High Voltage Scanning Transmission Electron Research Articles

Formation mechanism of dislocation network of cell structure in cyclically deformed near-[formula omitted] copper single crystals

As the initiation and propagation of fatigue crack is strongly correlated to the dislocations in cell structures of metals, investigating the formation of dislocation cell structures is essential for understanding the fatigue process. Although the cell boundary evolves from the dislocation network, the formation process of the dislocation network of fatigued copper single crystal is not completely realized. In this study, the dislocation structures in near-[1‾11] copper single crystals cyclically deformed at 293 K were investigated to analyze the formation mechanism of the dislocation network. The dislocation networks were characterized using a high-voltage scanning transmission electron microscope, where the virtual-scanning transmission electron microscope technique (virtual-STEM) was employed to conduct the Burgers vector analysis of the dislocations. The low-dislocation-density network was consistent with the three perfect screw dislocations (b = a/2 [1‾01], a/2 [1‾10], and a/2 [011‾]) in the (111) primary slip plane. For the high-dislocation-density region, the perfect and partial dislocations coexisted in the network. As the dislocation density increased, the perfect dislocations dissociated into Shockley partial dislocations (b = a/6 [1‾21‾], a/6 [112‾], and a/6 [2‾11]). This study provides a reference for future applications of the virtual-STEM technique in the field of dislocation analysis.

Characterization of dislocation microstructures in near-[formula omitted] single crystal copper using high-voltage scanning transmission electron microscopy

Metallic structures are prone to failure caused primarily by fatigue-related damage to the structural material. Fatigue behavior is strongly correlated with dislocation structures in metals. Therefore, investigating the formation of dislocation structures is essential for understanding the fatigue process. The dislocation structure in face-centered cubic crystals has a strong orientation dependence. However, the detailed characteristics and evolution process of dislocation structures of fatigued [1‾11] single crystal copper are still insufficient. This study investigated dislocation structures in near-[1‾11] single crystal copper cyclically deformed at room temperature under different constant plastic shear strain amplitude (γpl) to analyze the formation mechanism of dislocation structures and their relationship with fatigue behavior. Characteristic dislocation structures were characterized by adopting multiple planes using a high-voltage scanning transmission electron microscope. With increasing shear strain amplitude, a vein-like structure along the (1‾11) plane gradually developed into a wall structure. When γpl ≥ 1 × 10−3, deformation bands and cell bands were formed to accommodate the higher plastic strain. The state of cell-band development significantly affected the cyclic softening of the single crystal copper. A positive linear relationship existed between the shear stress amplitude and the reciprocal of the cell width.

Probing Threading Dislocations in a Micrometer-Thick GaN Film by High-Voltage Scanning Transmission Electron Microscopy

Probing Threading Dislocations in a Micrometer-Thick GaN Film by High-Voltage Scanning Transmission Electron Microscopy

High-Voltage Scanning Transmission Electron Microscopy: A Tool for Structural Characterization of Micrometer-Thick Specimens

Herein, the advantages of high-voltage scanning transmission electron microscopy (STEM) as a tool for structural characterization of micrometer-thick specimens are reported. Dislocations introduced in a wedge-shaped Si crystal were clearly observed by bright-field STEM operating at 1 MV. Many of the dislocations were straight and parallel to the 〈110〉, 〈112〉 or 〈113〉 directions. The widths of the dislocations in the STEM images were almost constant at 13–16 nm (i.e., 4–5 pixels) in the thickness range between 1 and 7.5 µm. The latest high-voltage STEM instrumentation is thus useful for imaging crystal defects in micrometer-thick materials, and enables multi-scale fields of view from a few nanometers squared to over 100 µm2.

Observation of Fatigue Dislocation Structures Formed in [001] Multi-slip Oriented Copper Single Crystals by High-voltage Scanning Transmission Electron Microscopy

Observation of Fatigue Dislocation Structures Formed in [001] Multi-slip Oriented Copper Single Crystals by High-voltage Scanning Transmission Electron Microscopy

Characterization of dislocation microstructures in cyclically deformed [001] copper single crystals using high voltage scanning transmission electron microscopy

Low-cycle fatigue tests of copper single crystals with the [001] stress axis were performed at room temperature under constant plastic shear strain amplitudes between γpl=3.5×10−4 and 1.0×10−2. Characteristic dislocation structures, which are highly dependent on a given plastic strain amplitude, were observed by high voltage scanning transmission electron microscopy. A vein-like dislocation structure, having a parallelepiped shape with two longitudinal (100) and (001) sides, was periodically formed along the [010] direction at around γpl=1.0×10−3. Labyrinth structure with the (100) and (001) dislocation walls was then gradually developed with increasing plastic strain amplitude.

Maximum usable thickness revisited: Imaging dislocations in Si by modern high-voltage scanning transmission electron microscopy

We have quantitatively evaluated the usable thickness of specimens in scanning transmission electron microscopy (STEM) at 1 MV using a wedge-shaped Si(110) single crystal including artificially introduced high-density dislocations. The width of dislocation images was employed as a criterion for the quantitative evaluation of usable thickness. Superior usable thickness in STEM than in TEM was found; the obtained results were 14.7 µm for STEM and 5.8 µm for TEM. In particular, in STEM, dislocations can be observed as thin lines with 10–15 nm width in the thickness range up to 10 µm. The latest high-voltage STEM is useful for imaging crystal defects in thick semiconductors.

Whole-cell imaging of the budding yeast Saccharomyces cerevisiae by high-voltage scanning transmission electron tomography

Electron tomography using a high-voltage electron microscope (HVEM) provides three-dimensional information about cellular components in sections thicker than 1 μm, although in bright-field mode image degradation caused by multiple inelastic scattering of transmitted electrons limit the attainable resolution. Scanning transmission electron microscopy (STEM) is believed to give enhanced contrast and resolution compared to conventional transmission electron microscopy (CTEM). Samples up to 1 μm in thickness have been analyzed with an intermediate-voltage electron microscope because inelastic scattering is not a critical limitation, and probe broadening can be minimized. Here, we employed STEM at 1 MeV high-voltage to extend the useful specimen thickness for electron tomography, which we demonstrate by a seamless tomographic reconstruction of a whole, budding Saccharomyces cerevisiae yeast cell, which is ~3 μm in thickness. High-voltage STEM tomography, especially in the bright-field mode, demonstrated sufficiently enhanced contrast and intensity, compared to CTEM tomography, to permit segmentation of major organelles in the whole cell. STEM imaging also reduced specimen shrinkage during tilt-series acquisition. The fidelity of structural preservation was limited by cytoplasmic extraction, and the spatial resolution was limited by the relatively large convergence angle of the scanning probe. However, the new technique has potential to solve longstanding problems of image blurring in biological specimens beyond 1 μm in thickness, and may facilitate new research in cellular structural biology.

Correlated micro-photoluminescence and electron microscopy study of a heterostructured semiconductor nanowire

Correlation between the optical and the structural properties of an individual heterostructured nanowire (NW) is crucial for optimising the NW synthesis and device design. In this work, low temperature (10 K) micro-photoluminescence (μ-PL), low and high voltage scanning transmission electron microscopy and conventional transmission electron microscopy are applied to the same individual NWs. The studied NWs, grown by Au-assisted molecular beam epitaxy, have a wurtzite GaAs core with a zincblende GaAsSb axial insert, enclosed with an AlGaAs radial shell and a GaAs capping layer. By subsequent analysis of one and the same NW by μ-PL and several electron microscopy techniques in different microscopes, we can relate the spectral features of a single NW to the structural features, such as different crystal phases, lattice defects and composition.

Strain Relaxation and In-Situ Observation of Voiding in Passivated Aluminum Alloy Lines.

ABSTRACTStrain relaxation in passivated Al-0.5% Cu lines was measured using X-ray diffraction coupled with in-situ observation of the formation and growth of stress induced voids. Samples of 1 μm thick Al-0.5% Cu lines passivated with Si3N4 were heated to 380ºC, then cooled and held at 150ºC. During the test, principal strains along the length, width, and height of the line were determined using a grazing incidence x-ray geometry. From these measurements the hydrostatic strain in the metal was calculated and strain relaxation was observed. The thermal cycle was duplicated in a high voltage scanning transmission electron microscope equipped with a backscattered electron detector. The 1.25 μm wide lines were seen to have initial stress voids. Upon heating these voids reduced in size until no longer observable. Once the samples were cooled to 150ºC, voids reappeared and grew. The measured strain relaxation is discussed in terms of void and θ-phase (Al2Cu) formation.

X-ray Determination and Finite-Element Modeling of Stress in Passivated Al-0.5%Cu Lines During Thermal Cycling.

AbstractWe have measured the principal strain state of AI-0.5%Cu lines passivated with silicon nitride directly and used it to calculate the stress state. The stress was determined as the lines were thermally cycled from room temperature to 450°C. The general stress-temperature behavior shows good fundamental agreement with that calculated using finite-element methods, although the magnitude of the stresses measured with x-rays is less than that predicted by modeling due to stressinduced voiding in the lines. This is shown with a high voltage scanning transmission electron microscope (STEM) operated in the backscattering mode.

Strain Relaxation and In-Situ Observation of Voiding in Passivated Aluminum alloy Lines

AbstractStrain relaxation in passivated Al-0.5% Cu lines was measured using X-ray diffraction coupled with in-situ observation of the formation and growth of stress induced voids. Samples of 1 μm thick Al-0.5% Cu lines passivated with Si3N4 were heated to 380°C, then cooled and held at 150°C. During the test, principal strains along the length, width, and height of the line were determined using a grazing incidence x-ray geometry. From these measurements the hydrostatic strain in the metal was calculated and strain relaxation was observed. The thermal cycle was duplicated in a high voltage scanning transmission electron microscope equipped with a backscattered electron detector. The 1.25 μm wide lines were seen to have initial stress voids. Upon heating these voids reduced in size until no longer observable. Once the samples were cooled to 150°C, voids reappeared and grew. The measured strain relaxation is discussed in terms of void and θ-phase (Al2Cu) formation.

Convergent-beam electron diffraction in the high-voltage electron microscope with continuously variable reference voltage for lenses and alignments

A device which provides a continuously variable reference voltage for lenses and alignments is developed for the high-voltage scanning transmission electron microscope H-1250ST. When continuously varying the operating voltage, compensation for all lenses and alignments is easily performed by the adoption of the device. The device is especially useful for forming the convergent-beam electron diffraction (CBED) patterns at any accelerating voltage. The performance of the device is demonstrated by observing the CBED patterns. Large-angle CBED patterns are taken for Si- and Y-based high- T c superconductors at various voltages. Systematic critical voltages of Y-based high- T c superconductors are measured for the 400 and 220 reflections.

A high-voltage scanning transmission electron microscope at Nagoya University.

A high-voltage scanning transmission electron microscope (STEM) H-1250ST of the maximum accelerating voltage of 1.25 MV was constructed at Nagoya University in 1983. The microscope, equipped with a field-emission gun, is designed with high-level STEM performance as well as conventional transmission microscopy mode operation. The aim of developing the microscope, basic design schemes, principal instrumentation, and techniques developed are described.

1 Mev Stem: A Progress Report

A high voltage scanning transmission electron microscope(HVSTEM) has a number of unique features: Compared with a high voltage conventional electron microscope, it can provide better resolution and contrast for thick specimens, because of the absence of the chromatic aberration contributed by energy-loss events; Compared with a low voltage STEM, it can collect inelastically scattered electrons more efficiently, since the the characteristic angle of single inelastic scattering varies inversely with energy; The size of the beam can be varied over a wide range for the purpose of selected-area-diffraction. The application of this promising technique in particular to the study of biological molecules which form crystals too thick to be studied with low voltage electron microscopies awaits exploration. In addition, in theory, the spot size of a HVSTEM is better than one Ångstrom – which undoubtly could not be achieved by a low voltage STEM unless the spherical aberration is corrected.

Investigation of Semiconductor Materials and Devices by High Voltage STEM Techniques

AbstractThis paper reviews work carried out in‐situ on semiconductor devices by means of a high voltage electron microscope equipped with facilities for scanning transmission electron microscopy, electron beam induced conductivity and energy loss spectrometry. These combined techniques have allowed simultaneous and unambiguous correlations to be established between crystallographic, electrical and elemental properties of semiconductor materials and devices. Examples of these correlations are given, and mention is made to facilities (scanning deep level transient spectroscopy and cathodoluminescence) which may further increase the potential of high voltage scanning transmission electron microscopy.

Imaging of weak Lorentz objects (p-n junctions) by high voltage Fresnel TEM and STEM

The very large (>or=15 cm) out of focus electron microscope techniques used for detecting electric and magnetic fields producing angular deflections of 10-5-10-6 rad are considered. Observations made on weak Lorentz objects, such as the electric field of p-n junctions, (i) by high voltage transmission electron microscopy (TEM) and by high voltage scanning transmission electron microscopy (STEM) performed by means of an attachment to a TEM instrument; (ii) by low and high voltage TEM are compared. The results show that high voltage TEM is very satisfactory-better than conventional TEM and far superior to STEM in imaging very weak Lorentz (phase) objects even in thick specimens, at least with the present state of development of STEM attachments. The operational conditions are given.