Ah Hyun Park

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.

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.

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Improving the graphene electrode performance in ultra-violet light emitting diode using silver nanowire networks

This paper reports a systematic study on the characteristics of silver nanowires (AgNWs) coated graphene and its application as a transparent current spreading electrode in ultra-violet light emitting diodes (UV-LEDs). The optimized values of optical transmittance and sheet resistance of AgNWs covered graphene were 87.7% at 375 nm and 50 ± 5 Ω/sq, respectively. Upon applying the AgNWs on graphene electrode, the UV-LED exhibited uniform bright light emission with a reduction in the forward voltage and about four-fold increase in the electroluminescence intensity. We attribute the observed performance improvements to a reduction in the sheet and contact resistances.

Enhanced Light Output Power of GaN Light-Emitting Diodes with Graphene Film as a Transparent Conducting Electrode

We report the enhanced light output power of GaN-based light-emitting diode (LED) by using graphene film as a transparent conducting electrode. Monolayer graphene was synthesized on copper foil by using chemical vapor deposition method and directly transferred onto the GaN-LED as a top electrode. Compared to the conventional LEDs using indium tin oxide (ITO) layer for an electrode material, the light output power of LED with graphene electrode was improved by ~25%. This was attributed excellent graphene characteristics of high electrical conductivity, high optical transmittance of nearly 97% over a wide range of infrared, visible, and ultraviolet region and large area uniformity with fewer defects.

The role of graphene formed on silver nanowire transparent conductive electrode in ultra-violet light emitting diodes

This paper reports a highly reliable transparent conductive electrode (TCE) that integrates silver nanowires (AgNWs) and high-quality graphene as a protecting layer. Graphene with minimized defects and large graphene domains has been successfully obtained through a facile two-step growth approach. Ultraviolet light emitting diodes (UV-LEDs) were fabricated with AgNWs or hybrid electrodes where AgNWs were combined with two-step grown graphene (A-2GE) or conventional one-step grown graphene (A-1GE). The device performance and reliability of the UV-LEDs with three different electrodes were compared. The A-2GE offered high figure of merit owing to the excellent UV transmittance and reduced sheet resistance. As a consequence, the UV-LEDs made with A-2GE demonstrated reduced forward voltage, enhanced electroluminescence (EL) intensity, and alleviated efficiency droop. The effects of joule heating and UV light illumination on the electrode stability were also studied. The present findings prove superior performance of the A-2GE under high current injection and continuous operation of UV LED, compared to other electrodes. From our observation, the A-2GE would be a reliable TCE for high power UV-LEDs.

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.

Influence of controlled growth rate on tilt mosaic microstructures of nonpolar a-plane GaN epilayers grown on r-plane sapphire

The influence of the epitaxial growth rate on the surface morphology, as well as the crystallinity and optical properties of nonpolar a-plane GaN epilayers grown on r-plane sapphire substrate, has been investigated by x-ray diffraction and photoluminescence studies. The a-plane GaN epilayer grown at faster growth rates revealed a horizontal-pillar-shaped morphology with triangular-pits having sharp corners. While the epilayer grown at a faster growth rate showed a greater difference in the ω tilt-offset angle and line-width value between the on- and off-axes from x-ray diffraction, the a-plane GaN grown at the relatively slower growth rate showed a flat morphology with few pits and small ω tilt-offset difference. Growth of nonpolar a-plane GaN epilayer was optimized, and the effect of the growth rate of the a-plane GaN epilayer and the reason for the difference in the ω tilt-offset line-width value between the c- and m-direction mosaicity of x-ray diffraction were analyzed.

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.