Electron Attenuation Research Articles

Attenuation of excited electrons at crystal surfaces

Attenuation of the electron current determines surface sensitivity of electron spectroscopies. Pronounced energy and directional dependencies of the electron attenuation in crystals differ strongly from those commonly used for amorphous solids. Quantum descriptions of the electron attenuation can be obtained from the complex band structure of crystal surfaces at energies above the vacuum level. Contributions, specific for concrete processes, are obtained from theoretical description of photoelectron spectroscopy and of electron diffraction.

Measurement of the hot electron attenuation length of copper

Ballistic electron emission microscopy is utilized to investigate the hot-electron scattering properties of Cu through Cu/Si(001) Schottky diodes. A Schottky barrier height of 0.64±0.02 eV and a hot-electron attenuation length of 33.4±2.9 nm are measured at a tip bias of 1.0 eV and a temperature of 80 K. The dependence of the attenuation length with tip bias is fit to a Fermi liquid model that allows extraction of the inelastic and elastic scattering components. This modeling indicates that elastic scattering due to defects, grain boundaries, and interfaces is the dominant scattering mechanism in this energy range.

Electronic structure at the perylene-tetracarboxylic acid dianhydride/Ag(111) interface studied with two-photon photoelectron spectroscopy

The electronic structure of the prototype metal/organic contact 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on a Ag(111)-surface has been investigated using time- and angle-resolved two-photon photoelectron spectroscopy (2PPE). Our analysis addresses particularly the nature of the interface state (IS) emerging at the interface due to the substrate-adsorbate interaction [C. H. Schwalb, S. Sachs, M. Marks et al., Phys. Rev. Lett. 101, 146801 (2008)]. Its free-electron-like dispersion and a possible backfolding at the surface Brillouin zone boundaries are discussed. Time-resolved pump-probe experiments reveal the inelastic electron lifetime along the dispersion parabola and show its decrease for increasing parallel momentum. The temperature dependence of the peak linewidth indicates a coupling of the IS to molecular vibrations. Moreover, additional aspects are addressed, such as the determination of the electron attenuation length of photoelectrons for low kinetic energy originating from the IS and the work function change of the sample upon PTCDA adsorption with very high energy resolution.

Determination of the Absolute Thickness of Ultrathin Al2O3 Overlayers on Si (100) Substrate

The thickness of nanometer Al(2)O(3) films was studied by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The thickness was determined from mutual calibration of the XPS and TEM measurements. The thickness offset of Al(2)O(3) films was proved to be close to zero by in situ XPS analysis. The thicknesses of a series of Al(2)O(3)/Si (100) films with different Al(2)O(3) thicknesses could be determined by mutual calibration from the thicknesses measured by TEM and XPS. The electron attenuation length of the Al 2p electron was determined as 2.4334 nm in the Al(2)O(3) matrix.

Electron attenuation anisotropy at crystal surfaces from LEED

Dynamical theory of electron scattering is used to describe the electron transport in the surface regions of crystals. The angle resolved attenuation length of electrons is derived from the transmitted LEED electron current decay. Electron attenuation length energy dependence and anisotropy in polar angle are found for crystalline Cu(1 1 1) for two high symmetry azimuths. Pronounced anisotropy in polar angle distributions of attenuation lengths is found to be in qualitative agreement with the results obtained from the photoelectron diffraction. Comparison with the attenuation lengths obtained from semiclassical simulations for amorphous copper is given. This comparison demonstrates that simple transfers of the smoothly behaving surface sensitivity from amorphous materials oversimplifies the electron attenuation process and can lead to incorrect results in quantitative analyses of crystalline surfaces.

XPS topofactors: determining overlayer thickness on particles and fibres

AbstractWe introduce the concept of an XPS ‘Topofactor’, which can be used in conjunction with the XPS ‘Thickogram’, to provide overlayer thicknesses on topographic samples of known geometry. The concept is essentially simple; analysis is performed with the sample normal directed towards the XPS analyser, the equivalent planar thickness is calculated from the Thickogram and the Topofactor applied to the result to provide the actual thickness. The Topofactor is thus defined as the ratio of the true overlayer thickness to the apparent overlayer thickness obtained by assuming the sample is ideally planar. Within this paper we describe how Topofactors can be calculated and consider cylinders (fibres) and spheres (particles) in some detail. For spheres, a Topofactor of 0.67 is a useful average value and for cylinders, a Topofactor of 0.79 is useful, (i.e. ∼2/3 and 4/5, respectively) both values would typically provide overlayer thicknesses within 10% of the actual value. An analytical description of cylindrical and spherical Topofactors to within 1% error is provided which accounts for variations in thickness and relative electron attenuation lengths. The Topofactors are useful for macroscopic and microscopic samples, but not for nanoscopic samples such as nanofibres and nanoparticles. In this case, we provide an analytical model that can predict the relative XPS signals from the core and the shell of nanomaterials to an error that is typically better than 10%. © Crown copyright 2009. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.

Characteristic energy band values and electron attenuation length of a chemical-vapor-deposition diamond (0 0 1)2 × 1 surface

The characteristic energy band values such as the Fermi-level position with respect to valence band top for a boron-doped p-type hydrogen-terminated chemical-vapor-deposition (CVD) diamond (0 0 1)2 × 1 surface and for a clean CVD diamond (0 0 1)2 × 1 surface have been determined by a new method with an accuracy of ±0.02 eV. The electron attenuation length for the clean diamond (0 0 1)2 × 1 surface for the electron kinetic energy of C 1s X-ray photoemission peak by Mg Kα excitation is experimentally determined to be 2.1–2.2 nm. These values are compared and discussed with the previously reported experimental and simulation values.

Revision of quantum efficiency formula for negative electron affinity photocathodes

Quantum efficiency （QE） is one of the most important characteristics of photoemission performance of negative electron affinity （NEA） photocathodes. The quantum efficiency formulas for NEA photocathodes show the influences of photocathode parameters on photoemission performance and also provide theoretical guidance for optimum design of photocathodes. The photoemission process in bulk and surface of NEA photocathodes is firstly analyzed. Surface electron escape probability and attenuation length are respectively revised considering the effects of incident photon energy，surface band bending region and surface barrier on photoemission. The quantum efficiency formulas for NEA photocathodes are established by integration method. The theoretically estimated QE curve is consistent with the experimental one，which validates the practicability of the revised QE formula. This work can provide theoretical references for research of NEA photocathodes.

Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p 3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.

Hot electron transport and a quantitative study of ballistic electron magnetic imaging on Co/Cu multilayers

We present a study of ballistic hot electron transport in Co/Cu/Co spin valves, using the ballistic electron emission microscope. By comparing samples with various Co thicknesses, we determine the hot electron attenuation length for both the minority- and majority-spin populations in the 1--2 eV energy range. Results are compared to recent calculations in order to qualitatively understand the hot electron attenuation lengths. Using the scanning ability of the microscope, we also present the imaging of different magnetic structures in the Co layers. In particular, we present a detailed study of the variation in $360\ifmmode^\circ\else\textdegree\fi{}$ domain-wall morphology versus applied field. The images are compared with micromagnetic simulations. Good agreement is found that ultimately allows for an estimation of the resolution limits of this microscope.

Monte Carlo simulation of low energy electron injection and scattering in insulating layers

Low energy electrons scattered in the conduction band of a dielectric solid should behave like Bloch electrons and will interact with perturbations of the atomic lattice, i.e. with phonons. Optical as well as acoustic phonons are included. Moreover, the inelastic scattering is described by the dielectric energy loss function Im ( − 1 / ϵ ) especially reflecting the excitation of valence band electrons, i.e. secondary electrons (SE). With these collective scattering models we have performed the simulation of electron injection and excited electron relaxation and attenuation in the insulator SiO 2. After secondary electron excitation to a mean initial energy of several eV their energy relaxation occurs within a short time interval of 200 fs to full thermalization. There is a very rapid cooling by impact ionization connected with cascading of electrons at the beginning during the first 20 fs, followed by much slower attenuation due to phonon losses in wide-gap dielectrics and insulators. These attenuation times are connected with SE escape depths, even with and against the direction of an electric field within the nonconductive isolating sample.

Ballistic electron attenuation length in titanylphthalocyanine films grown on GaAs

In this work we present an experimental investigation of hot electron transport in thin titanylphthalocyanine films grown on GaAs. Ballistic electron emission microscopy/spectroscopy is used as the experimental method. The transmission through a Au/titanylphthalocyanine/GaAs heterostructure is evaluated. In addition, an approach to calculate the attenuation length in titanylphthalocyanine is presented.

Functional hierarchy of two L domains in porcine endogenous retrovirus (PERV) that influence release and infectivity

The porcine endogenous retrovirus (PERV) Gag protein contains two late (L) domain motifs, PPPY and P(F/S)AP. Using viral release assays we demonstrate that PPPY is the dominant L domain involved in PERV release. PFAP represents a novel retroviral L domain variant and is defined by abnormal viral assembly phenotypes visualized by electron microscopy and attenuation of early PERV release as measured by viral genomes. PSAP is functionally dominant over PFAP in early PERV release. PSAP virions are 3.5-fold more infectious in vitro by TCID50 and in vivo results in more RNA positive tissues and higher levels of proviral DNA using our human PERV-A receptor (HuPAR-2) transgenic mouse model [Martina, Y., Marcucci, K.T., Cherqui, S., Szabo, A., Drysdale, T., Srinivisan, U., Wilson, C.A., Patience, C., Salomon, D.R., 2006. Mice transgenic for a human porcine endogenous retrovirus receptor are susceptible to productive viral infection. J. Virol. 80 (7), 3135–3146]. The functional hierarchies displayed by PERV L domains, demonstrates that L domain selection in viral evolution exists to promote efficient viral assembly, release and infectivity in the virus–host context.

Is MR-guided Attenuation Correction a Viable Option for Dual-Modality PET/MR Imaging?

Habib Zaidi, PhD This is an exciting time for molecular imaging because molecular medicine is expected to lead to a revolutionary paradigm shift in health care. While the nuclear medicine community is witnessing a revolution in the practice of this specialty with the introduction of dual-modality positron emission tomography (PET) and computed tomography (CT), many groups in academic and corporate settings are focusing their efforts on the development of promising multimodality imaging technologies that will combine magnetic resonance (MR) imaging and PET components in a single gantry. One such preclinical system is described by Judenhofer et al (1) in this issue of Radiology, and different design trends have been reported in the literature (2–8). Similar design concepts are also being investigated for combined single photon emission CT (SPECT) and MR imaging instrumentation (9). The recent interest in simultaneous PET/MR imaging is not the consequence of controversies surrounding the role and clinical benefits of PET/CT (10,11) but will likely be the subject of similar critique and debates. The development of PET/MR imaging has been motivated by various factors and has several important incentives (12). First, MR imaging is used to obtain anatomic and structural images with submillimeter spatial resolution that offers better soft-tissue contrast sensitivity than that offered by CT. MR imaging has excellent contrast between white and gray matter and allows for functional imaging in brain studies, and, more importantly, it can be used to assess flow, diffusion, perfusion, and cardiac motion in one examination (13). In addition, MR imaging can be combined with MR spectroscopy to measure the regional biochemical content and to assess the metabolic status or the presence of neoplasia and other diseases in specific tissue areas. Finally, MR imaging does not involve the use of ionizing radiation; thus, it can be used without restrictions in serial studies, for pediatric studies, and in many other situations where radiation exposure is a concern. PET imaging is used to record the regional distribution of radiolabeled tracers; however, unlike MR spectroscopy, it cannot be used to distinguish the specific molecular species to which the radionuclide is attached, and, unlike MR imaging, it provides little anatomic information. While many technical problems have recently been solved, there are still several important challenges to the implementation and operation of a PET/MR imaging system that must be overcome. In comparison with x-ray CT, MR imaging typically is more costly, involves longer examination times, and produces anatomic images from which it is more difficult or at least not as straightforward to derive attenuation maps for photon correction of the emission data (14). The latter issue was not described by Judenhofer et al (1), as it has received only limited attention in the scientific literature and few investigators have addressed the problem of the use of segmented MR data to construct an attenuation map for attenuation correction purposes with PET (15). However, interest in PET/MR imaging has been the driving force behind many worthwhile research efforts recently undertaken by different research groups. The major difficulty lies in the fact that the MR signal or tissue intensity level is not directly related to electronic attenuation, which renders conversion of MR images to attenuation maps less obvious when compared with CT. It is worth emphasizing that the optimal transmission scanning technique for PET/CT (ie, CT versus radionuclide sources) is still an open issue that remains to be addressed (16). This is not an issue for PET/MR imaging because of the limited space available, and thus, placement of external Published online 10.1148/radiol.2443070092

X-ray standing wave analysis of overlayer-induced substrate relaxation: The clean and Bi-covered (110) GaP surface

The relaxation of the surface P atoms, for both the clean and Bi-covered GaP(110) surface, was studied with x-ray standing wave (XSW) spectroscopy using surface-sensitive x-ray photoelectron as the XSW modulated signal. The photoemission signal of the outermost surface layer is mixed with the signal from the remaining near surface of the underlying substrate, so further analysis is required to calculate the geometry of the relaxation of the surface atoms. We present a general analysis method for extracting the geometry of the surface reconstruction that minimizes the propagation of the uncertainties associated with fitting XSW data. It takes advantage of the fact that the coherent distance may be more accurately determined than the coherent fraction in XSW data analysis. This method makes use of the electron attenuation length, and shows that the relaxation is only weakly dependent on the uncertainties of this parameter. Results indicate that, for the clean GaP surface, P relaxes with a small outward rotational displacement, with the axis of the rotation located at the second-layer Ga site, whereas, for the Bi-covered case, relaxation consists of a rotation in the opposite direction. The magnitude of the contraction is not negligible, and might be important in the interpretation of low-energy electron diffraction data and in ab initio calculations.

Spin tunnelling phenomena in single-crystal magnetic tunnel junction systems

A brief theoretical review points out the specific aspects of electronic transport insingle-crystal magnetic tunnel junctions employing bcc(100) Fe electrodes and a MgO(100)insulating barrier. The theoretical predictions are compared to the experimental reality inboth equilibrium and out-of-equilibrium regimes. For extremely small MgO thickness, weillustrate that the equilibrium tunnel transport in Fe/MgO/Fe systems leads toantiferromagnetic interactions. Artificial antiferromagnetic systems based on coupling byspin polarized tunnelling have been elaborated and studied. In the out-of-equilibriumregime and for large MgO barrier thickness, the tunnel transport validates specificspin filtering effects in terms of symmetry of the electronic Bloch function andsymmetry-dependent wavefunction attenuation in the single-crystal barrier. Within thisframework, we explain the experimental giant tunnel magnetoresistive effects at roomtemperature, up to 180%, measured in our simple or double barrier tunnel junctionsystems. Moreover, we illustrate that the magneto-transport properties of the junctionsmay be skilfully engineered by adjusting the interfacial chemical and electronic structure.

Electronic properties of Cu/SiO 2/Cu structures at different temperatures

In this experimental work, some electronic properties of vacuum evaporated Cu/SiO 2/Cu structures such as circuiting I c and emission I e currents versus the applied voltage, electron attenuation lengths in both copper and SiO 2 layers and the role of the latter layers have been investigated. Experimental results show that these devices undergo an electroforming process leading to resistivity decrease of several orders of magnitude along with a negative resistance region in their current–voltage characteristics. By decreasing the temperature, both I c and I e are decreased and at low temperatures the negative resistance region disappears completely. Electron attenuation lengths are measured between 6 and 14 V for copper and SiO 2 layers and their significance are discussed on the base of electron-impurity and electron-defect scatterings. The conduction mechanism is also discussed on the base of a filamentary model.

Probing the hot-electron transport properties and interface band structure ofFe∕Si(001) andFe81C19∕Si(001) Schottky diodes

Ballistic electron emission microscopy (BEEM) has been performed on both $\mathrm{Au}∕{\mathrm{Fe}}_{81}{\mathrm{C}}_{19}∕\mathrm{Si}$ (001) and $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{Si}$ (001) Schottky diodes at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The Schottky heights were measured to be $0.68\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $0.70\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the ${\mathrm{Fe}}_{81}{\mathrm{C}}_{19}∕\mathrm{Si}$ (001) and $\mathrm{Fe}∕\mathrm{Si}$ (001) interfaces, respectively. In addition, a second threshold voltage was observed for the $\mathrm{Fe}∕\mathrm{Si}$ (001) interface at $1.29\ifmmode\pm\else\textpm\fi{}0.04\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and attributed to an additional conduction band minimum at the interface that arises from the bonding of the Fe to the Si. The hot electron attenuation lengths at $1.25\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ were measured to be $3.5\ifmmode\pm\else\textpm\fi{}1.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $3.0\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for ${\mathrm{Fe}}_{81}{\mathrm{C}}_{19}$ and Fe, respectively. The attenuation length of the ${\mathrm{Fe}}_{81}{\mathrm{C}}_{19}$ showed a decrease with increasing energy consistent with the universal curve for electron-electron scattering. However, the attenuation length for the Fe showed this decrease only until the onset of the second threshold after which it increased. It is proposed that this increase is attributed to the parallel momentum distribution of the additional conduction band minimum at the $\mathrm{Fe}∕\mathrm{Si}$ (001) interface.

Electron attenuation lengths in fullerene and fullerides

Using X-ray photoemission measurements, we have determined the attenuation length of C 1s photoelectrons in C 60 film to be 21.5 Å with the incident photon energy of Mg Kα radiation. The inelastic mean free path calculated with the TPP-2M algorithm coincides fairly well with the experimentally determined attenuation length, indicating the validity of the algorithm to fullerene and fullerides. The inelastic mean free paths for some fullerides, i.e. K 3C 60, K 6C 60, Ba 4C 60, Sm 2.75C 60 and Sm 6C 60 are calculated to help the quantitative analyses of the photoemission spectra for these compounds.

Ballistic electron transport properties of Fe-based films on Si(001)

Thickness dependent ballistic electron emission microscopy (BEEM) studies have been performed on Au∕Fe81C19∕Si(001) and Au∕Si(001) Schottky diodes at 80K. The Schottky height was measured to be 0.70±0.02eV for the Fe81C19∕Si(001) interface. Electron attenuation lengths were extracted from the slope of the semilog BEEM current versus the thickness of the Fe81C19 layers for electron energies ranging from 1.0to1.5eV. In this range the attenuation length was found to decrease with increasing energy from 4.1±0.9to2.5±0.6nm, which indicates that some electron-electron scattering is occurring in the metal overlayer. This decrease is slightly greater than predicted for a free electron gas system, resulting from the complex structure of the Fe81C19 film.