Hyun Jeong

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.

Carrier localization in In-rich InGaN/GaN multiple quantum wells for green light-emitting diodes

Carrier localization phenomena in indium-rich InGaN/GaN multiple quantum wells (MQWs) grown on sapphire and GaN substrates were investigated. Temperature-dependent photoluminescence (PL) spectroscopy, ultraviolet near-field scanning optical microscopy (NSOM), and confocal time-resolved PL (TRPL) spectroscopy were employed to verify the correlation between carrier localization and crystal quality. From the spatially resolved PL measurements, we observed that the distribution and shape of luminescent clusters, which were known as an outcome of the carrier localization, are strongly affected by the crystalline quality. Spectroscopic analysis of the NSOM signal shows that carrier localization of MQWs with low crystalline quality is different from that of MQWs with high crystalline quality. This interrelation between carrier localization and crystal quality is well supported by confocal TRPL results.

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.

Suppressing spontaneous polarization of p-GaN by graphene oxide passivation: Augmented light output of GaN UV-LED

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. Therefore, improving hole injection rate is a key step towards high performance UV-LEDs. Here, we report a new method of suppressing spontaneous polarization in p-type region to augment light output of UV-LEDs. This was achieved by simply passivating graphene oxide (GO) on top of the fully fabricated LED. The dipole layer formed by the passivated GO enhanced hole injection rate by suppressing spontaneous polarization in p-type region. The homogeneity of electroluminescence intensity in active layers was improved due to band filling effect. As a consequence, the light output was enhanced by 60% in linear current region. Our simple approach of suppressing spontaneous polarization of p-GaN using GO passivation disrupts the current state of the art technology and will be useful for high-efficiency UV-LED technology.

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.

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.