Electron energy loss spectra and reflection images from surfaces

There is renewed interest in the interaction of high energy electron beams with surfaces at glancing angles of incidence, due partially to recent improvements in, and consequential resurgence of, reflection electron microscopy. In this paper electron energy loss of beams running parallel to a surface or at glancing incidence to it are studied and compared with predictions arising from the classical theory of dielectric excitation by a moving charge. There is good agreement for both the Cu and MgO surfaces studied here. It is pointed out that small angle energy loss from high energy electrons is insensitive to surface reconstructions or surface steps. Reflection energy filtered micrographs of the Cu surface are shown and the similarity between the zero and various energy loss images shows the preservation of contrast mechanisms for elastic and inelastic electrons.     

Efficient spin resolved spectroscopy observation machine at Hiroshima Synchrotron Radiation Center

Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.     

High resolution microanalysis in materials science using electron energy loss measurements

The physical basis of microanalysis using measurements of electron energy losses associated with atom ionization or plasmon excitation in thin electron microscope specimens is explained in a simple manner. In addition the equipment used to resolve both the high and low energy regions of the loss electron spectrum is described. It is shown that ionization loss analysis is still in its infancy, but plasmon loss analysis has now been providing quantitative microanalytical data on light metal alloys for 8 years. The results obtained from both techniques and their application to specific metallurgical problems are reviewed. Conclusions are drawn concerning the future use of these techniques in high resolution microanalysis.     

Performance and applications of electron energy loss spectroscopy in stem

When coupled in the image mode to a VG-HB501 microscope, the spectrometer designed by O. Krivanek and manufactured by Gatan Inc. is well suited for resolving analytical problems with a high spatial resolution. It actually records energy loss spectra from areas as small as 0.5 nm with a typical energy resolution of 1 eV over the energy loss range and with a good efficiency in collecting inelastic electrons. During the last few months, this high performance combination of microscope and spectrometer has been used to investigate (a) detection limits in EELS which are presently estimated of the order of ten atoms in a test situation such as metallic clusters deposited on a very thin carbon layer; (b) quantitative chemical analysis of representative nanovolumes of complex oxide specimens, emphasizing several aspects of elemental segregation in the neighborhood of grain boundaries and within vitreous areas; (c) changes of fine structures close to the K-oxygen threshold, due to different bonding states; and (d) efficient Z-contrast imaging modes on sections of embedded biological material without metallic staining.     

Parallel recording of electron energy loss spectra

Two silicon photo diode array devices were tested as parallel recording detectors for electron energy loss spectrometry (EELS). The direct bombardment of a Reticon photodiode array detector with high energy electrons (80 keV) causes an irreversible increase in diode dark current. The dark current saturates the detector amplifier after a dose of 10^?6 C/diode making it unsuitable for EELS. A scintillator coupled SIT vidicon is sensitive enough to count two high energy electrons with a spatial resolution of 100 μm, corresponding to 5 eV energy resolution with the electron optical system described. The large pixel-to-pixel gain variation inherent in the scintillator and vidicon can be reduced by averaging the spectrum over a large area of the target perpendicular to the dispersion direction. The L-edge of calcium for a 4 × 10^?3 weight fraction concentration biological specimen is observable in a 40 s parallel recorded spectrum. The minimum detectable concentration of calcium is estimated tobe ten times better for EELS than EDS X-ray analysis.     

Spin and energy analyzed photoemission: a feasibility analysis

New scientific opportunities, particularly for investigation of surface magnetism, will be provided by spin and energy analyzed photoemission. Electron-optical conservation laws and phase space concepts are summarized and applied to determine the feasiblity of an experiment consisting of a photoemitter in a magnetic field, a photoelectron energy analyzer and an electron spin analyzer. For the example of photoemission from a Ni crystal using He I resonance radiation and typical parameters for the energy and spin analyzers, a final signal count rate of approximately 220 counts/s is calculated. Ways to increase the count rate by orders of magnitude are described. In particular, a new experimental configuration is suggested which may avoid the large reduction in count rate caused by the magnetic field.     

Coherence and incoherence of inelastically scattered electron waves

Whether the elastically and inelastically scattered electron waves are mutually coherent is a key question for the quantitative evaluation of the contrast of a high resolution transmission electron microscope image and for confirming the energy filtering ability of electron holography. Using a simplified object model composed of only two scattering centers the elastically and inelastically scattered electron waves and their detection are discussed. The only used proof for the main results of this paper is the orthogonality of the object states. The reasoning can be extended straightforward to a general case, e.g. a complicated object as well as complicated interaction Hamiltonians as long as they remain time independent. The results of this paper are: (1) The elastically and inelastically scattered waves which have different energy losses are mutually incoherent. The reason for that is simply that the corresponding excited object states are mutually orthogonal; (2) the inelastically scattered waves which have the same energy loss and are scattered by the same object state are coherent, whereas they are incoherent if they are scattered by different object states though they have the same energy; (3) the coherence degree of the scattered electron waves is proportional to the modulus of the scalar product of the corresponding object states.     

The response of a fast phosphor screen scintillator (ZnO:Ga) to low energy ions (0-60 keV)

ZnO:Ga is a promising, high time resolution candidate for use as a fast-ion-loss detector in TJ-II. We compare its ionoluminescence with that of the standard fast-ion-loss detector material, SrGa(2)S(4):Eu (also known as TG-Green), when irradiated by H(+) ions with a range of energies E≤60?keV using a dedicated laboratory setup. It is found that ZnO:Ga is a reasonably good candidate for detecting low energy (E<60?keV) ions as it has excellent time resolution; however, its sensitivity is about 100 times lower than TG-Green, potentially limiting it to applications with high energy ion loss signals.     

Surface analyses by low energy SEM in ultra high vacuum

The design and performance of an ultra high vacuum SEM with a field emission gun for solid surface are described. In situ observations of an Si surface are demonstrated for processes of surface cleaning by flashing, step formation of Si(111) surface by annealing, epitaxial growth of metal-deposited layers on Si surfaces by heat treatment, surface segregation of Ni-contaminated layer on the Si(110) by heat treatments and phase transitions of reconstructed structures by electron energy loss spectroscopy.     

Combined experimental setup for spin- and angle-resolved direct and inverse photoemission

We present a combined experimental setup for spin- and angle-resolved direct and inverse photoemission in the vacuum ultraviolet energy range for measurements of the electronic structure below and above the Fermi level. Both techniques are installed in one ultrahigh-vacuum chamber and, as a consequence, allow quasisimultaneous measurements on one and the same sample preparation. The photoemission experiment consists of a gas discharge lamp and an electron energy analyzer equipped with a spin polarization detector based on spin-polarized low-energy electron diffraction. Our homemade inverse-photoemission spectrometer comprises a GaAs photocathode as spin-polarized electron source and Geiger-Muller counters for photon detection at a fixed energy of 9.9 eV. The total energy resolution of the experiment is better than 50 meV for photoemission and better than 200 meV for inverse photoemission. The performance of our combined direct and inverse-photoemission experiment with respect to angular and energy resolutions is exemplified by the Fermi-level crossing of the Cu(111) L-gap surface state. Spin-resolved measurements of Co films on Cu(001) are used to characterize the Sherman function of the spin polarization detector as well as the spin polarization of our electron source.     

A sub-50meV spectrometer and energy filter for use in combination with 200kV monochromated (S)TEMs

A high-energy resolution post-column spectrometer for the purpose of electron energy loss spectroscopy (EELS) and energy-filtered TEM in combination with a monochromated (S)TEM is presented. The prism aberrations were corrected up to fourth order using multipole elements improving the electron optical energy resolution and increasing the acceptance of the spectrometer for a combination of object area and collection angles. Electronics supplying the prism, drift tube, high-tension reference and critical lenses have been newly designed such that, in combination with the new electron optics, a sub-50 meV energy resolution has been realized, a 10-fold improvement over past post-column spectrometer designs. The first system has been installed on a 200 kV monochromated TEM at the Delft University of Technology. Total system energy resolution of sub-100 meV has been demonstrated. For a 1s exposure the resolution degraded to 110 meV as a result of noise. No further degradation in energy resolution was measured for exposures up to 1 min at 120 kV. Spectral resolution measurements, performed on the pi* peak of the BN K-edge, demonstrated a 350 meV (FWHM) peak width at 200 kV. This measure is predominantly determined by the natural line width of the BN K-edge.     

Real space maps of magnetic moments on the atomic scale: Theory and feasibility

The recently discovered EMCD technique (energy loss magnetic chiral dichroism) can detect atom specific magnetic moments with nanometer resolution, exploiting the spin selectivity of electronic transitions in energy loss spectroscopy. Yet, direct imaging of magnetic moments on the atomic scale is not possible. In this paper we present an extension of EMCD that can overcome this limit. As a model system we chose bcc Fe. We present image simulations of the _L3${\mathrm{L}}_3$ white line signal, based on the kinetic equation for the density matrix of the 200 kV probe electron. With actual progress in instrumentation (high brightness sources, aberration corrected lenses) this technique should allow direct imaging of spin moments on the atomic scale.     

Developments in the dynamical theory of high energy electron reflection.

High energy electron reflection (HEER) is an important technique in surface science and uses the information carried by high energy electrons reflected from surfaces to study surface structures and surface electronic states. With the development of reflection high energy electron diffraction (RHEED), high energy electron microscopy (REM), and high energy electron energy loss spectroscopy (EEL) in surface science, the usefulness of HEER has been widely recognized and demonstrated. However, a stationary dynamical solution for an arbitrary surface for HEER has not been obtained yet. In this paper, some developments in understanding the dynamical theory of HEER, particularly in recent years, are reviewed: 1. The introduction of the concept of current flow for a semi-infinite crystal model has removed the confusion around the wave points in the "band gap." 2. The consistency between the Bloch wave and multislice in the Bragg case has verified the validity of the argument of current flow and led to the emergence of the BMCR method (Bloch wave + Multislice Combined for Reflection). 3. The failure of the Bloch Wave-Only solution (the BWO solution) on Au (110) surfaces in the Bragg case revealed by the BMCR method implies that previous BWO calculations in the Bragg case might be at fault. 4. The 2-D dependence of the electron wave fields and Picard iteration-like character of multislice calculation in the Bragg case has led to the emergence of an Edge Patching method in Multislice-mode-Only (the EPMO method). The new method yields an infinitely convergent stationary dynamical solution for an arbitrary surface.     

Spatial resolution and energy filtering of backscattered electron images in scanning electron microscopy.

Numerical simulations of energy filtering effects on backscattered electron images of semiconductor multilayers are reported. The theoretical investigation has been performed for a wide range of energies, 1-40 keV, and for beam incidence angles between 90 degrees (normal incidence) and 20 degrees. Quite a general purpose of this research concerns the investigation of the optimum energy conditions and of their implications. It will be shown that the optimum energy defines an operating context suitable to ensure a compositional contrast enhancement; i.e. a minimum threshold current and a maximum resolution, without energy filtering, independent of the beam incidence angle. This optimum energy, depending on the specimen and its details, is, however, of the order of a few keV or less for specimen details having a size of the order of few nm. When the performance of the electron gun does not allow to work at low energy it is necessary to operate at an energy higher than the optimum one, the energy filtering can produce positive effects. Yet in those circumstances there is an optimum energy loss window suitable to minimise the threshold current. It spreads from 10-30%, depending on the primary energy and size of the compositional detail, for normal incidence, to a few per cent for high incidence angles and high energy. The simulation results for these last conditions are in agreement with the well-known experimental results obtained with the low-loss methods.     

Unconventional immuno double labelling by energy filtered transmission electron microscopy

A new method of immuno double labelling of biological specimens with a very high spatial resolution is presented. The advantage over conventional techniques is the possibility of using two very small labels leading to higher labelling efficiency, better penetration into the specimen and reduced steric hindrance between labels at closely spaced sites. The two labels are distinguished by their electron energy loss spectra using principal component analysis and then identified by comparison with an external standard using discriminant function analysis. The method is tested on samples of insect flight muscle labelled with 8 nm colloidal gold and silver and the statistical reliability of the classification is assessed. Extensions of the method are suggested and its potential for biological research is discussed.     

Inverse photoemission with energy resolution better than 200 meV

We present a spectrometer for inverse photoemission in the vacuum ultraviolet range with variable energy resolution between 400 and 165 meV full width at half maximum. The energy distribution of the electron beam used for excitation can be adjusted between 300 and 125 meV by the use of a toroidal 90 degrees electrostatic deflector combined with a slit aperture. The emitted photons are detected by Geiger-Muller counters filled with either acetone or iodine as counting gas. The optical bandpasses of the detectors can be tuned between 100 and 330 meV by varying the temperature of their entrance windows. The overall resolution of the spectrometer is determined by measuring the Fermi-level onset in inverse-photoemission data of polycrystalline gold. Furthermore, the resolution enhancement is demonstrated by spectra of image-potential-induced surface states at Cu(001).     

High resolution microanalysis of aluminium solid solution alloys using combined electron microscopy and energy analysis

The technique of combined electron microscopy and energy analysis is described, together with the main energy loss process that is used to analyse composition. It is shown that the spatial resolution of the technique is ～ 14 nm which is ten times better than any other technique available at present. The composition resolution in aluminium alloys is < 1 at.%, the exact value depends upon which alloy system is used. Applications of the technique to the measurement of fine scale concentration gradients are described for Al-Cu, Al-Mg and Al-Zn-Mg alloys.     

Very low energy microscopy in commercial SEMs

Minimum necessary adaptations are described that are sufficient for obtaining very low energy electron micrographs (VLEEMs) from commercially available routine scanning electron micrographs (SEMs) with the electrons accelerated to an energy of the order of tens of keV. A cathode lens inserted into the specimen chamber enables one to decelerate electrons in front of the specimen surface to a desired low landing energy, which can be freely varied even down to zero. When a potential slightly more negative than the accelerating voltage is applied, a scanning mirror electron microscopy mode can be effected. The achievable point resolution at very low energies proves to be not too dependent on the objective lens parameters, so that the physical limit of aberrations of the homogeneous field of the cathode lens is nearly attainable. The detection efficiency for the standard Everhart-Thornley secondary electron detector is discussed, and results for the routine Tesla BS 340 SEM are presented.     

Demonstration of lanthanum in liver cells by energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy and high-resolution transmission electron microscopy

The appearance of lanthanum in liver cells as a result of the injection of lanthanum chloride into rats is investigated by advanced transmission electron microscopy techniques, including electron energy loss spectroscopy and high‐resolution transmission electron microscopy. It is demonstrated that the lysosomes contain large amounts of lanthanum appearing in a granular form with particle dimensions between 5 and 25 nm, whereas no lanthanum could be detected in other surrounding cellular components.