Yik-Khoon Ee, Ronald A. Arif, Nelson Tansu, Pisist Kumnorkaew, and James F. Gilchrist Citation: Applied Physics Letters 91, 221107 (2007); doi: 10.1063/1.2816891 View online: http://dx.doi.org/10.1063/1.2816891 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/22?ver=pdfcov Published by the AIP Publishing

Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using Si O 2 /polystyrene microlens arrays

In this study, nanoparticles (NPs) of various types and sizes are arranged to enhance both the omnidirectional light harvesting of solar cells and the light extraction of light emitting diodes (LEDs). A graded-refractive-index NP stack can minimize reflectance, not only over a broad range of wavelengths but also at different incident angles; the photocurrent of silicon-based solar cells an also be significantly improved omnidirectionally. In addition, the optical gradient of an NP stack can also enhance the light-extraction efficiency of LEDs, due to both the graded refractive index and the moderate surface roughness. Large particles having sizes on the same order of the wavelength of the incident light roughen the LED surfaces further and extract light from beyond the critical angle, as supported by three-dimensional finite-difference time-domain simulations. Using this approach, the photoluminescence intensity can be increased by up to sevenfold. The developed technique: arranging sequences of different NPs in graded-refractive-index stacks, and considering their ability to scatter light due to their sizes and optical constants, may also significantly improve the performance of various optoelectronic devices.

Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light‐Emitting Diodes

In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.

GaN-based light-emitting diodes with photonic crystals structures fabricated by porous anodic alumina template

The current-voltage characteristics of n-type GaN Schottky diodes have been measured in the extrinsic region (that is, the temperature range of 100–300 K). The effective density of states in the conduction band decreases with decreasing temperature and is close to the electron concentration at 100 K, leading to a reduction in the energy difference between the conduction band minimum and Fermi level and an increase in the probability of tunneling. Therefore, changes in the effective density of states in the conduction band and the probability of tunneling at low temperature are responsible for the decrease of the barrier height and increase in the ideality factor on the basis of the thermionic emission model. The mechanism of forward current flow has been clearly established in this study, leading to the thermionic emission (thermionic field emission) interpretation of the I-V characteristics at 300 K (100 K).

Electronic transport and Schottky barrier heights of Pt/n-type GaN Schottky diodes in the extrinsic region

Photonic crystal patterns on the indium tin oxide layer of an InGaN/GaN light-emitting diode are fabricated via nanosphere lithography in combination with dry etching. The silica spheres acting as an etch mask are self-assembled into a hexagonal closed-packed monolayer array. After etching, the photonic crystal (PhC) pattern is formed across the indium-tin-oxide (ITO) films so that the semiconductor layers are left intact and thus free of etch damages. Despite slight degradation to the electrical properties, the ITO-PhC light-emitting diodes (LEDs) exhibit enhancements of their optical emission power by as much as 64% over an as-grown LED. The optical performances and mechanisms of the photonic crystal LEDs are investigated with the aid of rigorous coupled wave analysis and finite-difference time-domain simulations.

/pdf/ingan-light-emitting-diodes-with-indium-tin-oxide-photonic-3su9720cuj.pdf

InGaN light-emitting diodes with indium-tin-oxide photonic crystal current-spreading layer

Surface-textured InGaN/GaN light-emitting diodes (LEDs) coated with transparent Al2O3 powder were fabricated by natural lithography combined with inductively coupled plasma etching. For surface texturing, 300 nm size Al2O3 powder is used as an etching mask by simply coating the surface using a spin-coating process. Also, the powders are left on the surface after surface texturing to further increase extraction efficiency. At 20 mA, the light output power of the textured indium tin oxide (ITO) InGaN/GaN LEDs coated with the Al2O3 powder is enhanced by ∼112% compared with the conventional nontextured ITO LED. The enhanced light output power is attributed to the improved extraction efficiency resulting from an overall decrease in the total internal reflection due to the textured surface and the Al2O3 powder coating.

GaN-based light-emitting diode with textured indium tin oxide transparent layer coated with Al2O3 powder

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GaN-based light-emitting diode with textured indium tin oxide transparent layer coated with Al2O3 powder