Performance Enhancement of GaN-Based Light-Emitting Diodes with Magnesium Nitride Inter-Layers.

High quality GaN epilayers were obtained by using a magnesium nitride (MgxNy) inter-layer. X-ray photoelectron spectroscopy (XPS) reveals Mg 2p core-level spectra from the MgxNy inter-layers. The roughness of the MgxNy layers increased with the growth time, though a prolonged processing time resulted in a decrease in the roughness. A high-resolution X-ray diffraction ω-scan rocking curve was used to reveal that the screw dislocation density (TDD) of GaN with an MgxNy inter-layer was reduced and the crystalline quality of the GaN epitaxial layer was enhanced. Furthermore, the luminous efficiency of an LED with the MgxNy layers was increased by 20% relative to a reference LED.

Impact of Al Pre-Deposition Layer on Crystalline Quality of GaN Grown on Si(111) Substrates.

The effects of Al metal pre-deposition under different conditions on GaN grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated. Al pre-deposition improves surface morphology and crystal quality of GaN grown on Si. The surface morphology of Al pre-deposition layer, AlN, and GaN vary depending on Al pre-deposition temperature. With the increase of Al pre-deposition temperature, Al cluster size is observed to increase in the Al predeposition layer due to increased lateral mobility of Al atoms. The Al pre-deposition carried out at about 750 °C enables to grow pit-free GaN layer on Si(111) substrate.

Enhanced optical output power of InGaN/GaN light-emitting diodes grown on a silicon (111) substrate with a nanoporous GaN layer.

We report the growth of InGaN/GaN multiple quantum wells blue light-emitting diodes (LEDs) on a silicon (111) substrate with an embedded nanoporous (NP) GaN layer. The NP GaN layer is fabricated by electrochemical etching of n-type GaN on the silicon substrate. The crystalline quality of crack-free GaN grown on the NP GaN layer is remarkably improved and the residual tensile stress is also decreased. The optical output power is increased by 120% at an injection current of 20 mA compared with that of conventional LEDs without a NP GaN layer. The large enhancement of optical output power is attributed to the reduction of threading dislocation, effective scattering of light in the LED, and the suppression of light propagation into the silicon substrate by the NP GaN layer.

Enhanced optical output performance in InGaN/GaN light-emitting diode embedded with SiO2nanoparticles.

We demonstrated the InGaN/GaN-based light-emitting diodes (LEDs) with SiO2 nanoparticles embedded in nanopillar GaN template. With the SiO2 nanoparticles placed between the GaN nanopillars, subsequent overgrowth of GaN layer started only on the exposed tips of the nanopillars and rapidly switched to the lateral growth mode. This resulted in a high quality GaN layer "sitting" on the nanopillars and the layer of pores formed over the SiO2 nanoparticles. For multi-quantum-well LEDs grown on top of such template, ~3 fold increase in optical output was observed compared to reference samples. The effect is attributed mainly to the improved light extraction efficiency due to additional scattering in the nanopillars-SiO2-pores portion of the structure, also to the increased internal quantum efficiency caused by a decreased dislocation density and relaxed strain due to the GaN nanopillars.

Dislocation density dependent electroabsorption in epitaxial lateral overgrown InGaN/GaN quantum structures.

We study electroabsorption (EA) behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger EA performance, with a maximum EA enhancement factor of 4.8 compared to the reference without ELOG. The analyses show that the EA performance follows similar trends with decreasing dislocation density as the essential parameters of the photoluminescence spectra (peak position, width and intensity) together with the photoluminescence lifetimes. While keeping the growth window widths constant, compared to photoluminescence behavior, however, EA surprisingly exhibits the largest performance variation, making EA the most sensitive to the mask stripe widths.

Enhanced luminous efficacy in phosphor-converted white vertical light-emitting diodes using low index layer.

We demonstrated improved luminous efficacy for GaN-based vertical light emitting diodes (VLEDs) employing a low index layer composed of silicon dioxide (SiO(2)) on the top surface. Three-dimensional ðnite-difference time-domain simulations for the fabricated VLED chip show that the penetration ratio of the emitted/reflected light into the VLED chip decreased by approximately 20% compared to a normal VLED chip. This result is in good agreement with an empirical study stating that white VLEDs having a SiO(2) layer exhibit an 8.1% higher luminous efficacy than white VLEDs with no layer at an injection current of 350 mA. Photons penetrating into the VLED chip, which become extinct in the VLED chip, are reflected from the SiO(2) layer due to the index contrast between the SiO(2) layer and epoxy resin containing phosphor, with no degradation of the light-extraction efficiency of the VLED chip. As such, this structure can contribute to the enhancement of the luminous efficacy of VLEDs.

Efficiency enhancement of white light-emitting diodes via nano-textured silicone encapsulant.

We textured the surface of a silicone encapsulant to increase the extraction efficiency of white light-emitting diodes (LEDs) by using a plasma treatment. Here, the surface morphology could be controlled by changing the plasma condition and texturing morphology of the silicone encapsulant were proportion to the increased ratio of white LEDs. The luminous efficacy of the surface textured LEDs were increased 9.70% relative to the reference LED. Furthermore, the Fourier transform infrared spectroscopy spectrum showed that the chemical bonds of the silicone encapsulant were not changed by the argon-nitrogen plasma treatment, thereby reducing degradation of the optical characteristics and improving the reliability of LEDs.

Structural and optical investigation of GaInP quantum dots according to the growth thickness for the 700 nm light emitters.

We investigate Ga0.33In0.67P quantum dot structures appropriate for special lighting applications in terms of structural and optical behaviors. The Ga0.33In0.67P materials form from 2-dimentional to 3-dimensional dots as the nominal growth thickness increases from 0.5 nm to 6.0 nm, indicating a Stranski-Krastanov growth mode. As the ambient temperature is increased to 300 K, the PL spectrum of the B-type dots is annihilated quickly because the large dot size induces a defect-related nonradiative recombination process. In contrast, the PL spectrum of the A-type dots is well maintained to 300 K. These data indicate that the Ga0.33In0.67P material is appropriate for an active layer of 700 nm light emitters.

Improved photoluminescence efficiency in UV nanopillar light emitting diode structures by recovery of dry etching damage.

In this study, we have fabricated 375-nm-wavelength InGaN/AlInGaN nanopillar light emitting diodes (LED) structures on c-plane sapphire. A uniform and highly vertical nanopillar structure was fabricated using self-organized Ni/SiO2 nano-size mask by dry etching method. To minimize the dry etching damage, the samples were subjected to high temperature annealing with subsequent chemical passivation in KOH solution. Prior to annealing and passivation the UV nanopillar LEDs showed the photoluminescence (PL) efficiency about 2.5 times higher than conventional UV LED structures which is attributed to better light extraction efficiency and possibly some improvement of internal quantum efficiency due to partially relieved strain. Annealing alone further increased the PL efficiency by about 4.5 times compared to the conventional UV LEDs, while KOH passivation led to the overall PL efficiency improvement by more than 7 times. Combined results of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) suggest that annealing decreases the number of lattice defects and relieves the strain in the surface region of the nanopillars whereas KOH treatment removes the surface oxide from nanopillar surface.

Enhanced light output of InGaN/GaN blue light emitting diodes with Ag nano-particles embedded in nano-needle layer.

2.7 times increase in room temperature photoluminescence (PL) intensity and 3.2 times increase in electroluminescence (EL) intensity were observed in blue multi-quantum-well (MQW) GaN/InGaN light emitting diodes (LEDs) as a result of introduction of nano-needle structure embedded with Ag nanoparticles (NPs) into n-GaN film underlying the active MQW region and thick p-GaN contact layer of LEDs. The nano-needle structure was produced by photoelectrochemical etching. Simultaneously a measurable decrease in room temperature decay time from 2.2 ns in control samples to 1.6 ns in PL was observed. The results are explained by strong coupling of recombination in GaN/InGaN MQWs with Ag NPs related localized surface plasmons.

Localized surface plasmon enhanced quantum efficiency of InGaN/GaN quantum wells by Ag/SiO2 nanoparticles.

Optical properties of InGaN/GaN multi-quantum-well (MQW) structures with a nanolayer of Ag/SiO2 nanoparticle (NP) on top were studied. Modeling and optical absorption (OA) measurements prove that the NPs form localized surface plasmons (LSP) structure with a broad OA band peaked near 440-460 nm and the fringe electric field extending down to about 10 nm into the GaN layer. The presence of this NP LSP electrical field increases the photoluminescence (PL) intensity of the MQW structure by about 70% and markedly decreases the time-resolved PL (TRPL) relaxation time due to the strong coupling of MQW emission to the LSP mode.

Opposite carrier dynamics and optical absorption characteristics under external electric field in nonpolar vs. polar InGaN/GaN based quantum heterostructures.

We report on the electric field dependent carrier dynamics and optical absorption in nonpolar a-plane GaN-based quantum heterostructures grown on r-plane sapphire, which are surprisingly observed to be opposite to those polar ones of the same materials system and similar structure grown on c-plane. Confirmed by their time-resolved photoluminescence measurements and numerical analyses, we show that carrier lifetimes increase with increasing external electric field in nonpolar InGaN/GaN heterostructure epitaxy, whereas exactly the opposite occurs for the polar epitaxy. Moreover, we observe blue-shifting absorption spectra with increasing external electric field as a result of reversed quantum confined Stark effect in these polar structures, while we observe red-shifting absorption spectra with increasing external electric field because of standard quantum confined Stark effect in the nonpolar structures. We explain these opposite behaviors of external electric field dependence with the changing overlap of electron and hole wavefunctions in the context of Fermi's golden rule.

Optimization of the number of quantum well pairs for high-brightness AlGaLnP-based light emitting diodes.

We investigated high-brightness light emitting diodes (LEDs) appropriate for general lighting applications in terms of their temperature dependent photoluminescence characteristics and device performance according to the change of quantum well pairs (QWs). As the number of QWs was increased from 2 to 35 pairs, radiative recombination efficiency and device performances significantly improved, due to the suppression of carrier overflow by decreasing the carrier density in the active region and shortening the carrier transfer time from barrier to well. At a further increase in the number of QWs to 50 pairs, however, the optical and device performances started to degrade because of the increase in internal loss in the active region, such as the well volume itself acting as light absorbing layer and due to the aluminum oxide complexes in the barrier.

High performance of GaN-based flip-chip light-emitting diodes with hole-shape patterned ITO ohmic contact layer and AgIn reflector.

The enhancement of light extraction efficiency is observed when the hole-shape patterned ITO ohmic contact layer and AgIn reflector is adopted in GaN-based flip-chip (FC) light emitting diodes (LEDs). ITO layer (140 nm) and AgIn (200 nm) was deposited on the top of p-GaN by in-line DC sputtering and electron beam evaporating system, respectively. The ITO ohmic contact layer showed a low specific contact resistance of 2.66 x 10(-5) Omega cm(-2) and high transmittance of >85% at visible spectral regions. The AgIn reflector exhibited a low specific contact resistance of 1.90 x 10(-5) Omega cm(-2) and high reflectance of approximately 84% at visible spectral regions. Comparing with unpatterned ITO/AgIn layer, the optical output power of GaN-based FC LEDs improves approximately 30% by the adoption of micro size hole-shape patterned ITO ohmic contact layer and AgIn reflector.

Nanopatterned aluminum nitride template for high efficiency light-emitting diodes.

Nanopatterned aluminum nitride (NP-AlN) templates were used to enhance the light extraction efficiency of the light-emitting diodes (LEDs). Here, the NP-AlN interlayer between the sapphire substrate and GaN-based LED was used as an effective light outcoupling layer at the direction of bottom side and as a buffer layer for growth of GaN LEDs. The cross-sectional transmission electron microscopy (TEM) analysis showed that the formation of stacking faults and voids could help reduce the threading dislocations. Micro Raman spectra also revealed that the GaN-based epilayer grown on the NP-AlN template had smaller residual stress than that grown on a planar sapphire substrate. The normalized electroluminescence (EL) spectra at the top and bottom sides of device revealed that the enhancement of the bottom side emission of the LED with the NP-AlN interlayer was more notable than a planar sapphire substrate due to the graded-refractive-index (GRIN) effect of the NP-AlN.

Insertion of two-dimensional photonic crystal pattern on p-GaN layer of GaN-based light-emitting diodes using bi-layer nanoimprint lithography.

Nanoimprint lithography (NIL) was adapted to fabricate two-dimensional (2-D) photonic crystal (PC) pattern on the p-GaN layer of InGaN/GaN multi quantum well light-emitting diodes (LEDs) structure to improve the light extraction efficiency. For the uniform transfer of the PC pattern, a bi-layer imprinting method with liquid phase resin was used. The p-GaN layer was patterned with a periodic array of holes by an inductively coupled plasma etching process, based on SiCl4/Ar plasmas. As a result, 2-D photonic crystal patterns with 144 nm, 200 nm and 347 nm diameter holes were uniformly formed on the p-GaN layer and the photoluminescence (PL) intensity of each patterned LED samples was increased by more than 2.6 times, as compared to that of the un-patterned LED sample.

Investigating carrier localization and transfer in InGaN/GaN quantum wells with V-pits using near-field scanning optical microscopy and correlation analysis.

The V-pits and potential fluctuations in InGaN/GaN multiple quantum wells (MQWs) are key factors for understanding the performance of InGaN/GaN-based light-emitting diodes (LEDs). However, photoluminescence (PL) measurements using conventional optical microscopy only provide ensemble information due to the spatial resolution limit, known as the diffraction barrier, which hinders the analysis of dislocations and potential fluctuations. Here, in order to investigate the influence of the V-pits and potential fluctuations on local optical properties, we performed nanoscopic luminescence mapping for standard and V-pit InGaN/GaN MQWs samples with different sized V-pits using near-field scanning optical microscopy (NSOM) with illumination mode (I-mode) at various laser excitation powers. From the nanoscopic PL mapping data, we could clearly observe luminescence features associated with dislocations and potential fluctuations in the InGaN/GaN MQWs. We also employed correlation analysis to quantitatively analyze the nanoscopic PL mapping data for the different MQWs samples. Based on the results of NSOM PL with I-mode and correlation analysis, we could demonstrate that carrier transfer in the MQWs sample with large sized V-pits is suppressed by deeper potential fluctuations and higher energy barriers compared to the standard sample.

Investigation of Trapezoidal Well for Improving the Light Efficiency in AlGaInP-Based Light-Emitting Diodes.

We investigated high-brightness light emitting diodes appropriate for general lighting applications in terms of their optical behaviors and device performances according to the insertion of the sloped barrier between the well and the barrier and changing the sloped barrier thickness. As the sloped barrier thickness was increased from 0 to 5 nm, radiative recombination efficiency and device performances significantly improved, due to the suppression of carrier overflow by the stronger capture of carriers and the shortening of the carrier lifetime in the active region owing to the built-in quasi-electric field. At a further increase in the sloped barrier thickness to 10 nm, however, the optical and device performances started to degrade because of the loosening of the quantum confinement effect in the active region and due to the saturation of the improvement of the carrier capture by the sloped barrier region.

Air-Hybrid Distributed Bragg Reflector Structure for Improving the Light Output Power in AlGalnP-Based LEDs.

We investigated air gap-induced hybrid distributed Bragg reflectors (AH-DBRs) for use in high brightness and reliable AlGalnP-based light emitting diodes (LEDs). An air gap was inserted into the side of DBRs by selectively etching the Al(x),Ga1-xAs DBR structures. With the AH-DBR structures, the optical output power of LEDs was enhanced by 15% compared to LEDs having conventional DBRs, due to the effective reflection of obliquely incident light by the air gap structures. In addition, the electrical characteristics showed that the AH-DBR LED is a desirable structure for reducing the leakage current, as it suppresses unwanted surface recombinations.

Vertical-Injection AlGaInP LEDs with n-AlGaInP Nanopillars Fabricated by Self-Assembled ITO-Based Nanodots.

The light output power of AlGaInP-based vertical-injection light-emitting diodes (VI-LEDs) can be enhanced significantly using n-AlGaInP nanopillars. n-AlGaInP nanopillars, ~200 nm in diameter, were produced using SiO2 nanopillars as an etching mask, which were fabricated from self-assembled tin-doped indium oxide (ITO)-based nanodots formed by the wet etching of as-deposited ITO films. The AlGaInP-based VI-LEDs with the n-AlGaInP nanopillars provided 25 % light output power enhancement compared to VI-LEDs with a surface-roughened n-AlGaInP because of the reduced total internal reflection by the nanopillars at the n-AlGaInP/air interface with a large refractive index difference of 1.9.