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The efficiency of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on uniformity of hole distribution between the QWs. Typically, transport between the QWs takes place via carrier capture into and thermionic emission out of the QWs. In InGaN/GaN QWs, the thermionic hole transport is hindered by the high quantum confinement and polarization barriers. To overcome this drawback, hole injection through semipolar QWs located at sidewalls of V-defects had been proposed. However, in the case of the V-defect injection, strong lateral emission variations take place. In this work, we explore the nature of these variations and the impact of the V-defects on the emission spectra and carrier dynamics. The study was performed by mapping electroluminescence (EL) and photoluminescence (PL) with a scanning near-field optical microscope in LEDs that contain a deeper well that can only be populated by holes through the V-defects. Applying different excitation schemes (electrical injection and optical excitation in the far- and near-field), we have shown that the EL intensity variations are caused by the lateral nonuniformity of the hole injection. We have also found that, in biased structures, the PL intensity and decay time in the V-defect regions are only moderately lower that in the V-defect-free regions thus showing no evidence of an efficient Shockley-–Read–Hall recombination. In the V-defect regions, the emission spectra experience a red shift and increased broadening, which suggests an increase of the In content and well width in the polar QWs close to the V-defects. [ABSTRACT FROM AUTHOR]

V-defects are morphological defects that typically form on threading dislocations during epitaxial growth of (0001) -oriented GaN layers. A V-defect is a hexagonal pyramid-shaped depression with six { 10 1 ¯ 1 } -oriented sidewalls. These semipolar sidewalls have a lower polarization barrier than the polarization barriers present between the polar c-plane quantum wells and quantum barriers and can laterally inject carriers directly into quantum wells in GaN-based light emitting diodes (LEDs). This is especially important, as the high polarization field in c-plane GaN is a significant factor in the high forward voltage of GaN LEDs. The optimal V-defect density for efficient lateral carrier injection in a GaN LED (∼109 cm−2) is typically an order of magnitude higher than the threading dislocation density of GaN grown on patterned sapphire substrates (∼108 cm−2). Pure-edge dislocation loops have been known to exist in GaN, and their formation into large V-defects via low-temperature growth with high Si-doping has recently been studied. Here, we develop a method for pure-edge threading dislocation half-loop formation and density control via disilane flow, growth temperature, and thickness of the half-loop generation layer. We also develop a method of forming the threading dislocation half-loops into V-defects of comparable size to those originating from substrate threading dislocations. [ABSTRACT FROM AUTHOR]

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6–1 μm and the hole diffusion coefficient ∼0.6 cm2/s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs. [ABSTRACT FROM AUTHOR]

The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990s. Strategic use of V-defects in pre-quantum well superlattices or equivalent preparation layers has enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semi-polar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The "large" V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector b = ± a ± c) character threading dislocations. In addition, "small" V-defects often form in the multi-quantum well region and are believed to be deleterious to high-efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast transmission electron microscopy that during V-defect filling, the threading dislocation, which runs up the center of the V-defect, will "bend" onto one of the six { 10 1 ¯ 1 } semi-polar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs. [ABSTRACT FROM AUTHOR]

The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs. [ABSTRACT FROM AUTHOR]

We report on the material challenges of the growth of highly conductive n-AlGaN in germicidal ultraviolet light emitting diodes (GUV LEDs), with the degradation of the surface morphology of thick highly doped n-AlGaN due to the Si anti-surfactant effect. Threading dislocation inclination, increasing relaxation, and eventual cracking were observed with epitaxial n-AlGaN films thicker than 400 nm, along with an increasing Ga composition with the same metalorganic flows. With the optimization of the n-AlGaN conductivity in previous works, thin n-AlGaN films with high conductivity along with a smoothing superlattice were incorporated in GUV LED devices, resulting in LEDs with 285 nm electroluminescence, a low forward voltage of 4.2 V with a peak external quantum efficiency (EQE) of 10.6% and a peak wall-plug efficiency of 8.6% below 1 A/cm2, and an EQE of 5.5% at 20 A/cm2. [ABSTRACT FROM AUTHOR]

Hole injection through V-defect sidewalls into all quantum wells (QWs) of long wavelength GaN light emitting diodes had previously been proposed as means to increase efficiency of these devices. In this work, we directly tested the viability of this injection mechanism by electroluminescence and time-resolved photoluminescence measurements on a device in which QW furthest away from the p-side of the structure was deeper, thus serving as an optical detector for presence of injected electron–hole pairs. Emission from the detector well confirmed that, indeed, the holes were injected into this QW, which could only take place through the 10 1 ¯ 1 V-defect sidewalls. Unlike direct interwell transport by thermionic emission, this transport mechanism allows populating all QWs of a multiple QW structure despite the high potential barriers in the long wavelength InGaN/GaN QWs. [ABSTRACT FROM AUTHOR]

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ∼495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. This work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs. [ABSTRACT FROM AUTHOR]

Atomic layer etching (ALE) was performed on (Al, In, Ga)N thin films using a cyclic process of alternating Cl2 gas absorption and Ar+ ion bombardment in an inductively coupled plasma etcher system. The etch damage was characterized by comparing photoluminescence of blue single quantum well light-emitting diodes before and after the etch as well as bulk resistivities of etched p-doped layers. It was found that etched surfaces were smooth and highly conformal, retaining the step-terrace features of the as-grown surface, thus realizing ALE. Longer exposures to the dry etching increased the bulk resistivity of etched surfaces layers slightly, with a damaged depth of ∼55 nm. With further optimization and damage recovery, ALE is a promising candidate for controlled etching with atomic accuracy. It was found that Al0.1Ga0.9N acts as an etch barrier for the ALE etch, making it a suitable etch to reveal buried V-defects in III-nitride light emitting diodes. [ABSTRACT FROM AUTHOR]

A III-nitride red LED with an active region temperature of 835 °C on a Si substrate utilizing a strain-relaxed template (SRT) is demonstrated. The peak wavelength blueshifts from 670 nm at 1 A/cm2 to 636 nm at 150 A/cm2. The on-wafer external quantum efficiency was 0.021% at 7 A/cm2 with an emission wavelength of 655 nm. The LED grown on a Si substrate exhibited a 116 nm redshift when compared to a co-loaded LED grown on sapphire. This is attributed to the difference in strain state for the III-nitride layers grown on Si compared to sapphire, allowing for more indium to be incorporated in the LED grown on Si. This suggests efficient III-nitride red LEDs and µLEDs on Si with a SRT can be realized with further material, device structure, and processing optimizations. [ABSTRACT FROM AUTHOR]

Nitride based light emitting diodes (LEDs) have demonstrated very high efficiency in the blue and green wavelength ranges. For full color LED based displays, efficient and small sized devices emitting in the three primary colors - red, green and blue are required. While phosphide based red LEDs are highly efficient, high surface recombination velocities limit their scaling to small dimensions. Nitride emitters can be scaled to smaller sizes, however for longer emission wavelength such as yellow, orange, and red, the indium content in the active region of the devices needs to be >30%. Due to the large lattice mismatch between GaN and InN, the needed high indium content introduces a large strain in the structure, often resulting in poor material quality. Recently, a novel technique to mitigate this lattice mismatch induced strain was demonstrated with the use of compliant porous GaN underlayer based substrates. In our previous reports, we have discussed the surface morphology, strain state evolution and device demonstrations using these substrates, however a detailed discussion regarding the pore cross-sections and dislocation propagation for each generation of these porous GaN substrates was yet to be reported. In this work, we will discuss the cross-sectional transmission electron microscopy (TEM) results of these substrates across multiple stages of the substrate technology development. [ABSTRACT FROM AUTHOR]

AlGaN germicidal ultraviolet (GUV) light emitting diodes (LEDs) are one of the most promising disinfection technologies in fighting the COVID-19 pandemic; however, GUV LEDs are still lacking in efficiency due to low p-type doping efficiency in p-AlGaN. The most successful approach for producing conductive p-type AlGaN is the implementation of a polarization-enhanced short period Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N/Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N superlattice (SL) structure, which enhances hole injection and reduces device operating voltage. In this report, we investigated different aspects of the superlattice including the Al $_{\mathbf {x}}$ Ga $_{\mathbf {1-}\mathbf {x}}$ N and Al $_{\mathbf {y}}$ Ga $_{\mathbf {1-}\mathbf {y}}$ N alloy constituent compositions, ${x}$ and ${y}$ , period thickness, total thickness, and Mg dopant concentration in terms of LED performance as well as electrical, optical, and morphological characteristics. The polarization-enhanced p-type doping in the AlGaN superlattice was also investigated computationally, giving excellent agreement with experimental results. Highly efficient UVC LEDs (279 nm) with EQE of 2% at 5 A/cm2 were demonstrated. A maximum output power of 5.5 mW (56 mW/mm2) was achieved at 100 mA. [ABSTRACT FROM AUTHOR]

The recent progress in III-nitride tunnel junction (TJ) contacts for optoelectronic devices is summarized in this review paper. Due to the rapid developments of various III-nitride based optoelectronic devices for different emerging applications, TJs are of interest for these devices to achieve more efficient device performance. This review paper first briefly describes the working principles of TJs and the advantages of employing TJs in optoelectronic devices. The significant advancements in III-nitride TJs realized by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are addressed, as well as their TJ device performances. Lastly, the critical challenges for employing III-nitride TJs in wide bandgap ultraviolet (UV) light emitters are discussed for future investigations. [ABSTRACT FROM AUTHOR]

InGaN-based red micro-size light-emitting diodes (μLEDs) have become very attractive. Compared to common AlInGaP-based red µLEDs, the external quantum efficiency (EQE) of InGaN red µLEDs has less influence from the size effect. Moreover, the InGaN red µLEDs exhibit a much more robust device performance even operating at a high temperature of up to 400 K. We review the progress of InGaN red μLEDs. Novel growth methods to relax the strain and increase the growth temperature of InGaN red quantum wells are discussed. [ABSTRACT FROM AUTHOR]

Red micro-size light-emitting diodes (μLEDs) less than 10 × 10 μm2 are crucial for augmented reality (AR) and virtual reality (VR) applications. However, they remain very challenging since the common AlInGaP red μLEDs with such small size suffer from a dramatic reduction in the external quantum efficiency. In this work, we demonstrate ultra-small 5 × 5 μm2 607 nm amber μLEDs using InGaN materials, which show an EQE over 2% and an ultra-low reverse current of 10−9 A at −5 V. This demonstration suggests promising results of ultra-small InGaN μLEDs for AR and VR displays. [ABSTRACT FROM AUTHOR]