Enhanced light extraction efficiency of an LED package by a surface-mounted amorphous photonic structure

In this study, we propose a low-cost, simple and feasible post-processing approach to improve the light extraction efficiency (LEE) of LED packages. Amorphous photonic structures (APSs) with only short-range order are fabricated from anodic aluminum oxide (AAO) and transferred to intermediate polymer stamp (IPS) by nanoimprint technology. The IPS with APSs is directly mounted onto the surface of an LED package, where the LEE is achieved as 94.6%. The scanning electron microscope (SEM) images of AAO templates and imprinted IPS are analyzed by radial distribution function and diameter histogram. The far-field patterns of APS-mounted LED packages are measured in electroluminescence (EL). The three-dimensional finite-difference time-domain (3D-FDTD) calculations of transmittance of APSs confirm that they improve the light extraction above the critical angle. Two-dimensional Fourier power spectra from SEM images of APSs are also calculated. The LEE enhancement is attributed to that the APSs have short-range order on a length scale comparable to emission wavelength of LED. We provide novel multistage simulations in a simplified FDTD model for the LED package. Finally, we discuss the influence of the morphology of APSs on the LEE of the APS mounted LEDs.

Spectroscopic investigation of Cux Mg0.2-x Zn0.8 S (x = 0, 0.05, 0.10, 0.15) thin films for deep and dilute blue LED applications

The effect of copper (Cu) doping on the luminescent properties of the spray deposited Mg_0.2 Zn_0.8 S thin films were investigated for the first time. The Mg_0.2 Zn_0.8 S film is an excellent luminescent material with strong blue emissions. In the current investigation, we doped Mg_0.2 Zn_0.8 S with Cu by taking (Cu + Mg) as 20 at% by keeping other element ratios constant. Among the different samples in the series, Cu_0.05 Mg_0.15 Zn_0.8 S has shown promising results with dark blue emission. Also, these films showed good structural formation with lower or no other impurities, which is evident from the X-ray diffraction (XRD). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and atomic force microscopy (AFM) confirmed the improved material quality of Cu_0.05 Mg_0.15 Zn_0.8 S as compared to the pristine. Raman and X-ray photoelectron spectroscopy (XPS) studies have been carried out for the samples. Various defects induced in the films were investigated by recording the photoluminescence (PL) spectra and Cu:(Mg_0.2 Zn_0.8 S) films exhibited the capability to produce dilute blue luminescence by absorbing ultraviolet (UV) light. The Cu_0.05 Mg_0.15 Zn_0.8 S film showed promising material property, which is suitable for light-emitting diode (LED) applications.

Optimal Selection of Backside Roughing Parameters of High-Value Components Using Abrasive Jet Processing

This paper mainly presents a set of new Sapphire Backside Roughing technology. Presently, the associated Sapphire Backside Roughing technology is still concentrated on chemical etching, as its yield rate and efficiency are often limited by lattice structures, and the derived chemical waste fluid after etching is most likely to cause ecological contamination. In this research, refined abrasive jet processing technology is adopted, and in the meantime, the Taguchi experiment design method is taken for detailed experimental planning. Through processing parameter conditions and abrasive selection and development, proper surface roughing and processing uniformity are obtained so as to improve the various weak points of the abovementioned traditional etching effectively. It is discovered that abrasive blasting processing technology is, respectively, combined with wax-coated #1000 SiC particles and wax-coated #800 Zirconium particles to process the sapphire substrate with initial surface roughness 0.8–0.9 μmRa from the experiment. A 1.1–1.2 μmRa surface roughness effect can be achieved about two minutes later. The experimental results show that the actual degree of sapphire substrate surface roughing obtained in the AJM process depends on the gas pressure, impact angle, wax-coated abrasives, and additives. The new Sapphire Backside Roughing technology has high flexibility, which not only meets the requirements for sapphire surface roughing specification but can also effectively reduce the sapphire substrate roughing time and related cost.

Efficiency Models for GaN-Based Light-Emitting Diodes: Status and Challenges

Light-emitting diodes (LEDs) based on Gallium Nitride (GaN) have been revolutionizing various applications in lighting, displays, biotechnology, and other fields. However, their energy efficiency is still below expectations in many cases. An unprecedented diversity of theoretical models has been developed for efficiency analysis and GaN-LED design optimization, including carrier transport models, quantum well recombination models, and light extraction models. This invited review paper provides an overview of the modeling landscape and pays special attention to the influence of III-nitride material properties. It thereby identifies some key challenges and directions for future improvements.

A thin transferable blue light-emitting diode by electrochemical lift-off

We demonstrate a transferable blue light-emitting diode (LED) fabricated using a cost-effective approach. By means of solution-based electrochemical etching, an ultrathin free-standing membrane can be obtained from a commercial III-nitride LED wafer. The membrane, containing a full LED structure (including p-/n-type layers and multiple quantum wells) epitaxially grown on a sapphire substrate, is transferable to foreign substrates with a simple lift-off process facilitated by electrochemical etching. After fabrication, optical properties of the thin film are massively improved, accompanied by a 17-fold enhanced photoluminescence normal to the film surface. Prototype transferable blue LEDs are realized on both a copper-coated glass substrate and a polypropylene substrate. The devices exhibit a high performance with bright emission at 447 nm under electrical injection at room temperature.

Impact of roughening density on the light extraction efficiency of thin-film flip-chip ultraviolet LEDs grown on SiC

Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (~294-310 nm) grown on SiC-with reflective contacts and roughened emission surface-to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d ~λ_EL; 42-52 pyramids/µm²), but the LEE enhancement diminished when d was much larger than λ_EL (d ~5.5λ_EL; 2-3 pyramids/µm²). The UV LEDs had a 10 nm p-GaN contact layer, and the forward voltage was ~6 V at ~8 A/cm², with a voltage efficiency (VE) of ~70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN LEDs.

Moth eye-inspired anti-reflective surfaces for improved IR optical systems & visible LEDs fabricated with colloidal lithography and etching

Near- and sub-wavelength photonic structures are used by numerous organisms (e.g. insects, cephalopods, fish, birds) to create vivid and often dynamically-tunable colors, as well as create, manipulate, or capture light for vision, communication, crypsis, photosynthesis, and defense. This review introduces the physics of moth eye (ME)-like, biomimetic nanostructures and discusses their application to reduce optical losses and improve efficiency of various optoelectronic devices, including photodetectors, photovoltaics, imagers, and light emitting diodes. Light-matter interactions at structured and heterogeneous surfaces over different length scales are discussed, as are the various methods used to create ME-inspired surfaces. Special interest is placed on a simple, scalable, and tunable method, namely colloidal lithography with plasma dry etching, to fabricate ME-inspired nanostructures in a vast suite of materials. Anti-reflective surfaces and coatings for IR devices and enhancing light extraction from visible light emitting diodes are highlighted.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl₂/N₂-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

Photoelectrochemical liftoff of LEDs grown on freestanding c-plane GaN substrates

We demonstrate a thin-film flip-chip (TFFC) process for LEDs grown on freestanding c-plane GaN substrates. LEDs are transferred from a bulk GaN substrate to a sapphire submount via a photoelectrochemical (PEC) undercut etch. This PEC liftoff method allows for substrate reuse and exposes the N-face of the LEDs for additional roughening. The LEDs emitted at a wavelength of 432 nm with a turn on voltage of ~3 V. Etching the LEDs in heated KOH after transferring them to a sapphire submount increased the peak external quantum efficiency (EQE) by 42.5% from 9.9% (unintentionally roughened) to 14.1% (intentionally roughened).

Light extraction from luminescent light sources and application to monolithic ceramic phosphors

An extension of a theorem for light extraction [Adv. Opt. Technol.2, 291 (2013)] from a higher index luminescent body (LED or phosphor) through an extracting surface into a lower index output medium is derived. The result is valid for both geometric and diffractive surface structures. Using this bound and radiation transport calculations, we show that extraction from LEDs or phosphors requires a combination of cavity effects to enhance radiance behind the extracting surface and scattering or diffraction to couple trapped total-internal-reflection modes to propagating modes. The treatment applies to macroscopic luminescent sources whose thickness exceeds the longitudinal coherence length of the luminescent radiation.