Density Of Basal Stacking Faults Research Articles

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).

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.

Atomic-level study of {[formula omitted]} deformation twinning in pure Ti and Ti-5at.% Al alloy

In this work, pure Ti and Ti-5at.%Al alloy are both cold rolled, and then characterized to observe deformation twins {101¯1}. Many two-layer steps are observed at the {101¯1} twin boundary (TB) in pure Ti by high-resolution transmission electron microscopy (HRTEM). However, in addition to two-layer steps, a large number of four-layer steps are observed at the {101¯1} twin boundary in Ti-5at.%Al alloy. To understand the experimental observations, molecular dynamics (MD) simulations are performed to analyze the {101¯1} deformation twinning at the atomic scale. MD simulations are also showing similar results to the experimental observations in pure Ti and Ti-5at.%Al alloy. The two-layer and four-layer steps are twinning dislocations (TDs) with the Burgers vectors of 12×4γ2−93+4γ2<101¯2¯> ±16<12¯10> (b⇀2) and 4γ2−93+4γ2<101¯2¯> (b⇀4) by topological analysis, respectively. For pure Ti, the twin growth is carried out only by the slip of TDs b⇀2. For Ti-5at.% Al alloy, twinning is carried out by the gliding of TDs b⇀2 and TDs b⇀4. Due to the poor mobility of TDs b⇀4, the nucleation of the twinning dislocations b⇀4 (4-layer steps) requires some special nucleation sites, such as intersections between the stacking fault (SF) and TB (SF-TB intersection) in Ti-5at.% Al alloy. The SF-TB intersection is a sessile imperfect disconnection, and this defect is immobile, which leads to high stress at the SF-TB intersections to facilitate the nucleation of TD b⇀4. The Al element enhances the high density of basal stacking faults (BSFs) and SF-TB intersections at the twin-tip, and indirectly promotes the nucleation of TD b⇀4 in Ti-5at.% Al alloy. In addition, Al element also facilitates the nucleation of matrix dislocations, resulting in twin-dislocation interactions that promote the generation of TDs.

Magnesium doped semipolar (11–22) p-type gallium nitride: Impact of dopant concentration variants towards grain size distributions and crystalline quality

In this study, the effect of biscyclopentadienyl magnesium flow rate for the growth of p-type gallium nitride thin films was varied from 20 to 40 standard cubic centimeter per minute to acquire the optimum doping concentration level. The growth of semi-polar (11–22) p-type gallium nitride thin films on m-plane sapphire was accomplished via metal organic chemical vapor deposition. Atomic force microscopy reveals the occurrence of dissimilar grain size distributions upon the utilization of different dopant flow rates. Statistical data shows that the utilization of optimized biscyclopentadienyl magnesium flux of 30 standard cubic centimeter per minute would significantly reduce the grain size to as low as ∼ 550 nm (±1 nm), yielding a root mean square roughness of 6.00 nm. X-ray rocking curve analysis observed that a moderate biscyclopentadienyl magnesium flow rate of 30 standard cubic centimeter per minute leads to narrowing in full width at half maximum, indicating an enhanced crystallinity with a reduction of the basal stacking fault density of 8.18 × 104 cm−1 with the use of 30 standard cubic centimeter per minute of biscyclopentadienyl magnesium. Moreover, the mobilities and carrier concentrations were improved to as high as 1.8 cm2.V − 1.s − 1 and 5.5 × 1017 cm−3 with a moderate biscyclopentadienyl magnesium flow rate of 30 standard cubic centimeter per minute. The analysis suggests that with the optimum biscyclopentadienyl magnesium flow rate (30 standard cubic centimeter per minute), preferential sites for magnesium incorporation can be enhanced.

Semi-polar (11–22) AlN epitaxial films on m-plane sapphire substrates with greatly improved crystalline quality obtained by high-temperature annealing

This study addresses the difficulty of obtaining relatively low-cost semi-polar (11–22) AlN films epitaxially grown on m-plane sapphire substrates with high crystalline quality by applying a cost-effective high-temperature annealing process conducted at 1700 °C to films grown via standard-temperature metal organic chemical vapor deposition. The X-ray diffraction rocking curves of the annealed films along the [11–23] and [10–10] directions obtain full width at half maximum values of 0.152° and 0.193°, respectively. Atomic force microscopy results demonstrate the occurrence of a significant recrystallization process in the columnar formations of AlN films during annealing, resulting in increased AlN crystal grain size. Raman spectroscopy measurements reveal that the semi-polar (11–22) AlN films are subject to increased compressive stress after the annealing process. Transmission electron microscopy analyses confirm that our semi-polar AlN films provide appropriately low dislocation and basal stacking fault densities of 5.6 × 109 cm−2 and 1.05 × 105 cm−1, respectively. The proposed approach therefore obtains low-cost AlN films that are suitable for the mass manufacture of efficient optoelectronic devices.

Orientation-tunable InxGa1−xN nanowires with a high density of basal stacking faults for photoelectrochemical/photocatalytic applications

InxGa1−xN nanowires grew along the m-direction (A-NWs) or semipolar-direction (B-NWs) with the presence of a high density of BSFs.

Toward heteroepitaxially grown semipolar GaN laser diodes under electrically injected continuous-wave mode: From materials to lasers

III-nitrides based light-emitting diodes and laser diodes (LDs) have shown great success as solid-state lighting sources, but the development of common c-plane (0001) polar GaN emitters is facing limitations due to quantum-confinement Stark effect, efficiency drop, low efficiency at green range, and peak wavelength blue-shift. Efficient semipolar or nonpolar GaN light emitting diodes and LDs have been successfully demonstrated by growing on semipolar or nonpolar free-standing GaN substrates. The small size and high cost of high crystal quality semipolar or nonpolar free-standing GaN substrates, which are sliced from hydride vapor phase epitaxy grown c-plane bulk GaN substrate, have severely limited their commercial development and application. Achieving scalable heteroepitaxial semipolar GaN materials with a very low density of basal-stacking faults (BSFs) on a foreign substrate remains very challenging. The recent breakthrough in the demonstration of continuous-wave (CW) semipolar (202¯1) LDs at room-temperature on semipolar GaN/sapphire template marks a milestone in exploring high crystal quality heteroepitaxial semipolar GaN materials and low-cost semipolar emitters. Here, we review the key progress through the past years about the development of heteroepitaxial semipolar GaN materials including epitaxial lateral overgrowth, orientation controlling epitaxy, BSFs burying by neighboring Ga-polar (0001) GaN with air voids, facet-engineering orientation control epitaxy, resulting in a low density or free of basal stacking faults. Furthermore, we discuss the heteroepitaxially grown pulsed semipolar (112¯2) blue LDs and CW semipolar (202¯1) LDs.

Spontaneous CVD growth of InxGa1-xN/GaN core/shell nanowires for photocatalytic hydrogen generation

InxGa1-xN nanowires (NWs) have drawn great attentions as a promising photocatalyst for H2 generation via solar water splitting. In this article, m-axial NWs with InxGa1-xN/GaN core/shell structure can be formed spontaneously on the n-type Si (111) substrate via Ni-assisted chemical vapor deposition (CVD) method. CVD growth conditions were adjusted by tuning the duration and pressure. The microstructures, surface chemistry, and optical properties of the obtained NWs were characterized using various techniques. The growth mechanism is identified as a two-step process. The InxGa1-xN cores with a high density of basal stacking faults were grown by a vapour-liquid-solid (VLS) mode, while the GaN shells were grown on the sidewalls of InxGa1-xN core through a vapor-solid (VS) mode. The photocatalytic activities of the NWs were evaluated by the amount of H2 generated from water splitting. A thicker GaN shell was revealed to decrease the photoactivity by hindering the charge separation and transfer of electron-hole pairs generated inside the InxGa1-xN cores. A strategy can be proposed to improve the photocatalytic performance of NWs by terminating the VS growth of the GaN shells during the CVD process.

Temperature-dependent electrical and optical studies on nonpolar a-plane GaN thin films with various Si-doping levels

The electrical and optical properties of nonpolar a-plane GaN films with various silane flow rates were studied intensively utilizing temperature-dependent Hall-effect and photoluminescence measurements. The electron Hall mobility was found to change from increase to decrease while the silane flow beyond 16.06 nmol/min. It was inferred that the primary scattering mechanism of Si-doped GaN transferred from ionized impurity to thermal lattice vibration with the temperature over 450 K as more disordered lattice vibration was caused by Si doping. Meanwhile, the band-gap of nonpolar a-plane GaN film demonstrated a 32.56 meV narrowing owing to the Si-doping induced band-gap narrowing effect. Moreover, the basal stacking faults (BSFs) and Ga vacancy-related emission were evidently suppressed, and a decrease of 12.02% for BSFs density was achieved with Si-doping.

Atomic configurations of basal stacking faults and dislocation loops in GaN irradiated with Xe20+ ions at room temperature

The general features in ion-irradiated GaN are identified as basal stacking faults and dislocation loops. Understanding the atomic configuration of those lattice defects are important to reduce implantation damage during ion doping in GaN devices. In this study, however, due to the poor understanding of the basic mechanisms involved in such a process, Wurtzite GaN film bombarded with 5 MeV Xe ions was studied by using Transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Analyzing the microstructure by TEM, we show the formation of a high density of basal stacking faults, pyramidal dislocation loops and point defect clusters. These defects were carefully investigated by high resolution TEM (HRTEM), we notice the growth of basal stacking faults accompanied with the formation of pyramidal and prism dislocation loops. The most of observed stacking faults and dislocation loops are identified as gallium interstitials. This work is expected to provide insights into the understanding mechanisms controlling the irradiation damage of GaN exposed to ion irradiation and will benefit the fabrication of GaN devices.

Overgrowth and characterization of (11-22) semi-polar GaN on (113) silicon with a two-step method

A two-step approach has been developed for the growth of semi-polar (11–22) GaN on patterned (113) silicon substrates, which effectively eliminates Ga melt-back etching at a high temperature, one of the most challenging issues. A (113) Si substrate is patterned into groove trenches by means of using a standard photolithography technique and then anisotropic chemical etching, forming (111) facets with an inclination angle of 58˚ with respect to c-axis in addition to the un-etched (113) facets. A thick AlN layer is subsequently epitaxially grown on the patterned silicon to cover all the facets ensuring to eliminate the melt-back, followed by selectively depositing SiO2 masks on the (113) facets only. Further GaN overgrowth is performed only on the exposed (111) facets, forming (11–22) semi-polar GaN with high crystal quality along the vertical direction. Stimulated emission at room temperature has been observed with a low threshold. Low-temperature photoluminescence measurements confirm a significant reduction in basal stacking faults density. This method provides a promising approach to effectively suppress the Ga melt-back etching issue, which is particularly important for Al(Ga)N growth on semi-polar GaN that requires a high growth temperature. The presented results are crucially important for developing monolithic on-chip integration of electronics and photonics on silicon.

MOVPE growth and high-temperature annealing of (10[formula omitted]0) AlN layers on (10[formula omitted]0) sapphire

Heteroepitaxial growth of AlN layers on (101¯0) m-plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different nucleation temperatures were used to achieve single phase (101¯0) AlN layers. The crystallinity of the layers further increased with increasing annealing temperature and annealing time. The full-width at half maximum of the symmetric (101¯0) X-ray rocking curves decreased from about 2900/1940 arcsec to 1030/800 arcsec along [0001]/[112¯0]AlN. The density of basal stacking faults of the annealed layers was found to decrease from ∼2.8 × 105 to ∼1.5 × 104 cm−1. The annealed layers had a larger effective optical bandgap energy of ∼6.15 eV than ∼6.04 eV of the as-grown layers due to their better crystallinity and structural order.

High-temperature thermal annealing of nonpolar (1 0 [formula omitted] 0) AlN layers sputtered on (1 0 [formula omitted] 0) sapphire

Thermal annealing at high temperatures of nonpolar (101¯0) m-plane AlN layers directly sputtered on m-plane sapphire was investigated. The crystallinity of the layers increased with increasing annealing temperature. The full-width at half maximum of the symmetric (101¯0) X-ray rocking curves along [0001]/[112¯0]AlN decreased from about 3.5/2.0° to 0.24/0.19°. The density of basal stacking faults of the annealed layers was found to decrease from ∼1 × 105 to ∼5 × 103 cm−1. The annealed layers had a larger optical bandgap energy than the as-sputtered layers due to their better crystallinity and structural order.

Annealing of the sputtered AlN buffer layer on r‐plane sapphire and its effect on a‐plane GaN crystalline quality

We demonstrated how annealing of the sputtered AlN buffer layer (sp‐AlN) on r‐plane sapphire could be used to produce a high‐crystalline‐quality a‐plane GaN (a‐GaN). The sp‐AlN with large grains was confirmed by annealing at 1600 °C in N2 ambient, consequently its crystalline orientation and quality were significantly improved. Moreover, it was found that a‐GaN grown on annealed sp‐AlN showed better crystalline quality, including a reduction of basal stacking fault density, than a‐GaN grown on untreated sp‐AlN (as sputtered). The implication is that the a‐GaN growth method using annealed sp‐AlN is an effective way to obtain high crystalline quality.

Structural characterization of [formula omitted] twin boundaries in deformed cobalt

Deformation twinning is one of the most important strain accommodation mechanisms for deformed hexagonal close-packed materials. During plastic deformation, interfaces such as twinning boundaries usually play a critical role to affect the mechanical properties of many materials with hexagonal structure. As one kind of significant twinning modes, 101̅1 contraction twin usually occurs at the final stage of plastic deformation and serves to relax the stress concentration. Therefore, it is very crucial to understand the interfacial structure of twinning boundaries of 101̅1 twin at the atomic scale if we are to properly tailor twins for microstructural design and applications. In the present work, by means of high-resolution transmission electron microscopy, the 101̅1 deformation twin in deformed cobalt has been investigated. The results show that the twinning boundaries are not straight, but actually consist of 101̅1 TBs and {0002}||1̅011 basal-pyramidal interfaces. In addition, a high density of basal stacking faults is also observed experimentally within the 101̅1 twin. According to these experimental features, the possible mechanism for twinning boundary migration and for the emergence of such abundant basal SFs will be discussed.

Defect reduction in overgrown semi-polar (11-22) GaN on a regularly arrayed micro-rod array template

We demonstrate a great improvement in the crystal quality of our semi-polar (11-22) GaN overgrown on regularly arrayed micro-rod templates fabricated using a combination of industry-matched photolithography and dry-etching techniques. As a result of our micro-rod configuration specially designed, an intrinsic issue on the anisotropic growth rate which is a great challenge in conventional overgrowth technique for semi-polar GaN has been resolved. Transmission electron microscopy measurements show a different mechanism of defect reduction from conventional overgrowth techniques and also demonstrate major advantages of our approach. The dislocations existing in the GaN micro-rods are effectively blocked by both a SiO2 mask on the top of each GaN micro-rod and lateral growth along the c-direction, where the growth rate along the c-direction is faster than that along any other direction. Basal stacking faults (BSFs) are also effectively impeded, leading to a distribution of BSF-free regions periodically spaced by BSF regions along the [-1-123] direction, in which high and low BSF density areas further show a periodic distribution along the [1-100] direction. Furthermore, a defect reduction model is proposed for further improvement in the crystalline quality of overgrown (11-22) GaN on sapphire.

Three-dimensional reciprocal space mapping with a two-dimensional detector as a low-latency tool for investigating the influence of growth parameters on defects in semipolar GaN

A rapid nondestructive defect assessment and quantification method based on X-ray diffraction and three-dimensional reciprocal-space mapping has been established. A fast read-out two-dimensional detector with a high dynamic range of 20 bits, in combination with a powerful data analysis software package, is set up to provide fast feedback to crystal growers with the goal of supporting the development of reduced defect density GaN growth techniques. This would contribute strongly to the improvement of the crystal quality of epitaxial structures and therefore of optoelectronic properties. The method of normalized three-dimensional reciprocal-space mapping is found to be a reliable tool which shows clearly the influence of the parameters of the metal–organic vapour phase epitaxial and hydride vapour phase epitaxial (HVPE) growth methods on the extent of the diffuse scattering streak. This method enables determination of the basal stacking faults and an exploration of the presence of other types of defect such as partial dislocations and prismatic stacking faults. Three-dimensional reciprocal-space mapping is specifically used in the manuscript to determine basal stacking faults quantitatively and to discuss the presence of partial dislocations. This newly developed method has been applied to semipolar GaN structures grown on patterned sapphire substrates (PSSs). The fitting of the diffuse scattering intensity profiles along the stacking fault streaks with simulations based on a Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. Three-dimensional reciprocal-space mapping is shown to be a method sensitive to the influence of crystallographic surface orientation on basal stacking fault densities during investigation of semipolar (11{\overline 2}2) GaN grown on an r-plane (1{\overline 1}02) PSS and semipolar (10{\overline 1}1) GaN grown on an n-plane (11{\overline 2}3) PSS. Moreover, the influence of HVPE overgrowth at reduced temperature on the quality of semipolar (11{\overline 2}2) GaN has been studied.

Effect of indium accumulation on the characteristics of a-plane InN epi-films under different growth conditions

This study investigated the influence of indium accumulation happened on the surface of a-plane InN grown under different growth conditions. Three different growth rates with N/In ratio from stoichiometric to N-rich were used to grow a-plane InN epifilms on GaN-buffered r-plane sapphires by plasma-assisted molecular beam epitaxy. When a-plane InN was grown above 500°C with a high growth rate, abnormally high in-situ reflectivity was found during a-plane InN growth, which was resulted from indium accumulation on surface owing to In-N bonding difficulty on certain crystal faces of a-plane InN surface. Even using excess N-flux, indium accumulation could still be found in initial growth and formed 3-dimension-like patterns on a-plane InN surface which resulted in rough surface morphology. By reducing growth rate, surface roughness was improved because indium atoms could have more time to migrate to suitable position. Nonetheless, basal stacking fault density and crystal anisotropic property were not affected by growth rate.

Successive selective growth of semipolar (11-22) GaN on patterned sapphire substrate

Thanks to the use of two successive selective growths by metal organic chemical vapor deposition reactor, high quality semipolar (11-22) GaN with a homogenous defect repartition over the surface was achieved. The procedure starts with a first selective growth on a patterned sapphire substrate, leading to continuous stripes of three dimensional (3D) GaN crystals of low defect density. Then, a second selective growth step is achieved by depositing a SiNx nano-mask and a low temperature GaN nano-layer on the top of the GaN stripes. Hereby, we demonstrate an original way to obtain a homoepitaxial selective growth on 3D GaN crystals by taking advantage of the different crystallographic planes available. Basal stacking faults (BSFs) are generated during this second selective growth but could be eliminated by using a three-step growth method in which elongated voids are created above the defective area. For a fully coalesced sample grown using the 2 step method, dislocation density of 1.2 × 108 cm−2 and BSFs density of 154 cm−1 with a homogenous distribution have been measured by cathodoluminescence at 80 K. Consequently the material quality of this coalesced semipolar layer is comparable to the one of polar GaN on c-plane sapphire.

(Invited) High Efficiency Green-Yellow Emission from InGaN/GaN Quantum Well Structures Grown on Overgrown Semi-Polar (11-22) GaN on Regularly Arrayed Micro-Rod Templates

Last two decades have seen major developments in the field of III-nitride materials and optoelectronics, which is mainly based on polar GaN on sapphire. So far, the growth of c-plane GaN on sapphire has almost approached its theoretical limits and the major limit to the performance of GaN-based LEDs is due to the growth on c-plane. Consequently, this polar orientation leads to a number of fundamental issues, such as efficiency droop, and green and yellow gaps in wavelength coverage. One clear way forward, meeting the fundamental challenges, is to grow along semi-polar/non-polar direction. Semi-polar orientations, in particular, (11-22), offer another major advantage, namely, it can also significantly enhance indium incorporation into GaN, which cannot be achieved using c-plan GaN and thus is extremely important for growth of long wavelength emitters, such as green and even better for yellow. However, the current challenge is due to the crystal quality of semi-/non-polar GaN, which is far from satisfactory. So far, the best performing semi-polar emitters are grown on semi-polar GaN substrates which are typically ~ 10x10 mm in size. Therefore, it is necessary to develop a cost-effective approach to developing large size semi-polar GaN templates with major improvement in crystal quality but still based on sapphire substrates. The Sheffield team has developed an overgrowth approach for 2" (11-22) semi-polar GaN with significantly enhanced crystal quality on nanorod templates fabricated by self-organised nickel nano-mask approach. Very recently, a new overgrowth approach based on optical lithography mask pattern technique has been developed, where regularly arrayed micro rod templates can be cost-effectively fabricated and the size of the micro rods can be well controlled. The new approach has led to further improvement in both crystalline quality and surface morphology. This has been confirmed by high resolution transmission microscopy (TEM) studies, demonstrating further significant reduction in both dislocation density and basal stacking fault density. InGaN/GaN multiple quantum well structures with high In compositions have been grown on such semi-polar GaN templates, and we have achieved strong emission at long wavelengths, even down to ~600 nm. For the green-yellow emission at 555 nm, we have obtained an internal quantum efficiency (IQE) of up to 14%, and 7-10% for the pure yellow emission at ~570 nm. Con-focal photoluminescence (PL), micro-PL, time-resolved PL (TRPL), and micro-TRPL have been performed