Hee Yun Kim

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

Effect of periodic deflector embedded in InGaN∕GaN light emitting diode

This letter proposes a concept of InGaN∕GaN light emitting diodes with periodic deflector embedded structure (PDE-LED). The PDE-LED was grown on a sapphire substrate with SiO2 hexagonal patterned mask using selective metal-organic chemical deposition. More than 200 artificial inverted polygonal pyramids (AIPPs), which included six R planes and six N planes deflectors with inclined angles of 57° and 61°, respectively, were formed and periodically distributed on masked area. These AIPP deflectors revealed a superior capability of enhancing light extraction efficiency mainly because the AIPP deflector structure could provide multiple chances for photons to escape from the LED sidewall as opposed to a rectangular conventional LED. Thus, the light output power of the PDE-LED was 1.51 times higher than that of a conventional LED at an injection current of 20mA, while forward bias voltage and leakage current were compatible to those of conventional LEDs.

Comparison of various surface textured layer in InGaN LEDs for high light extraction efficiency

The various surface texturing effects of InGaN light emitting diodes (LEDs) have been investigated by comparison of experimented data and simulated data. The single-layer and double-layer texturing were performed with the help of ITO nanospheres using wet etching, where the ITO ohmic contact layer and the p-GaN layer are textured using ITO nanospheres as an etch mask. In case of single-layer texturing, p-type GaN layer texturing was more effective than ITO ohmic contact layer texturing. The maximum enhancement of wall-plug efficiency of double-layered textured LEDs is 40% more than conventional LEDs, after packaging at an injected current of 20 mA. The increase of light scattering at the textured GaN surfaces is a major reason for increasing the light extraction efficiency of LEDs.

Impact of two-floor air prism arrays as an embedded reflector for enhancing the output power of InGaN/GaN light emitting diodes

This paper reports on the impact of two-floor air prism (TFAP) arrays InGaN/GaN light emitting diodes (LEDs) as an embedded reflector with low refractive indices. The reflectance spectra, measured over the entire visible spectral region, shows strong reflectance modulations due to the TFAP arrays in GaN. The light output powers of TFAP LEDs is seen to be 2.62 times higher than that of conventional LEDs, and 1.55 times higher than that of LEDs with one floor air prism arrays, respectively, at injection currents of 100 mA. This significant enhancement is attributable to a combined effect of effective shaping with 62° angled sidewalls and high refractive index difference between the embedded air prism and the GaN.

Improvement of Light Output Power in InGaN/GaN Light-Emitting Diodes with a Nanotextured GaN Surface Using Indium Tin Oxide Nanospheres

An etching process and indium tin oxide (ITO) nanospheres were used to fabricate nanotexturing on a GaN surface in InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) to improve the light output power. ITO nanospheres can be easily obtained by a simple wet etching process owing to the relatively weak binding energy at the grain boundary and at the interface between the ITO layer and the GaN surface. Inductively coupled plasma (ICP) treatment is carried out with the help of ITO nanospheres, used as a patterning mask, to nanotexture the GaN surface. A simple process is proposed for GaN-texturing on all possible sides, including the sidewall. The light extraction efficiency of InGaN/GaN LEDs is significantly improved owing to the possibility of a high level of light scattering at the textured surface. Thus, the light output power of the nanotextured LEDs was increased by 33% compared with that of conventional LEDs at an injection current of 20 mA.

Improved GaN-Based LED Light Extraction Efficiencies via Selective MOCVD Using Peripheral Microhole Arrays

GaN-based light-emitting diodes with peripheral microhole arrays (PMA-LEDs) have been grown, and fabricated, on SiO2 hexagonal pattern masks using selective metal-organic chemical vapor deposition. The PMA-LED structure promises to enhance the light extraction efficiency via simple fabrication processes, as compared to conventional LED structures. The geometrical shape of the peripheral microhole structure serves to enhance the light extraction efficiency due to the effect of the PMA in facilitating multiple chances for photons to escape. Thus, the light output intensity of PMA-LED was 30% higher than that of conventional LEDs.

Effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes

We report on the effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes (LEDs). The silica nanospehres were coated on the selectively etched GaN using a spin-coating method. With the embedded silica nanospheres structures, we achieved a smaller reverse leakage current due to the selective defect blocking-induced crystal quality improvement. Moreover, the reflectance spectra show strong reflectance modulations due to the different refractive indices between the GaN and silica nanospheres. By using confocal scanning electroluminescence microscopy, a strong light emission from silica nanospheres demonstrates that the silica nanospheres acted as a reflector. We found that the optimized embedded silica nanospheres structure, whose the average size of the etched pits was about 3.5 μm and EPD was 3 x 10(7) cm(-2), could enhance light output power by a factor of 2.23 due to enhanced the probability of light scattering at silica nanospheres.

Enhanced air-cavity effect of periodically oriented embedded air protrusions for high-efficiency InGaN/GaN light-emitting diodes

We report on the development of periodically oriented embedded air protrusion (EAP) structures at the GaN-sapphire interface in InGaN/GaN LEDs. A specific SiO(2) mask pattern and a simple wet etching process were utilized for the fabrication of EAP structures. A strong coupling between closely proximate air cavities and the multiple quantum wells promoted spontaneous emission due to the high-index contrast at the GaN-air interface. As a result, the light output power of the EAP LED was 2.2 times higher than that of a conventional LED at an injection current of 20 mA.

Enhancement of Light Output Power in InGaN/GaN LEDs with Nanoroughed Hemispherical Indium Tin Oxide Transparent Ohmic Contacts

The light output power of light emitting diodes (LEDs) has been enhanced using nanoroughed hemispherical indium tin oxide (ITO) transparent ohmic contacts. The hemispherical shape is easily achieved by evaporating ITO film on an ITO nanosphere, and the aspect ratio of an individual hemispherical ITO is determined by the size of the ITO nanosphere. The diameter of the ITO nanospheres can be easily controlled by controlling the thickness of the ITO films. The light extraction efficiency of InGaN/GaN LEDs is significantly improved due to light incident at an angle of approximately 90° at the hemispherical nanoroughed prominence―air interface. Also, a high surface area of ITO prominences increases the light extraction efficiency. Thus, the light output power of the nanoroughed hemispherical LEDs is increased by 30% compared to conventional LEDs at an injection current of 20 mA.

Enhanced light output power of GaN-based light-emitting diodes by nano-rough indium tin oxide film using ZnO nanoparticles

We demonstrate the performance improvement of GaN-based light-emitting diodes (LEDs) using zinc oxide (ZnO) nanoparticles inserted between the p-GaN and the indium tin oxide (ITO) layers. Upon deposition of an ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain a nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the fabricated LEDs with ZnO nanoparticles is 39% higher than that of conventional LEDs at an injection current of 20 mA. This is attributed to the improved light extraction favored by the light scattering tendency of ZnO nanoparticles and the nano-roughened ITO film. In addition, the intermediate refractive index (n ∼2) of ZnO materials between those of the p-GaN (n ∼2.5) and the ITO (n ∼1.9) results in a broader critical angle and a reduction of total internal reflection.