The light output power (LOP) of vertical-type GaN-based light emitting diodes (LED) with surface roughness (texture) can be changed by texture size, density, and thickness of GaN film or by the combined effects of texture formation and thickness of GaN film. We have investigated these changes experimentally and note that the enhancement of the LOP by a factor of 2.4 can be improved with optimum texturing parameters as compared to that without texturing. In addition, the LOP of GaN-based LEDs under the same texture density increase slightly as thickness of GaN film decreases. Base on these results, we have evidently demonstrated that the enhancement factors of LOP are related to the correlation between texture size (density) and thickness of GaN film.

This study examines the influence of the defect properties on internal quantum efficiency (IQE) in blue InGaN-based light emitting diodes (LEDs). The defect parameter is introduced for estimating defect properties among defect density, size, and defect type that are strongly correlated with IQE in InGaN LED. The value of IQE can be expressed by the value of the defect parameter, which was obtained from transmission electron microscopy and cathodoluminescence measurement.

Optical properties of strain-compensated InGaN/InGaN quantum well (QW) structures using a InGaN substrate are investigated using the multiband effective mass theory. These results are compared with those of conventional InGaN/GaN QW structures using a GaN substrate. The strain-compensated QW structure shows that a smaller well thickness is needed to obtain the transition wavelength of 530 nm than the InGaN/GaN QW structure. The spontaneous emission peak of a strain-compensated QW structure is shown to be much larger than that of a conventional QW structure. This is mainly attributed to the fact that the internal field is reduced due to the decrease in the lattice mismatch with a substrate. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optical properties of graded InGaN/GaN quantum well (QW) lasers are analyzed as improved gain media for laser diodes emitting near 500 nm. These results are compared with those of conventional InGaN/GaN QW structures. The heavy-hole effective mass around the topmost valence band is found to nearly not be affected by the inclusion of the graded layer. The graded InGaN/GaN QW structure shows a much larger matrix element than the conventional InGaN/GaN QW structure. The radiative current density dependences of the optical gain are similar to each other. However, the graded QW structure is expected to have lower threshold current density than the conventional QW structure because the former has a lower threshold carrier density than the latter.

The effect of selective area growth (SAG) on wafer bowing of GaN-based light-emitting diodes (LEDs) is investigated. The SAG of LED structures was carried out on a silicon dioxide (SiO2) mask pattern with periodic 1000×1000μm openings, along the sapphire 〈1−100〉 and 〈−1−120〉 directions. The morphology of a selectively-grown n-GaN epilayer was examined in relation to various growth parameters such as temperature, pressure, and V/III ratio. Under optimized growth conditions, formation of a ridge-shaped epilayer with a v-pit free smooth surface was realized. Furthermore, the ridge-shaped vertical LED structure, after the removal of the sapphire substrate by laser lift-off (LLO) showed less wafer bowing compared with conventional vertical LED structures. This is attributed to the suppression of lateral strain and dislocations during the site-selective growth process, due to a reduction in the lateral dimensions.

In this paper we present the results of experiments and simulations for the light output power from LEDs for various refractive indices and the geometrical structures of the LED encapsulants. InGaN-based LED chips were fabricated and were bonded in Ag reflector cups within polyphthalamide (PPA) chip carriers; then, encapsulants with various refractive indices and the geometrical structures were fabricated onto them by using a dispensing method. The light output power with the encapsulant was shown to increase with the refractive index of the encapsulant materials in the case of a spherical encapsulant while it decreased in the case of a fiat geometry encapsulant. We performed ray tracing simulations for the LED light output and confirmed that the simulation results were consistent with our experimentally measured results. In addition, the light output with the encapsulant rapidly increased with the sidewall angle of the chip carrier in the case of the flat encapsulant while it was not affected by the sidewall angle, remaining constant, in the case of the spherical geometry.

In this paper we report on the selective area growth (SAG) of vertical GaN-based light-emitting diodes (LEDs) by low-pressure metal-organic chemical vapor deposition (MOCVD). SAG, under optimized growth conditions, leads to ridge-shaped epilayers with a smooth top surface, devoid of any surface defect structures. The final ridge-shaped vertical LED structures, after the removal of the sapphire substrate by laser lift-off (LLO), exhibit a smaller bowing effect than conventional vertical LED structures. The suppression of lateral strain in the epilayers is responsible for the smaller bowing effect because of the reduction in lateral dimensions. Consequently, the use of SAG LEDs can achieve a 21% higher light output power than conventional vertical LEDs, indicating a significant improvement in light extraction efficiency due to the light guiding pathways offered by the ridge-shaped geometry of the LED structures.

This paper reports the degradation characteristics of blue GaN-LED (Gallium Nitride-light emitting diode) chip related to various packages. Three types of package have been considered to investigate degradation characteristics in this study. One package is our proposed new concept of metal package by which we can estimate the lifetime of blue LED chip (chip size: 1mm x 1mm) with high accuracy. In the proposed package, components in conventional LED package which may obscure the degradation behaviour of LED chip itself were removed or replaced by other materials or components. And as a control group, two types of commercial packages were used (3W package and low power package). We investigated and analyzed the light-output degradation and changing of package materials of high-power blue LED chip packaged with these three types of package to aging time of 5,000 hours.[GRAPHICS](C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A vertical light-emitting diode (LED) with a chip size of 500 x 500 mu m(2) was fabricated by the laser lift-off (LLO) process of an InGaN-based blue LED wafer. After the LLO process, photonic crystal patterns by UV nano-imprint lithography were formed on the n-GaN top layer of the vertical LED over the entire area with a diameter of 2 inches. As the result of n-GaN patterning, light output power of the vertical LED with photonic crystals was increased by up to 44% compared to that of the vertical LED without a photonic crystal at a driving current of 1000 mA.

We demostrate the efficient p-type reflector for high performance vertical InGaN/GaN light emitting diodes (LEDs) with 1x1mm(2) chip size. The reflector consists of Indium-Tin-Oxide (ITO) and Ag-Pd-Cu (APC) alloy. The ITO was inserted between p-GaN layer and APC alloy using RF magnetron sputtering to prevent inter-diffusion of APC into GaN layer. Transmission electron microscopy (TEM) result shows that ITO plays an impotant role as a diffusion barrier to APC alloy. In addition, the contact resistivity of ITO to p-GaN layer was measured to be 1.32x10(-3) Omega cm(2) at annealing temperature of 600 degrees C for 1 minute. APC alloy was adpoted to acheive a higher reflectance for improvement of a light extraction efficiency. The APC alloy reflector appeared to have a higher reflectivity compared to conventional Ni/Ag film reflector. The verical LEDs with ITO/APC alloy reflectors showed the light-output power of 295 mW at an injection current of 350 mA, which is 15% higher than that with Ni/Ag reflectors. The output power enhancement is attributed to the increase of light extraction efficiency due to high reflectivity of APC alloy. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electronic and optical properties of strain-compensated InGaN/InGaN/MgZnO quantum well (QW) structures using a MgZnO substrate are investigated using the multiband effective mass theory. A strain-compensated InGaN/InGaN/MgZnO QW structure with a larger strain shows larger matrix element than that with a smaller strain. The spontaneous emission peak rapidly increases with increasing compressive strain because the matrix element is enhanced for the strain-compensated QW structure with a larger strain. In addition, we find that the strain-compensated QW structure with the larger Mg composition in the substrate has greater spontaneous emission peak than the strain-compensated QW structure with the smaller Mg composition in the substrate.

For the case of the white LED (phosphor converted), the temperature affects phosphor in addition to chip and package. The heat can cause the decrease of the phosphor performance as well as the irreversible damage to the phosphor. In this study, the effect of phosphor thermal configuration on the degradation rate has been investigated experimentally and verified by the theoretical simulations. Two different configurations have been tested. One is die-contact phosphor layer and the other is remote case. The experimental result shows that the LEDs of the die-contact phosphor layers exhibit lower degradation rate than those of the remote phosphor layers. The thermal simulation considering the heat generation from the phosphor (energy converting loss) shows similar results. The temperature of remote phosphor layer is higher than the die-contact case. High temperature causes rapid degradation of phosphor. As the power consumption of LEDs increases, the heat generation from the phosphor becomes no more negligible. For the higher performance and longer lifetime, we have to consider phosphor layer as the heat generator and design the layer with sufficient thermal paths. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We have proposed a new concept of metal package by which we can estimate the lifetime of blue light-emitting diode (LED) chips with high accuracy. Components in conventional LED package which may obscure the degradation behavior of LED chip itself were removed or replaced by other materials or components. Three kinds of chips from different manufacturers were analyzed in this study using proposed metal packages. In this paper, the optical and electrical characteristics such as light-output degradation and reverse leakage current of high-power blue LED chip were investigated and analyzed. Also, the relationship between light-output degradation and electrical characteristics of LED chip was described. With aging time of 5000 h, only one kind of blue LED chip shows enough light-output degradation to estimate life-time.

High-power white light-emitting diodes (LEDs) are fabricated by combining blue LEDs and green (Ba,Sr)(2)SiO(4):Eu(2+) and red CaAlSiN(3):Eu(2+) phosphors with varying phosphor geometry. The white LED having separated the phosphor layer by the silicone gel layer between green and red phosphor layers shows superior optical properties. The luminous efficiency (eta(L)) is improved by a decrease of reabsorption of green light by red phosphor owing to a difference among refractive indices. The white LED shows very high eta(L) of 51 lm/W and a high color rendering index of 95 under 350 mA. In addition, improved luminous properties of the white LED including a separated phosphor layer are confirmed via ray-trace simulation.

A detailed study of photoluminescence (PL) of GaN(1 nm)/Al0.2Ga0.8N(3.3 nm) twenty periods superlattice grown via metal-organic chemical vapor deposition is presented. The dependence of the PL emission energy, linewidth, and intensity on temperature, in the low temperature regime, is consistent with recombination mechanisms involving bandtail states attributed to a small degree of interfacial disorder. The activation energy of the nonradiative centers in our superlattice agrees well with the value we derive for the width of the tail-state distribution. Moreover, we find that the average phonon energy of the phonons that control the interband PL energy at high temperatures is larger for the superlattice than for a high-quality GaN film. This observation is consistent with model calculations predicting the phonon mode properties of GaN–AlN-based wurtzite heterostructures.