. N⁺(E) = R⁺(E) N(E). Experimentally, the dependence of R⁺(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.     

Rates of desorption from solid surfaces: Coverage dependence

The recently developed Classical Stochastic Diffusion Theory is applied to obtain the coverage dependence of desorption rates for Xe on W(111). Using the attractive Xe-Xe potential from gas phase experiments, we find a strong coverage dependence in the desorption rates and calculate Temperature Programmed Desorption Spectra (for a potential with reduced attractiveness) that are in excellent qualitative agreement with experimental results. We also investigated the effect of purely repulsive adsorbate-adsorbate interactions where we find, for some coverage ranges, that two different adsorption configurations can be stable (at different temperatures) leading to a marked change in the corresponding desorption rates and to distinct non-Arrhenius behavior.     

Secondary-ion emission from silicon bombarded with atomic and molecular noble-gas ions

The emission of Si⁺ from a clean silicon surface has been studied for bombardment with various atomic and molecular noble gas ions at energies between 1.5 and 30 keV. It was found that the degree of ionization of Si⁺ depends strongly (l?inearly) on the projectile energy but only weakly on the projectile mass. These results suggest that the degree of ionization is heavily affected by the (dynamic) perturbation of the bulk properties of the bombarded area which increases with increasing nuclear energy deposition.     

Diffraction from stepped surfaces: I. Reversible surfaces

In electron or atom diffraction experiments on surfaces, the angular shapes of the diffracted beams depend upon the distribution of steps over the surface. In this paper we analyze diffracted beam profiles from stepped surfaces that are reversible. A reversible surface is one in which the pair correlation function over the surface is symmetric with respect to positive and negative directions. We show that the intensity profile across a diffracted beam can be separated into a sharp central spike due to the limit of the correlation function at large separation plus wings or shoulders due to the finite extent of the step disorder. Simple functional expressions for these angular profiles are obtained by a Markov method of treating a one-dimensional geometric distribution of steps. The result explicitly displays the deep structure found for the general case. The method reduces the calculation to a simple eigenvalue problem so that even the continuously changing step distributions that occur in epitaxial growth can be treated easily. As in the general case, the resulting intensity profile is a sharp central spike plus a step-broadened term which now is a sum of Lorentzians. The widths of the Lorentzians are the logarithms of the eigenvalues of the matrix of probabilities which describe the step distribution over the surface. This matrix method, which treats the surface as a Markov chain, also points the correct way to account for correlations between surface atoms for two-dimensional distributions of steps. For a two-dimensional surface one must consider a Markov Random Field as opposed to a simple multiplication of two one-dimensional results. We compare the results of the general calculation to the Si epitaxy experiments of Gronwald and Henzler. The coverage and momentum transfer dependencies of the shapes of the calculated profiles agree with their measurements. The calculation is also applied to the RHEED measurements of Van Hove et al. during GaAs MBE. The measured intensity oscillations can be accounted for by a cyclically changing one-dimensional geometric distribution of steps among three layers in which the third-layer scattering increases with time.     

Temperature dependence of ion-induced auger electron emission from (111) silicon: I. Experiments

Abstract

Measurements of both secondary electron emission coefficient γ and SiL₂₃ Auger yield ρ_A obtained from (111) Si target bombarded by high fluence of noble gas ions were performed. For Si irradiated at room temperature at doses more than 10¹⁷ ions per cm², monotonous increasing variation of γ and ρ_A versus incidence angle i was observed. For Si irradiated at a temperature more than a critical value, γ(i) and ρ_A(i) curves exhibited, superimposed to monotonous variation, some minima when the ion beam penetrates the crystal along low index directions. In the range 20–650°C, the Auger yield temperature dependence showed a sharp variation around a critical value depending on the ion mass for a given incident energy. These results are linked to an amorphous-crystalline phase transition.     

Yield curves for photo-proton emission from carbon and beryllium

The yields of photo-protons at laboratory angles of 40° and 70° from carbon irradiated with bremsstrahlung of end-point energies between 125 and 1000 MeV are given. From these data and a more limited set for beryllium, the shapes of the cross-sections for photo-proton emission by monochromatic photons are deduced. From a separate series of measurements on carbon at 70° lab with bremsstrahlung of end-point energies 380 and 420 MeV, the energy spectrum of photo-protons emitted by photons of energies 400±20 MeV has been derived.     

Creep of beryllium. I

Creep characteristics of beryllium have been determined in the temperature range 600–800°C and the stress range 0.25–5 kgf/mm². The rate of the process is controlled by the Herring —Nabarro mechanism in the range of stresses less than 1 kgf/mm². The creep activation energy (39±1 kcal/mole) hence agrees with the energy of self-diffusion. The creep rate for stresses greater than 1 kgf/mm² is determined by the simultaneous progress of dislocation creep and slip, where the slip contribution grows with the increase in stress. An approximate picture of the deformation mechanisms of creep is constructed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 68–73, March, 1976.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces I. Clean and hydrogen-covered

The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C-I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C-I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH₂I₂ adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent β-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C₃ and C₄ production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation-dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved.     

Electron energy-loss spectra of clean and hydrogen-covered Cr(110) surfaces

Electron energy-loss spectra of clean and hydrogen-covered Cr(110) surfaces have been investigated in the spectral range of 0–80 eV for primary energies of 60–500 eV. The observed peaks for the clean surface are at the loss energies of 2, 3.5, 5.5, 9.6, 23, 35, 42, 48 and 55 eV. The 3p-excitation spectra for high primary energies (> 250 eV) are in good agreement with the corresponding optical spectrum. The edge at & 42 eV observed for low primary energies is tentatively attributed to the onset of the transition from the 3p subshell to the local 4s-band in the vicinity of the core hole. A characteristic energy-loss at ≈ 15.5 eV is observed after hydrogen adsorption. The 3p-spectrum is not influenced by hydrogen adsorption, indicating that the excitation is of a localized character.     

THz emission from semiconductor surfaces

We provide a review of experimental and theoretical work on electromagnetic terahertz pulse emission from semiconductor surfaces excited by femtosecond laser radiation. The main terahertz emission mechanisms are analysed. The terahertz emission from InAs and Ge is explained by the photo-Dember effect and electric field induced optical rectification. Electronic band structure and carrier scattering mechanisms are investigated by means of terahertz emission and absorption spectroscopy in InAs, InSb and Ge. To cite this article: V.L. Malevich et al., C. R. Physique 9 (2008).     

Angular dependence of auger electron emission from Cu (111) and (100) surfaces

Experimental angular emission profiles of the M_2,3M_4,5M_4,562 eV Cu Auger electrons from clean copper (100) and (111) surfaces in several azimuths are presented. A simple single scattering theory to account for elastic scattering of the Auger electrons by other ion cores in the solid is presented, and calculations have been performed to assess the importance of this process in contributing to the observed angular dependence. These calculations produce angular structure having a similar magnitude and temperature dependence to that observed experimentally, and some featural similarity in peak positions or shapes. It appears that elastic scattering is an important source of angular dependence, and that studies of adsorbed species on surfaces should provide a very sensitive method of surface structure determination.     

Electron emission from charged surfaces of ferroelectrics-electrets: Part 1. Properties of the electron emission

The present paper describes properties of the electron emission from negatively charged surfaces of poled ferroelectrics-electrets. This emission results from the irradiation of the surface with X-rays of weak intensity. It has been shown both experimentally and theoretically that the mechanism of its emergence differs from the photoemission mechanism, it rather roots in the ferroelectric state of the surface layer.     

Orientation dependence of oxygen adsorption on a cylindrical GaAs sample: I. Auger measurements

The orientation dependence of oxygen adsorption has been investigated by AES on the surface of a cylindrically shaped GaAs single crystal with [111̄0] being its axis. It thus exposes the main low index orientations (001), (111)Ga, (110), and (111̄)As, as well as all their vicinal surfaces and intermediate orientations on its surface. It is shown that it is possible to prepare all these orientations simultaneously and with reasonable quality by ion bombardment and annealing (IBA). The orientation dependence of the amount of adsorbed oxygen in the range (001)(111)Ga(110)(111̄)As can be understood in terms of different sticking coefficients on the different types of terrace site and of enhanced adsorption on edge-adjacent sites. These edge-adjacent sites show saturation at about 4 × 10⁵ L. Starting from (110) towards (111)Ga, at first, steps one atomic layer high are found, changing to a height of two layers when approaching (331). This behaviour can be understood in terms of the known relaxation on (110). A deep minimum in the amount of adsorbed oxygen between (111̄)As and (001̄) is interpreted to be due to an As stabilized low sticking coefficient phase between (112̄) and (113̄). Early saturation (at～10⁵ L) on (001) and (111̄)As is consistent with the fact that these surfaces usually do not reach their room temperature equilibrium phase upon preparation by IBA. Sudden and accidental oxygen induced composition changes towards As-richer substrate compositions further confirm this.