Search Results (28)

A comprehensive X-ray topography analysis of two selected aluminum nitride (AlN) bulk crystals is presented. We compare surface inspection X-ray topography in back-reflection geometry with high-energy transmission topography in the Lang and Laue configuration using the monochromatic K_α1 excitation wavelength of copper, silver, and tungsten, respectively. A detailed comparison of the results allows the assessment of both the high- and low-energy X-ray topography methods with respect to performance and structural information, giving essential feedback for crystal growth. This is demonstrated for two selected AlN freestanding faceted crystals up to 8 mm in thickness grown in all directions using the physical vapor transport (PVT) method. Structural defects of all facets of the crystals are determined using the X-ray topography in back-reflection geometry. The mean threading dislocation densities are 480 ± 30 cm⁻² for both crystals of either the Al- or N-face. Clustering of dislocations could be observed. The m-facets show the presence of basal plane dislocations and their accumulation as clusters. The integral transmission topographs of the $10\overline{1}0$ (m-plane) reflection family show that basal plane dislocations of the screw type in ${\displaystyle \frac{1}{3}}⟨\overline{1}2\overline{1}0⟩$ directions decorate threading dislocation clusters. Three-dimensional section transmission topography reveals that the basal plane dislocation clusters mainly originate at the seed boundary and propagate in the ${\displaystyle \frac{1}{3}}⟨\overline{1}2\overline{1}0⟩$ direction along the growth front. In newly laterally grown material, the Borrmann effect has been observed for the first time in PVT-grown bulk AlN, indicating very high structural perfection of the crystalline material in this region. This agrees with a low mean FWHM of 10.6 arcsec of the $10\overline{1}0$ reflection determined through focused high-energy Laue transmission mappings. The latter method also opens the analysis of the $∆2\theta$-shift correlated to the residual stress distribution inside the bulk crystal, which is dominated by dislocation clusters. Contrary to Lang transmission topography, the de-focused high-energy Laue transmission penetrates the 8 mm-thick crystal enabling a defect analysis in the bulk. Full article

After the thermal-mechanical processing of Mg alloys, extensive 30°⟨0001⟩ grain boundaries (GBs) are present in the recrystallized structure, which strongly affects the mechanical properties via interactions with lattice dislocations. In this work, we systematically investigate how the 30°⟨0001⟩ GBs influence the slip transmission during plastic deformation. We reveal that basal dislocations can be transmuted into its neighboring grain and continue gliding on the basal plane. The prismatic dislocation can transmit the GB remaining on the same Burgers vector. However, a mobile pyramidal $⟨c+a⟩$ dislocation can be absorbed at GBs, initiating the formation of new grain. These findings provide a comprehensive understanding on GB-dislocation interaction in hexagonal close-packed (HCP) metals. Full article

The compressive deformation of the extruded binary Mg-Gd with gadolinium in solid solution has been studied in situ by combining synchrotron diffraction and acoustic emission techniques during compression tests. These two techniques are useful in investigating the evolution of twinning in all its stages. The extruded bars develop a fiber texture with the basal plane parallel to the extrusion direction. Moreover, the quenching of the magnesium bars immediately after the extrusion process ensured the production of the solid solution of gadolinium in the magnesium matrix. The solid solution of gadolinium solute atoms is the main strengthening mechanism of alloys and has a strong influence in plastic deformation. Tensile twinning controls the macroscopic yielding under compressive modes, although the activation of basal and non-basal dislocation systems has been also detected by in situ techniques. The presence of gadolinium atoms in solid solution tends to inhibit tensile twinning and, therefore, the twin volume fraction decreases with the increase in the gadolinium content. The compressive work hardening curve shows a maximum peak at intermediate plastic strain which is related to the interaction of dislocations within twins. The maximum value and the position of the peak decreases with the increase in the gadolinium content. Full article

Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the {$10\stackrel{\mathrm{-}}{1}2$} twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the {$10\stackrel{\mathrm{-}}{1}1$} twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing. Full article

The basal plane dislocation (BPD) density is one of the most important defects affecting the application of SiC wafers. In this study, numerical simulations and corresponding experiments were conducted to investigate the influence of cooling processes, seed-bonding methods, and graphite crucible materials on the BPD density in an 8-inch N-type 4H-SiC single crystal grown by the physical vapor transport (PVT) method. The results showed that the BPD density could be effectively reduced by increasing the cooling rate, optimizing the seed-bonding method, and adopting a graphite crucible with a similar coefficient of thermal expansion as the SiC single crystal. The BPD density in the experiments showed that a high cooling rate reduced the BPD density from 4689 cm⁻² to 2925 cm⁻²; optimization of the seed-bonding method decreased the BPD density to 1560 cm⁻². The BPD density was further reduced to 704 cm⁻² through the adoption of a graphite crucible with a smaller thermal expansion coefficient. Full article

The body diode degradation in SiC power MOSFETs has been demonstrated to be caused by basal plane dislocation (BPD)-induced stacking faults (SFs) in the drift region. To enhance the reliability of the body diode, many process and structural improvements have been proposed to eliminate BPDs in the drift region, ensuring that commercial SiC wafers for 1.2 kV devices are of high quality. Thus, investigating the body diode reliability in commercial planar and trench SiC power MOSFETs made from SiC wafers with similar quality has attracted attention in the industry. In this work, current stress is applied on the body diodes of 1.2 kV commercial planar and trench SiC power MOSFETs under the off-state. The results show that the body diodes of planar and trench devices with a shallow P+ depth are highly reliable, while those of the trench devices with the deep P+ implantation exhibit significant degradation. In conclusion, the body diode degradation in trench devices is mainly influenced by P+ implantation-induced BPDs. Therefore, a trade-off design by controlling the implantation depth/dose and maximizing the device performance is crucial. Moreover, the deep JFET design is confirmed to further improve the body diode reliability in planar devices. Full article

We have investigated the interface dislocations in In_xGa_1−xN/GaN heterostructures (0 ≤ x ≤ 0.20) using diffraction contrast analysis in a transmission electron microscope. The results indicate that the structural properties of interface dislocations depend on the indium composition. For lower indium composition (up to x = 0.09), we observed that the screw-type dislocations and dislocation half-loops occurred at the interface, even though the former do not contribute toward elastic relaxation of the misfit strain in the InGaN layer. With the increase in indium composition (0.13 ≤ x ≤ 0.17), in addition to the network of screw-type dislocations, edge-type misfit dislocations were generated, with their density gradually increasing. For higher indium composition (0.18 ≤ x ≤ 0.20), all of the interface dislocations are transformed into a network of straight misfit dislocations along the <10–10> direction, leading to partial relaxation of the InGaN epilayer. The presence of dislocation half-loops may be explained by a slip on basal plane; formation of edge-type misfit dislocations are attributed to punch-out mechanism. Full article

The dynamics of dislocations introduced through indentation or scratching at room temperature into a few GaN layers that were grown using the HVPE, MOCVD and ELOG methods and had different dislocation densities were studied via the electron-beam-induced current and cathodoluminescence methods. The effects of thermal annealing and electron beam irradiation on dislocation generation and multiplication were investigated. It is shown that the Peierls barrier for dislocation glide in GaN is essentially lower than 1 eV; thus, it is mobile even at room temperature. It is shown that the mobility of a dislocation in the state-of-the-art GaN is not entirely determined by its intrinsic properties. Rather, two mechanisms may work simultaneously: overcoming the Peierls barrier and overcoming localized obstacles. The role of threading dislocations as effective obstacles for basal plane dislocation glide is demonstrated. It is shown that under low-energy electron beam irradiation, the activation energy for the dislocation glide decreases to a few tens of meV. Therefore, under e-beam irradiation, the dislocation movement is mainly controlled by overcoming localized obstacles. Full article

A high-performance Mg-10Gd-4Dy-1.5Ag-1Zn-0.5Zr (wt.%, EQ142X) alloy was designed by multi-element composite addition in this work, obtaining a high yield strength (~396 MPa) and ultimate tensile strength (~451 MPa) after hot extrusion and ageing. The high strength is mainly related to fine grains and nano-precipitates, especially the latter. β′ and γ″ nano-precipitation with high fractions are the main strengthening phases, leading to a strengthening increment of ~277 MPa. Moreover, the multi-element alloying in this study promotes the basal-prismatic network strengthening structure, composed of β′ nano-precipitation with (1-210) habit planes, γ″ nano-precipitation with (0001) habit planes, basal plane stacking faults and 14H-long period stacking ordered phase. In addition, the dislocations and fine grains introduced by the hot-extrusion process not only accelerate the precipitation rate of nanostructure and thus improve the ageing hardening efficiency, but also facilitate the formation of more uniform and finer nano-precipitation. Thus, it is proposed that introducing nano-precipitates network into fine-grained structure is an effective strategy for developing high-strength Mg alloys. Full article

This work presents an overview of the mechanical response and microstructure evolution of specifically oriented pure magnesium single crystals under plane strain compression at room temperature. Crystals of ‘hard’ orientations compressed along the c-axis exhibited limited room temperature ductility, although pyramidal ⟨c + a⟩ slip was readily activated, fracturing along crystallographic $\left\{11\overline{2}4\right\}$ planes as a result of highly localized shear. Profuse $\left\{10\overline{1}2\right\}$ extension twinning was the primary mode of incipient deformation in the case of orientations favorably aligned for c-axis extension. In both cases of compression along ⟨$11\overline{2}0$⟩ and ⟨$10\overline{1}0$⟩ directions, $\left\{10\overline{1}2\right\}$ extension twins completely converted the starting orientations into twin orientations; the subsequent deformation behavior of the differently oriented crystals, however, was remarkably different. The formation of $\left\{10\overline{1}2\right\}$ extension twins could not be prevented by the channel-die constraints when c-axis extension was confined. The presence of high angle grain boundaries and, in particular, $\left\{10\overline{1}2\right\}$ twin boundaries was found to be a prerequisite for the activation of $\left\{10\overline{1}1\right\}$ contraction twinning by providing nucleation sites for the latter. Prismatic slip was not found to operate at room temperature in the case of starting orientations most favorably aligned for prismatic slip; instead, cooperative $\left\{10\overline{1}2\right\}$ extension and $\left\{10\overline{1}1\right\}$ contraction twinning was activated. A two-stage work hardening behavior was observed in ‘soft’ Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was attributed to changes in the microstructure rather than the interaction of primary dislocations with forest dislocations. Full article

The creep behavior of a binary Mg-15 wt.% Gd alloy was investigated over the temperature range from 523 K to 743 K, i.e., in both the single-phase region (the hexagonal close-packed solid solution of Gd in Mg) and the two-phase region (the solid solution plus Mg₅Gd precipitates). The alloy was prepared by the squeeze casting technique. In the higher temperature range, at 723 and 743 K, the specimens were solution treated by in situ annealing prior to testing. At the temperature of 673 K and below, the alloy was tested in the cast state. In the higher temperature range, the behavior was interpreted in terms of the viscous glide, where the dislocation motion was constrained by the presence of solute atmospheres. The dislocation motion was controlled by the rate of the cross slip from the basal to the prismatic planes. At the temperatures of 623 K and 673 K, the creep behavior was rationalized by introducing the threshold stress concept. At the temperatures of 523 K and 573 K, the stresses required to achieve experimentally measurable creep rates were such that dislocations broke away from the atmospheres of foreign atoms. Comparison with a series of magnesium alloys prepared by squeeze casting and creep-tested by the same technique showed that gadolinium can be a favorable creep-resistance enhancing element. Full article

To solve the problem of poor formability of magnesium alloys, the bending and straightening process was used to successfully introduce large-volume $\left\{10\overline{1}2\right\}$ tensile twins and dynamic recrystallization into the plates, and the comprehensive mechanical properties of the plates were improved, in which the anisotropy index (Lankford value: $\overline{r}$) decreased by 77%, and the corresponding Erishen value (IE) increased by 88%. The research shows that most of the continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) inherit the grain orientation of the parent grains, and a few have deviations from the parent grains. The twinning-assisted dynamic recrystallization (TDRX) can effectively inherit the grain orientation of the parent grain and retain the orientation relationship of the $\left\{10\overline{1}2\right\}$ tensile twin. The cooperation of the pre-set tensile twinning and various dynamic recrystallization processes leads to the deflection of the basal plane, which effectively weakens the basal texture and promotes the activation of various non-basal slip systems. Combined with grain refinement strengthening and dislocation strengthening, the magnesium alloy plate, after bending and straightening, obtains good comprehensive mechanical properties. Full article

We report a comparative spectroscopic study on the thin films of epitaxial aluminum nitride (AlN) on basal plane sapphire (Al₂O₃) substrates grown in hydrogen (H₂) and nitrogen (N₂) gas reaction environments. AlN films of similar thicknesses (~3.0 µm) were grown by metal-organic chemical vapor deposition (MOCVD) for comparison. The impact of the gas environment on the AlN epilayers was characterized using high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS), secondary ion mass spectroscopy (SIMS), cathodoluminescence (CL), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The study showed that AlN layers grown in a N₂ environment have 50% less stress (~0.5 GPa) and similar total dislocation densities (~10⁹/cm²) as compared to the films grown in a H₂ environment. On the other hand, AlN films grown in a H₂ gas environment have about 33% lesser carbon and 41% lesser oxygen impurities than films grown in a N₂ growth environment. The possible mechanisms that influenced the structural quality and impurity incorporation for two different gas environments to grow AlN epilayers in the MOCVD system on sapphire substrates were discussed. Full article

Despite more than two decades of intensive research, ion implantation in group III nitrides is still not established as a routine technique for doping and device processing. The main challenges to overcome are the complex defect accumulation processes, as well as the high post-implant annealing temperatures necessary for efficient dopant activation. This review summarises the contents of a plenary talk, given at the Applied Nuclear Physics Conference, Prague, 2021, and focuses on recent results, obtained at Instituto Superior Técnico (Lisbon, Portugal), on ion implantation into non-conventional GaN structures, such as non-polar thin films and nanowires. Interestingly, the damage accumulation is strongly influenced by the surface orientation of the samples, as well as their dimensionality. In particular, basal stacking faults are the dominant implantation defects in c-plane GaN films, while dislocation loops predominate in a-plane samples. Ion implantation into GaN nanowires, on the other hand, causes a much smaller density of extended defects compared to thin films. Finally, recent breakthroughs concerning dopant activation are briefly reviewed, focussing on optical doping with europium and electrical doping with magnesium. Full article

Ultrasonic nanocrystal surface modification (UNSM) was applied to hot-rolled Mg-Li alloys (LAE361 and LA106). The microstructure, mechanical properties, deformation mechanisms, and corrosion resistance properties of these alloys after UNSM treatment were systematically studied. Significant improvement in surface hardness and decrease in surface roughness were achieved by UNSM treatment. Meanwhile, the basal texture intensity of the Mg-Li alloys reduced significantly, and several deformation twins appeared on the surface layer. The α phase of the surface layer underwent twin deformation and basal plane slip. The fibre textures in the β phase of LA106 Mg-Li alloy changed from γ and η to α and ε, which mainly resulted in the dislocation slip. More importantly, UNSM treatment exhibited enhanced strength and improved plasticity of LAE361 and LA106 Mg-Li alloys. The corrosion current density of LAE361 Mg-Li alloy reduced approximately 29.3% by UNSM treatment, while it increased the corrosion current density of LA106 Mg-Li alloy by 189.7%. These studies show that the application of UNSM to improve the corrosion resistance of duplex phases of LA106 Mg-Li alloy needs further investigation. Full article