Simulation and fabrication of highly efficient InGaN‐based LEDs with corrugated interface substrate

Cho, Jaehee; Kim, Hyunsoo; Kim, Hyungkun; Lee, Jeong Wook; Yoon, S. D.; Sone, Cheolsoo; Park, Yongjo; Yoon, Euijoon

doi:10.1002/pssc.200461337

Cited by 50 publications

(34 citation statements)

References 5 publications

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“…Much effort has been devoted to improve the brightness and efficiency of white LEDs by developing phosphors, which convert the source light into the desired emission. [25] It is expected that there are many advantages of using NCs in blue or UV LEDs instead of conventional rare-earth-based phosphors because the NCs have a high QE, broad absorption spectrum, and good color saturation. As a primary result to show the potential applicability of NCs as color-converting materials on LEDs, Bawendi's group disclosed a photograph of an NC-polymer down-conversion light-emitting device, emitting at 590 nm.…”

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Preparation of Highly Luminescent Nanocrystals and Their Application to Light‐Emitting Diodes

et al. 2007

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Preparation of Highly Luminescent Nanocrystals and Their Application to Light‐Emitting Diodes

et al. 2007

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“…Majority of the photons are likely to be guided through the air/GaN/sapphire waveguide structure, either to emanate in less useful directions or to be reabsorbed by the multiple-quantum-well (MQW) active layer. To overcome this intrinsic limitation, several approaches have been proposed and demonstrated, including surface-roughening [1], GaN growth on a patterned sapphire substrate [2,3], and integration of twodimensional photonic crystal patterns [4]. The idea of these approaches is to interfere the TIR phenomenon by integrating micro-or nano-structures in one of the GaN interfaces.…”

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Fabrication of reflective GaN mesa sidewalls for the application to high extraction efficiency LEDs

Lee

Kim

et al. 2007

Phys. Status Solidi (c)

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We proposed and demonstrated a modified GaN light-emitting diode (LED) structure. The structure was made by integrating angled sidewalls to an otherwise conventional LED structure. Various GaN sidewall angles were obtained by adjusting etch conditions and the etched surface roughness was ~ 2 nm. Nearfield emission patterns indicated that the angled sidewalls efficiently deflect photons that are initially guided laterally within the GaN epilayer into the off-surface direction. For sidewall angle of 30°, total surface emission strength was improved by a factor exceeding 3.1 Introduction GaN-based light-emitting diodes (LEDs), which emits near UV to infrared light, have been extensively researched and developed. As a result of the development, their application areas and demands are still growing. As internal quantum efficiency in the device are quickly approaching to one, however, one of the challenging issues en route to high-brightness and high-efficiency GaN-LEDs has been identified as how to extract more photons out of device that are otherwise trapped inside the chip. Total internal reflection (TIR) is mainly responsible for the photon trapping. This permit only a small fraction of the photons generated inside a GaN LED device escaping to the outer medium. A rough estimation for the GaN/ air single interface leads to an extraction efficiency of about 4%. Majority of the photons are likely to be guided through the air/GaN/sapphire waveguide structure, either to emanate in less useful directions or to be reabsorbed by the multiple-quantum-well (MQW) active layer. To overcome this intrinsic limitation, several approaches have been proposed and demonstrated, including surface-roughening [1], GaN growth on a patterned sapphire substrate [2,3], and integration of twodimensional photonic crystal patterns [4]. The idea of these approaches is to interfere the TIR phenomenon by integrating micro-or nano-structures in one of the GaN interfaces.We propose and demonstrate here a LED structure that utilizes the TIR phenomenon in positive manner. The underlying idea of the proposed GaN LED structure is redirecting the photons guided within the GaN epilayer at once by deflection. This structure is obtained by involving a modification to the LED mesa sidewalls; smooth mesa sidewalls are prepared at an oblique angle, all the way through the entire GaN epilayer. A schematic diagram of the LED structure is illustrated in Fig. 1(a). After its physical shape, we call it hereafter 'Sidewall-Deflector-Integrated (SDI)' LED structure. Upon encountering the slanted mesa sidewalls, the guided photons get deflected and emerge through the sapphire substrate. Figure 1(b) shows SEM images of a produced SDI-LED having its angled mesa sidewall.The sidewall profile effect on light extraction enhancement was discussed in other papers [5,6]. The results had some limits in practical use which were complexity in fabrication or low enhancement. But

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“…Figure 1 shows the scanning electron microscopy (SEM) image of the fabricated HPS. The fabrication process of the HPS is as follows [6,7]. After a photoresist (PR) of 3.5 µm thickness was coated on a c-plane sapphire substrate, the PR pattern was reflowed during a hard baking process at 140 °C to make hemispheric shape.…”

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Improvement of luminous intensity of InGaN light emitting diodes grown on hemispherical patterned sapphire

Lee

Park

et al. 2006

Phys. Status Solidi (c)

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To improve the external quantum efficiency, high quality GaN film was grown on hemispherical patterned sapphire by controlling the V/III ratio during the initial growth stage. The luminous intensity of white flash light emitting diode (LED) grown on hemispherical patterned sapphire (HPS) was estinated to be 5.8 cd at a forward current of 150 mA, which is improved by 20 % more than that of LED grown on conventional sapphire substrate. The improvement of luminous intensity was explained by considering not only an increase of the extraction efficiency via the suppressed total internal reflection at the corrugated interface but also a decrease of dislocation density. 1 Introduction A great deal of research has been devoted to develop prominent solid-state light emitting diodes (LEDs) which are a prerequisite for practical applications in the area of liquid crystal display (LCD) back light, full color display, traffic display, and general illumination [1]. The white light LEDs are the most promising solid-state lighting method to replace the conventional incandescent and fluorescent lamps. It is well known that high density threading dislocations are inherent in the epitaxial GaN films on sapphire substrates due to the large difference in the lattice constant between the epitaxial layer and sapphire substrate. Therefore, how to further reduce the dislocation density is an important issue for fabricating high performance LEDs. Moreover the reflective index of nitride films is higher than that of air and the sapphire substrate. Most of the generated lights in the active layer is absorbed by the electrode at each reflection and gradually disappear due to total internal reflection (TIL). It has been reported that one can reduce the TIL by forming ordered micro-scale hexagonal or rectangular shaped pattern on sapphire substrate which effectively scatters the emission light from the active layer [2,3]. However, growth of high quality GaN film on patterned sapphire substrate is more difficult than that of conventional sapphire substrate (CSS). In order to improve the quality of GaN epitaxial layer, various growth techniques have been devised by adjusting growth parameters, such as III/V ratio, growth pressure, growth temperature, and annealing time [4,5].In this work, we proposed and investigated the metal organic chemical vapour deposition (MOCVD) growth of undoped GaN flims on a hemispherical patterned sapphire (HPS) substrate by controlling the V/III ratio during the initial growth stage. The fabricated white flash LEDs grown on HPS showed an improved luminous intensity and injection current due to both the reduction of the dislocation density and increase of the external efficiency compare to the samples grown on CSS.

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Simulation and fabrication of highly efficient InGaN‐based LEDs with corrugated interface substrate

Cited by 50 publications

References 5 publications

Preparation of Highly Luminescent Nanocrystals and Their Application to Light‐Emitting Diodes

Preparation of Highly Luminescent Nanocrystals and Their Application to Light‐Emitting Diodes

Fabrication of reflective GaN mesa sidewalls for the application to high extraction efficiency LEDs

Improvement of luminous intensity of InGaN light emitting diodes grown on hemispherical patterned sapphire

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