Diffraction Of Incident Beam Research Articles

Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications.

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modelling of the energy- and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

Generation of optical vortex light beams by volume holograms with embedded phase singularity

Special features of the optical-vortex (OV) beams generated by thick holographic elements (HE) with embedded phase singularity are considered theoretically. The volume HE structure is based on the 3D pattern of interference between an OV beam and a standard reference wave with regular wavefront. The incident beam diffraction is described within the framework of a linear single-scattering model in which the volume HE is represented by a set of parallel thin layers with the “fork” holographic structure. An explicit integral expression is derived for the complex amplitude distribution of the diffracted paraxial beam with OV. The numerical analysis demonstrates that the HE thickness may essentially influence not only selectivity and efficiency of the OV beam generation but also the amplitude and phase profile of the diffracted beam as well as regularities of its propagation. We have studied the generated OV morphology and laws of its evolution; in particular, the possibility of obtaining a circularly symmetric OV beam regardless of the diffraction angle is revealed.

X-ray absorption anisotropy for polychromatic illumination—Crystal views from inside

We review an atomic resolution imaging method based on the analysis of the fine structure in X-ray absorption anisotropy, which results from incident beam diffraction. For a polychromatic X-ray beam, due to the suppression of higher order diffraction fringes, X-ray absorption anisotropy patterns can be interpreted as distorted real-space projections of the atomic structure around absorbing atoms. A qualitative method for analysis of X-ray absorption anisotropy patterns is presented, based on modeling of X-ray patterns with ray-traced images calculated for clusters around absorbing atoms.

Electron Collisions with Complex Targets: Diffraction Effects in Stimulated Desorption

We describe a new electron-scattering technique involving incident beam diffraction in electron-stimulated desorption (ESD) to probe ground-state surface bonding configurations of adsorbates and excited state configurations which lead to cation ESD. We present a qualitative theoretical description of diffraction in electron-stimulated desorption (DESD) and then demonstrate this process by studying the ESD of Cl+ from Cl terminated Si(111) (1 × 1) surfaces. The fine structure in the Cl+ ESD yields as a function of azimuth is due to interference of the incident electron-beam with the elastically scattered fraction. This interference forms a surface standing wave in the initial state of the desorption process. Comparison of the experimental data to the qualitative theory indicates that desorption is primarily initiated by an excitation localized on the Si atom bonded to Cl. The ~17 eV threshold energy is consistent with an ESD mechanism involving ionization of the Si 3s level followed by Auger cascading from the σ-bonding surface state and shake-up of an electron in the non-bonding surface π-level. The 2-hole, 1-electron final state produced can lead to the ESD of Cl+ due to the hole-hole Coulomb repulsion. Direct ionization of the Si–Cl σ-bonding state can also contribute to Cl+ ESD. In general, DESD provides a useful probe of the geometry and dynamics governing low-energy electron interactions with ordered complex targets.

Incident beam diffraction in electron stimulated desorption

We describe a new experimental technique utilizing incident beam diffraction in electron stimulated desorption (ESD) to determine the initial atomic bonding structure of desorbates. Diffraction in ESD can provide detailed information about the ground state surface bonding geometry of the absorber with sub-angstrom resolution of bond vectors. We discuss the role of electron diffraction in ESD, and demonstrate these ideas with experimental measurements of Cl + ion ESD from chlorinated Si(111)-(7×7) and (1×1) surfaces.

Incident beam effects in AED (Auger Electron Diffraction): the case of Cu(001)

The incident beam effects in the Auger Electron Diffraction of the Cu(001) surface have been studied through the anisotropy emission of the L 2,3VV and the M 2,3VV transitions. The spectra have been collected using a screened CMA in a polar scan for the two main direction [100], [110] and in normal emission azimuthal scan. The low and high kinetic energy Auger lines show anisotropies spectra with very different behaviour all over the polar angle scan, in particular around the 0° emission we observe that the anisotropy signal is in total antiphase. This is in very good agreement with the theoretical and the experimental X-ray photoelectron diffraction. Although incident beam diffraction should be sizeable, when the collection of the spectra is made with a standard CMA, our experiments evidence that incident beam diffraction plays a negligible role when the electrons collection is made through a very small detection angle.

PLVV Auger lineshape modulation by incident beam diffraction in InP

The electron excited PL 2,3VV Auger spectrum of InP has been investigated as a function of the incident beam direction. Phosphorus lineshape has been found to be modulated by the scattering-interference of primary electrons via changes in the relative intensity of surface- and bulk-specific spectral features.

PLVV Auger lineshape modulation by incident beam diffraction in InP

The electron excited PL 2,3VV Auger spectrum of InP has been investigated as a function of the incident beam direction. Phosphorus lineshape has been found to be modulated by the scattering-interference of primary electrons via changes in the relative intensity of surface- and bulk-specific spectral features.

Incident-beam effects in electron-stimulated Auger-electron diffraction.

We have examined incident-beam effects in electron-stimulated Auger-electron diffraction (AED) on a cleaved GaAs(110) surface. The results indicate that incident-beam diffraction is significant in an AED experiment, and that the dissipative nature of the incident beam in contributing to the Auger process must be accounted for. We have developed a qualitative model that describes the trend of the polar-angle dependence of the Auger intensity for both the incident and exit beams. In calculating the diffraction features, we used a zeroth-order approximation to simulate the dissipation of the incident beam, which is found to adequately describe the experimental data.

Incident beam diffraction effects in Auger electron emission from crystal surfaces

We have studied the anisotropy of Auger electron excitation by incident electrons in the near-surface layer of a crystal. By separate detection of the Auger electrons from the crystal substrate (Mo) and a submonolayer Si film we have determined the relative contributions due to diffraction of the incident beam and to the orientational dependence of the backscattering factor. By comparing the experimental data with the theoretical calculations made in many-wave dynamic approximation we have estimated the effect of thermal atomic vibrations and of the Coulomb nonlocality of ionization on the orientational anisotropy of the Auger signal. We have revealed a decrease of the orientation-averaged Auger electron yield compared to a disordered target of the same composition.

Incident beam effects in medium-energy backscattered electron diffraction.

We have examined the angular distributions of medium-energy (500--1500 eV) elastically backscattered electrons from bulk Cu(001) and Ni(001) crystals and from pseudomorphic Cu on Ni(001). The observed diffraction features are largely governed by forward scattering of the outgoing electrons, and incoming beam scattering appears to play a relatively minor role. This conclusion is reached by comparing these angular distributions with theoretical angular distributions, and with those obtained via angle-resolved Auger electron and x-ray photoelectron diffraction spectroscopies involving electrons of comparable kinetic energies. We find that the three experimental techniques yield very similar results, indicating that incident beam effects and the mode of excitation play a minor role. In the calculations, we have both included and neglected incident beam diffraction. By including incoming beam diffraction, we have for simplicity ignored the phase relationship between incoming and outgoing beams. The result is that inclusion of incoming beam scattering does not particularly improve the level of agreement between theory and experiment relative to calculations which neglect incoming beam diffraction. Moreover, the level of agreement between experiment and theory is quite good for the simpler theory which only treats outgoing electron scattering. Finally, we find that azimuthal-angle medium-energy backscattered electron diffraction scans for pseudomorphic Cu/Ni(001) are not as sensitive to elastic overlayer strain as are polar-angle scans. The latter measurement provides a highly sensitive method of measuring strain which is quite straightforward to interpret.