Papers by Keyword: STEM

Abstract: It is well known that the work hardening process of low-carbon steels is highly dependent on the movement and accumulation of dislocations in the crystal grains, which affect the stress and strain magnitudes and their distribution. The aim of this paper is to explain the importance of dislocation movement and density on the temperature, i.e. stress and strain changes during cold plastic deformation of low-carbon steels. Therefore, tests were carried out in this paper using the methods of static tensile testing, thermography, digital image correlation (DIC) and microstructural analysis. The microstructure analysis was carried out using a light and transmission electron microscope (TEM). The transmission electron microscope analysis was performed in two different modes, the TEM and scanning TEM (STEM). The results of static tensile testing, thermography and digital image correlation (DIC) are related to the microstructural changes that occur during the work hardening process of low-carbon steel. At the moment of maximum work hardening (immediately before fracture), significant grain elongation and high dislocation density of low-carbon steel were observed.

Abstract: Two types of carrot defects with and without a shallow pit were found by mirror projection electron microscopy (MPJ) inspection in 4H-SiC epi wafer. Surface morphology and cross-sectional structure of prismatic stacking faults (PSFs) were investigated using MPJ and atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM). The depths of the surface grooves due to the PSFs, the stacking sequences around the PSFs and the structure of the Frank-type stacking faults which were connected to the PSFs were different. We discuss the difference between the two types of carrot defects.

Abstract: Ga₂O₃ is a hopeful wide-band-gap semiconductor material for a next-generation power semiconductor. We performed crystal structure analysis on Ga₂O₃ film on sapphire substrate using cross-sectional transmission electron microscope (TEM) and atomic resolution plan-view scanning transmission electron microscopy (STEM). The TEM analysis suggested that the main Ga₂O₃ film is composed of κ-Ga₂O₃ or mixed crystal of κ-Ga₂O₃ and ε-Ga₂O₃. But, it is difficult to distinguish these two possibilities only by cross-sectional TEM. Contrast modulation of Ga atomic columns in the atomic resolution HAADF-STEM image showed that the main part of the Ga₂O₃ film was κ-Ga₂O₃ monolayer grown along the c-axis direction, and twins are formed.

Abstract: Dislocations and stacking faults in 4H-SiC (0001) _si epitaxial wafer was inspected by mirror projection electron microscopy (MPJ) with the aid of low-energy SEM and FIB-STEM. MPJ observation found dislocation conversion near the wafer surface, and the conversion was confirmed by micro etch pit and low energy SEM method. Another conversion occurred in the epitaxial layer on array of TED half loops, which were detected by MPJ, was also observed by cross-sectional STEM.

Abstract: The effect of varying growth rate on the formation of defects in homo-epitaxially grown cubic silicon carbide (3C-SiC) is studied. Three growth rates are considered (30, 60 and 90 μm/hr) demonstrating that as the growth rate increases the density of point defects, as demonstrated by photo- luminescence, and stacking faults (SFs), as measured by a KOH etching procedure, increase. Scanning transmission electron microscopy images demonstrate generation, annihilation and closure of SFs as a function film thickness. High resolution X-ray diffraction is used to uncover the higher quality of homo-epitaxial with respect hetero-epitaxial films through the examination of the sample mosaicity and SF density.

Abstract: A Barium-rich interface process provides SiO₂/SiC interface conditions suitable for obtaining SiC field-effect (FE) channel mobility twice that of a nitric oxide (NO) passivation anneal. The temperature dependence of the field-effect mobility indicates clear differences in their interface properties. Secondary-ion mass spectrometry (SIMS) indicates that Ba remains predominantly at the SiO₂/SiC interface, with only ~1×10¹⁷ cm^-3 Ba in the oxide.The interface structure and chemistry of the Ba-modified MOS devices was investigated using scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS). High-angle annular dark-field (HAADF) imaging reveals that the Ba interface layer results in an oxide-interface region not present in the NO annealed control sample. EDS mapping shows that this is a Ba-rich oxide interface layer. Using a new technique “revolving STEM” (RevSTEM) to correct drift and image distortion, SiC strain maps were generated. With an NO anneal there is tensile strain within SiC at the SiO₂/SiC interface, along the C-axis direction. With the Ba interlayer, however, there is no observable strain relative to the bulk SiC. This interface strain may correlate with the inversion layer mobility, with an unstrained interface preferred.

Abstract: Using a fractal analysis approach to study plant leaf venation and stem sections, we find that plants use very intelligent scaffolding strategies to tune mechanical strength of leaves and stems. Within plant organs, specialized types of tissues with different mechanical properties have evolved. Ideally, the biopolymers cellulose, hemicelluloses and lignin present in plant cell walls confer mechanical rigidity to plant tissues, but our studies reveal that the manner these biopolymers are distributed in the tissue matrix hold the key to the mechanical rigidity of the tissues. We have developed an algorithm to determine fractal dimension of the scaffolding matrix and the well-known box counting algorithm to calculate fractal dimensions of leaf venation in high resolution images of reticulate–veined leaves and optical microscope image of cellulose, hemicellulose, and lignin-stained cross sections of Turbina corymbosa. We found that in leaves with reticulate venation, veins form a scaffolding matrix imparting mechanical rigidity to leaves, and have a fractal dimension close to 1.0 for leaves which have less bending resistance, compared to fractal dimensions close to 1.7 for leaves which have higher bending resistance. Deriving this idea from plants, we use evaporation instability to develop scaffolding matrix with fractal dimensions higher than 1.5 in polymer films. This can form the basis of an efficient strategy to devise thin, stand-alone polymer films with tunable bending stiffness.

Abstract: This paper presents an investigation of the morphological and structural properties of graphene (Gr) grown on SiC(000-1) by thermal treatments at high temperatures (from 1850 to 1950 °C) in Ar at atmospheric pressure. Atomic force microscopy and micro-Raman spectroscopy showed that the grown Gr films are laterally inhomogeneous in the number of layers, and that regions with different stacking-type (coupled or decoupled Gr films) can coexist in the same sample. Scanning transmission electron microscopy and electron energy loss spectroscopy shoed that a nm-thick C-Si-O amorphous layer is present at the interface between Gr and SiC. Basing on these structural results, the mechanisms of Gr growth on the C-face of SiC under these annealing conditions and the role of this disordered layer in the suppression of epitaxy between Gr and the substrate have been discussed.

Abstract: We observed fine surface morphology of silicon carbide wafers using a low energy scanning electron microscope (LESEM). Typical kinds of surface defects were observed by LESEM. After low temperature KOH treatment, it is confirmed that positions of etch pits are the same positions of these defects. Correlation between LESEM imaging and cross-sectional scanning transmission electron microscopy (STEM) of the same defects reveals threading dislocations and basal plane dislocations at the core of the defects.

Abstract: Successful employment of advanced tool steel in engineering applications is related to its ability in terms of meeting service life requirements and fabrication with proper dimensions. Deep cryogenic treatment may be used to produce advanced tool steel by simultaneously increasing toughness, strength, and hardness. Twelve sets of specimens were tested in this paper, 9 of which were deep cryogenic treated and then tempered. Tensile properties, hardness, X-ray diffraction, and scanning transmission electron microscopy were applied for macroscopic and microscopic investigations. The best results of simultaneous improvement in toughness, hardness, and strength were obtained for 36 h soaking time and 1 h tempering time.