Basal Stacking Faults Research Articles

Recent Advances in Nonpolar and Semipolar InGaN Light-Emitting Diodes (LEDs).

The III-nitrides have attracted much attention because of their applicability in optoelectronic devices, whose emission wavelengths range from green to ultraviolet light due to their wide band gap. However, conventional c-plane GaN-based devices are influenced significantly by spontaneous and piezoelectric polarization effects, which could pose a limitation for increased luminous efficiency as a result of the quantum confined stark effect. Since the early 2000s, many groups have tried to solve these problems by examining the growth of GaN on non- or semipolar surface planes. High power non- and semipolar LEDs can be realized by the growth of a thick active layer. In addition, it is expected that it is possible to grow nonpolar InGaN LEDs with high quality p-GaN layers due to lower hole activation energy, and also long-wavelength semipolar InGaN LEDs because of the capacity for high indium incorporation in the quantum wells (QWs). However, non- and semipolar structures grown on sapphire substrate usually contain a high density of basal stacking faults and threading dislocations. For this reason, the growth of non- and semipolar GaN-based LEDs on a sapphire substrate has been attempted through the introduction of defect reduction techniques such as epitaxial lateral overgrowth, patterned sapphire substrate and re-growth techniques on a porous GaN layer, etc. Also, some researchers have grown high quality non- and semipolar GaN-based LEDs using non- and semipolar freestanding GaN substrates. In this review paper, we introduce and discuss recent progress in the development of non- and semipolar GaN-based LEDs and freestanding GaN substrates.

A model for 〈c+a〉 dislocation transmission across nano-spaced parallel basal stacking faults in a HCP alloy

We develop a model for recently observed 〈c + a〉 dislocation transmission across nano-spaced parallel basal stacking faults in hexagonal close-packed (HCP) Mg microcrystalline grains. The model theoretically predicts nanospace-dependent yield strength that is in good agreement with experimental data. Additionally, it theoretically predicts activation volume and strain rate sensitivity.

Enhancing the performance of Mg-based implant materials by introducing basal plane stacking faults.

One of the keys to allowing Mg alloys to serve as biodegradable materials is how to balance their degradation behaviours and mechanical properties in physiological environment. In this study, a novel Mg-6Ho-0.5Zn alloy (wt%) containing profuse basal plane stacking faults (SFs) is prepared. This newly-developed alloy with SFs exhibiting uniform corrosion behaviour, low corrosion rate and high mechanical properties, as compared to the classic Mg-Ho based alloys (Mg-6Ho and Mg-6Ho-1.5Zn). Furthermore, the Mg-6Ho-0.5Zn alloy shows no significant toxicity to Saos-2 cells. An original uniform corrosion mechanism is proposed by combining the special defect structure, orientation of SFs and promptly effective corrosion film. The development of the new microstructure for Mg-Ho based alloys with desirable corrosion performance has important implications in developing novel degradable Mg-based implant materials.

Characterization of a-plane InGaN light-emitting diodes with a SiNx interlayer grown on a patterned sapphire substrate by metal–organic chemical vapor deposition

Nonpolar, a-plane InGaN light-emitting diodes (LEDs) were grown on a hemispherical r-plane patterned sapphire substrate (HPSS) by the in situ deposition of SiNx masks and on a planar sapphire substrate by metalorganic chemical deposition (MOCVD). The use of the SiNx interlayer together with HPSS resulted in a reduced density of defects, such as basal plane stacking faults (BSFs) and partial dislocations (PDs) in the GaN epilayers, as compared with the use of GaN on a conventional planar substrate. The optical power of the InGaN LEDs with the SiNx layer and HPSS was approximately 2.5- and 10-fold higher than those of LEDs on SiNx and on a planar substrate at a current of 100 mA, respectively.

Single phase (112¯2) AlN grown on (101¯0) sapphire by metalorganic vapour phase epitaxy

Heteroepitaxial growth of AlN buffer layers directly on (101¯0) sapphire substrates by metalorganic vapour phase epitaxy has been investigated. A single-step growth procedure without a sapphire nitridation was employed resulting in mirror-like crack free ≈1.1–1.6μm thick AlN layers of single phase (112¯2) orientation. Trimethylaluminum pre-dose time and reactor pressure were optimized for surface roughness and crystal quality. The crystal quality was found to degrade with increasing pre-dose time and also reactor pressure. The smallest full width at half maximum value for on-axis X-ray rocking curve of the (112¯2) AlN layers was about 610arcsec and 1480arcsec along [1¯1¯23]AlN and [11¯00]AlN, respectively. The surface roughness, measured by atomic force microscopy using a 10×10μm2 area, was in the range 2.6–3.5nm. A basal stacking fault density of (7±1)×105cm−1 was estimated by transmission electron microscopy.

Connections between the basal I1 “growth” fault and 〈c + a〉 dislocations

Recently published experimental results have suggested a connection between the I1 basal stacking fault and the non-basal 13〈112¯3〉 (i.e. 〈c+a〉) dislocation, particularly in Mg–Y and Mg–Y–Zn alloys. Deformed Mg–Y alloys contain more 〈c+a〉 dislocations and I1 faults than pure Mg, and the I1 fault energy has been shown to be significantly reduced by the addition of Y solute atoms using ab initio modeling. The TEM evidence of a possible connection between the two crystal defects is reanalyzed to reveal: (i) a non-planar dissociation of the pyramidal 〈c+a〉 edge dislocation into the basal plane, resulting in an I1 fault bounded by 16〈202¯3〉 partial dislocations; and/or (ii) a new source mechanism involving nucleation of the pyramidal 〈c+a〉 dislocation from a pre-existing I1 fault. The former reaction is energetically favorable, but non-conservative. The latter is shown to be energetically conceivable for a wide range of fault geometries. The concepts hypothesized in this paper provide explanations for the frequent TEM observation of rectilinear, edge 〈c+a〉 dislocations in Mg alloys, as well as a possible explanation for the yield strength anomaly observed in Mg and Mg alloy single crystals. Implications for radiation damage in hexagonal close packed metals are also suggested.

Evaluation of growth methods for the heteroepitaxy of non-polar [formula omitted] GAN on sapphire by MOVPE

Non-polar a-plane gallium nitride (GaN) films have been grown on r-plane (11¯02) sapphire by metal organic vapour phase epitaxy (MOVPE). A total of five in situ defect reduction techniques for a-plane GaN are compared, including two variants with a low temperature GaN nucleation layer (LTNL) and three variants without LTNL, in which the high-temperature growth of GaN is performed directly on the sapphire using various crystallite sizes. The material quality is investigated by photoluminescence (PL), X-ray diffraction, cathodoluminescence, atomic force and optical microscopy. It is found that all layers are anisotropically strained with threading dislocation densities over 109cm−2. The PL spectrum is typically dominated by emission from basal plane stacking faults. Overall, growth techniques without LTNL do not yield any particular improvement and even result in the creation of new defects, i.e. inversion domains, which are seldom observed if a low temperature GaN nucleation layer is used. The best growth method uses a LTNL combined with a single silicon nitride interlayer.

Structure and strain relaxation effects of defects in InxGa1−xN epilayers

The formation of trench defects is observed in 160 nm-thick InxGa1−xN epilayers with x ≤ 0.20, grown on GaN on (0001) sapphire substrates using metalorganic vapour phase epitaxy. The trench defect density increases with increasing indium content, and high resolution transmission electron microscopy shows an identical structure to those observed previously in InGaN quantum wells, comprising meandering stacking mismatch boundaries connected to an I1-type basal plane stacking fault. These defects do not appear to relieve in-plane compressive strain. Other horizontal sub-interface defects are also observed within the GaN pseudosubstrate layer of these samples and are found to be pre-existing threading dislocations which form half-loops by bending into the basal plane, and not basal plane stacking faults, as previously reported by other groups. The origins of these defects are discussed and are likely to originate from a combination of the small in-plane misorientation of the sapphire substrate and the thermal mismatch strain between the GaN and InGaN layers grown at different temperatures.

Defect reduction of SiNx embedded m-plane GaN grown by hydride vapor phase epitaxy

Nonpolar (10−10) m-plane GaN has been grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE). We studied the defect reduction of m-GaN with embedded SiNx interlayers deposited by ex-situ metal organic chemical vapor deposition (MOCVD). The full-width at half-maximum values of the X-ray rocking curves for m-GaN with embedded SiNx along [11−20]GaN and [0001]GaN were reduced to 528 and 1427arcs, respectively, as compared with the respective values of 947 and 3170arcs, of m-GaN without SiNx. Cross-section transmission electron microscopy revealed that the basal stacking fault density was decreased by approximately one order to 5×104cm−1 due to the defect blocking of the embedded SiNx. As a result, the near band edge emission intensities of the room-temperature and low-temperature photoluminescence showed approximately two-fold and four-fold improvement, respectively.

Effect of Ag doping on the microstructure and photoluminescence of ZnO nanostructures

ZnO nanostructures were obtained by metal-organic chemical vapor deposition via Ag catalyst-assisted growth in a temperature range of 200–500 °C. Growth at temperatures above 500 °C resulted in vertically aligned ZnO nanorods (NLs). Ag incorporation into ZnO up to 0.4 at.% promoted creation of basal plane stacking fault (BSF) defects and corrugation of the side facets of the NLs. The presence of BSFs give rise to an additional photoluminescence peak with a wavelength of ∼386 nm, which is slightly red-shifted compared to the commonly observed NBE emission at ∼375 nm. The observed emission was found to be specifically observed from the side facets of the NLs. It is suggested that this emission is due to a high concentration of BSFs in the ZnO as a result of an incorporation of Ag as acceptor dopant. SEM image of an Ag-doped ZnO nanorod with corrugated side facets. The observed corrugation is accompanied by a high concentration of basal plane stacking faults.

Growth and doping of semipolar GaN grown on patterned sapphire substrates

In order to achieve large area semipolar GaN layers with high crystal quality, we have etched trenches into n-plane and r-plane sapphire wafers exposing c-plane-like side-walls, from which GaN stripes can be grown by metalorganic vapor phase epitaxy mainly in c-direction, forming semipolar {101¯1} or {112¯2} surfaces after coalescence. Here, we describe how to improve such layers by optimizing the side-facet orientation and by including a SiN nanomask interlayer in situ into the growth process, eventually resulting in a basal plane stacking fault density below 5×103cm−1. Moreover, doping experiments have revealed a substantially lower Mg incorporation efficiency on the {112¯2} surface as compared to the c-plane, whereas Si does not show such differences.

Basal plane stacking faults in semipolar AlGaN: Hints to Al redistribution

We investigate the optical and structural properties of an AlGaN layer with low Al content grown on a semipolar GaN facet. The correlation of scanning transmission electron microscopy with high‐resolution cathodoluminescence recorded on the identical sample spot allows to identify the emission features of AlGaN defects. We assign a basal plane stacking fault of type to a spatially localized emission below the ( in AlGaN. This defect starts inside the GaN template and continues through the AlGaN layer. The energy spacing between the () related emission band and the corresponding ‐type basal plane stacking fault emission is larger than in the binary GaN case which indicates a modified Al content at the defect.

Defect reduction method in (11-22) semipolar GaN grown on patterned sapphire substrate by MOCVD: Toward heteroepitaxial semipolar GaN free of basal stacking faults

We report on the selective area growth of semipolar (11-22) GaN epilayers on wet etched r-plane patterned sapphire substrates (PSS) by metal organic chemical vapor deposition. Using a three-step growth method, planar (11-22) GaN epilayers on 2in. wafers with significant optical and structural quality improvements have been obtained. The filtering of basal stacking faults and dislocations was achieved by overlapping adjacent crystals and forming voids between them. These voids act as a barrier to defect propagation which results in reduced defect density at the surface of the epilayer. Cathodoluminescence measurements at 80K revealed a dislocation density of 5.1×107cm−2 and a basal stacking fault density below 30cm−1. Moreover, photoluminescence and X-ray diffraction measurements attested a material quality similar to conventional GaN on c-plane sapphire. Such large scale semipolar GaN templates are opening the way for efficient semipolar devices grown heteroepitaxially.

Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.

Formation of stacking faults for improving the performance of biodegradable Mg–Ho–Zn alloy

A novel Mg–6Ho–1Zn alloy was successfully prepared, which had a good combination of mechanical properties and corrosion resistance due to the formation of profuse nano-spaced basal plane stacking faults (SFs). The ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation were 325MPa, 224MPa and 17%, respectively. More importantly, the alloy with SFs presented a uniform corrosion behavior in simulated body fluid (SBF) with a low corrosion rate of about 0.5mm/year, which could avoid rapid reduction in mechanical integrity. The performance of the alloy with SFs could be further improved by optimizing the microstructure. The investigation suggests that the alloys with SFs are promising biodegradable implant materials.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112¯0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I1 intrinsic basal stacking faults, and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) conditions on the surface during growth, that result in formation of asymmetric a-plane GaN grains elongated along the c-axis. Such conditions enhance the anisotropy of gallium diffusion on the surface and reduce the GaN nucleation rate.

In-situ, ex-situ EBSD and (HR-)TEM analyses of primary, secondary and tertiary twin development in an Mg–4 wt%Li alloy

This study presents experimental evidence for the sequence of activation and the evolution of primary tension (TTW), secondary compression (CTW) and tertiary double twinning (DTW) in an extruded Mg–4wt%Li alloy during uniaxial compression by means of in-situ and ex-situ EBSD and (HR-)TEM. In-situ EBSD analysis reveals that the secondary CTWs propagate very slowly, while the tertiary DTWs propagate faster once nucleated. Thereby, when DTW-ing overtakes the CTW, CTW boundary migration stops, indicating that the thickness of the CTW is limited by the onset of internal DTW-ing. This finding confirms some prior proposals based on post-mortem analyses. High-resolution (HR-)TEM analysis of a −18% compressed sample reveals significant basal slip activity and a high amount of basal stacking faults within the primary TTW as well as within the secondary CTW. Additionally, the HR-TEM analysis showed a large amount of basal dislocations within the area of the CTW-DTW transition. These findings give strong experimental evidence that the observed predominance of type 1 DTW-ing is related to the dissociation of extended basal dislocations.

The structure of 〈c + a〉 type dislocation loops in magnesium

Mg (hcp) single crystals exhibit an unusually high work-hardening rate when compressed along the c-axis at room temperature. Post-deformation investigation using bright-field and weak-beam dark-field techniques in a transmission electron microscope (TEM) and by high-angle annular dark-field imaging (HAADF) using a scanning transmission electron microscope confirms the presence of sessile c + a type dislocation loops on the basal plane whose density increases with increasing plastic strain; their presence is thought to account at least in part for the high work-hardening rate. These loops appear to be often arranged as rows along the 〈10–10〉 directions, are hexagonal in shape with their sides coinciding with the 〈10–10〉, are faulted and are composed of two 1/6〈20–23〉 type partials loops bounding a basal stacking fault. Their presence is attributed to the decomposition reaction 1/3[2-1-13] → 1/6[2-203] + SF(0001) + 1/6[20-23].

Effect of temperature-induced solute distribution on stacking fault energy in Mg–X(X = Li, Cu, Zn, Al, Y and Zr) solid solution: a first-principles study

Stacking fault energies (SFEs) of Mg solid solutions at different temperatures are very significant for studying dissociation of dislocation, plasticity deformation and other mechanical properties. Our present work starts with the investigation of interactions between basal stacking faults (SFs) and solutes (Li, Cu, Zn, Al, Y and Zr) using first-principles calculations. It is found that the interactions between basal SFs and solutes can be extended to several closed-packed layers. Combined with Fermi–Dirac function of solute distribution at each layer, the temperature dependence of SFE for Mg–X(X = Li, Cu, Zn, Al, Y and Zr) solid solution has been investigated. This study predicts correctly the increase tendency observed in experiment. Then the SFEs of Mg solid solutions at room temperature of 300 K are investigated at different solute concentrations; the obtained concentration dependence of SFEs is in agreement with the available experimental values. So the solute distribution under finite temperature due to the basal SF–solute interactions plays a critical role in the variation of SFEs of Mg solid solutions.

Stacking faults and interface roughening in semipolar (202¯1¯) single InGaN quantum wells for long wavelength emission

The microstructure of InGaN single quantum wells (QWs) grown in semipolar (202¯1¯) orientation on GaN substrates was studied by transmission electron microscopy. Stress relaxation in the lattice mismatch InxGa1−xN layer was realized by forming partial misfit dislocations associated with basal plane stacking faults (BPSFs). For given composition x = 0.24, BPSFs formation was observed when the QW thickness exceeded 4 nm. The high density of partial threading dislocations that bound the BPSFs is detrimental to light-emitting device performance. Interface roughening (faceting) was observed for both upper and lower QW interfaces (more pronounced for upper interface) and was found to increase with the thickness of the QW. BPSFs had a tendency to nucleate at roughened interface valleys.