Basal Stacking Faults Research Articles

(10–13)-oriented semipolar GaN growth on (10–10) m-plane ScAlMgO4 substrate

Abstract We report the growth of a GaN layer on (10–10) m-plane ScAlMgO4 (SAM) substrate. The GaN layer demonstrated (10–13) preference growth orientation. The anisotropy of the crystalline quality was distinctly observed through X-ray rocking curve taken across the sample surface over various azimuths across the orthogonal directions [0001] and [1–210] of the SAM substrate. Notably, the crystalline quality exhibited gradual degradation as the substrate is rotated around its surface normal away from the c-direction [0001] of the SAM substrate toward the a-direction [11–20]. Additionally, basal stacking faults in the (10–13)-oriented GaN were observed along [0001] direction using scanning transmission electron microscopy. Moreover, the selected area electron diffraction analysis was utilized to confirm the (10–13) growth orientation that was found to be consistent with the X-ray diffraction result. The (10–13) GaN layer exhibited a metal face through integrated differential phase contrast imaging.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

Atomic scale investigation of the phase transformation path from HCP to FCC in high-purity hafnium during torsion deformation

Although the hexagonal close-packed (HCP) to face centered cubic (FCC) martensitic phase transformation has been frequently observed and discussed in many HCP metals and alloys, the detailed phase transformation path has been disputed. Here, we studied the potential phase transformation path from HCP to FCC phase in high-purity hafnium (Hf) under torsional deformation condition. Results indicated that the HCP to FCC phase transformation was not accomplished directly by the glide of a/3[11¯00] Shockley partial dislocations on every other {0001} planes, but progressively transitions to the FCC lattice through multiple intermediate states. The complete phase transformation process can be further refined as: HCP → body-centered tetragonal (BCT)-like structure → 12R long-period stacking-ordered (LPSO) structure → FCC, and each step was accommodated by basal stacking faults. This work is instructive on the phase transformation from HCP to FCC not only in Hf but also in other HCP metals.

Improved characteristics of (11 [formula omitted]) plane AlGaN films grown on SiNx interlayer

SiNx interlayer was introduced in the epitaxial process of (11 2‾2) AlGaN film, and the influences of SiNx on the properties of (11 2‾2) AlGaN were characterized. It was revealed that the morphologic properties and crystal quality of (11 2‾2) AlGaN could be refined remarkably by optimizing SiNx growth time. Especially, a low root mean square value of 1.02 nm was obtained for S3 with a SiNx growth time of 10 min, which was 56.7 % less than S0 that grown without SiNx. Meanwhile, compared with S0, the full width at half maximum values of X-ray rocking curves for S3 were reduced by 39.6 % along [11 2‾3‾] direction and 41.2 % along [1 1‾00] direction. Besides, the background electron concentration and basal stacking faults related emission of (11 2‾2) AlGaN could also be suppressed remarkably by optimizing SiNx growth time to 10 min. These improvements could be originated from the formation of SiNx islands, leading to the lateral growth of AlGaN overlayer, the relaxation of strain in the AlGaN, and the annihilation of dislocations.

Microstructural modifications induced by He implantation at elevated temperature in AlN

The elastic strain and damage build-ups induced by 50 keV He implantations at elevated temperature in (0001)AlN were studied using a combination of X-ray diffraction, transmission electron microscopy and elastic recoil detection analysis experiments. No long-range migration of He atoms occurs up to 700°C. The point defect recombination is found to be enhanced with the increase of implantation temperature. When He concentration exceeds a determined temperature-dependent threshold, the stabilization of vacancies through the formation of He-vacancy clusters of critical size strongly enhances the interstitial concentration and elastic strain in proportion with the local He concentration. Above this threshold, for a critical He concentration that decreases from several at% at room temperature to a few tenths of at% at 700°C, bubbles form and induce a strong increase of disorder. Specific features observed at 700°C, including the systematic superimposition of bubble clusters with basal stacking faults, suggest the triggering between 550 and 700°C of a mechanism promoting disorder, which is discussed.

Migration mechanism of sessile like [formula omitted] pyramidal edge-dislocations dissociated along the basal plane of magnesium

The pyramidal II 〈c+a〉 edge-dislocation in Mg causes a transformation into the basal-dissociated 〈c+a〉 dislocation, in which two partial 〈c+a〉 dislocations aligned along the ⟨a⟩ direction on a basal plane are connected by an I1 type basal stacking fault. This paper investigates the mobility of basal-dissociated dislocations, which are considered as sessile, through molecular dyanmics simulations. We demonstrate that the dislocation migrates smoothly along the 〈c+a〉 direction even at very low temperatures. We also clarify the unique migration mechanism: two terminating 1/2〈c+a〉 partial dislocations exchange a Shockley's partial dislocation at every 1/2〈c+a〉 migration. By revealing the glissile nature of the basal-dissociated 〈c+a〉 dislocation, we provide new insights into the plasticity mechanisms of Mg. Further decompositions with basal-dissociated ⟨c⟩ dislocations occur when the strain is applied perpendicularly to the slip plane. Under higher shear stress, the dissociated ⟨c⟩ dislocations cannot easily migrate and instead decompose into heterogeneous structures.

Improved characteristics of [formula omitted] Al0.62Ga0.38N film grown with pulsed flow growth technique

Pulsed flow growth (PFG) technique is a simple and effective method to improve the quality of aluminum gallium nitride (AlGaN) alloys. In this work, a reformed three-way PFG was applied to deposit (11 2‾2) plane Al0.62Ga0.38N film, and the separate ammonia (NH3) supply time in a PFG cycle was modulated carefully. It was revealed that the full width at half maximum (FWHM) values of on-axis X-ray rocking curves (XRCs) along [11 2‾3‾]- and [1 1‾00]-direction decreased conspicuously from 0.496 ° to 0.28 ° and from 0.674 ° to 0.451 ° by modifying the separate NH3 supply time to 3 s. Meanwhile, the FWHM values of basal stacking faults (BSFs)-related and prismatic stacking faults (PSFs)-related off-axis XRCs exhibited a reduction of 21.2 % and 17.2 %, respectively. Besides, the ratio of the intensity of BSF-related emission to that of near band emission (NBE) could be suppressed from 0.45 to 0.17. In other words, the crystal quality of (11 2‾2) AlGaN could be ameliorated significantly by optimizing separate NH3 supply time. Furthermore, the root mean square value was also reduced to 0.78 nm. These improvements could be interpreted as the reduced parasitic reaction between TMA and NH3 as well as the boosted migration of Al adatoms.

Fröhlich resonance splitting in hybrid GaN nanowire-Ag nanoparticle structures

Plasmonic nanoparticles (NPs) have attracted significant attention due to their unique optical properties and broad optoelectronic and photonic applications. We investigate modifications of emission in hybrid structures formed by 60 nm silver NPs and GaN planar nanowires (NWs). Bare GaN NWs exhibit photoluminescence (PL) spectra dominated by broad bands peaking at ∼3.44 eV and ∼3.33 eV, attributed to basal plane stacking faults. In hybrids, two new narrow PL lines appear at 3.36 and 3.31 eV, resulting in PL enhancement at these energies. While the 3.36 eV line in hybrid structures can be explained using the Fröhlich resonance approximation based on the electric dipole concept, the appearance of two features at 3.36 and 3.31 eV indicates the splitting of resonance lines. This phenomenon is explained in framework of theoretical model based on the interaction of the dipole with its charge image, taking into account the quadrupole moment of the silver sphere and the quadrupole field of the charge image. A good agreement is obtained between the calculated Fröhlich resonance frequencies and the experimental PL lines in hybrid structures.

Stability of Intensity Imbalance between Left‐ and Right‐Circulation Modes in GaN Hexagonal Microdisk

The mechanisms underlying the stability of the intensity imbalance between the left‐ and right‐circulating lasing modes in a GaN hexagonal microdisk resonator are investigated. Finite‐difference time‐domain simulations reveal that the imbalance is determined by the position of the point light source (seed light for lasing) inside the resonator. The coupling mechanism between the resonance modes and light from the source is clarified using numerical calculations. In addition, it is confirmed that light is highly coupled with one of the circulation modes when the source is placed off the bilateral symmetry axis of the mode near the side of the resonator, without any influence of the polarization of the source. The results indicate that the imbalance of the lasing modes observed in the experiments is determined by the position of the seed light near the side of the resonator, which must be spontaneous emission from impurities or defects, such as donor–acceptor pairs, donor‐bounded excitons, basal plane stacking faults, or the transition of free electrons in the conduction band to acceptor levels et al.

The Effect of AlN Epilayer Growth Rate on the Growth of Semipolar (11–22) InGaN/GaN Light Emitting Diode

This article presents the effect of aluminium nitride (AlN) epilayer (EPL) growth rate on the growth of semipolar (11–22) InGaN/GaN light emitting diode (LED). Three samples are grown with different AlN EPL growth rates, which are low (L1, 1.0 nm s−1), intermediate (L2, 1.3 nm s−1), and high (L3, 1.6 nm s−1) growth rate. The root‐mean‐square (RMS) and peak‐to‐valley roughness show an increasing trend with the increasing AlN EPL growth rate which is attributed to an increment in indium composition as justified by the photoluminescence results. The sample with AlN EPL growth rate of 1.3 nm s−1 (L2) exhibits an outstanding crystal quality in terms of defect densities reduction, with a minimum basal stacking fault density of 1.60 × 104 cm−1 as calculated. Besides that, the obvious fringes of InGaN/GaN peaks observed in 2θ−ω scan proves that a high abruptness multi‐quantum well is attained. Sample L2 also shows a single peak profile in both photoluminescence and electroluminescence spectra with green wavelength emission of 506 and 537 nm, respectively, indicating a homogeneous indium composition. As the injection current is swept from 5 to 40 mA for sample L2, a weak blueshift of ≈6 nm is observed which indicates a reduction in quantum‐confined Stark effect (QCSE).

High‐Quality Nonpolar a‐Plane AlGaN Film Grown on Si‐Doped AlN Template by Metal Organic Chemical Vapor Deposition

High crystalline quality and flat a‐plane aluminum gallium nitride (AlGaN) films are obtained on Si‐doped AlN templates with a moderate silane (SiH4) flow rate by metal‐organic chemical vapor deposition (MOCVD). The effects of the SiH4 flow rate on the surface morphology, crystalline quality, stress state, and optical property of a‐plane AlN templates and AlGaN films are comprehensively investigated. As the SiH4 flow rate increases from 0 to 7.0 nmol min−1, the full width at half maximum of X‐ray rocking curve values along [0001]/[1–100] directions for the AlGaN films are monotonically reduced to 1124/1143 arcsec. Meanwhile, the surface root mean square roughness value is decreased to 0.88 nm. These achievements are attributed to the suppression of the anisotropy degrees and in‐plane stress of AlN templates. In addition, an excess SiH4 flow rate leads to deteriorated surface morphologies and increased basal plane stacking fault (BPSF) densities for both AlN and AlGaN films. This work suggests that doping AlN layers with an appropriate SiH4 flow rate is a promising route to obtain high‐quality a‐plane AlGaN films for efficient nonpolar deep ultraviolet (DUV) devices.

Shock-induced plastic deformation of nanopowder Ti during consolidation and spallation.

Consolidating nanopowder metals via impact loading is a potentially significant method for synthesizing and processing bulk nanocrystalline materials. However, until now, the microstructural features, plastic deformation during consolidation, and corresponding mechanisms have been seldom revealed. Using molecular dynamics (MD) simulations, we have studied the plastic deformation, densification, spallation, and micro-jetting in nanopowder titanium (np-Ti) during shock. Upon impact, np-Ti undergoes a transition from heterogeneous plasticity, including basal stacking faults (SFs) and {101̄2} twinning, to homogeneous disordering, as the impact velocity increases. Then the nanopowder structure evolves into a bulk nanostructure after the final densification, contributed by pore collapse. The subsequent detwinning arises during the release and tension stage, conducing to a partial structural recovery. When the impact velocity up ≥ 1.0 km s-1, the spallation is following, prompted via GB-sliding and disordering. Upon shock impact, it also facilitates micro-jetting owing to the presence of nanopores, contributing to the pressure gradient and transverse velocity gradient.

Observation of structural defects in GaN/InGaN multi-quantum wells grown on semipolar (112¯2) substrate using cathodoluminescence in transmission electron microscopy

Defect structures of semipolar GaN thin film, grown on the m-plane of sapphire, were investigated using the home-built cathodoluminescence (CL) stage in TEM. The CL maps with high spatial resolution identify the type of basal plane stacking faults (BSFs) and visualize their distribution. I1-BSF type defects had the highest population and the I2-BSF type defect was revealed in the form of thin strips with less population. Partial dislocations and basal stacking fault-related defects existed in the form of segmented lines but with strong luminescence characteristics. TEM-CL was able to identify the distribution and the characteristic luminescence from the defects, which were difficult to discern in the typical defect analysis.

Extra-conventional strengthening mechanisms in non-recrystallized grains of an extruded Mg-Gd-Zr alloy

Mg-RE (RE = rare earth) based alloys generally exhibit outstanding mechanical properties. However, their high-strength seems to be unexplained using classic strengthening mechanisms in some cases. Herein, a Mg-13Gd-0.4Zr (wt%) alloy that was fabricated by a conventional differential thermal extrusion plus artificial aging treatment exhibits ultra-high yield strength over 510 MPa in both tension and compression. Characterizations using Cs-corrected scanning transmission electron microscopy (STEM) show two unusual microstructures in non-recrystallized grains as: a large density of basal stacking faults (SFs) and profuse distortion areas (DAs). Atomic-resolution STEM imaging indicates that basal SFs are consisted of two types of intrinsic SFs, namely I1 and I2, and DAs are self-assembled by 〈c〉 and 〈c + a〉 screw partials. Their strengthening mechanisms are analogous to grain boundary strengthening and dispersion strengthening, respectively, contributing satisfactory yield-strength increments of ∼46 MPa and ∼76 MPa, respectively. Moreover, DAs improved aging hardening by inducing novel clusters at DA-related boundaries, or increasing the number density of βH’ precipitate and promoting their distribution along a certain direction. This work supplements the strengthening mechanisms in traditional high-strength Mg-RE(-Zr) based alloys and provides novel insights in the development of ultra-high-strength Mg alloys.

The trigonal structure as a reference to access the spontaneous polarization of wurtzite crystals

The Berry-phase treatment of the spontaneous polarization defines this quantity as the polarization difference between the investigated system and an appropriate reference state. In the case of wurtzite crystals, the spontaneous polarization is usually referenced to the zinc blende structure. Here, we demonstrate that the closely-related trigonal structure can also be used as a reference to determine the spontaneous polarization of wurtzite materials. We compare the trigonal-based values for wurtzite nitrides XN (X = Al, Ga, In) and oxides YO (YBe, Zn) with the widely-known zinc blende results reported in the literature and find an excellent agreement. It is shown that the electronic contribution of polarization is greater than the ionic one. Furthermore, we reproduce the experimental value of the spontaneous polarization of wurtzite GaN reported by Lähnemann et al. [Phys. Rev. B 86, 081302(R) (2012)]. In order to do so, we calculate the spontaneous polarization for each type of basal stacking faults using periodic hexagonal supercells. This theoretical analysis leads to a value nearly identical to the experimental measurement.

Optimizing strength-ductility of laser powder bed fusion-fabricated Ti–6Al–4V via twinning and phase transformation dominated interface engineering

Titanium alloys produced by laser powder bed fusion (LPBF) are known to possess fine microstructures and multi-scale interfaces. Here, we introduce a high density of α'/β, α/β phase interfaces, {10 1‾1} twin boundaries, and basal stacking faults (BSFs) in a Ti–6Al–4V alloy through LPBF and annealing. The LPBF-fabricated alloy consists of fine acicular α′ martensite with numerous {10 1‾1} twins and BSFs, which results in ultrahigh strength (>1300 MPa), but very low ductility (<5%) due to the massive interface strengthening. Subsequent annealing treatments decrease the strength while increasing the ductility, as the α′ martensite partially or fully decomposes into a lamellar (α+β) structure. The 955 °C-annealed alloy possesses a good strength-ductility combination (yield strength of 1000 MPa, tensile strength of 1078 MPa, and total elongation of 20%). During the stages of uniform plastic strain (up to ∼12%), deformation occurs by dislocation slips, leading to significant dislocation accumulations around the α/β interfaces. At later stages (∼20% strain), when necking occurs, a high density of fcc-γ bands is observed in the hcp-α phase, indicating the onset of deformation-induced martensitic transformation (DIMT) from hcp-α to fcc-γ. The occurrence of DIMT may be attributed to the decreased stacking fault energy and the cohesive energy difference between the hcp and fcc phases, due to the segregation of Al elements in the hcp-α phase. Low-loss electron energy loss spectroscopy reveals that the deformation-induced fcc phase is not Ti-hydride, but a new allotrope of Ti.

Regrowth characteristics of semipolar (11–22) GaN epitaxial film on corroded semipolar GaN template

Semipolar (11–22) GaN epitaxial film was grown by metal-organic chemical vapor deposition (MOCVD) on a prismatic structure corroded (11–22) GaN template prepared by wet chemical etching method. The X-ray rocking curves (XRCs) of on-axis and off-axis (11–22) GaN reflection planes were measured, and the results demonstrate that (11–22) GaN epitaxial film grown on the corroded GaN template can improve the crystal quality by reducing the defects such as basal stacking faults (BSFs) and partial dislocations (PDs). The SEM analysis for the semipolar GaN epitaxial film grown on the corroded GaN template and untreated GaN template also show that the mosaic grain size of GaN epitaxial film grown on the corroded GaN template is larger than that untreated one. The photoluminescence (PL) spectrum of the semipolar GaN epitaxial film grown on the corroded GaN template may also indicate better crystal quality. Besides, we found that the recombination mechanism in BSFs defects was dominated by radiative recombination. In our case, our research demonstrates that the crystal quality of semipolar GaN epitaxial films can be improved by regrowing GaN film on corroded GaN template.

Unusual F3 stacking fault in magnesium

An unusual F3 basal stacking fault resulting from twin-dislocation interaction in magnesium is observed in molecular dynamics simulation. The F3 fault is produced in the twin lattice from the interaction between a migrating (101¯2) twin boundary and a partial dislocation of either a prismatic 〈c〉 edge, or a prismatic 〈c+a〉 mixed dislocation in the matrix. The condition is that the partial dislocation needs to have a negative sign and lie on a plane intersecting a compression site of the twin boundary. The F3 fault can also be generated when a positive basal 〈a〉 mixed dislocation in the twin lattice, with slip plane intersecting a compression site of the twin boundary, interacts with a basal-prismatic twinning disconnection. The F3 fault comprises two I1 faults that have the same character but are separated by two basal layers. It has one end connected to the twin boundary, and the other end bounded by a lattice defect with a Burgers vector identical to that of a 30° Shockley partial dislocation. The formation frequency of the F3 fault is higher at a lower shear stress (below ∼400 MPa) and/or a lower temperature (100 K and 200 K). The F3 fault can decompose into a glissile 30° Shockley and a T2 fault at a temperature above ∼400 K. The relationships between the F3 fault and other types of basal stacking faults such as I2, T2 or paired I1 faults that are separated by multiple basal layers are discussed.

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.