Surface excitation parameter for 12 semiconductors and determination of a general predictive formula

The surface excitation parameter (SEP) is theoretically calculated for 12 semiconductors (GaN, GaP, GaSb, GaAs, InSb, InAs, InP, SiC, ZnSe, ZnS, Si and Ge) and for Ni (which is usually used as a reference in experiments) for electron energies between 300 eV and 3400 eV, and for angles between 0° and 70° to the surface normal. We use our previous definition of SEP, as the change in excitation probability, for an electron, caused by the presence of the surface in comparison with an electron moving the same distance in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS‐ε(k, ω)‐REELS software determining the energy‐differential inelastic electron scattering cross‐sections for reflection‐electron‐energy‐loss spectroscopy (REELS), and for which the only input is the dielectric function of the medium. By fitting to these SEP values as well as our previous ones, i.e. from 27 materials, including metals, oxides, polymers and semiconductors, we also establish a simple equation depending on the generalized plasmon energy and the energy band gap of the material which allows to estimate the SEP when the dielectric function is not available. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface excitation parameter for selected polymers

The surface excitation parameter (SEP) is theoretically determined for different polymers, namely, polyethylene (PE), polystyrene (PS), polyacetylene (PA) and polymethyl methacrylate (PMMA), for electron energies between 300 and 5000 eV and for angles between 0 and 70^o to the surface normal. We use our previous definition of SEP as the change in excitation probability of an electron caused by the presence of the surface in comparison with an electron moving in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS‐ε(k, ω)‐ REELS software determining the energy‐differential inelastic electron scattering cross‐sections for reflection‐electron‐energy‐loss spectroscopy (REELS). More precisely, the volume component for an infinite medium is subtracted from the calculated REELS cross‐section and in this way the surface excitation component of the cross‐section is determined and the SEP calculated. We find that the presence of an energy band gap reduces the SEP values compared to those for metals, and this decrease is larger for polymers with larger gaps. Copyright © 2008 John Wiley & Sons, Ltd.     

Systematic calculation of the surface excitation parameters for 22 materials

Quantitative surface analysis requires knowledge of surface excitations by electrons. These excitations are characterized by the surface excitation parameter (SEP) which represents the surface excitation probability while an electron moves across a solid surface. In this work, a systematic calculation of a SEP database has been performed for 22 materials, including metals, oxides and semiconductors, for electron energies between 100 eV and 5000 eV, and for angles α of incidence/emission between 0.5^o and 89.5^o with respect to the surface normal. Surface excitations are considered for both sides of a solid–vacuum interface when an electron is incident on or emitted from a surface. These SEPs represent not only the appearance of surface excitations but also the inhibition of bulk excitations. Four common SEP formulas are evaluated, and we present best‐fit parameters for the most satisfactory formula. SEP can then be readily determined for about 22 materials and various energies and electron incidence or emission angles. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical study of the surface excitation parameter from reflection‐electron‐energy‐loss spectra

A theoretical method to determine the so‐called surface excitation parameter (SEP) is presented. This method is based on the modelling of reflection‐electron‐energy‐loss spectroscopy and more particularly on the analysis of energy‐differential inelastic electron scattering cross sections calculated within the model. The SEP is extracted from theoretical cross‐section spectrum by calculating the ratio between the surface loss component of the spectrum and the elastic peak intensity. The calculations have been performed entirely with the dielectric function, using the software QUEELS (Quantitative analysis of Electron Energy Losses at Surfaces) recently developed by Yubero and Tougaard [Surf. Interface Anal. 2004; 36 : 824]. The angular distribution of SEP is calculated for angles between 10° and about 70° for aluminium and silicon. We propose also an extension of the method for materials (e.g. copper) that do not present clear surface and volume plasmons. Copyright © 2005 John Wiley & Sons, Ltd.     

Determination of the relative sputtering yield of carbon to tantalum by means of Auger electron spectroscopy depth profiling

The relative sputtering yield of carbon with respect to tantalum was determined for 1 keV Ar⁺ ion bombardment in the angular range of 70°–82° (measured from surface normal) by means of Auger electron spectroscopy depth profiling of C/Ta and Ta/C bilayers. The ion bombardment‐induced interface broadening was strongly different for the C/Ta and Ta/C, whereas the C/Ta interface was found to be rather sharp, the Ta/C interface was unusually broad. Still the relative sputtering yields (Y_C/Y_Ta) derived from the Auger electron spectroscopy depth profiles of the two specimens agreed well. The relative sputtering yields obtained were different from those determined earlier on thick layers, calculated by simulation of SRIM2006 and by the fitting equation of Eckstein. The difference increases with increase of angle of incidence. Copyright © 2009 John Wiley & Sons, Ltd.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.     

Electron-impact coherence parameters for the excitation of the 51 P state of Sr

Electron-photon polarization correlation measurements have been carried out for the excitation of the 5¹P state of Sr at electron impact energies of 30.3 and 58.4 eV and electron scattering angles of 20°–130° for 30.3 eV and 20°–100° for 58.4 eV. The resulting Stokes parametersP ₁,P ₂,P ₃ are used to derive the usual complete scattering parameter sets λ, χ and γ,L _⊥,P _l. New FOMBT calculations for these parameters are reported alongside the measured data and show substantial agreement with the experiment and with recent calculations by Srivastava et al.     

Local inverse inelastic mean free path at interface

We calculated a local inverse inelastic mean free path (local-IIMFP) for electrons crossing a medium–medium interface, considering various incident electron energies, crossing angles and combinations of materials. We used an extension of a classical dielectric model developed by Li and co-workers for an electron crossing a surface (interface vacuum-medium). Moreover, the integration over the distance of the local-IIMFP allows to obtain the interface excitation parameter (or IEP) characterizing the change in excitation probability for an electron crossing an interface once caused by the presence of the interface in comparison with an electron for which only volume excitations are considered. We perform these calculations for angles between 0° and 80°, for electron energies between 500 and 2500 eV and for various pairs of materials, as Al/In for its academic interest or Au/Si and SiO₂/Si for their technological importance. Small but not negligible variations of the local-IIMFP and the IEP were observed for metal–metal or metal–semiconductor interfaces, while quite significant variations are obtained when one of the materials is a insulator.     

Triplet states in 1,3-butadiene

The electron impact excitation spectrum 0f 1,3-butadiene has been studied at 20, 35 and 55 eV impact energies and scattering angles of 10° to 80°. Two low lying states are observed with maxima at 3.2 and 4.9 eV, and are identified as the ³B_u state and ³A_g state respectively     

Surface transient effects in ultralow‐energy O2+ sputtering of silicon

It is known that by lowering the impact energy the sputter rate and surface transient width in SIMS will be reduced. However, few studies have been done at ultralow energies over a wide range of impact angles. This study examines the dependence of sputter rate and transient width as a function of O₂⁺ primary ion energy (E_p = 250 eV, 500 eV and 1 keV) and incidence angles of 0–70°. The instrument used is the Atomika 4500 SIMS depth profiler and the sample was Si with 10 delta‐layers of Si_0.7Ge_0.3. We observed that the lowest transient width of 0.7 nm is obtainable at normal and near‐normal incidence with E_p ～ 250 eV and E_p ～ 500 eV. There is no significant improvement in transient width going down in energy from E_p ～ 500 to ～250 eV. The onset of roughening is also not obvious at E_p ～ 250 eV over the whole angular range studied. Although the sputter rate during the surface transient is normally different from that at steady state, only at E_p ～ 250 eV was it observed that the sputter rate remained fairly independent of depth. We conclude that the best working ranges to achieve a narrow transient width and accurate depth calibration are at E_p ～ 250 eV/0° < θ < 20°and 500 eV/0°< θ < 10°. Copyright © 2005 John Wiley & Sons, Ltd.     

Electron-photon polarisation correlations in 140 eV electron impact excitation of helium

Electron-polarised photon coincidence techniques are used to determine linear and circular polarisation correlations from the differential electron impact excitation of the 2¹ P state of helium at an incident electron energy of 140 eV. At 30° and 45° electron scattering angles, all the Stokes parameters are determined, whereas at 52.5° onlyP ₂ is measured. Comparisons are made with the distorted wave (DW) calculations of Madison [11] and the first order many-body theory (FOMBT) of Cartwright and Csanak [4].     

In-plane polarisation correlation study of the 31 D state of helium excited by 40 eV electrons

Electron-photon polarisation correlations measured in the scattering plane defined by the incident and scattered electron momenta are reported for 40 eV electron impact excitation of the 3¹ D state of helium and for electron scattering angles in the range 40°≦?≦120°. When combined with the results of our earlier polarisation analysis on radiation emitted perpendicular to the scattering plane, detailed information on the shape and dynamics of the exicted state is obtained. In the small angular range of overlap (40–60°) there is excellent agreement with a pervious experimental study. The behaviour of the linear polarisation of the radiation emitted in the scattering plane is well reproduced by a multichannel eikonal theory for ?≦80°. Otherwise theories totally fail to described the excitation process.     

Ab initio study of Rg2I− (Rg = Ar,Kr, Xe)

The equilibrium geometries, vibrational frequencies, and dissociation energies of rare gas iodine clusters Rg₂I^?(Rg = Ar, Kr, Xe) were calculated at the Hartree–Fock (HF), second‐order Møller–Plesset (MP2), the coupled cluster method with single and double excitation and a noniterative correction for triple excitations method [CCSD(T)] levels. The title species have bent C_2v structure of about 60° angle. The electron correlation effects and relativistic effects on the geometry and stability were investigated at CCSD(T) level. Both effects stabilize title species. The calculated electron affinities are in good agreement with the experimental results available. The effect of high angular momentum functions (g and h) was studied. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Lyman-α excitation spectra in the photodissociation of the doubly excited states of H2

A Lyman-α excitation spectrum has been observed using synchrotron radiation in the energy region corresponding to the double electron excitation of H₂. There exist in the spectrum three thresholds at 26.6±0.5 eV, 29.2±0.7 eV and 30.9±0.6 eV, and a dip at 34.1±0.5 eV. A Lyman-α excitation spectrum in the energy region corresponding to the single electron excitation has been also observed using a detection system which works as a band pass filter for detecting of Lyman-α fluorescence. The cross section of Lyman-α fluorescence in the photodissociation of the doubly excited states is very small, e.g., in the order of 10^?20 cm² at 30 eV, in comparison with that from the single electron excitation.     

Calculation of Surface Excitation Parameters by a Monte Carlo Method

Electron inelastic mean free path (IMFP) is an important parameter for surface chemical quantification by surface electron spectroscopy techniques. It can be obtained from analysis of elastic peak electron spectroscopy (EPES) spectra measured on samples and a Monte Carlo simulation method. To obtain IMFP parameters with high accuracy, the surface excitation effect on the measured EPES spectra has to be quantified as a surface excitation parameter (SEP), which can be calculated via a dielectric response theory. However, such calculated SEP does not include influence of elastic scattering of electrons inside samples during their incidence and emission processes, which should not be neglected simply in determining IMFP by an EPES method. In this work a Monte Carlo simulation method is employed to determine surface excitation parameter by taking account of the elastic scattering effect. The simulated SEPs for different primary energies are found to be in good agreement with the experiments particularly for larger incident or emission angles above 60° where the elastic scattering effect plays a more important role than those in smaller incident or emission angles. Based on these new SEPs, the IMFP measurement by EPES technique can provide more accurate data.     

2H NMR spin relaxation study of the soft segment motion in alternating ethylene–propylene copolymers

We synthesized three partially deuterated polymer samples, namely a poly(ethylene‐alt‐propylene) (EP) alternating copolymer, a poly(styrene‐b‐EP) diblock copolymer (SEP) and a poly(styrene‐b‐EP‐b‐styrene) triblock copolymer (SEPS). The ²H spin–lattice relaxation time, T₁, of EP soft segments above their glass transition temperature was measured by solid‐state ²H NMR spectroscopy. It was found that the block copolymers had a fast and a slow T₁ component whereas EP copolymer had only a fast component. The fast T₁ components for SEP and SEPS are similar to the T₁ value of EP above ca 20°C. The slow T₁ component for SEP and SEPS exhibited a minimum at 60°C and approached the value of the fast component near the T_g of polystyrene. The motional behavior of the EP units for SEP is similar to that of SEPS over the entire range of temperature. Copyright © 2001 John Wiley & Sons, Ltd.     

Photovoltaic properties of lead telluride thin film

Polycrystalline PbTe thin film is prepared on glass substrate at 200 °C. PbTe thin film isN-type and the carriers are electrons. The incident energy of photons, 3.4 eV, generates more electron carriers as the distance decreases which give rise to photoelectric current. The density of donorsN _d was determined to be 1.1×10²⁰ cm^?3 which is consistent with theN-type conduction of PbTe. The activation energies ofN-type PbTe thin films are 0.139, 0.139 and 0.126 eV below 60 °C which change toP-type above 60 °C. This may be due to generation of Pb vacancies in the lattice. The piezoresistivity is measured, the increase of conductivity may be due to displacements of lattice defects under applied stress.     

Resonant scattering of slow electrons from naphthalene vapour

Resonant scattering of low energy electrons from naphthalene has been investigated using electron transmission spectroscopy. The transmission spectrum of naphthalene yields a value of ?0.20 ± 0.05 eV for the first electron affinity. The first and second excited states of C₁₀H^?₈ are detected at 0.96 ± 0.1 eV and 1.55 ± 0.13 eV. Resonant structure is also observed at 5.29 ± 0.1 cV and 7.55 ± 0.05 eV; it is suggested that the former resonance is the previously unidentified peak in the threshold electron excitation spectrum of Compton and co-workers.     

Ab initio calculations of relativistic and electron correlation effects in polyatomics using the universal Gaussian basis set: XeF2

Ab initio accurate all-electron relativistic molecular orbital Dirac–Fock self-consistent field calculations are reported for the linear symmetric XeF₂ molecule at various internuclear distances with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF₂ at various internuclear distances. The relativistic correction to the electronic energy of XeF₂ was calculated as ～ ?215 hartrees (?5850 eV) by using the Dirac–Fock method. The dominant magnetic part of the Breit interaction correction to the nonrelativistic interelectron Coulomb repulsion was included in our calculations by both the Dirac–Fock–Breit self-consistent field and perturbation methods. The calculated Breit correction is ～6.5 hartrees (177 eV) for XeF₂. The relativistic Dirac–Fock as well as the nonrelativistic HF wave functions predict XeF₂ to be unbound, due to neglect of electron correlation effects. These effects were incorporated for XeF₂ by using various ab initio post Hartree–Fock methods. The calculated dissociation energy obtained using the MP 2(full) method with our extensive basis set of 313 primitive Gaussians that included d and f polarization functions on Xe and F is 2.77 eV, whereas the experimental dissociation energy is 2.78 eV. The calculated correlation energy is ～ ?2 hartrees (?54 eV) at the predicted internuclear distance of 1.986 Å, which is in excellent agreement with the experimental Xe—F distance of 1.979 Å in XeF₂. In summary, electron correlation effects must be included in accurate ab initio calculations since it has been shown here that their inclusion is crucial for obtaining theoretical dissociation energy (D_e) close to experimental value for XeF₂. Furthermore, relativistic effects have been shown to make an extremely significant contribution to the total energy and orbital binding energies of XeF₂. © 1995 John Wiley & Sons, Inc.