Prismatic Stacking Faults Research Articles

Atomic-scale visualization of defect-induced localized vibrations in GaN.

Phonon engineering is crucial for thermal management in GaN-based power devices, where phonon-defect interactions limit performance. However, detecting nanoscale phonon transport constrained by III-nitride defects is challenging due to limited spatial resolution. Here, we used advanced scanning transmission electron microscopy and electron energy loss spectroscopy to examine vibrational modes in a prismatic stacking fault in GaN. By comparing experimental results with ab initio calculations, we identified three types of defect-derived modes: localized defect modes, a confined bulk mode, and a fully extended mode. Additionally, the PSF exhibits a smaller phonon energy gap and lower acoustic sound speeds than defect-free GaN, suggesting reduced thermal conductivity. Our study elucidates the vibrational behavior of a GaN defect via advanced characterization methods and highlights properties that may affect thermal behavior.

Molecular dynamics simulations of the shear and tensile mechanical properties of rare-earth metal erbium based on deep-learning potential

To gain atomic insights into the physical and mechanical properties of rare-earth metal erbium (Er), performing molecular dynamics (MD) simulations is highly advantageous but has been constrained for lacking of Er interatomic potential. In this work, the essential Er potential for MD simulations was developed by using the deep-potential (DP) method based on first-principles calculations. By systematically comparing the DP-predicated physical properties (melting and solidifying points, elastic properties, etc.) with the results of density functional theory (DFT), experimental or MEAM, we demonstrated that the developed DP model of Er exhibits satisfactory generalization ability and reasonable DFT accuracy on predicting these properties. From the DP-predicted results, we confirmed that the HCP-Er is mechanically stable. The melting point of HCP-Er is 1847 K, while the solidifying point is 1105 K at the cooling rate of 1×1011 K/s. Moreover, we found that the prismatic unstable stacking fault energy of HCP-Er is lower than the basal one, indicating that the Shockley partial dislocation along prismatic planes is easier to slip. The basal/prismatic and prismatic/basal interfaces were discovered during the tensile deformation of HCP-Er single-crystal along the [0001] direction, signifying the occurrence of the parent-to-‘twin’ lattice reorientation. The yield strength of HCP-Er decreases from 5.41 to 5.21 GPa with rising the temperature from 150 to 500 K. The results would be helpful for better understanding the mechanical behaviors of metal Er and further developing the binary or multinary Er-containing DP models.

On the Temperature and Composition Dependence of Non‐basal Stacking Faults in C14 Laves Phases

Activation of non‐basal slip is essential in improving the deformability of hexagonal crystals. However, for complex intermetallics such as Laves phase, the mechanism of non‐basal slip remains largely unknown. In this work, the prismatic slip systems of C14 Laves phase crystals and possible metastable states along the slip paths are assessed using atomistic simulations. Multiple prismatic stacking fault states with the same projected positions of atoms but different site occupancies and chemical distributions are identified at different inter‐atomic layers, temperatures and chemical compositions. The formation of energetically favorable prismatic stacking faults involves short‐range diffusion which implies the thermally activated nature of prismatic slip. The outcomes of this work advance the understanding of temperature and composition‐dependent non‐basal slip in Laves phases.

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.

Improving the strength and ductility of magnesium alloys by Sc alloying: An experimental and first-principles study

Mg–Sc alloys have great application potential due to the positive effect of Sc on improving corrosion resistance and mechanical properties. In this work, the effects of Sc content on the strength and ductility of Mg-xSc (x=0, 1, 2, 3 at.%) alloys were investigated by experiments and first-principles calculations. The results indicate that, with the increase of Sc content, the hardness, strength, and ductility of the alloys increase significantly. The improvement of strength is attributed to the solid solution strengthening and grain size refinement effect. The addition of Sc reduces the prismatic unstable stacking fault energy of the alloys, thereby promoting the activation of stacking faults and <c+a> dislocations, resulting in the significant improvement of ductility. The mechanism of the enhanced ductility of Mg-xSc solid solution alloys is revealed, which provides guidance for designing high-performance Mg alloys.

Enhanced plastic deformation capacity in hexagonal-close-packed medium entropy alloys via facilitating cross slip

• A novel Hf x (ZrTi) 100-x (x=0, 20, 33 and 40 at.%) MEAs with single phase HCP was developed. • Addition of Hf enhanced the strain hardening rate and tensile ductility. • Effects of Hf on stacking fault energy and dislocation slip behavior were revealed. • Facilitated cross slip and pyramidal dislocations is responsible for the outstanding mechanical properties. Alloys with a hexagonal close-packed (HCP) lattice often suffer from intrinsic brittleness due to their insufficient number of slip systems, which limits their practical uses. In this paper, nevertheless, we show that remarkably tensile ductility in HCP Hf-Zr-Ti medium entropy alloys (MEAs) was achieved, particularly in the MEAs with a higher content of Hf. Both first-principles calculation and experimental analyses reveal that addition of Hf increases basal I2 stacking fault energy and decreases prismatic stacking fault energy in these HCP MEAs, which promotes the source of pyramidal dislocations due to the facilitated cross slips of basal dislocations and eventually give rise to the observed large tensile ductility. Our current findings not only shed new insights into understanding deformation of HCP alloys, but also provide a basis for controlling alloying effects for developing novel HCP complex alloys with optimized properties.

Characteristics of [formula omitted] screw dislocations and their slip on prismatic and pyramidal planes in pure [formula omitted] titanium from atomistic simulations

Molecular statics and dynamics simulations were carried out to characterize the core structure and calculate the critical resolved shear stress (CRSS) of an 〈a〉 screw dislocation in pure α-Ti using a recent modified embedded atom method spline like potential developed for Ti-Nb alloys. Firstly, it is shown that the generalized stacking fault energy (GSFE) curves calculated using this potential agree well with density functional theory (DFT) and nudged elastic band + DFT computations for the basal, prismatic-I and pyramidal-I planes. In particular, this potential predicts that the 〈a〉 prismatic stacking fault (SF) is lower in energy than the basal intrinsic I2 SF. Two stable core structures for the 〈a〉 screw dislocations were identified by this potential, one dissociates on the pyramidal-I plane, and the other on the prismatic-I plane. The pyramidal-I core is found to be the ground state core structure, while the prismatic-I core is metastable with an excess energy of 18.8 meV/b, which is in good agreement with published DFT calculations. Also, it is shown that the basal dissociation of the 〈a〉 screw dislocation is not stable and upon relaxation this core reconstructs directly to the pyramidal-I ground state structure. Finally, the CRSS for the slip of a prismatic 〈a〉 screw dislocation core on the prismatic-I plane is computed as a function of temperature between 0 and 300 K. It is found that screw dislocations glide by kink-pair mechanism on the prismatic-I planes, and the calculated CRSSs are in good agreement with experimental measurements.

Formation of voids and their role in the recovery of sputtered AlN during high-temperature annealing

The structural recovery of AlN grown by reactive sputtering on a sapphire substrate during high-temperature annealing is studied by means of transmission electron microscopy and secondary ion mass spectrometry. The as-grown film shows high-density planar defects, such as basal and prismatic stacking faults, caused by the limited diffusion length of the adatoms and, thus, presents a columnar structure. The presence of high-density nanopipes is associated with the presence of unintentional oxygen impurities. Based on the atomic resolution transmission electron microscopy analysis, we show that basal and prismatic stacking faults vanish in the films via a climb mechanism and describe this process as the nucleation of jogs promoted by the diffusion of vacancies. The nanopipes present in the as-grown film transform into faceted voids and act as a beneficial source of excess vacancies that promote dislocation annihilation by climb. The transformation of nanopipes to faceted voids resembles the transition from open channel pores to close faceted pores, which has been observed in porous silicon and can be described in terms of a classical sintering theory.

Structures of boundaries and corners of fully-closed hexagonal domains in HVPE-AlN film

Prismatic stacking fault (PSF) is a common and important lattice defect in wurtzite nitrides. This work reports unusual nano-sized hexagonal-shaped domains in AlN films grown on (0001) SiC substrate by hydride vapor phase epitaxy (HVPE), based on atomic-resolution scanning transmission electron microscopy and first-principles calculations. The domain boundaries are all PSFs lying parallel to {112¯0} planes, and comprise alternating 4-membered-rings (tetra-rings) and 8-membered-rings (octa-rings). The six corners of a single domain, each defined by the intersection of two neighbouring PSFs, are observed for the first time to have well-assembled structures containing one, two or three octa-rings in different configurations. Distinct features include that each domain corner carries a 1/6<112¯0> corner partial dislocation (CPD) arranged tangentially there, and that a net zero of the Burgers vectors is obtained summing up all six CPDs of a domain. In addition, all six side PSF boundaries are found to simultaneously intersect with I1 basal stacking fault (BSF), and a mechanism is proposed to account for the formation of an edge-type CPD from two intersected screw-type stair-rod dislocations (SRDs). Hence, the fully-closed PSF domain is a unique self-assembled system not only as a textbook example to demonstrate a solution of the argued disconnection problem at the PSF-PSF intersection, but also as a defective system which can help relax the stress remained in the film growth process. Based on these facts, a hexagonal island growth mechanism is discussed for the formation of the observed domain.

Atomic scale simulation of the strain rate and temperature dependence of crack growth and stacking faults in zirconium

Molecular dynamics simulations of single crystal zirconium fracture were performed to study the deformation mechanisms active on the basal and prismatic planes. The effects of temperature (0 to 300 K) and strain rate (108–1010 s−1) were investigated. Crack tip orientation was found to strongly affect the fracture behaviour. On the basal plane twinning ({112̄1}<11̄26>) and emission of <c+a> type dislocations that then dissociated into partial dislocations around pyramidal I2 stacking faults were seen to occur during fracture. At higher strain rates (109 and 1010 s−1), twinning occurred. The emission of edge dislocations (13<12̄10> type) was prevalent on the prismatic plane and were found to be strongly affected by temperature. At higher temperature (150 and 300 K), the dislocation density increased. The crack grew further at 150–300 K than at 0 K and the shielding effect of dislocations was limited due to their movement away from the crack tip. The addition of iodine at basal I2, pyramidal I1 and I2 stacking faults was seen to decrease the energy of its formation whereas for the prismatic stacking fault it was found to increase it. The iodine also changed the order of favourability of the stacking faults with basal I2 and pyramidal I1 stacking faults becoming much more favourable and prismatic going from most to least favourable.

Defect characterization of {101¯3} GaN by electron microscopy

Advances in obtaining untwinned (101¯3)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (101¯3) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (101¯3) has the potential for higher radiative efficiency than established semi-polar orientations.

Defect Tolerance of Intersubband Transitions in Nonpolar GaN/(Al,Ga)N Heterostructures: A Path toward Low-Cost and Scalable Mid- to Far-Infrared Optoelectronics

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_{3}$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^{5}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ to ${10}^{6}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, 5 \ifmmode\times\else\texttimes\fi{} ${10}^{3}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{4}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, about 9 \ifmmode\times\else\texttimes\fi{} ${10}^{10}$ to 2 \ifmmode\times\else\texttimes\fi{} ${10}^{11}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, and about ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250--300 meV (wavelength range 4.1--4.8 \textmu{}m) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%--20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Light-Emitting V-Pits: An Alternative Approach toward Luminescent Indium-Rich InGaN Quantum Dots

Realization of fully solid-state white light emitting devices requires high efficiency blue, green, and red emitters. However, challenges remain in boosting the low quantum efficiency of long wavelength group-III-nitride light emitters through conventional quantum well growth. Here, we demonstrate a new direct metal–organic chemical vapor deposition approach to grow In-rich InGaN quantum dots on Si substrates using V-pits, bypassing the need for patterning or unconventional substrates. A correlative nanoscale study on the optical, compositional, and structural properties of intersecting V-pits reveals that the highly textured surface gives rise to localized high intensity red-shifted emission from the apexes of pyramids where InGaN quantum dots spontaneously form. We establish the origin of this efficient long wavelength luminescence to result from both spatially confined higher In-content deposition, as well as smaller bandgap energy basal stacking faults entrapped within a ring of low-emissivity prismatic stacking faults. Our monolithic growth approach on Si would open up new pathways toward attaining CMOS-compatible phosphor-free white light emitting solid-state devices.

Exceptional Deformability of Wurtzite Zinc Oxide Nanowires with Growth Axial Stacking Faults.

To ensure reliability and facilitate the strain engineering of zinc oxide (ZnO) nanowires (NWs), it is significant to understand their flexibility thoroughly. In this study, single-crystalline ZnO NWs with rich axial pyramidal I (π1) and prismatic stacking faults (SFs) are synthesized by a metal oxidation method. Bending properties of the as-synthesized ZnO NWs are investigated at the atomic scale using an in situ high-resolution transmission electron microscopy (HRTEM) technique. It is revealed that the SF-rich structures can foster multiple inelastic deformation mechanisms near room temperature, including active axial SFs' migration, deformation twinning and detwinning process in the NWs with growth π1 SFs, and prevalent nucleation and slip of perfect dislocations with a continuous increased bending strain, leading to tremendous bending strains up to 20% of the NWs. Our results record ultralarge bending deformations and provide insights into the deformation mechanisms of single-crystalline ZnO NWs with rich axial SFs.

Atomistic analyses of HCP-FCC transformation and reorientation of Ti in Al-Ti multilayers

Atomistic simulations have been performed in this study to examine the roles of interface characteristics, layer thickness, and heating rate on microstructural transformation of Al-Ti multilayers. The individual layer thickness varies from 4 nm to 55 nm, and eight different configurations that have different crystallographic orientation relationships are examined. All the multilayers are heated to 300 K with a heating rate varying between 1.5 × 1013 K·s−1 and 3.0 × 1010 K·s−1. No microstructural changes occur for the multilayers having the traditional orientation relationship (OR) (configuration A) observed experimentally. Similarly, the multilayers having other configurations that are defined by different degrees of misorientations (configurations 1–6) from the traditional OR also do not show any microstructural change. The transformation in multilayers having incoherent and high energy interface (configuration B) is heavily influenced by layer thickness and heating rate. The Al layer does not show any transformation for all the considered configurations, layer thicknesses, and heating rates. However, the Ti layer has shown the formation of FCC Ti and reorientation of parent HCP Ti in multilayers with configuration B. The HCP to FCC structural transformation of Ti occurs by creating a series of successive prismatic stacking faults in the Ti layer, and results in a decrease of energy by 4–5 meV/atom for complete transformation and 1–3 meV/atom for partial transformation. The HCP-FCC transformation of Ti results in a volume change in the range of 0.06–0.08 Å3/atom. At larger layer thicknesses, reorientation in the Ti layer, which is equivalent to an effective rotation of parent Ti about [12¯10] direction by 90°, occurs. The reorientation occurs via the formation of an intermediate BCC Ti phase resulting from the coordinated displacement of atoms on alternating (0001) planes in [101¯0] direction, accompanied by almost 10% compression along [101¯0] direction. The intermediate BCC Ti transforms to reoriented HCP Ti by displacements of atoms on alternating (110) planes in [11¯0] direction, accompanied by almost 2% compression along [110] direction. Reoriented Ti is less strained as compared to parent HCP Ti because of the change in the c/a ratio.

Structural Characterization of Prismatic Stacking Faults of Two Types of Carrot Defects in 4H-SiC Epi Wafers

Two types of carrot defects with and without a shallow pit were found by mirror projection electron microscopy (MPJ) inspection in 4H-SiC epi wafer. Surface morphology and cross-sectional structure of prismatic stacking faults (PSFs) were investigated using MPJ and atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM). The depths of the surface grooves due to the PSFs, the stacking sequences around the PSFs and the structure of the Frank-type stacking faults which were connected to the PSFs were different. We discuss the difference between the two types of carrot defects.

The effect of local chemical environment on the energetics of stacking faults and vacancy platelets in [formula omitted]-zirconium

The onset of breakaway irradiation growth in Zr and its alloys has been correlated with the nucleation and growth of faulted vacancy c-loops on basal planes. One theory for c-loop stabilization at high fluence is the reduction in stacking fault energy by solute or impurity segregation. This hypothesis is investigated through density functional theory calculations of the binding energy of Fe, Sn, Nb, Cr, and Ni to basal extrinsic, basal intrinsic or prismatic stacking faults in α-Zr. By means of a thermodynamic model, the ab initio binding energies have been correlated to segregation profiles and modified SFEs. Sn and Fe are shown to be tightly bound to basal stacking faults with only a weak interaction towards prismatic stacking faults. The corresponding segregation to faulted basal c-loops significantly reduces their formation energies and has the potential to stabilize these defects in the matrix. Based on the DFT calculations presented here, Nb, Cr, and Ni do not have a considerable effect on faulted c-loop formation energies. The binding energies of Fe, Sn, Nb, Cr, Ni, and H to a 19-vacancy platelet were additionally calculated to confirm the behavior of these elements with a nucleating c-loop defect. Fe and Cr in particular show strong binding to the periphery of this high-energy structure and correspondingly reduce the nucleation barrier for a vacancy c-loops.

Microstructure of Stacking Fault Complex/Carrot Defects at Interface Between 4H-SiC Epitaxial Layers and Substrates

A carrot defect with a shallow pit has been detected in a 4H-SiC epitaxial wafer using mirror projection electron microscopy inspection. The origin of the carrot defect and the microstructure of the conversion point at the interface between the epitaxial layer and the substrate were investigated using transmission electron microscopy and high-resolution scanning transmission electron microscopy. We found that two types of threading edge dislocations (TEDs) in the substrate, with $$ {\varvec b} = 1/3\left[ {\bar{2}110} \right] $$ and $$ {\varvec b} = 1/3\left[ {\bar{1}\bar{1}20} \right] $$ , generated the carrot defect with a shallow pit in the epitaxial layer. The two TEDs converted to basal plane dislocations (BPDs) at the conversion point. Furthermore, one of the two BPDs led to the pair generation of a threading dislocation running to the epi surface and four partial dislocations propagating on each basal plane. The other BPD joined one of the four partial dislocations at the conversion point; this merged dislocation was assumed to cause a prismatic stacking fault.

Crystal quality and surface structure tuning of semi-polar (11–22) GaN on m-plane sapphire via in-situ multiple ammonia treatment

The crystal quality and morphological properties of semi-polar (11–22) gallium GaN were enhanced by introducing the In-Situ Multiple Ammonia Treatment (I-SMAT) method. On- and off-axis x-ray rocking curve (XRC) analysis reveals that I-SMAT would reduce several types of crystal defect namely prismatic stacking faults, basal stacking fault type I and II, and partial/perfect dislocations. By implementing an optimized flow of ammonia (NH3) and number of treated GaN pairs, the arrowhead-like features of semi-polar (11–22) GaN was effectively reduced the root mean square roughness from 4.52 to 3.07 nm. High atomic force microscopy (AFM) magnification scan demonstrated the alteration of the grain structure on the surface to become more compact and orderly arranged resulting in narrowing/shallowing of the interfacial valleys between grains. Great influence of employed I-SMAT pairs and NH3 flux was observed from XRC and AFM analysis. Conversely, excessive NH3 flux during this approach would roughen the structural and morphological properties of the semi-polar epilayer whereby the alternating thin GaN epilayer would undergo the selective-area etching to the extreme.

Luminescence behavior of semipolar (101¯1) InGaN/GaN “bow-tie” structures on patterned Si substrates

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.