Tae Su Oh

Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode

We report a device that combines indium tin oxide (ITO) nanodot nodes with two-dimensional chemically converted graphene (CCG) films to yield a GaN-based light emitting diode (LED) with interesting characteristics for transparent and current spreading electrodes for the potential use in the ultraviolet region. The current-voltage characteristics and electroluminescence output power performance showed that CCG network on ITO nanodot nodes operated as a transparent and current spreading electrode in LED devices.

GaN-Based Light-Emitting Diodes on Micro-Lens Patterned Sapphire Substrate

GaN-based light-emitting diodes (LEDs) were fabricated on a micro-lens patterned sapphire substrate (ML-PSS). ML patterning on the sapphire substrate was carried out by using photolithography with photo-resist reflow technique and dry etching process using chlorine based inductively coupled plasma. The ML-PSS was prepared using a periodic ML pattern with diameters of 3 µm and spacing of 2, 4, and 5 µm, respectively, on the c-plane sapphire substrate. The leakage current of the LEDs fabricated on the ML-PSS greatly decreased compared to that of a conventional LED and it decreases with increasing ML-pattern spacing; it decreases from 1.8 to 0.2 µA at reverse voltage of 15 V as the ML-pattern spacing is increased from 2 to 5 µm. The output power of the LED with 5 µm spacing was about 155% higher than that of a conventional LED and about 10% higher than that of the LED on the PSS with spacing of 2 µm. This improvement of the output power is contributed not only by reduction of dislocation density depending on spacing of patterning but also by the enhancement of light extraction efficiency with outcoupling via the ML patterned facets on sapphire substrate.

Coupling of InGaN/GaN multiquantum-wells photoluminescence to surface plasmons in platinum nanocluster

We present the coupling of InGaN/GaN multiquantum-wells (MQWs)photoluminescence(PL) to surface plasmons (SPs) in platinum (Pt) nanoclusters (PNCs). To tune the extinction spectrum of Pt thin film through surface modification such as PNC, the thermal annealing method was employed. From conventional PL and time-resolved PL measurements, enhanced emission and faster luminescence decay time of the PNC-formed InGaN/GaN MQWs was observed with blueshifted emission behavior near the plasmon absorption band of PNC. A Purcell enhancement factor, which was calculated to describe the increase in spontaneous emission rate ( R se ) , revealed an approximate 2.2 times enhancement of R se at 425 nm. We believe that these phenomena result from efficient energy transfer in PNC-formed InGaN/GaN MQWs by SPs coupling.

Growth and properties of Al-rich InxAl1−xN ternary alloy grown on GaN template by metalorganic chemical vapour deposition

An Al-rich InxAl1−xN ternary alloy was grown on a GaN template by metal–organic chemical vapour deposition (MOCVD). The GaN template was fabricated on a c-plane sapphire with a low temperature GaN nucleation layer. The growth of the 300 nm thick InxAl1−xN layer was carried out under various growth temperatures and pressures. The surface morphology and the InN molar fraction of the InxAl1−xN layer were assessed by using atomic force microscopy (AFM) and high resolution x-ray diffraction, respectively. The AFM surface images of the InxAl1−xN ternary alloy exhibited quantum dot-like grains caused by the 3D island growth mode. The grains, however, disappeared rapidly by increasing diffusion length and mobility of the Al adatoms with increasing growth temperature and the full width at half maximum value of ternary peaks in HR-XRD decreased with decreasing growth pressure. The MOCVD growth condition with the increased growth temperature and decreased growth pressure would be effective to grow the InxAl1−xN ternary alloy with a smooth surface and improved quality. The optical band edge of InxAl1−xN ternary alloys was estimated by optical absorbance and, based on the results of HR-XRD and optical absorbance measurements, we obtained the bowing parameter of the InxAl1−xN ternary alloy at b = 5.3 eV, which was slightly larger than that of previous reports.

Light outcoupling effect in GaN light-emitting diodes via convex microstructures monolithically fabricated on sapphire substrate

GaN-based light-emitting diode (LED) was fabricated on the sapphire substrate with monolithic convex microstructures (CMs) array. Using confocal scanning electroluminescence (EL), we have directly observed the strong outcoupling phenomenon of the light confined in a LED via the CMs array. This outcoupled light could be efficiently converged on the convex center through consecutive reflections at the flat area and the curved slant area of the CMs array. Compared to the conventional LED, the ray tracing simulation and far field EL results of the LED with a CM array showed efficient light extraction toward the top surface, i.e., 0-5, 40-45 and 60-65 degree by the outcoupling effect. We conclude that the outcoupled optical path via CMs is the dominant factor of the enhanced light extraction in the LED with a CM array.

Enhanced Light Extraction in GaN-Based Light-Emitting Diodes with Holographically Fabricated Concave Hemisphere-Shaped Patterning on Indium–Tin-Oxide Layer

The enhanced light extraction of the GaN-based light-emitting diode (LED) with a two-dimensional concave hemisphere-shaped indium–tin-oxide (ITO) layer, fabricated using laser holographic lithography and inductively-coupled plasma etching techniques were studied. Compared to the LED with a planar ITO layer, it is found that the light output power of LEDs with concave hemisphere patterned ITO layers is improved by ∼34%. This enhanced light output power is attributed to the increase of light extraction by effective photon escape and self-divergence effect at the concave hemisphere-shaped ITO surface.

Structural and Optical Properties of In-Rich InAlGaN/InGaN Heterostructures for White Light Emission

The effect of trimethylaluminium (TMAl) flow rate and the reactor pressure on the structural and optical properties of In-rich InAlGaN quaternary alloy grown on InGaN/GaN by metal organic chemical vapor deposition (MOCVD) were investigated. The addition of Al atoms to InGaN layers at an optimized growth reactor pressure, 200 mbar in our case, resulted in a broad emission from green to red because of the spatial fluctuation of local In concentration and also the formation of three-dimensional (3D) island structures due to low surface mobilities of Al atoms. When the reactor pressure is decreased below 200 mbar, the In-rich phase separation and related green to red emission have not been observed. Combining the blue emission from InGaN layer with the green to red emission from In-rich InAlGaN alloy layer, white light emission has been obtained. The addition of small amount of Al atoms at an optimum reactor pressure was found to enhance the In-rich phase formation in InAlGaN quaternary alloy.

Submicro-electroluminescence spectroscopy of dodecagonal micropit-arranged blue light-emitting diodes

We investigate the local electroluminescence (EL) characteristics of c-plane blue InGaN/GaN light-emitting diodes (LEDs) with periodically arranged dodecagonal micropits (DMs) which have {1?0??1??1} and {1?1??2??2} facets using confocal scanning electroluminescence microscopy (CSEM). By increasing the injection current from 1 to 20?mA, the EL peak for c-plane LEDs is shifted to a shorter wavelength (by up to 5.5?nm), but the EL peaks for semi-polar {1?0??1??1} and {1?1??2??2} facets are nearly unchanged because of the substantially reduced polarization-related electric fields in the quantum wells. CSEM images show various emitting distributions due to the different emission origins as a function of peak position in the EL spectrum. The present observations can help one to improve the understanding of the spatial origin of increased external quantum efficiency in InGaN LEDs with DM arrays.

Growth of GaN Epilayers on Defect-Selective Etched Nanoporous GaN Templates Generated by Electrochemical Etching

In this work, a defect-selective etched nanoporous GaN was formed using electrochemical etching in oxalic acid and employed as a template for the overgrowth of high quality GaN epilayers by metal/organic chemical vapor deposition. To illustrate the influence of the nanoporous GaN on the subsequent overgrown GaN epilayer, high resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM) were measured. The linewidth values of the XRD rocking curve for an overgrown GaN epilayer decreased both for the (002) symmetry plane and (102) asymmetry plane. The TEM observations indicated that the GaN epilayers overgrown on the nanoporous GaN templates had fewer threading dislocations (TDs) due to dislocation bending and had annihilated in the form of dislocation loops. The results indicate that the use of nanoporous GaN as a template is an effective way to reduce the density of TDs in GaN epilayers.

Improved Strain-Free GaN Growth with a Nearly Lattice-Matched AlInN Interlayer by Metalorganic Chemical Vapor Deposition

Using metalorganic chemical vapor deposition, a strain-free GaN layer has been successfully grown by employing a 40-nm-thick nearly lattice-matched (NLM) Al1-xInxN as an interlayer. The Al1-xInxN interlayers having an InN molar fraction of x~0.11 and 0.13 led to crack-networking at the GaN surface due to excessive tensile strain by lattice-mismatching. In the case of the GaN layer with a NLM Al1-xInxN interlayer (x~0.18), however, strain-free GaN structure with improved structural and optical properties was demonstrated from the results of atomic force microscopy, Raman scattering and photoluminescence. By using transmission electron microscopy (TEM), the origin on strain-free state and improved properties of the GaN layer with the NLM AlInN interlayer was investigated. Based on TEM observations, we suggest that the faulted zone-like growth mechanism on roughed AlInN surface and partial compensation of tensile thermal stress are major factors on the improved strain-free GaN film.