Gallium Nitride-based Light-emitting Diodes Research Articles

Bidirectional light-emitting diode as a visible light source driven by alternating current

Gallium nitride-based light-emitting diodes have revolutionized the lighting market by becoming the most energy-efficient light sources. However, the power grid, in example electricity delivery system, is built based on alternating current, which raises problems for directly driving light emitting diodes that require direct current to operate effectively. In this paper, we demonstrate a proof-of-concept device that addresses this fundamental issue – a gallium nitride-based bidirectional light-emitting diode. Its structure is symmetrical with respect to the active region, which, depending on the positive or negative bias, allows for the injection of either electrons or holes from each side. It is composed of two tunnel junctions that surround the active region. In this work, the optical and electrical properties of bidirectional light emitting diodes are investigated under direct and alternating current conditions. We find that the light is emitted in both directions of the supplied current, contrary to conventional light emitting diodes; hence, bidirectional light-emitting diodes can be considered a semiconductor light source powered directly with alternating current. In addition, we show that bidirectional light-emitting diodes can be stacked vertically to multiply the optical power achieved from a single device.

Perspective on light-fidelity and visible light communication

Visible light communication (VLC) is an exciting research area. VLC offers a significantly greater spectrum than radio frequency and is ideal for data communication. In VLC, the visible light spectrum, from 400 to 700 nm, is used for both lighting and communication. Gallium nitride-based light-emitting diodes (LEDs) and laser diodes are the optical sources in visible light communications. Laser diodes have an advantage over LEDs in visible light communication. VLC has applications in underwater communication, vehicle-to-vehicle communications, among others.

Enhanced Heat Dissipation in Gallium Nitride-Based Light-Emitting Diodes by Piezo-phototronic Effect.

As a new generation of light sources, GaN-based light-emitting diodes (LEDs) have wide applications in lighting and display. Heat dissipation in LEDs is a fundamental issue that leads to a decrease in light output, a shortened lifespan, and the risk of catastrophic failure. Here, the temperature spatial distribution of the LEDs is revealed by using high-resolution infrared thermography, and the piezo-phototronic effect is proved to restrain efficaciously the temperature increment for the first time. We observe the temperature field and current density distribution of the LED array under external strain compensation. Specifically, the temperature rise caused by the self-heating effect is reduced by 47.62% under 0.1% external strain, which is attributed to the enhanced competitiveness of radiative recombination against nonradiative recombination due to the piezo-phototronic effect. This work not only deepens the understanding of the piezo-phototronic effect in LEDs but also provides a novel, easy-to-implement, and economical method to effectively enhance thermal management.

Gallium nitride micro-light-emitting diode structured light sources for multi-modal optical wireless communications systems.

Gallium nitride-based light-emitting diodes (LEDs) have revolutionized the lighting industry with their efficient generation of blue and green light. While broad-area (square millimetre) devices have become the dominant LED lighting technology, fabricating LEDs into micro-scale pixels (micro-LEDs) yields further advantages for optical wireless communications (OWC), and for the development of smart-lighting applications such as tracking and imaging. The smaller active areas of micro-LEDs result in high current density operation, providing high modulation bandwidths and increased optical power density. Fabricating micro-LEDs in array formats allows device layouts to be tailored for target applications and provides additional degrees of freedom for OWC systems. Temporal and spatial control is crucial to use the full potential of these micro-scale sources, and is achieved by bonding arrays to pitch-matched complementary metal-oxide-semiconductor control electronics. These compact, integrated chips operate as digital-to-light converters, providing optical signals from digital inputs. Applying the devices as projection systems allows structured light patterns to be used for tracking and self-location, while simultaneously providing space-division multiple access communication links. The high-speed nature of micro-LED array devices, combined with spatial and temporal control, allows many modes of operation for OWC providing complex functionality with chip-scale devices.This article is part of the theme issue ‘Optical wireless communication’.

Improving the luminous efficiency of gallium nitride-based light-emitting diodes using Ag nanograting structure

In order to improve the traditional gallium nitride (GaN)-light-emitting diode (LED) luminous efficiency, a structure of the GaN-LED is designed based on the local field enhancement of surface plasmon (SP). The principle of improving LED luminous properties using the SP features is described in detail. The factors that affect the light extraction efficiency of this LED structure are simulated with the COMSOL software. The thickness of p-GaN layer, the thickness of indium tin oxide (ITO) layer, the grating period T , the duty ratio r , as well as the shape of the interface between p-GaN and ITO are studied. The normalized radiated powers, the normalized absorbed powers, and the electric field distribution of this structure under different structure parameters have been achieved. The results show that when the thickness of p-GaN layer is 110 nm, the thickness of ITO buffer layer is 120 nm, the period T is 370 nm, the duty ratio is 0.5, and the value of D 1 / T is 0.6. The luminous intensity of LED is higher than that of the traditional LED nearly 43 times and is nearly 36 times higher than that of the Ag nanosphere structure, and SPs can be coupled with the active layer effectively to improve the luminous efficiency of LED, providing a theoretical guide for the preparation of high-performance GaN-LED chips.

Luminescence Spectra of High-Power Violet and Ultraviolet Gallium Nitride-Based LEDs

The electroluminescence spectra of high-power light-emitting diodes (LEDs) based on p–n heterostructures of the InGaN/AlGaN/GaN type emitting in the visible short-wavelength and ultraviolet spectral regions (the range from 370 to 460 nm) are investigated. The shape of spectra is described by a model taking into account a two-dimensional combined density of states and potential fluctuations. Additional long-wavelength peaks are found in emission spectra. The emission power of ultraviolet and violet LEDs reaches 233 mW at a current of 350 mA, and the external quantum yield reaches 23% at the maximum (near a current of 100 mA).

Light extraction enhancement of 265 nm deep-ultraviolet light-emitting diodes with over 90 mW output power via an AlN hybrid nanostructure

Deep-ultraviolet (DUV) aluminum gallium nitride-based light-emitting diodes (LEDs) on transparent aluminum nitride (AlN) substrates with high light extraction efficiency and high power are proposed and demonstrated. The AlN bottom side surface configuration, which is composed of a hybrid structure of photonic crystals and subwavelength nanostructures, has been designed using finite-difference time-domain calculations to enhance light extraction. We have experimentally demonstrated an output power improvement of up to 196% as a result of the use of the embedded high-light-extraction hybrid nanophotonic structure. The DUV-LEDs produced have demonstrated output power as high as 90 mW in DC operation at a peak emission wavelength of 265 nm.

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.

Numerical simulations of light‐extraction efficiencies of light‐emitting diodes on micro and nanopatterned sapphire substrates

Numerical simulations are performed to investigate the light-extraction efficiencies (LEEs) of gallium nitride-based light-emitting diodes (LEDs) on patterned sapphire substrates (PSSs), using the three-dimensional finite-difference time-domain method. PSSs with hexagonal arrays of cone-shaped patterns were used, and the pitch of the pattern array, pattern fill factor and pattern height were varied from nanoscale to microscale. The relative LEE for each PSS-LED was calculated, and the geometrical parameters were analysed for optimised light extraction enhancement of the PSS-LEDs.

Gallium nitride-based light-emitting diodes with embedded air voids grown on Ar-implanted AlN/sapphire substrate

GaN-based light-emitting diodes (LEDs) grown on sapphire with ex situ AlN nucleation layer prepared by radio-frequency sputtering were investigated. GaN-based epitaxial layers grown on the Ar-implanted AlN/sapphire (AIAS) substrates exhibited selective growth and subsequent lateral growth due to the difference of lattice constants between the implanted and implantation-free regions. Consequently, air voids over the implanted regions were formed around the GaN/AlN/sapphire interfaces. We proposed the growth mechanisms of the GaN layer on the AIAS substrates and characterized the LEDs with embedded air voids. With a 20 mA current injection, experimental results indicate that the light output power of LEDs grown on the AIAS substrates was enhanced by 25% compared with those of conventional LEDs. This enhancement can be attributed to the light scattering at the GaN/air void interfaces to increase the light extraction efficiency of the LEDs.

Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.