Group III-nitride light emitting diodes (LEDs) suffer from poor efficiency for longer wavelength emission. This is partly due to increased polarization-induced barriers to vertical carrier injection at InGaN/GaN interfaces in the polar c-plane, where higher In-content is required for long-wavelength emission. Polarization-induced barriers can be bypassed by lateral carrier injection through the semipolar sidewalls of V-defects, which form at the apex of threading dislocations (TDs) during kinetically limited growth. This increases wall-plug efficiency (WPE) through the reduction of forward voltage (VF). TD and resulting V-defect density can be controlled through the formation of edge dislocation half-loops prior to V-defect opening. In this work, we demonstrate a green V-defect LED with optimized AlGaN caps with high external quantum efficiency (EQE) and WPE. The green V-defect LED demonstrated here with such a device structure achieves a peak EQE and a peak WPE of 43.9% and 37.0%, respectively.

AlGaN-based ultraviolet light-emitting diodes (UV LEDs) are considered promising solid-state UV light sources; however, their low wall-plug efficiency (WPE) has remained a major barrier to practical implementation. In this work, we designed the epitaxial structure and growth conditions for MOCVD-grown AlGaN-based UV LEDs to achieve coherent growth of the AlGaN layers and the formation of low-areal-density p-GaN islands. We further developed a processing approach to address the limited light extraction associated with highly efficient AlGaN-based UV LED epitaxial structures. A mirror p-contact and an ohmic-like n-contact were co-optimized to achieve high reflectance and reliable electrical contacts in UV LEDs with low p-GaN areal coverage, and a flip-chip geometry was adopted to further improve light extraction. High WPE of the UV LEDs was demonstrated under continuous-wave (CW) current injection. A peak WPE of 19.0% at 0.09 A/cm(2) and a WPE of 7.4% at 10 A/cm(2) were achieved for UV LED emitting at 298 nm.

We report on the successful demonstration of an all metalorganic chemical vapor deposition (MOCVD) grown fully transparent tunnel junction (TJ) germicidal UV LED, resulting from the use of a lightly doped n--AlGaN contact layer enabling rapid MOCVD growth optimization. We found that the optimal condition for LED performance was a 3 nm p++-Al0.6Ga0.4N / 9 nm n++-Al0.65Ga0.35N TJ above a 20 period 1 nm p-Al0.8Ga0.2N/ 1 nm p-Al0.2Ga0.8N short-period superlattice (SPSL). We observed a peak external quantum efficiency (EQE) of the λ = 294 nm TJ UV LED of 12.1%, and an EQE of 10.4% at 20 A/cm2 and 9.1% at 35 A/cm2, with an excess voltage of 1.5 V at 1 A/cm2.

We report on the successful demonstration of an all metalorganic chemical vapor deposition (MOCVD) grown fully transparent tunnel junction (TJ) germicidal UV LED, resulting from the use of a lightly doped n--AlGaN contact layer enabling rapid MOCVD growth optimization. We found that the optimal condition for LED performance was a 3 nm p++-Al0.6Ga0.4N / 9 nm n++-Al0.65Ga0.35N TJ above a 20 period 1 nm p-Al0.8Ga0.2N/ 1 nm p-Al0.2Ga0.8N short-period superlattice (SPSL). We observed a peak external quantum efficiency (EQE) of the λ = 294 nm TJ UV LED of 12.1%, and an EQE of 10.4% at 20 A/cm2 and 9.1% at 35 A/cm2, with an excess voltage of 1.5 V at 1 A/cm2.

We report on the use of ultrathin low-coverage p-GaN hole injection islands on the top surface to improve the light extraction in UV LEDs, resulting in world-record UVB AlGaN UV LEDs emitting at 300 and 310 nm. With the optimization of the p-GaN island density, size, thickness, and doping, we demonstrated a 300 nm emitting device with a peak continuous wave (CW) external quantum efficiency (EQE) of 18.9%, a peak CW wall plug efficiency (WPE) of 16.8%, and a CW EQE of 8.8% at 20 A/cm2, and a 310 nm emitting device with a peak CW EQE of 20.3%, a peak CW WPE of 15.4%, and a CW EQE of 10.9% at 20 A/cm2.

AlGaN-based UV-A LEDs have wide applications in medical treatment and chemical sensing; however, their efficiencies are still far behind visible LEDs or even shorter wavelengths UV-C counterparts because of the large lattice mismatch between the low-Al-content active region and the AlN substrate. In this report, we investigated the composition and thickness of the quantum barrier in the active region in terms of LED performance. Due to the improved strain management and better carrier confinement, efficient UV-A LEDs (320 nm - 330 nm) with EQEs up to 6.8% were demonstrated, among the highest efficiencies at this wavelength range.

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.

AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.

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.

AlGaN-based deep ultraviolet LEDs have attracted significant interest as a reliable and efficient disinfection technology to combat the pandemic outbreaks. However, EQEs of commercial DUV LEDs is currently low. One of the bottlenecks that limit the overall efficiency is the low light extraction efficiency (LEE), which is <15% for conventional flip chip devices. Many attempts have been made to increases the LEE, such as using thin (or no) absorbing p-GaN layer or fabricating novel microstructures, which often result in significant increase in the forward voltage or involve costly processing steps. $\mu\text{LEDs}$ have been proved to increase the LEE in visible wavelength devices but there are a few reports on the size dependence characteristics in the DUV range [1]–[2]. Additionally, $\mu\text{LEDs}$ can prevent the current crowding present in large UV LEDs since growing a thick and conductive n-AlGaN on AlN templates without relaxation remains a challenge.

Highly conductive silicon-doped AlGaN and ohmic contacts are needed for deep-UV LEDs and ultrawide bandgap electronics. We demonstrate improved n-Al0.65Ga0.35N films grown by metal–organic chemical vapor deposition (MOCVD) on sapphire substrates using a low V/III ratio (V/III = 10). A reduced V/III ratio improves repeatability and uniformity by allowing a wider range of silicon precursor flow conditions. AlxGa1−xN:Si with x > 0.5 typically has an electron concentration vs. silicon concentration trend that peaks at a particular “knee” value before dropping sharply as [Si] continues to increase (self-compensation). The Al0.65Ga0.35N:Si grown under the lowest V/III conditions in this study does not show the typical knee behavior, and instead, it has a flat electron concentration trend for [Si] > 3 × 1019 cm−3. Resistivities as low as 4 mΩ-cm were achieved, with corresponding electron mobility of 40 cm2/Vs. AFM and TEM confirm that surface morphology and dislocation density are not degraded by these growth conditions. Furthermore, we report vanadium-based ohmic contacts with a resistivity of 7 × 10−5 Ω-cm2 to AlGaN films grown using a low V/III ratio. Lastly, we use these highly conductive silicon-doped layers to demonstrate a 284 nm UV LED with an operating voltage of 7.99 V at 20 A/cm2, with peak EQE and WPE of 3.5% and 2.7%, respectively.

The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 mΩ cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.

The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric and spintronic behaviours 1 – 5 . In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behaviour and superconductivity 3 , 4 . Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides and black phosphorus 6 , 7 . Twisted van der Waals structures are usually created using a transfer-stacking method, but this method cannot be used for materials with relatively strong interlayer binding. Facile bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. Here we demonstrate that the Eshelby twist, which is associated with a screw dislocation (a chiral topological defect), can drive the formation of such structures on scales ranging from the nanoscale to the mesoscale. In the synthesis, axial screw dislocations are first introduced into nanowires growing along the stacking direction, yielding van der Waals nanostructures with continuous twisting in which the total twist rates are defined by the radii of the nanowires. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles. We further show that the twisting topology can be tailored by controlling the radial size of the structure.

Engineering the structure of materials endows them with novel physical properties across a wide range of length scales. With high in-plane stiffness and strength, but low flexural rigidity, two-dimensional (2D) materials are excellent building blocks for nanostructure engineering. They can be easily bent and folded to build three-dimensional (3D) architectures. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal Bi2Se3 nanoplate and wavelike Bi2Te3 edges buckling up and down forming periodic ripples. Unlike 2D heterostructures directly grown on substrates, the solution-based synthesis allows the heterostructures to be free from substrate influence during the formation process. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled Bi2Se3/Bi2Te3 heterojunction broadens the horizon for the application of 2D materials heterojunction and the design and fabrication of 3D architectures based on them, which could provide a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electromechanic applications.