To overcome the low external quantum efficiency of ultraviolet light‐emitting diodes (UV LEDs) in the technologically significant wavelength range of 300–350 nm, a change of approach to device design may be required. Herein, room‐temperature electroluminescence (EL) at 335 nm is achieved from simple aluminum gallium nitride (AlGaN)‐based metal–insulator–semiconductor (MIS) light‐emitting diodes (LEDs), which do not contain any p‐doped material. Current–voltage and capacitance–voltage measurements indicate that electrons in the valence band of the n‐Al0.14Ga0.86 N layer efficiently tunnel via localized states in the thin, sputter‐deposited aluminum nitride (AlN) barrier of the MIS device to provide the source of holes needed for near‐band‐edge luminescence. The full width at half maximum (FWHM) in the UV emission peak is only around 18 nm and it is very close to the photoluminescence (PL) peak of the active n‐Al0.14Ga0.86N layer, confirming that the EL is derived from band‐to‐band radiative transitions in the simple non‐p‐doped MIS diode. This design is a potential option to overcome the problem of the poor thermal excitation of holes in the AlGaN‐based pn junction devices.

Photocatalysis has a potential to become a cost effective industrial process for water cleaning. One of the most studied photocatalysts is titanium dioxide which, as a wide band gap semiconductor, requires ultraviolet (UV) light for its photoactivation. This is at the wavelengths where the efficiency of present-day light emitting diodes (LEDs) decreases rapidly, which presents a challenge in the use of UV-LEDs for commercially viable photocatalysis. There is also a need for accurate photocatalysis measurement of remediation rates of water-borne contaminants for determining optimum exposure doses in industrial applications. In response to these challenges, this paper describes a UV-LED based photocatalytic test reactor that provides a calibrated adjustable light source and pre-defined test conditions to remove as many sources of uncertainty in photocatalytic analysis as possible and thereby improve data reliability. The test reactor provides a selectable intensity of up to 1.9 kW m^−2 at the photocatalyst surface. The comparability of the results is achieved through the use of pre-calibration and control electronics that minimize the largest sources of uncertainty; most notably variations in the intensity and directionality of the UV light emission of LEDs and in LED device heating.

This work presents the design, fabrication and measurement of gallium nitride (GaN) distributed Bragg reflector cavities integrated with input and output grating couplers. The devices are fabricated using a new, low-cost nanolithography technique: displacement Talbot lithography combined with direct laser writing lithography. The finite-difference time-domain method has been used to design all the components and measured and modelled results show good agreement. Such devices have applications in GaN integrated photonics and biosensing.

GaN grating couplers and a distributed Bragg reflector cavity are fabricated using displacement Talbot lithography on GaN-on-sapphire. Cavity resonances are simulated and measured for two devices with Q factors of ~200.

III-nitride nanostructures are of interest for a new generation of light-emitting diodes (LEDs). However, the characterization of doping incorporation in nanorod (NR) structures, which is essential for creating the p-n junction diodes, is extremely challenging. This is because the established electrical measurement techniques (such as capacitance–voltage or Hall-effect methods) require a simple sample geometry and reliable ohmic contacts, both of which are difficult to achieve in nanoscale devices. The need for homogenous, conformal n-type or p-type layers in core–shell nanostructures magnifies these challenges. Consequently, we demonstrate how a combination of non-contact methods (micro-photoluminescence, micro-Raman and cathodoluminescence), as well as electron-beam-induced-current, can be used to analyze the uniformity of magnesium incorporation in core–shell NRs and make a first estimate of doping levels by the evolution of band transitions, strain and current mapping. These techniques have been used to optimize the growth of core–shell nanostructures for electrical carrier injection, a significant milestone for their use in LEDs.

We report the ability to control relative InN incorporation in InGaN/GaN quantum wells (QWs) grown on the semi-polar and non-polar facets of a core-shell nanorod LED structure by varying the growth conditions. A study of the cathodoluminescence emitted from series of structures with different growth temperatures and pressures for the InGaN QW layer revealed that increasing the growth pressure had the effect of increasing InN incorporation on the semi-polar facets, while increasing the growth temperature improves the uniformity of light emission from the QWs on the non-polar facets.

III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1–100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1–100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.

Heat extraction is often essential in ensuring efficient performance of semiconductor devices and requires minimising the thermal resistance between the functional semiconductor layers and any heat sink. This paper reports the epitaxial growth of N-polar GaN films on polycrystalline diamond substrates of high thermal conductivity with metal-organic vapor phase epitaxy, by using a SixC layer formed during deposition of polycrystalline diamond on a silicon substrate. The SixC layer acts to provide the necessary structure ordering information for the formation of a single crystal GaN film at the wafer scale. It is shown that a three-dimensional island (3D) growth process removes hexagonal defects that are induced by the non-single crystal nature of the SixC layer. It is also shown that intensive 3D growth and the introduction of a convex curvature of the substrate can be deployed to reduce tensile stress in the GaN epitaxy to enable the growth of a crack-free layer up to a thickness of 1.1 mu m. The twist and tilt can be as low as 0.65 degrees and 0.39 degrees respectively, values broadly comparable with GaN grown on Si substrates with a similar structure.

The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing {1−101} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing {11−22} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109cm−2 to 3.0×107cm−2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

Ultraviolet light emitting diodes (UV-LEDs) are attracting the interest of researchers for the design of compact photoreactors due to their energy efficiency, life expectancy, design flexibility, and easily tuned intensity and emission wavelength. However, due to the quasi-point source nature and viewing angle dependence of these illumination sources, the light distribution in LED based reactors can be highly inhomogeneous if the locations of the LEDs in the reactor are not carefully designed. This work describes the design of a novel standardized reactor for accurate measurements of the efficiency of photocatalytic materials under well-controlled lighting conditions. For standardized kinetic studies, it is necessary to ensure that a homogeneous radiation distribution is achieved over the catalyst surface. UV irradiation calculations involving rigorous solution of the radiative transport equation have been performed to compute the incident radiation at each point of the reactor geometry. Homogeneity calculations over the catalytic surface have been analysed for a range of LED configurations, diameter and distance of the catalyst surface with excellent agreement with measurements. We demonstrate that for many of the configurations and distances examined a poor homogeneity over the catalyst surface is obtained if the LED configuration is not carefully designed. The optimized reactor was built and predictions of the numerical model were validated against spectrophotometric measurements. The designed reactor can be also operated for the determination of the activity of photocatalytic materials in a slurry under very high radiation fluxes. The reactor model was validated with rigorous inclusion of absorption and scattering phenomena under highly demanding conditions of high incident radiation intensities. The developed design provides a novel route for quantitative assessment of photocatalytic materials and reactions.

Microcavities based on group-III nitride material offer a notable platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers.Disk or tube geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes.In this article we present the fabrication of homogenous and dense arrays of axial InGaN/GaN nanotubes via a combination of displacement Talbot lithography (DTL) for patterning and inductively coupled plasma top-down dry-etching.Optical characterization highlights the homogeneous emission from nanotube structures.Power-dependent continuous excitation reveals a non-uniform light distribution within a single nanotube, with vertical confinement between the bottom and top facets, and radial confinement within the active region.Finitedifference time-domain simulations, taking into account the particular shape of the outer diameter, indicate that the cavity mode of a single nanotube has a mixed WGM-vertical Fabry-Perot mode (FPM) nature.Additional simulations demonstrate that the improvement of the shape symmetry and dimensions primarily influence the Q-factor of the WGMs whereas the position of the active region impacts the coupling efficiency with one or a family of vertical FPMs.These results show that regular arrays of axial InGaN/GaN nanotubes can be achieved via a low-cost, fast and large-scale process based on DTL and top-down etching.These techniques open a new perspective for cost effective fabrication of nano-LED and nano-laser structures along with bio-chemical sensing applications.

Strong room-temperature electroluminescence at 365 nm has been demonstrated from simple Au/AlN/n-GaN metal-insulator-semiconductor (MIS) light emitting diodes, which do not contain p-doped material. Current-voltage and electroluminescence data indicate that an AlN insulating layer thickness of 10 nm results in optimized diode behavior and maximum ultraviolet emission: At lower thicknesses carriers tunnel easily through the barrier, whereas at greater thicknesses the forward resistivity is excessively high. A decrease in emission intensity was observed at high injection currents due to Fowler-Nordheim tunnelling. However the device efficiency was found to improve by a factor of 10 when the AlN layer and the metal contact layer were deposited without breaking vacuum, thereby preventing any contamination or oxidation of the AlN surface. Additionally, this MIS device showed clear resonant tunnelling characteristics which are correlated with the enhanced light emission intensity.

GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume, and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would improve the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth, and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two-dimensional (2D) growth mode to three-dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the <10-10> directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region.

In this paper, a novel simulation tool has been developed to examine the statistics of polarization dependent loss (PDL) using short fibre links in the first instance. Hardware emulator which has been previously developed [1] was used to generate measurement results with the same settings as the simulator. The results show that the simulator works reliably, so it can now be used to simulate the statistical results for more realistic long distance communications systems. The results also prove that the PDL variance reaches a minimum when each of the fibre lengths is approximately equal.

This dataset contains the results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements carried out on core-shell nanostructures. The samples are highly regular arrays of GaN plasma etched cores onto which thick InGaN layers were grown using different metal organic vapour phase epitaxy (MOVPE) growth parameters. Three different InGaN growth conditions were considered with the following parameters: 750°C at 300 mbar, 700°C at 300 mbar and 750°C at 100 mbar. Statistical growth rates were determined on the non-polar crystal planes from measurements of increase in diameter using SEM images. TEM analysis was carried out on a single nanorod for greater detail.

Controlling the long-range homogeneity of core–shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as ∼7 meV·μm–1 along the m-plane facets from core–shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 μm. High-resolution transmission electron microscopy and spatially resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 μm length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short-range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core–shell layers is critical to enable them to compete with and replace conventional planar light-e...

Core-shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both {11-20} a-plane and {10-10} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the {10-10} m-plane facets, even at a reduced growth rate. The sample grown at 750 degrees C and 100 mbar had an InN mole fraction of 25% on the {10-10} facets of the NRs. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

High-aspect-ratio GaN-based nanostructures are of interest for advanced photonic crystal and core-shell devices. Nanostructures grown by a bottom-up approach are limited in terms of doping, geometry and shape which narrow their potential application areas. In contrast, high uniformity and a greater diversity of shape and design can be produced via a top-down etching approach. However, a detailed understanding of the role of etch process parameters is lacking for creating high-aspect ratio nanorods and nanopores. Here we report a systematic analysis on the role of temperature and pressure on the fabrication of nanorod and nanopore arrays in GaN. Our results show a threshold in the etch behaviour at a temperature of ~125°C, which greatly enhances the verticality of the GaN nanorods, whilst the modification of the pressure enables a fine tuning of the nanorod profile. For nanopores we show that the use of higher temperatures at higher pressures enables the fabrication of nanopores with an undercut profile. Such a profile is important for controlling the optical field in photonic crystal-based devices. Therefore we expect the ability to create such nanostructures to form the foundation for new advanced LED designs.