Quantification of High-Resolution Lattice Images and Electron Holograms

Progress towards the quantification of high-resolution electron microscopy and electron holograms has been achieved using digital acquisition with a slow-scan charge-coupled device (CCD) camera. Two applications are described: the precise measurement of lattice-fringe spacings and the determination of the mean inner potential. Lattice images can be characterized by a finite sum of two-dimensional sinusoids. A new method for measurement of the frequency, amplitude and phase of each sinusoid, based on an interpolation technique in reciprocal space, is presented. The method offers considerably higher precision for measurement of lattice fringes than the optical bench and is applicable to images recorded with an electron dose of less than 1 el / Å2 and specimen areas as small as 8 Å across. The attainable precision is dependent on specimen characteristics, electron dose and the size of the measured area, and ranges from 0.001 Å to 0.05 Å. An improved method has also been developed for measurement of mean inner potential using digital off-axis electron holograms from 90° crystal wedges. The value of (-14.21 ± 0.16) V obtained for GaAs represents the most accurate measurement yet reported for the mean inner potential

Nano-faceted stabilization of polar-oxide thin films : The case of MgO(111) and NiO(111) surfaces

Molecular beam epitaxy growth of polar MgO(111) and NiO(111) films demonstrates that surface stabilization of the films is achieved via the formation of neutral nano-faceted surfaces. First-principles modeling of the growth of polar MgO(111) films reveals that the growth does not proceed layer-by-layer. Instead, the Mg or O layers grow up to a critical sub-monolayer coverage, beyond which the growth of the next layer becomes energetically favorable. This non-layer-by-layer growth is accompanied by complex relaxations of atoms both at the surface and in the sub-surface, and leads to the experimentally observed surface nano-faceting of MgO and NiO (111) films through formation of neutral nano-pyramids terminated by {100} facets. These facets are limited in size by an asymptotical surface energy relation to their height; with the reconstruction being much more stable than previously reported surface terminations across a wide range of growth conditions. The termination offers access to lower coordinated atoms at the intersection of the neutral {100} planes whilst also increasing the surface area of the film. The unique electronic structures of these surfaces can be utilized in catalysis, as well for forming unique heterostructures for electronic and spintronic applications

Quantitative electron holography applied to crystal wedges and interfaces

Electron holography is unique among transmission electron microscopy techniques in that it allows retrieval of phase of the electron wave, information which is lost with other imaging techniques. With access to the phase, interactions of the incident electron wave with the electrostatic and magnetic potentials of the specimen can be studied directly with high spatial resolution. Besides resolution enhancement, the original goal for electron holography, applications to wide range of materials problems are currently under investigation. This work will address applications to measurements of mean inner potential of crystals and to epitaxial crystal interfaces.In non-magnetic materials, in the absence of external electrical fields and/or charge layers, the electrostatic potential which alters the phase of the exit surface wave results from the atomic potentials. For amorphous and polycrystalline samples of known thickness, the phase change depends on the volume average of the atomic potentials, i.e. the mean inner potential V0.</jats:p

Parabolas from Reconstructed Surfaces Observed in Convergent-Beam RHEED Patterns

Parabolas have been observed in the reflection high-energy electron diffraction (RHEED) patterns from surfaces of single crystals since the early thirties. In the last decade there has been a revival of attempts to elucidate the origin of these surface parabolas. The renewed interest stems from the need to understand the connection between the parabolas and the surface resonance (channeling) condition, the latter being routinely used to obtain higher intensity in reflection electron microscopy (REM) images of surfaces. Several rather diverging descriptions have been proposed to explain the parabolas in the reflection and transmission Kikuchi patterns. Recently we have developed an unifying general treatment in which the parabolas are shown to be K-lines of two-dimensional lattices. Here we want to review the main features of this description and present an experimental diffraction pattern from a 30° MgO (111) surface which displays parabolas that can be attributed to the surface reconstruction.</jats:p

Reflection electron microscopy of triangular quantum wells

Asymmetric triangular quantum wells (ATQW) have recently been developed to improve the absorption and emission characteristics of opto-electronic devices. A triangular compositional gradient is produced during epitaxial growth of AlxGa1-xAs on GaAs by either continuous variation of x or by alternate deposition of thin layers of the two compounds where the ratio of the layer thickness produces an average alloy composition. The knowledge of the compositional profile of the actual ATQW, as derived from HREM, has proven essential for the description of the photo-emission properties of the device. Recently, the technique of reflection electron microscopy has also been applied to multiple rectangular quantum wells of GaAs/AlxGa1-xAs with the encouraging result that layers with 1% Al can be distinguished from the GaAs layers. In this work we investigate the applicability of REM to characterization of ATQW.Triangular quantum wells were grown by MBE, employing the digital alloy method. Fig. 1 shows a schematic representation of the nominal thickness and composition of the studied ATQW.</jats:p

Wave-front phase retrieval in transmission electron microscopy via ptychography

There are many different strategies that allow the solving of the well-known phase problem corresponding to the loss of phase information during a physical measurement. In microscopy, and, in particular, in transmission electron microscopy, most of these strategies focus on the retrieval of high-resolution information with the importance of lower resolution data often overlooked. Ptychography offers a means to investigate such data. Ptychography is a robust diffractive imaging technique with fast convergence for phase retrieval but, until now, has not been applied at the nanoscale. In this paper, we use the ptychographical iterative engine to retrieve the phase change at the exit plane of metallic nanoparticles using a conventional transmission electron microscope. Ptychographical reconstructions yielded images with a phase resolution of π/10 and a spatial resolution of 1 nm. These results stand as a first step toward aberration-free lensless imaging. The technique lends itself to be an alternative to off-axis electron holography or focal series reconstructio