Screw Dislocations Research Articles

Dislocation-related leakage-current paths of 4H silicon carbide

Improving the quality of 4H silicon carbide (4H-SiC) epitaxial layers to reduce the leakage current of 4H-SiC based high-power devices is a long-standing issue in the development of 4H-SiC homoepitaxy. In this work, we compare the effect of different type of dislocations, and discriminate the effect of dislocation lines and dislocation-related pits on the leakage current of 4H-SiC by combining molten-KOH etching and the tunneling atomic force microscopy (TUNA) measurements. It is found that both the dislocation lines of threading dislocations (TDs) and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. Due to the different Burgers vectors of TDs, the effect of TDs on the reverse leakage current of 4H-SiC increases in the order to threading edge dislocation (TED), threading screw dislocation (TSD) and threading mixed dislocation (TMD). Basal plane dislocations (BPDs) are also found to slightly increase the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible. Our work indicates that reducing the density of TDs, especially TMDs and TSDs, is key to improve the quality of 4H-SiC epitaxial layers and reduce the reverse leakage current of 4H-SiC based high -power devices.

Influence of group IV element on basic mechanical properties of BCC medium-entropy alloys using machine-learning potentials

In order to elucidate the origin of excellent mechanical properties of high-entropy alloys (HEA), it is essential to develop the atomic-level depiction of defect structures, taking into account the influence of composition. Especially in body-centered cubic (BCC) HEA an alteration of constituent elements may lead to a dramatic change in the behavior on the macroscopic level. To study that effect, we employed a machine learning technique and constructed highly accurate robust potentials for two BCC medium-entropy alloys: MoNbTa and ZrNbTa. Even though they have close composition, the mechanical properties of the two alloys differ not only quantitatively, but also qualitatively. We show that the group IV element Zr decreases values of bulk and elastic constants. The influence of short-range order on stacking fault and twin boundary energies is discussed. Also, we show the difference in the screw dislocation core shapes between the two alloys. The cores of 〈111〉 screw dislocations in the ZrNbTa case demonstrate a non-compact shape substantially extended on the (110) plane.

Response embedded atom model potential of Pb at finite temperature: application on the dislocation mobility

Dislocation is a major carrier of plastic deformation for metal materials and are crucial. Understanding the mechanism of dislocation motion is beneficial for understanding the plastic deformation of materials under dynamic loading. In this work, a new response EAM (REAM) potential is developed for the applications under high pressure and finite temperature conditions. We use the REAM potential to investigate the behaviors of edge and screw dislocations in Pb by molecular dynamics (MD) simulations, and compare it with two commonly used EAM potentials. Specially, we examine the influence of the stacking fault energy and the temperature-dependent elastic constants on the dislocation motions. Our results show that the temperature-dependent elastic constants do not considerably affect the dislocation motion at the linear region of low stress, while the stacking fault energy plays a significant role. In the nonlinear region, the stacking fault energy and elastic constant together influence the dislocation motion. In subsonic and low transonic regimes, the dislocation width oscillates with time, but eventually fluctuates around equilibrium width.

Nucleation Mechanism of Nano-Iron Oxide in a Supercritical Water Reaction

Despite its ubiquitous application in various branches of engineering and materials science, nucleation with reactions from supercritical water remains elusive. In this work, molecular dynamics with a reactive force field was studied to demonstrate the nucleation process of nano-iron oxide crystals synthesized via the supercritical hydrothermal synthesis method using Fe2(SO4)3 as the precursor. It was shown that during the early stages of nucleation, high local density areas first occurred, making these sites primary candidates for nucleation. Furthermore, the amorphous intermediate reorganizes from the aggregation and coalescence of the prenucleation clusters (PNCs) and precedes the emergence of a crystalline nucleus rather than direct one-step nucleation from classical theory. Through calculation, it was found that the average nucleation rates increase generally with the increase in reaction temperature and system density, and the volumes of nano-iron oxide clusters are maintained at 10–12 nm3 when simulating for 3 ns. It was further confirmed that increasing the reaction temperature and density is conducive to enhancing the crystallinity of iron oxide nanocrystals within a certain range of increments, with larger clusters forming faster at higher temperatures and densities. It was demonstrated that the screw dislocation formation mechanism caused nano-iron oxide lattice defects. Although not all of the nano-iron oxide nuclei generated have lattice defects, all of the perfect nano-iron oxide nuclei are generated by adjusting the configuration of nano-iron oxide nuclei with lattice defects.

Automated analysis of X-ray topography of 4H-SiC wafers: Image analysis, numerical computations, and artificial intelligence approaches for locating and characterizing screw dislocations

The physical vapor transport (PVT) crystal growth process of 4H-SiC wafers is typically accompanied by the occurrence of a large variety of defect types such as screw or edge dislocations, and basal plane dislocations. In particular, screw dislocations may have a strong negative influence on the performance of electronic devices due to the large, distorted or even hollow core of such dislocations. Therefore, analyzing and understanding these types of defects is crucial also for the production of high-quality semiconductor materials. This work uses automated image analysis to provide dislocation information for computing the stresses and strain energy of the wafer. Together with using a genetic algorithm this allows us to predict the dislocation positions, the Burgers vector magnitudes, and the most likely configuration of Burgers vector signs for the dislocations in the wafer.Graphical abstract

Spontaneous formation of cubic phases: a molecular dynamics study for soft repulsive spherocylinders

ABSTRACTSpontaneous formation of structures with ‘cubic symmetry’ is observed by molecular dynamics simulation. We show that many phases with cubic symmetry with planes of edge dislocations appear when the Sm layers are allowed to break up at relatively low pressures. The phases obtained can be interpreted as bend-grain-boundary phases consisting of edge dislocations, the counterpart of the twist-grain-boundary phase consisting of screw dislocations.

Absence of Off-Diagonal Long-Range Order in hcp ^{4}He Dislocation Cores.

The mass transport properties along dislocation cores in hcp ^{4}He are revisited by considering two types of edge dislocations as well as a screw dislocation, using a fully correlated quantum simulation approach. Specifically, we employ the zero-temperature path-integral ground state (PIGS) method together with ergodic sampling of the permutation space to investigate the fundamental dislocation core structures and their off-diagonal long-range order properties. It is found that the Bose-Einstein condensate fraction of such defective ^{4}He systems is practically null (≤10^{-6}), just as in the bulk defect-free crystal. These results provide compelling evidence for the absence of intrinsic superfluidity in dislocation cores in hcp ^{4}He and challenge the superfluid dislocation-network interpretation of the mass-flux-experiment observations, calling for further experimental investigation.

Modeling of critical resolved shear stress for interaction between screw dislocation and prismatic dislocation loop in alpha-iron

Modeling of critical resolved shear stress for interaction between screw dislocation and prismatic dislocation loop in alpha-iron

Chirality and dislocation effects in single nanostructures probed by whispering gallery modes.

Nanostructures such as nanoribbons and -wires are of interest as components for building integrated photonic systems, especially if their basic functionality as dielectric waveguides can be extended by chiroptical phenomena or modifications of their optoelectronic properties by extended defects, such as dislocations. However, conventional optical measurements typically require monodisperse (and chiral) ensembles, and identifying emerging chiral optical activity or dislocation effects in single nanostructures has remained an unmet challenge. Here we show that whispering gallery modes can probe chirality and dislocation effects in single nanowires. Wires of the van der Waals semiconductor germanium(II) sulfide (GeS), obtained by vapor-liquid-solid growth, invariably form as growth spirals around a single screw dislocation that gives rise to a chiral structure and can modify the electronic properties. Cathodoluminescence spectroscopy on single tapered GeS nanowires containing joined dislocated and defect-free segments, augmented by numerical simulations and ab-initio calculations, identifies chiral whispering gallery modes as well as a pronounced modulation of the electronic structure attributed to the screw dislocation. Our results establish chiral light-matter interactions and dislocation-induced electronic modifications in single nanostructures, paving the way for their application in multifunctional photonic architectures.

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

A new medium-Mn steel was designed to achieve unprecedented tensile properties, with a yield strength beyond 1.1 GPa and a uniform elongation over 50%. The tensile behavior shows a heterogeneous deformation feature, which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands. Multiple strain hardening mechanisms for excellent tensile properties were revealed. Firstly, non-uniform martensite transformation occurs only within a localized deformation band, and initiation and propagation of every localized deformation band need only a small amount of martensite transformation, which can provide a persistent and complete transformation-induced-plasticity effect during a large strain range. Secondly, geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening. Lastly, martensite formed by displacive shear transformation can inherently generate a high density of mobile screw dislocations, and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multiplication due to the dynamic strain aging effect, and these two effects provide a high density of mobile dislocations for strong strain hardening.

Energy and velocity of sliding of edge and screw dislocations in austenite and Hadfield steel: Molecular dynamics simulation

The sliding of edge and screw dislocations in Hadfield steel and in pure HCC iron (austenite) depending on temperature and deformation rate was studied by the method of molecular dynamics. The complete dislocation appears in the present model immediately in the form of a split into a pair of partial Shockley dislocations separated by a packing defect. The distance between partial dislocations is several nanometres. As the shear rate increases, this distance decreases. According to the data obtained, the energies of edge and screw dislocations in steel are higher than in pure austenite. The energy of the total edge dislocation in γ-iron and Hadfield steel averages 2.0 and 2.3 eV/Å, helical – 1.3 and 1.5 eV/Å respectively. Dependences of the sliding velocity of the edge and screw dislocations on the shear rate and temperature were obtained. The sliding velocity of the edge dislocation is in all cases higher than the screw one, which is explained by the difference in the propagation velocity of longitudinal and transverse waves in the material. With an increase in the shear rate, the sliding speed increases to a certain limit, depending on the propagation velocity of the corresponding elastic waves. At low and normal temperatures, the sliding velocity of dislocations in Hadfield steel is significantly (about one and a half times) lower compared to pure HCC iron. In pure iron, the sliding velocity of dislocations decreases with increasing temperature. However, for Hadfield steel, this dependence is nonmonotonic: as the temperature increases to about 500 K, the dislocation rate increases. That is probably due to the intensification of diffusion of impurity carbon atoms; then, as in iron, it decreases.

Planar Gliding and Vacancy Condensation: The Role of Dislocations in the Chemomechanical Degradation of Layered Transition-Metal Oxides

Stacking faults driven by dislocations have been observed in layered transition-metal oxide cathodes both in cycled and uncycled materials. The reversibility of stacking-sequence changes directly impacts the material performance. Irreversible glide due to lattice invariance or local compositional changes can initiate a catastrophic sequence of degradation mechanisms. In this study we compare the chemomechanical properties of LiCoO2 and LiNiO2 by combining density functional theory and anisotropic linear elasticity theory. We calculate stacking fault energies as a function of Li content and quantify the extent to which excess Ni hinders stacking-sequence changes. We then characterize screw dislocations, which mediate stacking-sequence changes, and find a peculiarly compliant behavior of LiNiO2 due to the interaction of Jahn–Teller distortions with the dislocation strain field. Finally, we analyze the tendency of vacancies to segregate along dislocation lines. This study represents the first instance of explicit ab initio atomistic dislocation models in layered oxides and paves the way for the understanding and optimization of the chemomechanical behavior of cathode active materials during battery operation.

The Activation Energy of Strain Bursts during Nanoindentation Creep on Polyethylene.

In the present investigation, statistical characterization of strain bursts observed during the load-controlled deformation of high-density polyethylene (HDPE), which arise within the crystalline phase during plastic deformation, was carried out via high-resolution nanoindentation creep experiments. Discrete deformation processes occurred during the nanoindentation creep tests, which indicated that they arose from the break-off of dislocation avalanches, i.e., dislocation climb is a possible mechanism for indentation creep deformation. Characterization of the strain bursts, in terms of the associated height and number, demonstrated that these quantities followed a Gaussian distribution depending on the load and loading rate. This analysis enabled the accurate measurement of creep activation energy. Our method used nanoindentation tests to measure the creep activation energy of HDPE within both the crystalline and amorphous phases. The activation energy of the creep process within the crystalline phase was evaluated using two methods. The frequency of jumps within the crystalline phase, as a function of the strain rate, showed two peaks related to the 5 nm and 10 nm jump sizes that corresponded to the block size within the crystalline lamellae. The results indicated that the intervals coincided with the mean free path of dislocations and the block grain boundaries acted as dislocation barriers. From the dependence of burst frequency on the strain rate and temperature, the activation energy and thermally activated length of the dislocation segment for the plastic slip activation were determined to be 0.66 eV and 20 nm, respectively. Both numbers fit well to the Peterson's model for the nucleation and motion of thermally activated dislocation segments. A similar activation energy resulted from the differential mechanical analysis of the literature for the αI-transition, which occurred near room temperature in polyethylene. The transition was described as the generation of screw dislocation and its motion along a block grain boundary; therefore, this process is suggested to be the basic mechanism underlying the strain bursts observed in this study.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Growth of N-polar In-rich InAlN by metal organic chemical vapor deposition on on- and off-axis sapphire

Metal organic chemical vapor deposition is used to grow N-polar In-rich InAlN layers directly on on- and off-axis (misoriented by 4° towards a plane) c-plane sapphire substrates. During the InAlN growth, trimethylaluminum, ammonia, and the total flow was kept at 4.74 μmol/min, 3 slm and 10 slm, respectively, while trimethylindium (TMIn) flow was selected between 8.42 and 13.48 μmol/min. All samples were grown at about 706 °C however; TMIn flow of 10.95 μmol/min was also tested at the growth temperature of 686 °C. With increasing the TMIn flow, the In molar fraction increased from 0.55 to 0.69, irrespectively of the substrate miscut. For the moderate TMIn flow and In molar fraction of less than 0.63, density of screw and edge dislocations were from 0.3 to 12 ✕ 109 cm-2 and 5 to 7 ✕ 1010 cm-2, respectively, while decisively lower densities appeared on on-axis structures. On the other hand, InAlN surface RMS roughness was from 1 to 8 nm favouring the off-axis substrate and low TMIn flow. Structural and surface deterioration appeared with the highest TMIn flow of 13.48 μmol/min and In molar fraction of 0.69, which hold also for reduced temperature of the growth. Nevertheless, room temperature photoluminescence full-width at half maximum less than 270 meV appeared for all samples, with maxima between 1.9 and 1.5 eV. In-rich N-polar InAlN grown on on-axis sapphire can be used as a buffer layer in new types of heterostructure devices.

Three-dimensional deformation velocity field and kinematic characteristic of the middle and east parts of Haiyuan fault zone from InSAR and GPS observations

Three-dimensional deformation velocity field can reveal fine characteristics of surface deformation, and provide data support for the research on crustal motion mechanism and seismic hazard. In this study, the ascending and descending InSAR deformation velocity fields in the middle and east parts of Haiyuan fault zone are firstly obtained with Sentinel-1A data and SBAS InSAR technology. Then, by combining with GPS observations, the three-dimensional deformation velocity field is extracted and analyzed. Finally, the locking depths and deep slip rates at different locations are inverted by using the classical two-dimensional screw dislocation model, and the maximum shear-strain rate is investigated. The results indicate that there are significant differences in the InSAR deformation velocity field between both sides of the middle and east parts of Haiyuan fault zone. The east–west deformation velocity field clearly reveals the left-lateral strike-slip characteristic of the fault zone. The northward movement gradually changes to southward movement from northwest to southeast. The vertical deformation velocity field displays significant differences on both sides of the section between MMSF and the middle of LHSF, and the middle and eastern of HYE fault. For LHSF, the locking depth and deep slip rate are 9.7 km and 4.9 mm/a; for HYW fault, they gradually decrease from west to east, with the locking depths of 6.4 km, 3.6 km and 3.2 km, respectively, and the deep slip rates of 5.5 mm/a, 4.2 mm/a and 3.8 mm/a, respectively; for HYM fault, they are 7 km and 4.7 mm/a; for HYE fault, they are 5.8 km and 3.6 mm/a. There are two obvious extreme regions of maximum shear-strain rate along the fault strike. This study can provide a reference for investigating the mechanism of regional uplift and extension and seismic hazard.

Effect of Microstructure on the Mechanical Response of Hydrogen-Charged Pure Iron

In this paper, we investigate how two different microstructures in pure iron affect the dislocation mobility in hydrogen-charged and non-charged samples by conducting stress-relaxation tests. The effective activation volume of the pure iron for both types of microstructures (cold-rolled and annealed samples) has been determined for both H-charged and uncharged material. Information about the dislocation structures formed during stress relaxation is provided by conducting TEM analysis. We employ a self-consistent kinetic Monte-Carlo (SCkMC) model of the ½ [111] screw dislocation in Fe to investigate how hydrogen affects the mobility and behavior of the dominant mobile dislocation in Fe at different stresses and H concentrations. The results from our simulations show the following: (i) at low stresses the deviation from the primary slip plane in the presence of H is lower than the deviation in the uncharged Fe. The deviation angle decreases with increasing H concentration; (ii) at higher shear stresses, the higher probability for kink-pair formation in the secondary (110) planes in the presence of H, leads to an enhanced deviation from the primary slip plane, which increases with increasing H concentration. We use the results of stress-relaxation tests and SCkMC simulations to propose an explanation for the formation of dislocation cell structures in pure and hydrogen charged Fe in the cold-rolled and annealed samples.

Synthesis of Ligand-Induced Chiral Tellurium Nanocrystals

Chiral tellurium nanoparticles have recently garnered tremendous attention as emerging inorganic nanomaterials with intrinsically chiral space groups owing to their potential in next-generation stereosynthesis, spintronics, and optoelectronics. Inspired by the chiral ligand-mediated synthetic strategy, we herein present hydrothermal-assisted synthesis of chiral polyhedral tellurium nanoparticles that provides differed chirogenesis than that of particles fabricated by wet chemistry in recent studies; the thiolated cysteine molecules change the morphology of tellurium nanoparticles from fundamental two-dimensional shapes to chiral three-dimensional polyhedra owing to the screw dislocation effects observed only during nanoparticle growth. However, the nanoparticles do not exhibit chiral behaviors at the nucleation stage. Further investigation indicates that the growth of chiral polyhedral tellurium nanoparticles is overwhelmingly affected by parameters such as the hydrothermal reaction time, amount of polyvinylpyrrolidone, and species of chiral molecules. We believe that these findings can provide new insights into the fundamental relationships among structural chirality, chiral ligands, screw dislocations, and chiral space groups in principle.

Gradient microstructure response in different phases of Ti2AlNb alloy with laser shock peening

Ti2AlNb alloy is a new type of lightweight and heat-resistant structural material that has been an active research topic with multi-phases due to its improved mechanical properties. However, the phase response under severe plastic deformation (SPD) at high strain rates is unclear, which limits the understanding of its strengthening mechanism. Here, laser shock peening (LSP) was employed to investigate the microstructure response of α2, O, and B2/β phases in Ti2AlNb alloy. After LSP, the highest density dislocations were generated in the B2/β phases, followed by the O phases, and the least dislocations were observed in the α2 phases. In the B2/β phases, only dislocation tangles (DTs) were observed. Dislocation lines (DLs) and DTs were mainly distributed in the O phases. As for the α2 phases, DLs were generated while a small number of DTs were piled up at the grain boundaries. In addition, parallel dislocations (PDs) with a spacing of about 135 nm can be observed on the surface of the α2 phases, and the dislocations interspersed between PDs were composed of screw dislocations with long segments. In addition to dislocation structures, stacking faults (SFs) were observed in both α2 phases and O phases, and mechanical twins were formed in the O phases, which were generated to coordinate the SPD induced on the surface. These dislocation structures formed dislocation cells (DCs) and sub-grain boundaries which further developed into high-angle grain boundaries (HAGBs) together with mechanical twins, dividing the coarse grains and refining the microstructure. As the depth increased, the responses of the three kinds of phases decreased. The PDs in the α2 phases have widened spacing and could not penetrate the grains. The mechanical twins were gradually reduced until no obvious twin structure could be observed. Finally, dislocations, grain size, microhardness, and residual stress presented a gradient distribution. The improved microhardness (maximum 470 HV) and compressive residual stress (maximum −600 MPa) which were consistent with the refined grains and high-density dislocations gradually decreased to that of the base metal (BM).

Influence of ordering phenomena on carbon segregation in tungsten screw dislocation

Concentration profiles on the different sites of tungsten screw dislocation are calculated as a function of the nominal carbon atom concentration, temperature and dislocation density. Rigid lattice Monte Carlo simulations and associated mean-field modelling are performed based on interaction energies from ab initio calculations. We show that the repulsive interactions induce ordering phenomena that drastically change the concentration in the core of the dislocation and leads to ordered structures among the octahedral sites. We detail the domains of short-range ordering (SRO) and long-range ordering (LRO).