Photoluminescence and spin relaxation of MnZnO/GaN-based light-emitting diodes

This study investigates the spin relaxation of GaN-based light-emitting diodes with an MnZnO film by examining its photoluminescence (PL) and time-resolved magnetization modulation photoluminescence. PL measurements reveal that the application of a magnetic field produced a clear difference between the intensities of the right (σ⁺) and left (σ⁻) circular polarization components. The circular polarization was identified as P_circ = [I(σ⁺) − I(σ⁻)] / [I(σ⁺) + I(σ⁻)], where I(σ⁺) and I(σ⁻) are the intensities of the σ⁺ and σ⁻ components, respectively. The PL polarization was 3.6% in a 0.5 T magnetic field. In a magnetic field, the photo-ionized lifetime and spin-polarized lifetime values were approximately 13.64 and 54.54 ns, respectively. The right-circular-spin-polarization lifetime and the left-circular-spin-polarization lifetime values were about 39.09 and 40.01 ns, respectively.     

Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures

We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface.     

Critical Assessment 23: Gallium nitride-based visible light-emitting diodes

Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency ‘droop’ (the reduction in efficiency of nitride LEDs with increasing drive current density), the ‘green gap’ (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.     

Enhanced light output of angled sidewall light-emitting diodes with reflective silver films

A proof-of-concept of applying laser micro-machining to fabricate high performance GaN light-emitting diode (LED) was presented in this study. Laser micro-machining was applied to fabricate GaN LED chip with angled sidewalls (ALED). The inclined sapphire sidewalls were coated with highly reflective silver film which functions as an efficient light out-coupling medium for photons within the LED structure. Thus, more laterally-propagating photons can be redirected to the upward direction of the ALED with silver coating (Ag-ALED). Performances of the Ag-ALED, ALED and conventional planar GaN LED were evaluated. At an injection current of 30 mA, the light output intensity of Ag-ALED was significantly improved by 97% and 195% as compared to ALED and conventional planar LED, respectively. The corresponding wall-plug efficiency of Ag-ALED was remarkably increased by 95% and 193% as compared to ALED and conventional planar LED, respectively. The results of this study demonstrated that the Ag-ALED showed a pronounced increase in light output intensity compared to conventional planar LED, which may have many potential applications in the field of display engineering.     

Improving radiative recombination efficiency of green light-emitting diodes

Although the solid-state lighting industry has achieved huge successes in both red and blue part of the visible spectrum during the last 40 years, light-emitting diodes (LEDs) that emit green light consistently exhibit inferior efficiencies. Thanks to the use of down-conversion phosphors, white LEDs have been commercialised without using green LEDs. However, the efficiency problem of green LEDs still hinders many potential applications of solid-state lighting and limits the overall system efficiency. This review first attempts to conclude and comment on the complex factors that limit the performance of green LEDs with recent research progresses. Then the article focuses on reviewing various strategies to improve green light LED radiative recombination efficiencies.

This review was chosen as a runner up of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Polarized GaN-based light-emitting diode with an embedded metallic/dielectric subwavelength grating

Highly polarized light from an InGaN/GaN light emitting diode is proposed using an embedded multi-layer metallic/dielectric sub-wavelength grating and a dielectric transition layer. Transmission of transverse magnetic mode (TTM), reflection of transverse electric mode, and polarization extinction ratio (ER) were calculated using commercial “GSOLVER” software, based on a full vector implementation of Rigorous Coupled-Wave Analysis algorithm. TTM and ER were found to be largely enhanced by the presence of the transition layer, made of MgF₂ or SiO₂, placed between GaN and the grating section. TTM > 95% and ER > 34 dB for closely optimized Al/MgF₂ gratings were predicted. These values are significantly higher than those obtained by single-layer metallic gratings.     

Cubic zincblende gallium nitride for green-wavelength light-emitting diodes

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are highly energy efficient and their widespread usage in lighting can induce significant worldwide electricity savings. To achieve white and colour-tuneable lighting, the mixing of light from red-, blue- and green-wavelength LEDs is desired. At present, the efficiency of green-wavelength LEDs is only about half of that of red- and blue-wavelength LEDs, which is also known as the ‘green gap’ problem. Cubic zincblende GaN has the potential to bridge the ‘green gap’ due to the theoretical absence of internal electric fields that plague the commonly used c-plane hexagonal wurtzite structure. This review first looks at the various methods of achieving metastable zincblende GaN, and examines the crystal defects present. Then the different components towards a full zincblende GaN LED structure, including p- and n-type doping, zincblende InGaN and AlGaN on GaN heterostructures, together with the problems that need to be overcome to achieve green-wavelength emissions are reviewed.

This review was submitted as part of the 2017 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged     

Effects of nano-structured photonic crystals on light extraction enhancement of nitride light-emitting diodes

The light extraction efficiency of an InGaN/GaN light-emitting diode (LED) can be enhanced by incorporating nano-structured photonic crystals inside the LED structure. We employed plane wave expansion (PWE) method and finite difference time domain (FDTD) method to reveal the optical confinement effects with the relevant parameters. The results showed that band-gap modulation could increase the efficiency for light extraction at the lattice constant of 200 nm and depth of 200 nm for the 468-nm LED. Focused ion beam (FIB) using Ga created the desired nano-structured patterns. The LED device micro-PL (photoluminescence) results have demonstrated that the triangular photonic crystal arrays could increase the peak illumination intensity by 58%. The peak wavelength remained unchanged. The integrated area under the illumination peak was increased by 75%. As the patterned area ratio was increased to 85%, the peak intensity enhancement was further improved to 91%, and the integrated area was achieved at 106%.     

New stylbene-based arylamines with dehydroabietic acid methyl ester moieties for organic light-emitting diodes

New triarylamines possessing a dehydroabietic acid methyl ester moiety and a stylbene substituent were prepared and tested as hole-transporting materials in light-emitting diodes, LEDs, with Alq₃ as an emissive material. LEDs based on one of these new amines show similar performance to those where TPD, processed from solution, is used as hole-transporting material. In addition, while TPD films tend to crystallise upon storage at room temperature, the films of the new amines do not appear to crystallise under similar conditions. The stabilisation of an amorphous morphology is attributed to the presence of the bulky dehydroabietic acid methyl ester moiety, although it appears to reduce hole mobility.     

Chemical bond manipulation for nanostructure integration on wafer scale

In this paper, we have briefly summarized our activity in the area of chemical bond manipulation for the integration of nanostructures on a full wafer scale. Chemical bond manipulation involves a judicious combination of surface phenomena: reactions or diffusion, and growth process such as molecular beam epitaxy (MBE). Here, we present our results on oxidation, metallization and nitridation and their role in the formation of nanostructures. We find that oxygen changes the bonding partner from Ge to Si and this phenomenon can be controlled by controlling the annealing temperature. We have employed this phenomenon for the fabrication of novel, multiperiod Si/SiO₂/Ge layered structure which exhibits interesting light emitting properties. Further, by making use of selective diffusion of cobalt atoms through Ge layers it is possible to incorporate metallic features into Ge quantum dots. Moreover, it is possible to fabricate Si nanopillars through high temperature reaction of nitric oxide. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrence of these two simultaneous processes result in the formation of nanometre-sized Si pillars capped by silicon nitride. All these results emphasize the fact that we can extend information obtained through traditional surface science experiments for the fabrication of novel structures on a full wafer scale.