Reduction Of Dislocation Density Research Articles

Effect of Tempering Temperature on the Aqueous Corrosion Resistance of 9Cr Series Heat-Resistant Steel

In this investigation, the aqueous corrosion resistance of 9Cr series heat-resistant steel during tempering was investigated. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectrometer (EDS) were used to analyze the effect of tempering temperature on the microstructure and precipitation behavior of precipitates. The heat-resisting steel was heated to 1150 °C for 1 h, and then tempered at different temperatures between 680 °C and 760 °C for 2 h. The microstructure of the heat-resistant steel after tempering was composed of lath-tempered martensite and fine precipitates. The hardness decreased with increasing tempering temperature, ranging from HBW 261 to HBW 193. The aqueous corrosion resistance improved as the tempering temperatures increased from 680 °C to 720 °C but deteriorated at higher temperatures, such as 760 °C, which was obtained by an electrochemical corrosion performance test. The aqueous corrosion resistance was affected by the decrease in dislocation density and the decrease in Cr solution in the tempered martensite. With the increase in the tempering temperature, the aqueous corrosion potential first increases and then decreases, the self-corrosion current density first decreases and then increases, and the polarization resistance first increases and then decreases. Furthermore, the increase in corrosion resistance is attributed to the reduction in dislocation density and chromium depletion in the martensitic structure as the tempering temperature approaches 720 °C. This paper reveals the effect of tempering temperature on the corrosion resistance of 9Cr series heat-resistant steel, which is a further exploration of a known phenomenon.

Towards Strength–Ductility Synergy in Cold Spray for Manufacturing and Repair Application: A Review

Cold spray is a solid-state additive manufacturing technology and has significant potential in component fabrication and structural repair. However, the unfavourable strength–ductility synergy in cold spray due to the high work hardening, porosity and insufficient bonding strength makes it an obstacle for real application. In recent years, several methods have been proposed to improve the quality of the cold-sprayed deposits, and to achieve a balance between strength and ductility. According to the mechanism of how these methods work to enhance metallurgical bonding, decrease porosity and reduce dislocation densities, they can be divided into four groups: (i) thermal methods, (ii) mechanical methods, (iii) thermal–mechanical methods and (iv) optimisation of microstructure morphology. A comprehensive review of the strengthening mechanism, microstructure and mechanical properties of cold-sprayed deposits by these methods is conducted. The challenges towards strength–ductility synergy of cold-sprayed deposits are summarised. The possible research directions based on authors’ research experience are also proposed. This review article aims to help researchers and engineers understand the strengths and weaknesses of existing methods and provide pointers to develop new technologies that are easily adopted to improve the strength–ductility synergy of cold-sprayed deposits for real application.

Effect of trace grain boundary segregation element bismuth on stress relaxation behavior of copper

The stress relaxation resistance of copper-based materials determines the reliability of contact and stable transmission of connector plugs. This study investigates the effect of trace bismuth (110 wt ppm) on the stress relaxation resistance of pure copper at temperatures of 25 °C, 100 °C, and 200 °C. The relationship between microstructure and stress relaxation resistance was analyzed using SEM, EBSD, and HAADF-STEM. The results demonstrate that trace bismuth significantly enhances the stress relaxation resistance of pure copper, especially at higher temperatures of 100 °C and 200 °C, where the stress relaxation rates of Cu-0.011Bi samples after 48 h decreased by 21.36 % and 18.06 %, respectively, compared to Cu samples. Trace bismuth elements exist in copper mainly in the form of double atomic layers segregated at grain boundaries, which pin the grain boundaries, inhibit grain growth, and hinder dislocation slip. At room temperature (25 °C), bismuth mainly improves stress relaxation resistance by refining grains. At higher temperatures of 100 °C and 200 °C, the significant enhancement in stress relaxation resistance is primarily related to bismuth's inhibition of the reduction in dislocation density and recrystallization during the stress relaxation process.

Deformation and microregion fracture mechanisms in type 316L welded joint under isothermal and thermo-mechanical fatigue loadings

Isothermal (IF) and thermo-mechanical fatigue (TMF) tests were conducted on 316L welded joints to investigate the effects of phase angle (in-phase (IP), clockwise-diamond (CD), and out-of-phase (OP)) and mechanical strain amplitude (±0.4 %, ±0.5 %, ±0.6 %) on deformation and microregion fracture mechanisms. Results reveal that increasing strain amplitude induces a higher softening rate, reduced fatigue life, and more pronounced dynamic strain aging (DSA). Nevertheless, there is a complicated discrepancy in the cyclic deformation between various phase angles. At high strain amplitudes, local embrittlement along the austenite/δ-ferrite interface induces cracking in the weld metal (WM) under both IF and TMF loadings, with TMF life increasing with phase angle. At a lower strain amplitude of 0.4 %, however, fracture location migration and different TMF life were observed. Microstructural analysis reveals that during TMF a lower strain amplitude of 0.4 %, the microstructure in both base metal (BM) and WM evolves from simple planar structures to low-energy substructures, characterized by an increase in kernel average misorientation, geometrically necessary dislocations, and low-angle grain boundaries, together with a reduction in high-angle grain boundaries and twin boundaries. In the WM, increasing phase angle tends to reduce dislocation density and reproduces planar dislocation structures due to enhanced DSA, thereby lowering cellular structure maturity. Conversely, in the BM, dislocation proliferation and interaction intensify, enhancing cellular structure maturity and de-twinning effect, which reduces BM fracture resistance and causes fracture migration from the WM to the BM. Moreover, active DSA under CD loading improves deformation resistance in both microregions, prolonging fatigue life and contributing to the observed different TMF life.

Mechanical characterization of nanocrystalline Cu-Ag alloys subjected to shear deformation

ABSTRACT This study investigates the mechanical behaviour of nanocrystalline Cu-Ag alloys using molecular dynamics simulations combined with detailed analyses of dislocation densities, atomic populations, and deformation characteristics. Shear tests were conducted across three different atomic compositions at temperatures ranging from 10 K to 500 K. The influence of temperature and Ag impurities on the mechanical properties of nanocrystalline Cu is explored. Elevated temperatures are found to promote plastic deformation by increasing atomic mobility, resulting in reduced dislocation densities and flow stresses. Increasing Ag content correlates with lower dislocation densities, highlighting enhanced plastic activity between grain and grain boundaries. Structural population analysis underscores the significant impact of Ag species on the mechanical response of nanocrystalline Cu. Further investigations are warranted to explore additional deformation mechanisms. These findings contribute to a deeper understanding of nanocrystalline Cu-Ag alloys, informing the design of novel materials with tailored mechanical properties.

Breaking strength-plasticity trade-off in heterostructure CrFeNiAl0.28Si0.09Ti0.02Cu0.01 high entropy alloy by heat-treatment-induced nano-precipitation

In this study, heat treatment was employed to enhance the mechanical properties of an as-cast CrFeNiAl0.28Si0.09Ti0.02Cu0.01 high entropy alloy (HEA) with the heterostructure consisting of face-centered cubic (FCC), body-centered cubic (BCC) and B2 phases. It was observed that, after heat treatment at 873 K for 1 h followed by water-quenching, well-dispersed L12 and L21 nano-precipitates could be formed in FCC phase and BCC phase of this as-cast HEA, respectively. Furthermore, it is worth noting that despite a noticeable reduction in dislocation density, this heat treatment process significantly enhances yield strength, ultimate tensile strength, plasticity and hardness simultaneously, which could be attributed to the formation of nano-scale precipitates mentioned above. These findings provide a novel approach for fabricating high-performance HEAs by introducing complex multi-level heterogeneity.

Microstructure and mechanical properties of a low-alloyed Mg–Zn–Al–Ca alloy: Effect of extrusion speed

A low-alloyed Mg-1.2Zn-0.6Al-0.1Ca (wt.%) alloy was extruded at 200 °C with different ram speeds (0.5–4.0 mm/s), and the microstructure and mechanical properties were studied systematically. Heterostructures with fine dynamic recrystallized (DRXed) grains and coarse unDRXed grains were achieved at lower ram speeds of 0.5 mm/s and 1.0 mm/s, and fully-DRXed microstructure was attained at 4.0 mm/s. Increasing the extrusion speed resulted in an increase in DRXed grain size from 0.9 μm to 3.8 μm, and a transformation of the DRXed texture component from <10 to 10>−<11–20> to a new orientation that deviated by approximately 14°. The sample extruded at 0.5 mm/s presented an excellent tensile yield strength (TYS) of 369 MPa along with a 7.8% elongation, which was mainly due to the high hetero-deformation induced (HDI) strengthening provided by its heterostructures. Increasing ram speed resulted in an improved elongation despite a decreased TYS. The reasons for the decreased strength with increasing extrusion speed were mainly associated with grain growth, reduced dislocation density and weakened HDI strengthening. The reasons for the improved ductility with increasing extrusion speed were largely due to the increased DRXed grains fraction with soft orientations.

Eliminating the soft zone for grade 91 steel weldment via enhancing prior austenite grain size of the intercritical heat-affected zone

The soft zone is deemed unwanted for grade 91 steel weldments as it constitutes a cracking-sensitive region deleterious for long-term creep performance. In the present work, formation mechanisms of the soft zone in grade 91 steel weldments have been elucidated and an approach for its elimination has been proposed. It has been demonstrated that a soft zone forms within the intercritical heat-affected zone (ICHAZ) after post-weld tempering. Post-weld normalizing and tempering enables the elimination of the soft zone. Further analysis reveals that the ICHAZ of the as-welded specimen possesses a mixed structure, consisting of over-tempered martensite and fresh martensite with a fine prior austenite grain size. During post-weld tempering, the increase in grain size and the reduction in dislocation density are the main factors contributing to the decrease in hardness. The recrystallization rate of the ICHAZ is faster than that of other regions, resulting in a decrease in hardness from 350.8 HV0.3 to 204.6 HV0.3 and the formation of the soft zone. Increased prior austenite grain size in the ICHAZ, achieved through post-weld normalizing and subsequent tempering, facilitates the formation of tempered martensite and prevents the formation of the soft zone.

Improved mechanical properties and thermal conductivity of laser powder bed fused tungsten by using hot isostatic pressing

In this study, the microstructure evolution, mechanical properties and thermal conductivity of laser powder bed fused tungsten subjected to the subsequent hot isostatic pressing (HIP) processes with natural cooling (NC) and ultra-rapid quenching (URQ) were investigated. The results showed that the relative density was significantly improved after HIPping regardless of the cooling rates. The HIP-URQ treatment led to a near defect-free component with increased recrystallization and reduced dislocation density. The enhanced comprehensive mechanical properties, including the ultimate compressive strength of 1180 MPa without sacrificing microhardness was achieved for the HIP-URQ sample. Furthermore, the highest thermal conductivity of 168.6 W/(m·K) related to the densification behaviour and microstructure evolution during the HIP treatments was reported.

Atomic-scale understanding of microstructural evolution in electrochemical additive manufacturing of metallic nickel

Atomic-level manufacturing is fundamentally concerned with the precise removal, addition, and migration of material at the atomic and close-to-atomic scale (ACS). Tip-based electrochemical deposition, a quintessential ACS electrochemical additive manufacturing technique, offers promising prospects for achieving bottom-up fabrication of metallic micro/nano structures. However, the complex physicochemical reactions involved in electrodes lead to a limited understanding of the mechanisms underlying atomic electrodeposition and structural evolution. For the first time, this study proposes electric double-layer controlled electrochemical kinetics and investigates the effect of direct current (DC) and pulse current (PC) on nickel atomic electrodeposition using molecular dynamics (MD) simulations. The findings reveal that compared to DC electrodeposition, PC electrodeposition results in more orderly deposition morphology, improved surface smoothness, reduced dislocation density, and lower crystal distortion, with these effects being particularly pronounced under low pulse duty ratio conditions. In addition, the pulse frequency significantly influences the morphology and structure of the deposit. The high pulse frequency yields smoother surfaces with local protrusions, while the low frequency favors the formation of orderly and dense structures excepting slightly increased roughness. This study provides critical insights into understanding the microscopic mechanisms of atomic-scale electrodeposition processes and achieving atomically controlled tip-based electrochemical additive manufacturing of micro/nanodevices.

Effect of dislocation density on microstructure and mechanical properties of cold-rolled medium Mn steel

This study investigated the effect of dislocation density on microstructural transformation and mechanical properties of Fe-7.23Mn-4.42Al-0.09C-1.99Si (wt.%) steel. By the tempering treatment, the dislocation density was reduced. The effect of of dislocation density on the plasticity of the steel was analyzed. The dislocation density was reduced from 2.3 × 10−16 m−2 to 1.6 10−16m−2 by tempering. Reducing dislocation density in the metastable austenite resulted in a higher metastable austenite transformation rate. And the low dislocation density steels exhibited a longer strain length during tensile deformation. It was obvserved that the dislocation density of the test steel decreased by 30% after tempering, the austenite transformation rate increased to 53.3%, and tensile strength and elongation increased to 1010 MPa and 44% respectively.

High-temperature oxidation behavior of a novel γ′-strengthened superalloy manufactured by laser-beam powder bed fusion: Effect of post-heat treatment

This study systematically researched the high-temperature oxidation behavior of a γ′-strengthened alloy based on Haynes 230 prepared by laser-beam powder bed fusion (PBF-LB). The results indicate that the experimental alloy showed a higher oxidation resistance than Haynes 230 at 1000 °C. The oxidation products mainly consisted of the outmost MnCr2O4-TiO2 layer, intermediate Cr2O3 layer and inner TiO2 layer. Branched Al2O3 oxides were formed in the interior oxidation zone. Post-heat treatment can increase oxidation resistance by reducing the oxidation rates from 0.0152 mg2cm−4h−1 to 0.0126 mg2cm−4h−1. The decrease in dislocation density and the precipitation of ordered γ' phases can slow down the short-circuit diffusion and lattice diffusion rate of oxygen, and are the main reasons for this improvement. Furthermore, the γ' phase precipitation and dislocation density reduction can provide more sites for Cr2O3 nucleation and reduce the Cr supply required for grain growth. As a result, the Cr2O3 grains on the heat-treated sample surface were smaller.

Electroplasticity mechanism of Ti2AlNb alloys under hot compression with the application of a lower electric current

In this study, a lower electric current (1.0–2.0 A/mm2) was applied during hot compression of Ti2AlNb alloys, and the mechanical response, microstructure evolution, and electroplasticity mechanism were investigated. The results showed that the maximum flow stress drop could reach 18.7 % after applying a lower electric current of 2.0 A/mm2, which confirms the existence of electroplasticity. The stress drop increased with increasing electric current density. However, the flow stresses of the samples in the isothermal compression test and those subjected to a 1.0 A/mm2 electric current compression test were similar, indicating that there is a threshold value for the electroplasticity effect on flow stress. The lower strain hardening rate caused by the electric current confirmed the electroplasticity effect, promoting the softening effect. Additionally, the lower average activation energy Q, affected by the electric current, implied the improvement of the deformability of the alloy due to the deformation mechanism of dynamic recovery (DRV), dynamic globularization, or dynamic recrystallization (DRX). The gradual reduction of dislocation density with increasing electric current density resulted in the stress drop, which can be ascribed to the directional drifting electrons colliding with dislocations, which promoted the movement of dislocations by arranging the dislocations in parallel. The local high temperature induced by the local Joule heating effect at the O lamella had a strong promoting effect on the globularization of the O lamella by producing uniform lattice distortion/local strain at the O/B2 interface. With increasing furnace temperature, the drifting electrons accelerated the transformation of LAGBs to HAGBs which promoted the appearance of dynamic recrystallization grains.

Record High Thermoelectric Figure of Merit of a III-V Semiconductor InGaSb by Defects Engineering via the Addition of Excess Constituent Elements.

Materials with enhanced electron and reduced phonon transport properties are preferred for thermoelectric applications. The defect engineering process can optimize the interrelated electron and phonon transport properties to enhance thermoelectric performance. As the influence of various crystalline defects on the functional properties of materials is diverse, it is crucial to scale, optimize, and understand them experimentally. With this perspective, crystalline defects in InGaSb ternary alloys were engineered and their influence on the thermoelectric properties was studied experimentally. Crystalline defects such as point defects, dislocations, and compositional segregations were induced in In0.95Ga0.05Sb crystals by the addition of excess constituent elements, In, Ga, or Sb. The addition of excess Ga increased point defects, whereas excess Sb reduced dislocation densities. The thermoelectric figure of merit value (ZT) of In0.95Ga0.05Sb+Ga0.02 was recorded to be 0.87 at 573 K, which is the highest among other reported values of III-V semiconductors. The collective interactions of compositional segregations, point defects, and dislocations with electrons and phonons enhanced the ZT in this study.

Unveiling Thermal and Athermal Effects in Strain Hardening Removal of A6061 Aluminum Alloy

AbstractThis study explored the application of a high-density pulsed electric current (HDPEC) to mitigate strain hardening in a cold-rolled A6061 aluminum alloy while examining the simultaneous application of HDPEC with furnace heating to reveal the contributions of thermal and athermal effects. The results showed that significant strain-hardening relief was achieved through the HDPEC treatment, particularly at 300 A/mm² for 260 ms, resulting in a 23% reduction in strength and an 86% increase in ductility. Microstructural analysis revealed a shift to fine and equiaxed grains with reduced dislocation density, which was primarily attributed to thermal effects. HDPEC annealing exhibits superior efficiency compared to the conventional annealing treatment, offering cost and time advantages. In addition, this study validated the synergistic impact of HDPEC and furnace heating, with furnace heating supplementing energy requirements, facilitating practical HDPEC implementation. These findings suggest that the HDPEC method and the combined method with conventional heating are promising alternatives for strain-hardening alleviation in A6061 aluminum alloy manufacturing, supporting the development of an eco-friendly and efficient process. Graphical Abstract

Growth and characterization of micro-LED based on GaN substrate

As the diminution of micro-LED pixels advances, the pivotal role of dislocation phenomena becomes increasingly pronounced. This study provides insight into the key characteristics and dominant mechanisms of GaN-based micro-LEDs by comparing the homoepitaxial and heteroepitaxial configurations. Our findings reveal that variability in V-shaped pits distribution markedly influences the performance and uniformity of micro-LED chips. While the homoepitaxial micro-LEDs, alongside significantly reduced dislocation density and residual stress, effectively preclude the formation of them and thus ensuring superior uniformity both within and among micro-LED chips. Notably, the external quantum efficiency (EQE) peak of homoepitaxial micro-LEDs surpasses that of heteroepitaxial variants by 40%. Motivated by the realization that the reduced MQW thickness at the sidewalls of V-shaped pit aids carrier injection, a great enhancement in EQE from 7.9% to 14.8% (@ 10 A/cm2) was achieved by the optimization of homoepitaxial structure. Therefore, the growth of micro-LED with lower dislocation density, lower residual stress, and epi-structure of low-energy-barrier MQWs demonstrated the profound impact on advancing micro-LED technology to obtain the performance of high uniformity, high brightness, and low power consumption.

Development of photovoltaic and photodetection characteristics in CdS/P3HT devices through Al-doping

CdS thin films were deposited by chemical bath deposition onto FTO substrates with Al concentrations of 0, 1, 3, 5 and 6 %. X-ray diffraction revealed introduction of 1 % Al-doping reduced dislocation density and enhanced the crystal quality of CdS. Scanning electron microscopy confirmed a reduction in grain size in Al-doped CdS films compared to CdS. N3 and P3HT layers were spin-coated onto the prepared substrates, respectively. The fabrication of the solar cells was completed using Ag silver paste for the top contact. The lowest photoluminescence peak intensity was achieved for CdS (3 %Al):P3HT solar cell, indicating efficient exciton dissociation. 3 % Al-doped CdS-based device exhibited the highest efficiency at 0.210 %, nearly seven times that of reference device. CdS (3 %Al):P3HT device demonstrated the best photodetection characteristics, with a responsivity of 2.2 × 10-2 A/W, detectivity of 3.3 × 108 Jones, response time of 13 ms, and recovery time of 12 ms at zero bias voltage.

Recrystallization behavior and strengthening mechanism of friction stir welded T-joint of Ti80 titanium alloy

In this study, Ti80 titanium alloy T-joints were produced by friction stir welding (FSW), and the joint temperature and residual stress distribution of the joints were analyzed by numerical simulation based on the ABAQUS platform, and the correlation between microstructure and performance of FSWed T-joint was investigated. The results show that the SZ is lamellar α grains due to the fact that the peak temperature of all joints exceeds β phase transus. The deformation and dynamic recrystallization behavior take place in the β phase region. During the subsequent cooling stage, the acicular α/α' phase is precipitated from refined prior β phase. The non-uniform heating and cooling results in the significant microstructure differences in the weld thickness direction. The top and middle of the SZ are composed of basketweave structure, while the bottom of the SZ shows a bimodal structure. The microhardness distribution along the skin shows a higher value in the weld area, and the weakest area is located in the HAZ due to recovery, resulting in a significant reduction in dislocation density of the weld. The tensile specimen breaks in the HAZ when it is stretched along the skin direction, while it breaks in a direction parallel to the skin when it is stretched along the stringer. The fracture mechanism of the joint changes from a hybrid mode of ductile and brittle fracture to ductile fracture as the rotation speed increases from 600 rpm to 750 rpm.

(Invited) Scalable 2-Step Fabrication of Core-Shell GaN Micropillars

Group III-Nitride three-dimensional structures have gained substantial interest for application in optoelectronic, energy, and sensor devices with non-planar geometries [1-4]. The benefits of 3D structures, such as GaN-based nano-/micro-rods, compared to planar films include large surface area, high structural quality, non-polar-surface utilization, small footprint, mechanical flexibility, and enhanced light absorption/extraction efficiency. To advance the technology of non-planar III-Nitride semiconductors, it is necessary to optimize and scale up the fabrication of such structures with controlled geometries and desired electronic and optical properties. Here, we report the fabrication of wafer-scale periodic arrays of vertically aligned GaN core-shell micropillars using a combination of top-down etch and epitaxial overgrowth.The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from truncated hexagonal pyramids with the (1-101) semi-polar sidewalls to hexagonal prisms with the (1-100) non-polar sidewalls, by employing a post-ICP wet chemical etch and by tuning the HVPE growth temperature. Optimization of both ICP etching and epitaxial overgrowth reduced dislocation density in the GaN shells, thus enhancing the transport and optical characteristics of these device platforms. Photo- and cathodoluminescence showed a substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures, supported by Raman scattering, electron-backscatter-diffraction (EBSD), and X-ray-diffraction measurements. Transmission electron microscopy revealed improved crystal quality with reduced dislocation density in the epitaxially grown shells. This work demonstrates the feasibility of selective epitaxy on micro/nano-engineered 3D templates for realizing high-quality GaN-on-Si devices such as LEDs and p-i-n photodetectors.

(Invited) Ultrahigh Efficiency Excitonic Micro-LEDs

To date, it has remained challenging to achieve high efficiency micro-LEDs, which are desired for next generation displays and virtual/augmented reality. The performance degradation with reducing dimensions for conventional LEDs has been largely attributed to the plasma damage induced on the mesa sidewalls. Here, we show such a fundamental challenge can be overcome by developing nanowire excitonic LEDs. In this study, InGaN/GaN nanowire LED heterostructures were grown on N-polar GaN-on-sapphire substrates utilizing selective area epitaxy. Studies have shown that the exciton binding energy in strain-relaxed InGaN nanostructures can be dramatically enhanced. Due to the strong Coulombic interaction between electrons and holes, the impact of nonradiative SRH recombination on the performance of an excitonic LED is significantly reduced. By exploiting the large exciton oscillator strength of quantum-confined nanostructures, we have achieved an external quantum efficiency (EQE) exceeding 25% for a green-emitting device with lateral dimensions less than 1 micrometer, which is the highest value reported for any LEDs of this size to the best of our knowledge. It is further observed that the EQE and output power of the excitonic LED show a significantly faster rising trend with current injection, compared to conventional devices. We have identified several important factors for achieving high efficiency excitonic LEDs, including the epitaxy of nanostructures to achieve strain relaxation and reduced dislocation densities and the formation of three-dimensional quantum-confinement to enhance electron-hole wavefunction overlap. We have further investigated the effect of Mg incorporation in p-GaN layer on the device performance. We found out that an optimized Mg dopant incorporation in nanowire structures can contribute significantly to the efficiency of micro-LEDs. We have measured significantly enhanced performance by optimizing Mg dopant incorporation, which leads to reduced leakage current and improved EQE. For example, for a red-emitting micro-LED, a peak external quantum efficiency of >8% was measured for a device with a lateral dimension less than one micrometer by optimizing Mg-dopant incorporation. Through these studies, we show that by harnessing the exciton oscillator strength of nearly dislocation-free nanostructures, together with a careful optimization of p-doping in the contact layer, the efficiency bottleneck of micro-LEDs can be addressed.Conflict of interest: Some IP related to this work has been licensed to NS Nanotech, Inc., which was co-founded by Z. Mi. The University of Michigan and Mi have a financial interest in the company.