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STEM Optics
Published in Robert J. Keyse, Anthony J. Garratt-Reed, Peter J. Goodhew, Gordon W. Lorimer, Introduction to Scanning Transmission Electron Microscopy, 2018
Robert J. Keyse, Anthony J. Garratt-Reed, Peter J. Goodhew, Gordon W. Lorimer
At low magnifications, in a STEM with no post-specimen lenses, a view of the scanned raster pattern will appear because the scanned beams are not quite parallel to the axis (see Figure 2.3). If the electron probe is stationary on a crystalline part of a specimen then a convergent beam electron diffraction (CBED) pattern will be visible. If the scanning system is switched on to scan the probe over this area of the specimen (i.e. high magnification) the CBED pattern will still be visible, with only a slight loss of detail. Thus it is possible (since the DPOS often has a small hole in it) to view a high magnification bright field STEM image simultaneously with the CBED pattern.
Diffraction
Published in Peter E. J. Flewitt, Robert K. Wild, Physical Methods for Materials Characterisation, 2017
Peter E. J. Flewitt, Robert K. Wild
This class of diffraction patterns was first demonstrated in 1939 by Kossel and Mollenstedt, but their use and application have recently only become widespread following the ability to produce electron beams with a small diameter in the TEM and STEM. The effect has many similarities to Kikuchi diffraction. In convergent beam electron diffraction (CBED), the operating conditions in the microscope are arranged such that the electron beam is focused onto the crystal specimen. Thus, the specimen is traversed by a cone of electron beams. Convergent beam diffraction (CBD) differs from Kikuchi diffraction only in that for the convergent beam the cone of electrons is produced externally while in Kikuchi diffraction the cone of electrons arises from the diffuse scattering within the crystal. The principle is illustrated in Figure 3.72 where the cone of electrons is incident on the specimen surface at the plane BB′, the objective lens then focuses the diffracted beams onto the back focal plane at CC′ and DD′. The ray diagrams for the production of CBD are shown in Figure 3.73 for both the TEM and STEM modes. Here, the second condenser aperture determines the convergence angle and a large number with varying diameters is required. The objective lens focuses the electron beam onto the back focal plane after passing through the specimen. For a more detailed description of the technique, the reader is referred to Steeds (1979) and Tanaka and Terauchi (1985). These patterns display the variation in the intensity of transmitted or diffracted beams as a function of the angle between the incident electron beams and the crystal. These patterns have been given the generic name ‘Tanaka patterns’, and a typical bright-field pattern is shown in Figure 3.74 for a stainless steel specimen. This is essentially a map of the intensity of the direct beam variation with angle, over a range of angles in the general region of the [001] zone axis (Eades 1988).
Influence of transition elements (Zr, V, and Mo) on microstructure and tensile properties of AlSi8Mg casting alloys
Published in Canadian Metallurgical Quarterly, 2023
Zhan Zhang, Anil Arici, Francis Breton, X.-Gant Chen
Conventional metallographic polishing method was used to prepare the samples for microstructure observation. The polished samples were etched by 0.5% HF in deionised water for 35 s to reveal the distribution of dispersoids. A scanning electron microscope (SEM, JSM-6480LV) equipped with an energy dispersive X-ray spectrometer (EDS) was used to analyse the microstructure of these experimental alloys. A transmission electron microscope (TEM, JEM-2100) equipped with an energy dispersive EDS was applied to examine precipitates and dispersoids in detail. TEM foils were prepared in a twin-jet electropolisher using a solution of 30% nitric acid in methanol at −20°C. Convergent-beam electron diffraction (CBED) patterns were used to measure the thicknesses of TEM foils for the calculation of number density of dispersoids and precipitates. An optical image analyzer with CLEMEX JS-2000, PE4.0 software was employed to quantitatively characterise the phase features on optical, SEM and TEM images. For Si particles, 25 optical images with 500× magnifications for each sample were taken, and then the characteristics of the silicon particles in those optical images were statistically analysed using the image analyzer. For intermetallic particles, 25 backscattered SEM images with 500× magnification for each sample were statistically analysed with the image analyzer. The characteristics of needle-like β″-MgSi precipitates, such as the number density and size, were analysed with the methods developed in Ref. [19], and 6–8 TEM bright-field images with 50,000× magnification for each T6 sample were quantitatively analysed.
Diamond as the heat spreader for the thermal dissipation of GaN-based electronic devices
Published in Functional Diamond, 2022
The progress of GaN grown on the SCD (111) substrates was also achieved by the NTT Basic Research Laboratories. In 2011, Hirama et al. reported the AlGaN/GaN HEMTs with a low thermal resistance grown on SCD (111) substrate by using MOCVD [56]. Benefitting from a high temperature cleaning process at 1200 °C in the hydrogen ambient, the formation of the amorphous interfacial layer was prevented on the diamond substrate, which was a crucial process to obtain the atomically abrupt AlN/diamond heterointerface [57]. After the thermal cleaning, a 180 nm-thick AlN buffer was grown, followed by 20-period AlN/GaN multilayers, then the AlGaN/GaN heterostructure was grown with the GaN thickness of 600 nm. Although the surface morphology did not show atomic steps, the metal-polar was confirmed for the structure using convergent beam electron diffraction in this study. The two-dimensional electron gas (2DEG) was successfully achieved, with the sheet carrier density of 1.0 × 1013 cm−2 and mobility of 730 cm2/Vs. The AlGaN/GaN HEMTs with a 3 µm-gate length showed the maximum drain current of 220 mA/mm, cut-off frequency of 3 GHz and maximum frequency of oscillation of 7 GHz. The thermal resistance between HEMT and diamond is 4.1 K mm/W, as shown in Figure 8. The 0.4 µm-gate-length HEMT showed a dc drain-current density of 770 mA/mm and breakdown voltage of 165 V [58]. The RF power density of 2.13 W/mm was obtained (Figure 9). This is the first report on the RF power operation of the AlGaN/GaN HEMTs epitaxially grown on the diamond substrate.
Many-beam dynamical scattering simulations for scanning and transmission electron microscopy modalities for 2D and 3D quasicrystals
Published in Philosophical Magazine, 2019
Saransh Singh, William C. Lenthe, Marc De Graef
We have presented models of electron diffraction modalities for 2D and 3D quasicrystalline phases. Higher dimensional crystallography combined with appropriate occupation domains for different atom types are employed in our model. Specifically, forward models for conventional transmission electron diffraction, convergent beam electron diffraction, electron backscatter diffraction and electron channelling are derived. Simulations of different diffraction modalities for realistic quasicrystal structures are presented and compared with experimental diffraction patterns. For the AlNiCo example considered in this paper, the differences between the quasicrystalline and approximant phases manifest themselves in the mutual information value, providing a potentially automated way to classify quasicrystalline and approximant phases based on their EBSD patterns. Furthermore, the predicted diffraction patterns show strong dynamical scattering effects, both in the transmission and scattering microscopy modes. The forward model is used in a dictionary-based method to perform automated orientation mapping for quasicrystals. These models can also be generalised to forward models for defect imaging modalities such as scanning transmission electron microscopy diffraction contrast imaging (STEM-DCI) and electron channelling contrast imaging (ECCI).