GaN LED Research Articles

Facile fabrication of moisture resistance and thermally stable SrGa2S4:Eu2+ phosphor-in-glass microcubes for white LED

SrGa2S4:Eu2+ phosphor known for its moisture sensitivity was successfully fabricated into white light-emitting device (WLED) with enhanced moisture and thermal stability by the dicing of phosphor-in-glass (PiG) plate. Sulfide PiG plates were diced into 100×100μm cubes which showed enhanced moisture resistance and thermal stability due to high thermal resistivity of the glass-based PiG encapsulant and design of PiG microcube. Moisture sensitivity test revealed that 95% of emission intensity was retained for sulfide PiG microcubes compared to 88% for powder phosphor even after 48h. The WLED was fabricated with sulfide PiG microcubes under 450nm GaN LED chip and showed CIE coordinates of (0.39, 0.39) with high CRI of 85 under operating current of 40mA for conformal WLED packaging applications.

White thin-film flip-chip LEDs with uniform color temperature using laser lift-off and conformal phosphor coating technologies.

We fabricated a phosphor-conversion white light emitting diode (PC-WLED) using a thin-film flip-chip GaN LED with a roughened u-GaN surface (TFFC-SR-LED) that emits blue light at 450 nm wavelength with a conformal phosphor coating that converts the blue light into yellow light. It was found that the TFFC-SR-LED with the thin-film substrate removal process and surface roughening exhibits a power enhancement of 16.1% when compared with the TFFC-LED without a sapphire substrate. When a TFFC-SR-LED with phosphors on a Cu-metal packaging-base (TFFC-SR-Cu-WLED) was operated at a forward-bias current of 350 mA, luminous flux and luminous efficacy were increased by 17.8 and 11.9%, compared to a TFFC-SR-LED on a Cup-shaped packaging-base (TFFC-SR-Cup-WLED). The angular correlated color temperature (CCT) deviation of a TFFC-SR-Cu-WLED reaches 77 K in the range of -70° to + 70° when the average CCT of white LEDs is around 4300 K. Consequently, the TFFC-SR-LED in a conformal coating phosphor structure on a Cu packaging-base could not only increase the luminous flux output, but also improve the angular-dependent CCT uniformity, thereby reducing the yellow ring effect.

Heat Sink Cooling Fan and Rotation Speed Effect Analysis on Heat Dissipation of High Power GaN LED Package

In this work, thermal simulation analysis on high power LED is reported where the effect of the heat sink cooling fan and its rotation speed on the heat dissipation of the high power LED was evaluated. Ansys version 11 was utilized for the simulation. The thermal performance of the high power LED package was assessed in terms of operating junction temperature, von Mises stress and thermal resistance. The heat dissipation analysis was done under four types of convection condition:one natural convection conditionthree forced convection condition,. The forced convection condition was used to replicate the effect of a fan with various rotation speeds placed under the heat sink to increase the convective heat transfer coefficient. Results of the analysis showed that that the junction temperature, von Mises stress and thermal resistance of the GaN chip reduces with the increase of the fan rotation speed.

Spectroscopic evaluation of Zn(PO3)2:Dy3+ glass as an active medium for solid state yellow laser

A spectroscopic investigation of zinc phosphate glass activated with 1.0, 5.0 and 10.0mol% of Dy(PO3)3 is performed through absorption and luminescence spectra and decay times to study its potentialities for yellow laser operation upon excitation at 399nm, which fits to the requirements of GaN LEDs. In the 1.0mol% Dy(PO3)3-doped glass a quantum efficiency of 80±5% was estimated for the dysprosium 4F9/2 level luminescence, the 4F9/2→6H13/2 yellow emission shows greater intensity than the 4F9/2→6H15/2 blue emission, as well as a very high optical gain, which might make this glass phosphor a promising gain medium for solid state yellow laser pumped by GaN LEDs.

Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process.

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.

Visible Light Communication Using a Blue GaN $\mu $ LED and Fluorescent Polymer Color Converter

This letter presents a novel technique to achieve high-speed visible light communication (VLC) using white light generated by a blue GaN <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> LED and a yellow fluorescent copolymer. We generated white light suitable for room illumination by optimizing the ratio between the blue electroluminescence of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> LED and yellow photoluminescence of the copolymer color converter. Taking advantage of the components’ high bandwidth, we demonstrated 1.68 Gb/s at a distance of 3 cm (at 240 lx illumination). To the best of our knowledge, this is the fastest white light VLC results using a single blue LED/color converter combination.

Investigation on Polarization Induced Electro-Optical Property of GaN LED Using TEM-EBIC Combined with Cathodoluminescenc

Strained InGaN layers grown on a c-plane sapphire is known to have strong spontaneous and piezoelectric polarization field, where the discontinuity of polarization at heterojunction is to induce bounded surface charges [1]. These polarization-related phenomenon can affect the internal electric field of active region and induce the quantum-confined stark effect (QCSE) at quantum well. QCSE results in the spatial separation of electrons and holes in the quantum well, blue shift, and increase of carrier density [2]. To reduce the polarization induced QCSE, efforts were made by growing GaN-InGaN on semi-polar and non-polar substrates [3]. In this study, we report the polarization induced electro-optical property of polar, semi-polar, and non-polar GaN-InGaN LED in nano-scale.Electron-Beam Induced Current imaging (EBIC) in Transmission Electron Microscopy (TEM-EBIC) enables us to map the electrical response from region to region in nano-scale. When the EBIC is to combine with cathodoluminescence (CL) and microstructural analysis in transmission electron microscopy(TEM), emission characteristics can be directly correlated with the electrical properties and microstructure, possibly linked with the piezoelectric polarization and electrical junction property.Electrical responses with electron beam were mapped with microstructure and the point-to-point mapping of local emission characteristics using home-built TEM-EBIC/CL holder (Fig. 1). The holder was equipped with liquid nitrogen cooling finger to minimize the carrier diffusion. Spectrum collection and the manipulation of the spectrum was carried out using home-built software from the panchromatic and monochromatic spectrum. Ion-milling system, instead of using focused ion beam, was used to avoid the artifacts of unintentional noise coming from the surface recombination current. First, we measured I-V curve of 3 TEM samples of the polar, semi-, and non-polar specimen to acquire the built-in voltage, breakdown voltage, and leakage current. And then, we acquired STEM-EDS on each specimen to identify microstructure and thickness of active region. Second, EBIC signal was obtained from each points of target analysis area under reverse bias condition, which enables us to estimate the depletion region width and the polarization field across on the p-n junction. We also observed that the polarization filed was decreased from the sample grown on the c-plane to the one on the m-plane as expected. The minority carrier diffusion length showed dependency with polarization, which could arise from the space charge induced by polarization. CL spectra showed a tendency of blue-shift of emission in c-plane specimen as injection beam current increased, which might be originated from the bandgap fluctuation by the quantum-confined stark effect (QCSE) with strong polarization. CL spectra were obtained to confirm the polarization direction with the increase of reverse bias voltage. Peak shifts in CL might come from the cancellation of internal polarization with the external bias. Built-in voltage was also estimated with the polarization.[1] Fabio Bernardini et al., Phys. Rev. B 56, R10024 (1997)[2] Shigefusa F. Chichibu et al., Nature materials 5, 810 (2006)[3] Pan et al., APEX 5, 062103 (2012) Number 42.

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.

Investigation on Polarization Induced Electro-Optical Property of GaN LED Using TEM-EBIC Combined with Cathodoluminescence

Investigation on Polarization Induced Electro-Optical Property of GaN LED Using TEM-EBIC Combined with Cathodoluminescence

전류 차단 층을 갖는 LED의 향상된 광세기

Abstract Inserting a SiO 2 layer underneath the p-pad electrode as the current blocking layer (CBL) structure and extending p-metal finger patterns, the GaN LEDs using an indium–tin-oxide (ITO) layer show the improved light output intensity, resulting from better current spreading and reduced light loss on the surface of p-pad metal. The LEDs with an oxide layer of 100 μm-pad-width and 6 μm-finger-width have better light output intensities than those with an oxide layer of 105 μm-pad-width and 12 μm-finger-width. Using the ATLAS device simulator from Silvaco Corporation, the current density distributions on the active layer in CBL LEDs have been investigated. Key Words : GaN LED, Current Blocking Layer, SiO 2 Layer, Current Spreading, Simulation Received 2 June 2014, Revised 4 July 2014Accepted 20 July 2014Corresponding Author: Kee-Young Kwon (Kongju National University) Email: kky@kongju.ac.krⒸ The Society of Digital Policy & Management. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is ISSN: 1738-1916 properly cited.

The Emission Property of Phosphors LEDs Based on ZnCuInS/ZnSe/ZnS Quantum Dots

.In this research, ZnCuInS/ZnSe/ZnS quantum dots (QDs) have been studied as an excellent red emitting source for blue GaN LED because of its non-toxic deep red emmission, and large Stokes shift properties. In the paper ZnCuInS/ZnSe/ZnS core/shell quantum dots were prepared with the particle size of 4.5nm. According to the measurement of photoluminescence spectrum emitted by ZnCuInS/ZnSe/ZnS core/shell quantum dots, the emitting peak of 700 nm and the full was achieved as red emitter.It was found that absorption edge and photoluminescence peak shifted to shorter wavelength with decreasing the nanocrystal size due to quantum size effect.Meanwhile, we were prepared ZnCuInS/ZnSe/ZnS core/shell quantum dot light emitting diodes and their photoluminescence properties were studied. After the suitable bias was applied on the films, increasing the ZnCuInS/ZnSe/ZnS QDs concentration in the blue GaN chips, red emission increased with decreasing LED’s blue light.

Graphene interlayer for current spreading enhancement by engineering of barrier height in GaN-based light-emitting diodes.

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

Study of GaN LED ITO nano-gratings with standing wave analysis

Study of GaN LED ITO nano-gratings with standing wave analysis

A novel integrated structure of thin film GaN LED with ultra-low thermal resistance: erratum

A novel integrated structure of thin film GaN LED with ultra-low thermal resistance: erratum

(Invited) Improving the Performance of Quantum Dot Light-Emitting Diodes through Nanoscale Engineering

Semiconductor nanocrystal quantum dots (QDs) are extremely attractive fluorophores for use in lighting and display applications due to their narrow-band, highly efficient and widely tunable emission, and amenability toward low-cost processing and flexible substrates. As QDs begin to enter the display market as downconverters in back-lit displays, attention has increasingly turned to high-performance electrically pumped QD-based light-emitting diodes (QD-LEDs). Tremendous progress has been made since the first demonstration of QD-LEDs, as there have been numerous attempts to improve the device performance by optimizing both the materials synthesis and the device architecture.1 Even so, there is substantial room for improvement, particularly in terms of device stability, overall quantum efficiency, and in efficiency “roll-off”, in which efficiency falls at high drive currents. While device architectures can always be further optimized through sheer iteration/evolution, larger advances may require more fundamental insight. Indeed, accelerating the development of essentially all optoelectronic devices based on QDs is the increasingly deep understanding of the physics of quantum-confined nanomaterials and how they interact within devices. For instance, as the outcome of years of detailed study, the multicarrier phenomenon known as “carrier multiplication”2 has advanced beyond the laboratory to be observed in functional solar cells.3 However, even as the subtleties of QD-LED physics become clearer, the role of multicarrier effects including non-radiative Auger recombination, is still largely unclear. While Auger recombination has been established as an important factor in efficiency roll-off in GaN LEDs,4 its relevance to QD-LEDs remains the subject of ongoing controversy.Here we analyze the role of Auger recombination in QD-LEDs by conducting a systematic characterization of device performance in conjunction with studies of the dynamics of photoexcited carriers directly within the device structure.5 In our LEDs, we use a series of structurally engineered core/shell QDs that exhibit very similar single-exciton properties, but distinctly different rates of non-radiative Auger recombination.6 By correlating device performance with the multi-carrier photophysical properties of the QDs, we find that both LED efficiency and the onset for efficiency roll-off are strongly influenced by Auger recombination processes. We further show that device efficiency can be improved by use of either of two distinct types of specifically engineered heterostructured QDs, designed either to reduce the efficiency of Auger recombination, or to improve charge injection balance in the QD layer during operation. We demonstrate how both types of QD enhance peak emission efficiency, and significantly increases the threshold current of efficiency roll-off, and discuss how this insight can be applied to achieve future performance improvements.References (1) Bae, W. K.; Brovelli, S.; Klimov, V. I. MRS BULLETIN 2013, 38, 721.(2) Schaller, R. D.; Klimov, V. I. Physical Review Letters 2004, 92, 186601.(3) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Science 2011, 334, 1530.(4) Iveland, J.; Martinelli, L.; Peretti, J.; Speck, J. S.; Weisbuch, C. Physical Review Letters 2013, 110, 177406.(5) Bae, W. K.; Park, Y.-S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Nat Commun 2013, 4.(6) Bae, W. K.; Padilha, L. A.; Park, Y.-S.; McDaniel, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I. ACS Nano 2013, 7, 3411.

Recent Progress on Critical Cleaning of Sapphire Single-Crystal Substrates: A Mini-Review

Sapphire single-crystals are one of the industrial favorite substrates for GaN film deposition and LED application as well as a model system for oxides’ aqueous interfacial chemistry studies. Contamination control of sapphire substrates is critical to the device fabrication and experimental studies of interfacial properties. Here we reviewed various methods reported in the literature including conventional and new wet-chemical methods, and UV/plasma methods. Special attentions were paid to their cleaning performance in terms of reliably removing organic/particulate/heavy metallic

A novel integrated structure of thin film GaN LED with ultra-low thermal resistance.

This study proposes a novel packaging structure for vertical thin-GaN LED applications by integration of LED chip and silicon-based packaging process. The vertical thin film LED is directly mounted on package submount. The shortest thermal path structure from junction to package submount achieves the lowest thermal resistance of 1.65 K/W for LED package. Experimental results indicate that low thermal resistance significant improved forward current up to 4.6A with 1.125 × 1.125 mm² LED chip size.

Defect roles in the excitation of Eu ions in Eu:GaN

Eu ions in situ doped in GaN with V/III ratios varying from 3200 to 9600 have been investigated using resonant site-selective photoluminescence (PL), power dependent cathodoluminescence (CL), and a unique electron beam power dependent dual excitation experiment combining the techniques of PL and CL. The results of these experiments reveal the role of defects in the electronic excitation of Eu ions and the link between the GaN host and Eu ion dopants. The relative number of beneficial defects present in each sample for a majority Eu site (Eu1) and a specific secondary site (Eu2) are revealed. Also, a room temperature activated non-radiative recombination pathway linked to a specific, sample dependent Eu2 excitation pathway is identified. Unlike conventional GaN LEDs, Eu:GaN device performance does not rely completely on crystalline quality, but on the presence of specific excitation enhancing defects and the absence of non-radiative de-excitation channels.

High-voltage thin-film GaN LEDs fabricated on ceramic substrates: the alleviated droop effect at 670 W/cm^2

High-voltage thin-film GaN LEDs with the emission wavelength of 455 nm were fabricated on ceramic substrates (230 W/m · K). The high-voltage operation was achieved by three cascaded sub-LEDs with dielectric passivation and metal bridges conformally deposited on the side walls. Under the driving power of 670 W/cm(2), the high-voltage LEDs exhibit much alleviated efficiency droop and the operative temperature below 80 °C. The excellent performances were attributed to the improved current spreading within each sub-LED and the superior heat sinking of the ceramic substrate.

Optimization design of holographic photonic crystal for improved light extraction efficiency of GaN LED

A study on improved light extraction efficiency (LEE) of LEDs with photonic crystals formed by holographic lithography (HL) was present. The propagation and extraction of light in LEDs were simulated using FDTD method for LED structures with top HL PC and embedded HL PC respectively. By optimizing the design parameters of the HL PC, the best value of 88.3% enhancement of LEE were obtained. Result also revealed that the scattering and diffraction of photonic lattices play a more important role in the increased light extraction of HL PC LEDs than photonic band gap effects.