Conventional Dark-field Imaging Research Articles

Considerations for extracting moiré-level strain from dark field intensities in transmission electron microscopy

Bragg interferometry (BI) is an imaging technique based on four-dimensional scanning transmission electron microscopy (4D-STEM) wherein the intensities of select overlapping Bragg disks are fit or more qualitatively analyzed in the context of simple trigonometric equations to determine local stacking order. In 4D-STEM based approaches, the collection of full diffraction patterns at each real-space position of the scanning probe allows the use of precise virtual apertures much smaller and more variable in shape than those used in conventional dark field imaging such that even buried interfaces marginally twisted from other layers can be targeted. With a coarse-grained form of dark field ptychography, BI uses simple physically derived fitting functions to extract the average structure within the illumination region and is, therefore, viable over large fields of view. BI has shown a particular advantage for selectively investigating the interlayer stacking and associated moiré reconstruction of bilayer interfaces within complex multi-layered structures. This has enabled investigation of reconstruction and substrate effects in bilayers through encapsulating hexagonal boron nitride and of select bilayer interfaces within trilayer stacks. However, the technique can be improved to provide a greater spatial resolution and probe a wider range of twisted structures, for which current limitations on acquisition parameters can lead to large illumination regions and the computationally involved post-processing can fail. Here, we analyze these limitations and the computational processing in greater depth, presenting a few methods for improvement over previous works, discussing potential areas for further expansion, and illustrating the current capabilities of this approach for extracting moiré-scale strain.

High-SNR 3D dark-field microscopy based on orbital angular momentum extraction

A 3D dark-field microscopy with high signal-to-noise ratio (SNR) is realized by demodulating the orbital angular momentum (OAM) information of the compound signal from the sample. Under ordinary Gaussian beam illumination containing pure 0th-order OAM component, the scattering of the sample extends the input light energy to non-zero order ones. In order to extract the specific order of OAM component, a forked-grating hologram with opposite wavefront is applied in the detection path. The SNR with 4.5 times enhancement is observed experimentally compared with conventional dark-field imaging in that the demodulation of the dark-field signal in OAM domain realizes a comparatively more thorough separation of reflected and scattered light. A spatial resolution of better than 0.78 μm is achieved. The 3D sectioning ability inherited from confocal configuration is verified by volumetric imaging of the defects of a fused silica sample with topological charge m = 0, 1, 2. The analysis of multiple orders tends to provide indications for specific characteristics of the samples such as the structural chirality and symmetry.

Super-contrast-enhanced darkfield imaging of nano objects through null ellipsometry.

We rediscover the null ellipsometry principle for an outstanding image-contrast enhancement method for darkfield imaging. Simply by adding polarizers, compensators, and a photodiode sensor to a conventional darkfield imaging system and applying the null principle, Si nano-cylinder structures as small as D20nm (H20nm) on non-patterned wafer, and gap defects as small as 14.6nm and bridge defects as small as 21.9nm on 40nm line and 40nm space patterns (H40nm), which are invisible in conventional darkfield imaging, can be distinguished from scattered noise. To the best of our knowledge, no method has been successful for identifying such small non-metal (silicon) nanoscale objects with such low magnification (×20) optics.

Antiphase structures in a periodically inverted GaAs/AlGaAs waveguide investigated by transmission electron microscopy

We have performed transmission electron microscopy (TEM) observation of antiphase structures in a periodically inverted GaAs/Al0.1Ga0.9As waveguide fabricated on a (100) GaAs substrate intentionally misoriented toward by 2°. We have unambiguously confirmed that the artificial (011) boundaries between the inverted and noninverted domains are of antiphase nature by TEM observations including conventional dark-field imaging and a two-beam technique. We also investigated unintentionally formed self-annihilating non-inverted GaAs domains grown on a Ge interlayer introduced for growing inverted GaAs epilayers using the sublattice-reversal epitaxy technique.

Structure and possible origins of stacking faults in gamma-yttrium disilicate

Parallel stacking faults on (010) planes are frequently observed in hot-pressed Y2Si2O7. A combination of conventional dark-field imaging and high-resolution transmission electron microscopy was used to investigate the structure of these faults. It was found that they consisted of the repeat of one layer of the two-layer γ-Y2Si2O7 structure with an associated in-plane rigid body displacement. The resulting structure was confirmed by image simulation of high-resolution images from two perpendicular projections. A model for the formation of the stacking faults is proposed as a consequence of a transformation from β-Y2Si2O7 to γ-Y2Si2O7 in the hot pressing.

Electron microscopy nanoscale characterization of ball-milled Cu–Ag powders. Part I: Solid solution synthesized by cryo-milling

We present a study of nanostructured Cu–Ag material obtained by low temperature ball milling. The microstructural characterization is carried out using a wide variety of transmission electron microscopy (TEM) techniques from conventional dark-field imaging, selected area diffraction and energy dispersive X-ray spectrometry (EDS) to more modern techniques of scanning TEM high-angle dark-field imaging (HAADF), nanodiffraction and high resolution imaging (HREM). A novel method of HREM image analysis is also presented which consists of calculating geometric phase images by Fourier filtering. Real-space maps of lattice spacings and lattice rotations are thus obtained. The analysis addresses the following essential points of nanostructural characterization: dislocation densities, grain sizes and morphologies, grain boundaries and local lattice rotations, local textures, local variations in lattice parameters. A new description of the microstructure emerges from the observations, quite different to that expected. Analytical modeling suggests that large lattice rotations can be expected in nanomaterials produced by intense deformation.

Imaging of III-V Compound Superlattices by Hrem and Rem

ABSTRACTTwo techniques for the analysis of III-V compound superlattices are examined. It has been proposed that high-resolution TEM of [100]-oriented thin foils would give an improvement in layer contrast compared with [110]-oriented thin foils; it is shown here that the contrast of [100]-oriented superlattices is not necessarily better. Moreover, both high resolution and conventional dark-field imaging may be subject to significant diffraction contrast effects resulting from the bending of the reflecting planes near the surface of the sample. Reflection electron microscopy (REM) of cross-sectional (110) cleavage planes can also yield dark-field superlattice images and selected area RHEED patterns can in principle be used to determine reliably the superlattice strain as surface effects are minimized.

Determination of orientation of small crystallites in thin nickel films on UHV-cleaved mica

Because of the small size of thin film crystallites (<100 A) in the initial stages of growth, the application of electron diffraction methods such as small area diffraction to determine crystallographic orientation isnot possible. Direct lattice imaging techniques are limited to large lattice spacings and require improved electron-optical operation conditions, Conventional dark field imaging shows poor azimuthal angle resolution ~10° ) . A powerful but simple method can overcome these difficulties. It allows the determination of lattice planes according to Bragg image contrast 1 and with further improvement the evaluation of the exact and unique orientation of small crystals 2 is possible without the need to use very high resolution microscopes. The technique is based on the fact that the dark field image of a diffracting crystal is displaced by defocusing the objective lens (defocused dark field imaging : DDF). The amount of the shift depends on the defocus and the diffraction angle, and hence on the imaging lattice plane. Combining the amount and the direction of the shift (which is perpendicular to the lattice plane (hkl)) results in a vector proportional to the corresponding reciprocal lattice vector (h, k,/). The shift can be measured by doubly exposing an electron micrograph with a defocus difference between the two exposures. An azimuthal angle resolution of about 1 ° is obtainable.