Edge-type Dislocations Research Articles

Transmission electron microscopic study on rutile-type GeO2 film on TiO2 (001) substrate

Rutile-type GeO2 (r-GeO2) with an ultrawide bandgap of ∼4.7 eV has emerged as a promising material for next-generation power-electronic and optoelectronic devices. We performed transmission electron microscopy (TEM) observation to analyze the structural properties of r-GeO2 film on r-TiO2 (001) substrate at an atomic level. The r-GeO2 film exhibits a threading dislocation density of 3.6 × 109 cm−2 and there exist edge-, screw-, and mixed-type dislocations in the film as demonstrated by two-beam TEM. The edge-type dislocations have Burgers vectors of [100] and/or [110]. The bandgap of the r-GeO2 film is 4.74 ± 0.01 eV as determined by electron energy loss spectroscopy.

Reverse Hall–Petch Effect of Nano-Bainite in a High-Carbon Silicon-Containing Steel

High-strength steels are widely used in various mechanical production and construction industries for their low cost, high strength and high toughness. Among these, bainitic steels have better comprehensive performance relative to martensite and ferrite. In this paper, from the point of view of its microscopic fine structure and mechanical properties, the high-carbon silicon-containing steel Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni was austenitized at high temperature after a brief isothermal treatment at 280 °C and is briefly reviewed. We have used EBSD, TEM and 3D-APT to observe a unique transformation in which high-carbon silicon-containing steels form nanostructured bainite with nanometer widths. Intriguingly, as the isothermal duration decreases, the beam bainite width becomes increasingly finer. When the beam bainite width falls below 50 nm, there is a sudden shift in defect type from the conventional edge-type dislocations to a defect characterized by the insertion of a semi-atomic surface in the opposite direction, which leads to different degrees of reduction in the micro- and macro-mechanical properties of high-carbon silicon-containing steels from 1754 MPa to 1667 MPa. This sudden change in the sub-structural properties is typical of the reverse Hall–Petch effect.

Growth of Ga0.70In0.30N/GaN Quantum-Wells on a ScAlMgO4 (0001) Substrate with an Ex-Situ Sputtered-AlN Buffer Layer.

This study attempted to improve the internal quantum efficiency (IQE) of 580 nm emitting Ga0.70In0.30N/GaN quantum-wells (QWs) through the replacement of a conventional c-sapphire substrate and an in-situ low-temperature GaN (LT-GaN) buffer layer with the ScAlMgO4 (0001) (SCAM) substrate and an ex-situ sputtered-AlN (sp-AlN) buffer layer, simultaneously. To this end, we initially tried to optimize the thickness of the sp-AlN buffer layer by investigating the properties/qualities of an undoped-GaN (u-GaN) template layer grown on the SCAM substrate with the sp-AlN buffer layer in terms of surface morphology, crystallographic orientation, and dislocation type/density. The experimental results showed that the crystallinity of the u-GaN layer grown on the SCAM substrate with the 30 nm thick sp-AlN buffer layer [GaN/sp-AlN(30 nm)/SCAM] was superior to that of the conventional u-GaN template layer grown on the c-sapphire substrate with an LT-GaN buffer layer (GaN/LT-GaN/FSS). Notably, the experimental results showed that the structural properties and crystallinity of GaN/sp-AlN(30 nm)/SCAM were considerably different from those of GaN/LT-GaN/FSS. Specifically, the edge-type dislocation density was approximately two orders of magnitude higher than the screw-/mixed-type dislocation density, i.e., the generation of screw-/mixed-type dislocation was suppressed through the replacement, unlike that of the GaN/LT-GaN/FSS. Next, to investigate the effect of replacement on the subsequent QW active layers, 580 nm emitting Ga0.70In0.30N/GaN QWs were grown on the u-GaN template layers. The IQEs of the samples were measured by means of temperature-dependent photoluminescence efficiency, and the results showed that the replacement improved the IQE at 300 K by approximately 1.8 times. We believe that the samples fabricated and described in the present study can provide a greater insight into future research directions for III-nitride light-emitting devices operating in yellow-red spectral regions.

Low Al-content n-type Al[formula omitted]Ga[formula omitted]N layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic chemical vapor deposition to deliver high-quality n-AlxGa1−xN (x = 0–0.12, [Si] = 1×1017 cm−3) epitaxial layers on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13–0.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5×1015 cm−3 and carbon of 2×1016 cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while screw dislocations only raise for x above 0.077. The room temperature electron mobility of the n-AlxGa1−xN remain in the range of 400–470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties.

Effects of dislocation arrangement and character on the work hardening of lath martensitic steels

Work-hardening behavior of a lath martensitic Fe–18Ni alloy during tensile deformation is discussed based on the Taylor's equation. The dislocation characteristics are monitored using in situ neutron diffraction. In the specimens of as-quenched (AQ) and tempered at 573 K (T573), the dislocations are extremely dense and randomly arranged. The dislocations in AQ and T573 form dislocation cells as deformation progresses. Consequently, a composite condition comprising cell walls and cell interiors is formed, and the coefficient α in the Taylor's equation increases. Cells are already present in the specimen tempered at 773 K (T773), which has a low dislocation density and a large fraction of edge-type dislocations. As deformation continues, the dislocation density of T773 increases, its cell size decreases, and its composite condition become stronger. Simultaneously, the edge-type dislocation fraction decreases, keeping α unchanged. Thus, both the dislocation arrangement and character affected α, thereby affecting the work-hardening behavior.

Transition from Screw-Type to Edge-Type Misfit Dislocations at InGaN/GaN Heterointerfaces

We have investigated the interface dislocations in InxGa1−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.

High crystallinity N-polar InGaN layers grown on cleaved ScAlMgO4 substrates

We have grown high-crystallinity InGaN layers on ScAlMgO4 (SAM) substrates using metalorganic vapor-phase epitaxy. We have prepared atomically flat SAM substrates by cleaving them along the c-plane and have utilized direct InGaN growth without any low-temperature buffer layer. The resulting InGaN layer has a distinct hexagonal hillock morphology and remarkable crystalline quality. The x-ray rocking curve measurements showed that (0002̄) and (10–1–2) peaks full widths at half-maximum are as good as 384 and 481 arcsec, respectively. The calculated threading dislocations densities are as low as 2.9 × 108 and 1.6 × 109 cm−2 in the case of screw-type and edge-type dislocations, respectively.

Influence of nano-indentation depth on the elastic–plastic transformation of 6H-SiC simulated

To investigate the effect of nano-indentation depth on the elastic–plastic transition mechanism of 6H-SiC, the loading process of nano-indentation on a 6H-SiC⟨0001⟩ crystal surface is analyzed by using the molecular dynamics method. Diamond indenters and 6H-SiC amorphous nanolayers are constructed; Tersoff and Vashishta potential functions are established to describe the interactions between atoms; and simulated environmental factors, such as relaxation conditions, ensemble, and loading speed, are optimized. The stress distribution and internal deformation characteristics are analyzed by combining load–displacement curves, radial distribution curves, dislocation nucleation, and their evolutive processes. The load–displacement curve deviates from the theoretical value at a depth of 3.8 nm and enters the elastic to plastic transition phase, where the shear stresses generated by friction on the contact surfaces cause the atoms on the stacked layers in the elastic phase to move in a directional manner and form dislocations: edge-type dislocations connecting the two layer dislocations and horizontal spiral dislocations. The plastic deformation phase starts at a depth of 5.9 nm, the atoms keep migrating and reorganizing, the original dislocation rings break to form incomplete dislocations, and the new dislocations formed by the combination of partial edge-type dislocation rings expand internally until fracture. The formation and expansion of dislocations contribute to the elasto-plastic transformation of 6H-SiC, and amorphous states are found to be involved in the deformation process at both the indentation contact surface and the deformation layer. Moreover, there are differences in the direction of dislocation expansion due to the inhomogeneous stress distribution at the contact surface.

Step pinning and hillock formation in (Al,Ga)N films on native AlN substrates

The influence of edge-type threading dislocations (TDs) on the epitaxial growth of AlGaN on native AlN substrates was investigated theoretically and experimentally. In the step flow growth regime, we find that pure edge-type TDs cause a pinning of surface steps, resulting in curved step segments. Theoretical calculations reveal that this pinning mechanism is solely mediated by the altered surface potential due the strain field imposed by the TD. Within the curved step segment, the step width is subject to changes resulting in an altered Ga/Al incorporation rate. According to the density functional theory calculation, this effect is related to the different surface diffusion length of Ga and Al and represents a further destabilization mechanism during step flow growth. Another consequence of surface step pinning is the occurrence of areas where the step width is increased. These areas serve as precursors for 2D nucleation due to an increased adatom density. Once nucleated, these nuclei grow along the c-direction via continuous 2D nucleation, while lateral expansion occurs due to adatom incorporation on the m-facets.

Electron transport properties in thin InN layers grown on InAlN

We present a comprehensive analysis of structural and charge transport properties of high-quality N-polar InN/In0.57Al0.43N heterostructures grown by metalorganic chemical vapour deposition (MOCVD) on on-axis c-plane sapphire substrate and off-axis c-plane sapphire substrate misoriented by 4° toward the a-plane. Instead of a typical GaN template, we included an In-rich InAlN buffer layer to mitigate the lattice mismatch between InN and sapphire. We observed a strong correlation between the structural quality of InN and the electron mobility that was measured in a broad temperature range. Experimental results showed that InN grown at 550 °C had a higher density of edge-type dislocations, weaker temperature dependence of electron mobility, and higher electron concentration compared with InN grown at 600 °C. Our results suggest that acoustic phonon piezoelectric scattering dominated between 150 and 300 K, while dislocations and interface scattering prevailed from 10 to 150 K. The highest electron mobility of 714 cm2/Vs at room temperature was obtained for InN grown at 600 °C on the on-axis substrate, which corresponded with the lowest density of edge-type dislocations (2.7 ✕ 1010 cm−2) and the best surface morphology with a root-mean-square roughness of 1.4 nm.

Defect generation behavior in Czochralski-grown ScAlMgO4 crystal using synchrotron X-ray topography

Synchrotron X-ray topography was employed to characterize the defect structure and investigate the defect generation process in Czochralski (CZ)-grown ScAlMgO4 (SAM) crystals. In the X-ray topographs of the <2¯ 0 2 0>, <2¯ 0 2 22>, and 0 0 0 24 diffractions of a SAM(0 0 0 1) in-plane wafer, straight- and stream-type dislocations were analyzed using the extinction rule. These dislocations were ascertained to have both <1¯1¯ 2 0> and <0 0 0 1> components of the Burgers vector. A SAM(0 1¯ 1 0) cross-section wafer was then observed by X-ray topography to investigate the generation process of the defect structure. Straight-type dislocations along [1¯ 0 1 0], [0 1 1¯ 0], and [1 1¯ 0 0] were generated at the early-growth stage, to release accumulated large thermal strain derived from the solid–liquid phase change. Dislocation-free regions and low-dislocation density regions, including stream- and edge-type dislocations, appeared in the late-growth stage. The weak contrast of the growth-striation in the late-growth stage indicates the reduction in strain. The reduced strain is considered to have generated dislocation-free and low-dislocation density regions. The dislocation structure in SAM reflected the behavior of the thermal strain during the CZ-growth.

Analysis of Residual Stresses and Dislocation Density of AA6082 Butt Welds Produced by Friction Sir Welding

The Friction Stir Welding (FSW) method was employed to join AA6082 sheets. The welds were produced with different tool traverse speed (200 and 250 mm/min), rotational speed (1000 and 1250 RPM) and tool tilt angle (0 and 2 deg). Based on the analysis of XRD patterns, the total precipitation volume fractions in the nugget zones and the base material were calculated. The FSW process resulted in a reduction in the fraction of precipitates up to 64 pct compared to the parent material. Based on the Williamson–Hall analysis and indentation tests, the residual stresses were calculated. The highest tensile residual stresses of − 89.09 ± 6.19 MPa were observed for the base material, and the welding process reduced the residual stresses. The calculated dislocation density in the parent material AA6082 was equal to 8.225 × 1013 m−2, while in the welds a decrease was observed up to the value of 1.419 × 1013 m−2. In addition, the FSW process changed the nature of dislocations with edge-type dislocations dominating, while screw dominating character of dislocations were prevalent in the parent material. The mobility of dislocations in the studied welds was higher and reached the value of 16.78 × 10–7frac{m}{s}, while the dislocation mobility in the parent material was equal to 3.19 × 10–7frac{m}{s}. Process parameters during welding have a crucial effect on the amount of heat and strains introduced during the process, and thus influence the residual stresses, dislocation density and mobility, which might have a fundamental impact on the properties of the produced welds.

In(Ga)N 3D Growth on GaN-Buffered On-Axis and Off-Axis (0001) Sapphire Substrates by MOCVD.

In(Ga)N epitaxial layers were grown on on-axis and off-axis (0001) sapphire substrates with an about 1100 nm-thick GaN buffer layer stack using organometallic chemical vapor deposition at 600 °C. The In(Ga)N layers consisted of a thin (~10–25 nm) continuous layer of small conical pyramids in which large conical pyramids with an approximate height of 50–80 nm were randomly distributed. The large pyramids were grown above the edge-type dislocations which originated in the GaN buffer; the dislocations did not penetrate the large, isolated pyramids. The large pyramids were well crystallized and relaxed with a small quantity of defects, such as dislocations, preferentially located at the contact zones of adjacent pyramids. The low temperature (6.5 K) photoluminescence spectra showed one clear maximum at 853 meV with a full width at half maximum (FWHM) of 75 meV and 859 meV with a FWHM of 80 meV for the off-axis and on-axis samples, respectively.

Observation of threading dislocations and misfit dislocation half-loops in GaN/AlGaN heterostructures grown on Si using electron channeling contrast imaging

We implemented invisibility criterion and black–white contrast orientation analysis into low-tilt electron channeling contrast imaging (ECCI) for dislocation-type discrimination in GaN and AlGaN layers grown on a Si(111) substrate. Our ECCI and x-ray diffraction (XRD) analysis attained consistent threading dislocation densities for GaN and AlGaN grown on Si, but demonstrated drastic discrepancy in the percentage of edge-type dislocations, potentially due to the lack of appropriate consideration of mixed-type (a→+c→) dislocations in XRD. Further ECCI analysis of GaN/AlGaN heterointerface revealed mixed-type (a→+c→) dislocation half-loops and dislocation bending due to compressive strain relaxation, validating that not all the dislocations originated from the mosaic or columnar structure. As a result, XRD analysis based on the mosaic block model does not give reliable edge-to-screw dislocation ratio. The observation of classic van der Merwe–Matthews-type dislocation half-loop nucleation and dislocation gliding could be associated with potential GaN/AlGaN optoelectronic device degradation issues.

Initial growth mechanism of high-quality CVD diamond on Ir/sapphire substrate compared with Ir/MgO substrate

We demonstrate the growth of the highest-quality 2-in-diameter (001) diamond on a (112¯0) sapphire substrate. To elucidate the mechanism of the formation of this diamond, we observe the surfaces of the Ir buffer layer, bias-enhanced nucleation (BEN)-treated Ir surfaces, and diamond surfaces grown at the initial stage with different diamond growth times on both the (112¯0) sapphire and (001) MgO substrates. The (001) Ir buffer surface exhibits atomically flat terraces with atomic steps. However, after the BEN treatment, a ridge surface appears and diamond island growth begins from the bottom of the ridges during the diamond chemical vapor deposition growth. On Ir/sapphire, quadrangular-pyramid diamond three-dimensional (3D) islands with {111} sidewalls grow first. Subsequently, the sidewall facet changes from {111} to {011} and preferentially coalesced in the 〈010〉 direction. In contrast, on Ir/MgO, quadrangular-pyramid diamond 3D islands with {111} sidewalls grow first. Subsequently, the columnar growth proceeds. Consequently, the coalescence of diamond 3D islands on Ir/MgO requires a longer time than that on Ir/sapphire. We detect Ir on the top and sidewalls of the diamond 3D islands. This suggests that the Ir buffer can promote diamond growth at the initial stage as a catalytic effect. The X-ray rocking curve full width at half maximum for the 311 Bragg reflection is roughly proportional to the etch pit density. This is in accord with the transmission electron microscopy observation that the etch pits are of point-bottom-type and originated from a pure edge-type dislocation.

Large-area total-thickness imaging and Burgers vector analysis of dislocations in β -Ga2O3 using bright-field x-ray topography based on anomalous transmission

Using bright-field x-ray topography based on anomalous transmission (AT), we have demonstrated the first large-area total-thickness imaging of dislocations in β-Ga2O3 at the substrate scale. The dislocation images were acquired from the entire 10 mm × 15 mm × 680 μm (001)-oriented substrate prepared by edge-defined film-fed growth (EFG) by stitching together hundreds of topographic images, each recorded with the forward-diffracted beam in the Laue geometry for g = 020, 0–20, 022, and 400, under the conditions in which AT occurred. Dislocations distributed over the entire crystal volume were imaged as long as their Burgers vectors (b) were not orthogonal to the g-vectors. The results of the g·b analysis of the dislocation contrasts clearly revealed three major dislocation types that were numerically dominant in the EFG crystal: (i) b-axis screw-type dislocations with b∥ξ∥[010] (ξ is the unit vector of line direction), (ii) b-axis edge-type dislocations with b∥[001] and ξ∥[010], and (iii) curved mixed-type dislocations lying on the (001) planes with b∥[010]. Based on their b- and ξ-vectors, types (i) and (ii) were attributed to dislocations that propagated during EFG pulling up along the [010] direction, while type (iii) was attributed to dislocations generated through glide in the [010](001) slip system under stress. The extent to which AT can manifest itself is explained by using the effective absorption coefficient calculated for the above g-vectors based on dynamical x-ray diffraction theory.

The Effect of Heavy Fe-Doping on 3D Growth Mode and Fe Diffusion in GaN for High Power HEMT Application.

The high electron mobility transistor (HEMT) structures on Si (111) substrates were fabricated with heavily Fe-doped GaN buffer layers by metalorganic chemical vapor deposition (MOCVD). The heavy Fe concentrations employed for the purpose of highly insulating buffer resulted in Fe segregation and 3D island growth, which played the role of a nano-mask. The in situ reflectance measurements revealed a transition from 2D to 3D growth mode during the growth of a heavily Fe-doped GaN:Fe layer. The 3D growth mode of Fe nano-mask can effectively annihilate edge-type threading dislocations and improve transfer properties in the channel layer, and consequently decrease the vertical leakage current by one order of magnitude for the applied voltage of 1000 V. Moreover, the employment of GaN:C film on GaN:Fe buffer can further reduce the buffer leakage-current and effectively suppress Fe diffusion.

Microstructure and crystallographic structure of ferritic steel subjected to stress-corrosion cracking

The article presents the results of studies on the microstructure and crystallographic texture of ferritic steel under stress-corrosion cracking (SCC). Using scanning electron microscopy (SEM), the size and type of non-metallic inclusions, the elemental composition of corrosion products, and the cracking mode in the SCC zone were determined. As a part of X-ray diffraction (XRD) analysis, during which the shape and size of grains-crystallites, crystallographic texture, atomic displacements, the Debye-Waller factor and instrumental broadening of lines using the Caliotti function for LaB6 were taken into account, parameters of the microstructure in the SCC zone were assessed. It is shown that the SCC zone is characterized by high density of introduced edge-type dislocations, strong elastic microdistortions of the crystalline lattice and a relatively small size of coherent scattering domains (CSD). It is found that the processes of texture formation in the pipeline during SCC are characterized by a set of Cube {001}<100>, Brass {110}<11¯1>, Copper {112}<1¯11¯>, rotated copper H {001}<11¯0>, Goss {001}<110> and СН {001}<21¯0> orientations. It is shown that when approaching the zone where there are no SCC traces, orientations of Goss {001}<110>, Copper {112}<1¯11¯> and Rot G {011}<01¯1> type increase and orientations of Cube {001}<100>, H {001}<11¯0> and СН {001}<21¯0> type decrease.

Dislocation-enhanced electrical conductivity in rutile TiO2 accessed by room-temperature nanoindentation

Dislocation-enhanced electrical conductivity is an emerging topic for ceramic oxides. In contrast to the majority of present studies which focus on large-scale crystal deformation or thin film fabrication to introduce dislocations, we use a nanoindentation “pop-in stop” method to locally generate 〈011〉 edge-type dislocations at room temperature, without crack formation, on the (100) surface of a rutile TiO2 single-crystal. Ion beam assisted deposition of microcontacts allowed for both deformed and non-deformed zones to be locally probed by impedance spectroscopy. Compared to the dislocation-free region, a local enhancement of the electrical conductivity by 50% in the dislocation-rich regions is found. The study paves the way for local “mechanical-doping” of ceramics and oxide materials, allowing for the use of dislocations to tune the local conductivity with high spatial resolution.

Microscopic origin of thermal droop in blue-emitting InGaN/GaN quantum wells studied by temperature-dependent microphotoluminescence spectroscopy.

To elucidate the microscopic origin of the thermal droop, a blue-emitting indium gallium nitride (InGaN) quantum well grown on epitaxially laterally overgrown gallium nitride was investigated using temperature-dependent microphotoluminescence spectroscopy. Below 300 K, the sample exhibited a well-known dislocation-tolerant luminescence behavior. However, as temperature increases from 300 K to 500 K, the near band-edge emission at the wing region (with lower threading dislocation densities) was stronger than that at the seed region (with higher threading dislocation densities), indicating that threading dislocations are the microscopic origin of the thermal droop. Considering the carrier diffusion length, edge-type threading dislocations should play a major role in the thermal droop of heteroepitaxially grown InGaN-based LEDs.