Screw Dislocations Research Articles

Chemical Vapor Deposition of Er-Doped WS2 Flakes with Near-Infrared Emission and Enhanced Near-Infrared Photoresponse

The electronic and optical properties of two-dimensional transition metal dichalcogenides can be tuned by a doping strategy. Herein, spiral and monolayer Er-doped WS2 (WE) flakes have been synthesized by chemical vapor deposition using ErCl3, WO3, and S as precursors. The growth and properties of WE flakes have been systematically studied based on experimental and theoretical analysis. The morphology, size, and thickness of WE flakes can be regulated by adjusting the molar ratio of ErCl3/WO3 and the growth time. It has been found that the ErCl3 precursor not only acts as the promoter of the WE flakes during the growth but also drives the formation of screw dislocations owing to the internal strain in the lattice induced by Er3+ doping. The WE flakes exhibit near-infrared (NIR) emission at ∼1530 nm under 980 nm excitation, which corresponds to the energy transition from 4I13/2 to 4I15/2 of the Er3+ dopants. In addition, the WE-based device exhibits improved NIR photoresponse with a 16.9-fold enhancement in photocurrent compared with that of the WS2-based device. Our work provides a simple method for the preparation of highly crystalline WE flakes with NIR emission and enhanced NIR photoresponse.

Simulated nanoindentation into single-phase fcc Fe_{x}Ni_{1-x} alloys predicts maximum hardness for equiatomic stoichiometry

We investigate by molecular dynamics simulation the mechanical behavior of concentrated alloys under nanoindentation for the special example of single-phase fcc Fe_{x}Ni_{1-x} alloys. The indentation hardness is maximum for the equiatomic alloy, x=0.5. This finding is in agreement with experimental results on the strength of these alloys under uniaxial strain. We explain this finding with the increase of the unstable stacking fault energy in the alloys towards x=0.5. With increasing Fe content, loop emission from the plastic zone under the indenter becomes less pronounced and the plastic zone features a larger fraction of screw dislocation segments; simultaneously, the length of the dislocation network and the number of atoms in the stacking faults generated in the plastic zone increase. However, the volume of twinned regions in the plastic zone is highest for the elemental solids and decreases for the alloys. This feature is explained by the fact that twinning proceeds by the glide of dislocations on adjacent parallel lattice planes; this concerted motion is less efficient in the alloys. Finally, we find that surface imprints show increasing pile-up heights with increasing Fe content. The present results will be of interest for hardness engineering or generating hardness profiles in concentrated alloys.

Objective molecular dynamics study of cross slip under high-rate deformation

In this paper, we investigate high-strain-rate deformations in crystalline materials using a novel implementation of Objective Molecular Dynamics (OMD). The OMD method is exact and has a rigorous foundation based on the fundamental invariance of the underlying potential energy surface: all atoms out to infinity satisfy the equations of molecular dynamics to high accuracy. Using this OMD method, we compute how dislocations filling all of the space in a crystalline material undergo time-dependent, three-dimensional motions during deformation. We apply this method to investigate the dynamics of screw dislocations in FCC nickel. Our key finding is that the macroscopic motion (i.e., loading conditions) and initial conditions greatly affect the atomic scale deformation mechanisms—such as the formation, motion, multiplication, annihilation, and abrupt changes of the slip plane and Burgers vector of dislocations. Small changes in the macroscopic loading conditions generate a rich variety of atomic deformation pathways. In certain macroscopic motions, we observe the growth of a stacking fault into a mechanical twin, which subsequently thickens by a process of step motion. In other macroscopic motions, we observe the initiation and subsequent development of cross-slip by the Friedel–Escaig (FE) or Fleischer mechanisms (FL). Under mixed loading conditions, a novel mechanism, with a combination of both FE and FL mechanisms was also observed. Our findings on the effect of external strain rate and temperature on the critical stress for homogeneous cross-slip quantitatively agree with a version of transition state theory with a stress-dependent activation barrier. Beyond dislocation motion, we demonstrate the modeling of sliding surfaces using the OMD framework. These examples highlight potential applications of the OMD framework to the mechanisms underlying plastic deformation and friction in material systems.

Narushenie kiral'noy simmetrii i neodnorodnye sostoyaniya v deformirovannykh ferromagnetikakh

The general form of chiral terms associated with deformation of a ferromagnet is determined. The possibility of a transition from the helical state to the vortex state is demonstrated for a ferromagnetic rod subjected to elastic torsional strain. In the case of a single screw dislocation, the formation of the skyrmion state due to elastic strains and the helical distribution of magnetization induced by strains in the dislocation core is indicated. The conditions in which a transition from one chiral magnetization distribution to another distribution in an ensemble of identical dislocations are determined.

The etching behaviour of dislocations in N-doped 4H-SiC substrate

The substrate-induced dislocations in the homo-epitaxial film could affect the quality of the epitaxial layer and lead to degradation of the device performance and stability. Effective evaluation of dislocations in 4H-SiC substrate is crucial for the quality control of the material. This work reported the etching behaviours of threading screw dislocation (TSD), threading edge dislocation (TED), and basal plane dislocation (BPD) in 4H-SiC substrate. Potassium hydroxide (KOH) and sodium peroxide (Na2O2) with different mass ratios were used as etchants. It was found that the average size of etching pits for each kind of dislocation increased linearly with the increase in the mass ratios of Na2O2. Interestingly, the slope of the linear increase for TSD was significantly larger than that of TED or BPD, which implies that the etching behaviour of TSD is more sensitive to the oxidant content in etchants, compared to TED and BPD. This outcome is related to the larger Burgers vector in TSD. In addition, the slope of TED was nearly equal to that of BPD due to the same Burgers vectors in them. The results in the work are helpful for understanding the etching mechanism and improving the etching technology for 4H-SiC substrate and epitaxial film.

Atomistic determination of Peierls barriers of dislocation glide in nickel

The Peierls barrier measures the lattice resistance to dislocation glide in crystalline solids. We use the nudged elastic band (NEB) method to calculate the Peierls barriers for screw and edge dislocation glide in a face-centered cubic (FCC) metal of Ni. The minimum energy paths (MEPs) across single or sequential Peierls barriers are determined under shear loading. The NEB results show the decreasing Peierls barrier with increasing shear stress, giving the Peierls stress at which the Peierls barrier vanishes. The effects of boundary condition and system size on Peierls barriers are studied by comparing strain- and stress-controlled NEB results. Furthermore, the free-end NEB methods are applied to determine MEPs with improved computational efficiency. The NEB results are also used to evaluate the energetic driving force of dislocation glide, which is consistent with that determined from the Peach-Koehler force. The accuracy of the present NEB results based on an empirical interatomic potential is assessed by comparison with a machine-learning potential. This work demonstrates the robust and efficient quantification of Peierls barriers to dislocation glide in an FCC metal, and it lays a solid foundation for the atomistic determination of Peierls barriers in compositionally complex alloys with the FCC structure in future studies.

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.

Formation and propagation mechanism of complex stacking fault in 180 μm thick 4H-SiC epitaxial layers

The formation mechanism and propagation behavior of a complex stacking fault (SF) in thick silicon carbide epitaxial layers was investigated using depth-resolved section X-ray topography and ultraviolet photoluminescence (UVPL) imaging. The SF is initiated in the substrate by deflection of a threading screw dislocation onto two basal half planes creating a Frank fault. This fault propagates into the epitaxial layer and interacts with several threading mixed dislocations, which creates both Frank and Shockley SFs on the same basal plane. UVPL images using different spectral filters show various SF type conversions occurring during propagation of the complex SF.

Implementation of Large Scale Deep Learning Non-Destructive Methods for Characterizing 4H-SiC Materials

A whole wafer method for industrial high volume, non-destructive characterizing of extended defects is demonstrated for 150 mm and 200 mm 4H-SiC wafers. Deep learning (DL) coupled with non-destructive techniques (NDT, DL-NDT) involving high volume, fast optical microscopy methods correlates industry accepted chemistry and physics-based etch and diffraction techniques for defect characterization. The application of the DL-NDT method is shown to reproduce defect distributions achieved by accepted etch techniques for extended defects of threading dislocations (TD), basal plane dislocations (BPD), and threading screw dislocations (TSD). An example of algorithm development is described to show progress toward implementing the method, as well as DL-NDT defect density compared to etch density for multiple wafers. The development status for implementing this technique for large-scale industrial wafer production includes etch validation of the results to ensure the technique is consistent and reliable. The ability to use this non-destructive technique ultimately will result in better correlation with device behavior and provide feedback to crystal growth processes to improve substrate wafers, while reducing the need for etch methods.

Topological field-effect transistor with quantized on/off conductance of helical/chiral dislocation states

Topology is a key ingredient driving the emergence of quantum devices. The topological field-effect transistor (TFET) has been proposed to outperform the conventional field-effect transistor by replacing the on state with topology-protected quantized conductance, while the off state has the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET, having both on and off quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured, with one acting as a channel and the other as a gate controlled by a local magnetic field. Reversible field switching is achieved with the on and off conductance of $2{e}^{2}/h$ and ${e}^{2}/h$, respectively, as shown by tight-binding quantum transport calculations. Furthermore, $\mathrm{BaBi}{\mathrm{O}}_{3}$ is shown as one candidate material that has the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices.

Rapid epitaxy of 2-inch and high-quality α-Ga2O3 films by mist-CVD method

High thickness uniformity and large-scale films of α-Ga2O3 are crucial factors for the development of power devices. In this work, a high-quality 2-inch α-Ga2O3 epitaxial film on c-plane sapphire substrates was prepared by the mist-CVD method. The growth rate and phase control mechanisms were systematically investigated. The growth rate of the α-Ga2O3 films was limited by the evaporation of the microdroplets containing gallium acetylacetonate. By adjusting the substrate position (z) from 80 to 50 mm, the growth rate was increased from 307 nm/h to 1.45 μm/h when the growth temperature was fixed at 520 °C. When the growth temperature exceeded 560 °C, ε-Ga2O3 was observed to form at the edges of 2-inch sapphire substrate. Phase control was achieved by adjusting the growth temperature. When the growth temperature was 540 °C and the substrate position was 50 mm, the full-width at half maximum (FWHM) of the rocking curves for the (0006) and (10-14) planes were 0.023° and 1.17°. The screw and edge dislocations were 2.3 × 106 and 3.9 × 1010 cm-2, respectively. Furthermore, the bandgaps and optical transmittance of α-Ga2O3 films grown under different conditions were characterized utilizing UV-visible and near-IR scanning spectra.

Atomic scale defect formation and phase transformation in Si implanted β-Ga2O3

Atomic scale details of the formation of point defects and their evolution to phase transformation in silicon (Si) implanted β-Ga2O3 were studied using high resolution scanning transmission electron microscopy (STEM). The effect of Si implantation and the formation of defects was studied as a function of the dose of implanted atoms, and the detailed mechanism of lattice recovery was observed using both in situ and ex situ annealing of the implanted β-Ga2O3. The implantation created nanoscale dark spots in STEM images, which we identified as local γ-Ga2O3 inclusions generated by the relaxation of lattice due to ⟨010⟩ screw dislocations created by the implantation. The number and size of γ-Ga2O3 regions increased as the Si dose increased, and eventually the γ-Ga2O3 crystal phase (with stacking defects) took over the entire implanted volume when the peak Si concentration was over ∼1020 cm−3. Annealing above 1100 °C disintegrates the local γ-Ga2O3 phase and returns the structure to defect-free, single crystal β phase, likely indicating that point defects (such as Si interstitials and cation vacancies) are spatially redistributed by the annealing. However, when the structure is completely transformed to γ-Ga2O3 by the implantation, post-annealing leaves a high concentration of dislocations within the β phase, which relates to the inhomogeneous distribution of Si atoms detected by secondary ion mass spectrometry.

Dislocation arrangements in 4H-SiC and their influence on the local crystal lattice properties.

Two wafers of one 4H-silicon carbide (4H-SiC) bulk crystal, one cut from a longitudinal position close to the crystal's seed and the other close to the cap, were characterized with synchrotron white-beam X-ray topography (SWXRT) in back-reflection and transmission geometry to investigate the dislocation formation and propagation during growth. For the first time, full wafer mappings were recorded in 00012 back-reflection geometry with a CCD camera system, providing an overview of the dislocation arrangement in terms of dislocation type, density and homogeneous distribution. Furthermore, by having similar resolution to conventional SWXRT photographic film, the method enables identification of individual dislocations, even single threading screw dislocations, which appear as white spots with a diameter in the range of 10 to 30 µm. Both investigated wafers showed a similar dislocation arrangement, suggesting a constant propagation of dislocations during crystal growth. A systematic investigation of crystal lattice strain and tilt at selected wafer areas with different dislocation arrangements was achieved with high-resolution X-ray diffractometry reciprocal-space map (RSM) measurements in the symmetric 0004 reflection. It was shown that the diffracted intensity distribution of the RSM for different dislocation arrangements depends on the locally predominant dislocation type and density. Moreover, the orientation of specific dislocation types along the RSM scanning direction has a strong influence on the local crystal lattice properties.

Characteristics and formation mechanism of serrated markings on the hydrogen-related quasi-cleavage fracture in as quenched low-carbon martensitic steel

The present paper investigated the characteristics of serrated markings on the hydrogen-related quasi-cleavage fracture in an as quenched low-carbon martensitic steel. The serrated markings corresponded to the ridges between {011} micro-facets. The ridges were formed along lath/block/packet boundaries or nearly parallel to 〈211〉 or 〈011〉 directions within the lath. In addition, the angle between {011} micro-facet and tensile axis was the largest of the six crystallographically equivalent {011} planes in each variant, demonstrating that the resolved normal stress imposed on the {011} plane plays a key role on the quasi-cleavage fracture. The above characteristics are common to martensitic steels and ferritic steels. We propose that the coalescence of vacancies/voids induced by hydrogen-enhanced screw dislocation motion needs to be considered to understand the fundamental mechanism of the hydrogen-related quasi-cleavage fracture along {011} planes in BCC steels.

Spiral Square Nanosheets Assembled from Ru Clusters.

Spiral two-dimensional (2D) nanosheets exhibit unique physical and chemical phenomena due to their twisted structures. While self-assembly of clusters is an ideal strategy to form hierarchical 2D structures, it is challenging to form spiral nanosheets. Herein, we first report a screw dislocation involved assembled method to obtain 2D spiral cluster assembled nanosheets (CANs) with uniform square morphology. The 2D spiral Ru CANs with a length of approximately 4 μm and thickness of 20.7 ± 3.0 nm per layer were prepared via the assembly of 1-2 nm Ru clusters in the presence of molten block copolymer Pluronic F127. Cryo-electron microscopy (cryo-EM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) demonstrate the existence of screw dislocation in the spiral assembled structure. The X-ray absorption fine structure spectrum indicates that Ru clusters are Ru3+ species, and Ru atoms are mainly coordinated with Cl with a coordination number of 6.5. Fourier-transform infrared (FT-IR) spectra and solid-state nuclear magnetic resonance hydrogen spectra (1H NMR) indicate that the assembly process of Ru clusters is formed by noncovalent interactions, including hydrogen bonding and hydrophilic interactions. Additionally, the Ru-F127 CANs exhibit excellent photothermal conversion performance in the near-infrared (NIR) region.

Detection of Common Anatomical Landmarks and Vertical Trajectories for Freehand Pedicle Screw Placement

It is clinically important for pedicle screws to be placed quickly and accurately. Misplacement of pedicle screws results in various complications. However, the incidence of complications varies greatly due to the different professional titles of physicians and surgical experience. Therefore, physicians must minimize pedicle screw dislocation. This study aims to compare the three nail placement methods in this study, and explore which method is the best for determining the anatomical landmarks and vertical trajectories. This study involved 70 patients with moderate idiopathic scoliosis who had undergone deformity correction surgery between 2018 and 2021. Two spine surgeons used three techniques (preoperative computed tomography scan [CTS], visual inspection-X-freehand [XFH], and intraoperative detection [ID] of anatomical landmarks) to locate pedicle screws. The techniques used include visual inspection for 287 screws in 21 patients, preoperative planning for 346 screws in 26 patients, and intraoperative probing for 309 screws in 23 patients. Observers assessed screw conditions based on intraoperative CT scans (Grade A, B, C, D). There were no significant differences between the three groups in terms of age, sex, and degree of deformity. We found that 68.64% of screws in the XFH group, 67.63% in the CTS group, and 77.99% in the ID group were placed within the pedicle margins (grade A). On the other hand, 6.27% of screws in the XFH group, 4.33% in the CTS group, and 6.15% in the ID group were considered misplaced (grades C and D). The results show that the total amount of upper thoracic pedicle screws was fewer, meanwhile their placement accuracy was lower. The three methods used in this study had similar accuracy in intermediate physicians (P > 0.05). Compared with intermediate physicians, the placement accuracy of three techniques in senior physicians was higher. The intraoperative detection group was better than the other two groups in the good rate and accuracy of nail placement (P < 0.05). Intraoperative common anatomical landmarks and vertical trajectories were beneficial to patients with moderate idiopathic scoliosis undergoing surgery. It is an optimal method for clinical application.

Advanced processing of differential phase contrast data: Distinction between different causes of electron phase shifts

Differential phase contrast, in its high resolution modification also known as first moment microscopy or momentum resolved STEM [1–7] , basically measures the lateral momentum transfer to the electron probe due to the beam interaction with either electrostatic and/or magnetic fields, when the probe transmits the specimen. In other words, the result of the measurement is a vector field p→(x,y) which describes the lateral momentum transfer to the probe electrons. In the case of electric fields, this momentum transfer is easily converted to the electric field E→(x,y) causing the deflection, and from ϱ=ɛ0∇⋅E→ the local charge density can be calculated from the divergence of the electric field.However, from experimental data it is known that also the calculation of the vector field’s curl ∇→×p→ in general yields non-zero results. In this paper, we use the Helmholtz decomposition (Wikipedia contributors, 2022), also known as the fundamental theorem of vector calculus, to split the measured vector fields into their curl-free and divergence-free components and to interpret the physical meaning of these components in detail. It will be shown, that non-zero curl components may be used to measure geometric phases occurring from irregularities in crystal structure such as a screw dislocation.

Structural investigation of triangular defects in 4H-SiC epitaxial layers as nucleation source for bar shaped stacking faults (BSSFs)

The formation of recombination-induced bar shaped stacking faults (BSSFs) during forward voltage operation of SiC devices, can lead to increased voltage drop and enhanced device degradation. In this study, a triangular epitaxial defect is identified as a nucleation source for the growth of BSSF in forward-biased 4H-SiC p-n diode test structures. We performed low and high voltage current emission microscopy measurements in order to detect the position of BSSFs in the active area of the device and in-depth structural analysis to locate their nucleation source. It was found that basal plane dislocations that converted into threading screw dislocations, close to the surface of the epitaxial layer and included in the triangular defect, act as nucleation source for the BSSFs. Those BSSFs expand from the top towards the bottom of the epitaxial layer, which is a newly reported expansion mechanism compared to the already reported BSSFs growing from the substrate/epitaxial layer interface towards the epitaxial layer surface.

High‐Brightness GaN‐Based Blue Light‐Emitting Diodes Using AlON Buffer Layer on 4 In.‐Patterned Sapphire Substrate: Low‐Dislocation‐Defect Epitaxy, Growth Mechanisms, and Higher Device Performance

AlN is a common buffer layer in GaN‐based light emitting diode (LED) devices. However, application of AlN on industry‐relevant patterned sapphire substrate (PSS) causes unwanted screw dislocations in the epitaxial layer during GaN nucleation. Industry experience suggests that AlON buffer layer improves the quality of GaN epitaxial layer, but the low‐dislocation‐defect epitaxy mechanisms remain unclear. Herein, high‐brightness GaN‐based blue LEDs with the sputtered AlON buffer layer on industry‐grade four‐inch PSS is demonstrated. The effect of oxygen addition to AlN is revealed by growing GaN epitaxial layers on AlON buffer layers with different oxygen contents and testing these GaN layers in LED devices. Compared with conventional AlN, our AlON buffer layer reduces the density of screw dislocations, thereby improving the quality of the GaN epitaxial layers and the performance of GaN‐based LEDs. These insights allow the explanation of the growth mechanisms of GaN on PSS with AlN and AlON buffer layers and can guide the development of next‐generation optoelectronic technologies.

High quality AlN film assisted by graphene/sputtered AlN buffer layer for deep-ultraviolet-LED

The advantages of van der Waals epitaxial nitrides have become a research hot topic. It is worth noting that graphene plays an important role in the research of epitaxial AlN epitaxial layer. In this work, we demonstrate a method to obtain high-quality and low-dislocation AlN epitaxial layer by combining graphene and sputtered AlN as the nucleation layer on the C-sapphire substrate via metal organic chemical vapor deposition, and successfully fabricated a 277 nm AlGaN-based deep ultraviolet light emitting diode (DUV-LED) based on the obtained AlN epitaxial layer. The presence of graphene promotes the stress release of AlN. Compared with the AlN epitaxial layer directly grown on graphene/sapphire substrate, the exist of sputtered AlN/graphene nucleation layer facilitates most of the threading dislocations in AlN can annihilate each other in the range of about 100 nm. Thus, as grown AlN epitaxial layer shows the decreasing of the screw dislocation from 2.31 × 109 to 2.08 × 108 cm−2 significantly. We manufacture an DUV-LED with 277 nm emission wavelength by using high-quality AlN films, which shows that magnitude of the leakage current is only on the order of nanoamperes and the forward turn on voltage is 3.5 V at room temperature. This study provides a meaningful strategy to achieve high-quality AlN film and high-performance DUV-LED.