Buem Joon Kim

Design rule of nanostructures in light-emitting diodes for complete elimination of total internal reflection.

Cone-shaped nanostructures with controllable side-wall angle are success- fully fabricated with a SiO(2) nanosphere lithography (NSL) etching mask. Vertical LEDs with cone-shaped nanostructures with a 24.1° side-wall angle provide 6% more light output power compared to those using hexagonal pyramids formed by photochemical etching. This achievement is attributed to effective elimination of total internal reflection by angle-controlled nanostructures.

Effect of anions in Au complexes on doping and degradation of graphene

In this study, the role of anions in Au complexes was investigated as a dopant for graphene. Au(OH)3, Au2S, AuBr3, and AuCl3 were used as dopants. The average sheet resistance of the graphene decreased from 950 Ω sq−1 to 820, 600, 530, and 300 Ω sq−1 and the work function increased from 4.3 eV to 4.6, 4.8, 5.0 and 4.9 eV with Au(OH)3, Au2S, AuBr3, and AuCl3 dopants, respectively. However, thermal annealing of graphene increased the sheet resistance and decreased the work function. Furthermore, the G and 2D bands in the Raman spectra were recovered and the Au3+ peak in the Au 4f spectra decreased after thermal annealing, indicating the aggregation of metal particles. These results suggested that the degree of doping was related to the electronegativity of the anion in the Au complex and degradation of graphene was related to the bond strength between the Au cation and the counter anion. Therefore, anions with a high electronegativity and high bond strength are adequate for use as a p-type dopant in graphene.

Enhancement of wall-plug efficiency in vertical InGaN/GaN LEDs by improved current spreading.

We present the enhancement of wall-plug efficiency in vertical InGaN/GaN light-emitting diodes (V-LEDs) by improved current spreading with a novel Al2O3 current blocking layer (CBL). The Al2O3 CBL deposited by electron-beam evaporation shows high transmittance and good corrosion resistance to acidic solutions. V-LEDs with an Al2O3 CBL show similar light output power but lower forward voltage as compared to those with a SiO2 CBL deposited by plasma-enhanced chemical vapor deposition. As a result, the wall-plug efficiency of V-LEDs with an Al2O3 CBL at 500 mA was improved by 5% as compared to those with a SiO2 CBL, and by 19% as compared to those without a CBL.

Role of ionic chlorine in the thermal degradation of metal chloride-doped graphene sheets

The degradation mechanism of graphene sheets doped with metal chloride was investigated as a function of the annealing process. The sheet resistance of doped graphene increased from 500–700 Ω sq−1 to 10 kΩ sq−1 and the transmittance at 550 nm decreased from 95% to 87–91% after annealing at 400 °C. Furthermore, the work function of doped graphene decreased from 4.7–5.1 eV to 4.2–4.5 eV after annealing. The G and 2D band peaks in the Raman spectra were shifted to lower wavenumbers by annealing at 400 °C, regardless of the kind of dopant, and reached nearly the same level as that of the pristine graphene. X-ray photoemission spectroscopy showed that the chlorine anions and chlorine atoms disappeared after thermal annealing. The scanning electron microscopy images revealed the capability of annealing to gather the unstable metal cations, thereby inducing the aggregation of metal particles. The degree of doping of the graphene sheets was strongly related to not only metal cations but also chlorine anions. Therefore, aggregation of metal particles and adsorption of chlorine ions degraded the properties of graphene as a function of annealing temperature.

Visible Color Tunable Emission in Three-Dimensional Light Emitting Diodes by MgO Passivation of Pyramid Tip

We demonstrated visible color tunable three-dimensional (3D) pyramidal light emitting diodes by depositing the MgO on and near the tip of the pyramid as an insulating layer. Here, we show that the degradation of the materials (i.e., p-GaN) crystallinity and the built-in electric field due to the nanoscale geometry of the tip region is responsible for the large leakage current observed in LEDs. Confocal scanning electroluminescence microscopy images clearly showed that the intensity of the light emitted out of the side facet of the pyramid is much higher than that of the light extracted out of the tip surface, indicating that the MgO layer prohibited the carrier injection to the MQWs layer, suppressing the leakage occurring at or near the tip region of the pyramids. The color range of the LEDs can be also tuned by using the MgO layer, a blue-shift by 10.3 nm in the wavelength. This technique is simple and scalable, providing a promising solution for developing 3D pyramidal LEDs with low leakage current and controllable light emission.

Effects of W diffusion barrier on inhibition of AlN formation in Ti/W/Al ohmic contacts on N-face n-GaN

We investigate the effect of W diffusion barrier in Ti/W/Al ohmic contacts formed on N-face n-GaN. The contacts exhibit contact resistivity of as low as 2.3 × 10−4 Ω cm2 and better thermal stability than Ti/Al contacts. Cross-sectional transmission electron microscopy micrographs reveal that in-diffused Al atoms on the n-GaN surface react with N atoms to form an AlN layer in Ti/Al contacts, resulting in upward band bending, and consequently, a high contact resistivity. The use of a 10-nm-thick W layer suppresses the in-diffusion of Al atoms to n-GaN, thereby preventing the formation of AlN and enhancing the thermal stability of Ti/W/Al contacts.

Effect of reflective p-type ohmic contact on thermal reliability of vertical InGaN/GaN LEDs

AbstractWe report on the enhanced thermal reliability of vertical-LEDs (VLEDs) using novel reflective p-type ohmic contacts with good thermal stability. The reflective p-type ohmic contacts with Ni/Ag-Cu alloy multi-layer structure shows low contact resistivity, as low as 9.3 × 10−6 Ωcm2, and high reflectance of 86% after annealing at 450°C. The V-LEDs with Ni/Ag-Cu alloy multi-layer structure show good thermal reliability with stress time at 300°C in air ambient. The improved thermal stability of the reflective ohmic contacts to p-type GaN is believed to play a critical role in the thermal reliability of V-LEDs.

Nanolithography for 3-dimentional nanostructures: enhancement of light output power in vertical light emitting diodes

We review the recent developments to enhance the external quantum efficiency (EQE) in GaN based vertical light-emitting diodes (V-LEDs). The controlling of side-wall angle by SiO2 nanosphere lithography significantly improved the light extraction efficiency (LEE) of V-LEDs; this result is 6% higher than the photo chemical etching (PCE) method, which is known to have the highest light extraction, and 300% higher than flat surface V-LEDs. Nanostructured V-LEDs with a very dense forest of vertically aligned ZnO nanowires on the surface of N-face n-GaN induce the dramtic improvement in LEE. The structural transformation at the nanolevel by the UV radiation and Ozone (UV-O) treatment contributes the high density of Zn seed on GaN, and then this approach shows an extreme enhancement in LEE (>2.8x) compared to flat V-LEDs. The enhanced LEE was also demonstrated by depositing a spontaneously formed MgO nano-pyramids and ZnO refractive-index modulation layer on the surface of V-LEDs, resulting in the increase of output power by 49 %, comparing with the V-LEDs with a flat n-GaN surface. The enhancement of light output power by the nanotexturing of n-face n-GaN was remarkably influenced by 3-dimentional nanostructures.