Ya Ju Lee

Study of the Excitation Power Dependent Internal Quantum Efficiency in InGaN/GaN LEDs Grown on Patterned Sapphire Substrate

The mechanisms of the excitation power dependent internal quantum efficiency in InGaN/GaN multiple quantum wells (MQWs) LEDs grown on the planar and the patterned sapphire substrates (PSS) at temperature of 15 and 300 K were investigated. From observation the tendency of emission peak energy and carrier lifetime variation in MQWs with different excitation power for both LED samples, we conclude the internal quantum efficiency would increase as coulomb screening effect dominates at lower carrier injection stage and decrease due to the band-filling effect at higher density stage. At room temperature, the majority of the initial injected carriers would be first consumed by the thermal activated nonradiative centers that hinder the further achievement of high-efficiency LED devices. Experimentally, the internal quantum efficiency of the LED grown on the PSS is ~70% and that of the LED grown on the planar sapphire substrate is ~62%. For the LED grown on the PSS, the observed higher internal quantum efficiency is due to the larger activation energy Therefore, the reduction of dislocation defects and the prevention of injected carriers escaping from extended states would be a promising prospective for InGaN/GaN MQWs LEDs to achieve high internal quantum efficiency.

High output power density from GaN-based two-dimensional nanorod light-emitting diode arrays

Here we propose and realize a scheme for making a direct contact to a two-dimensional nanorod light-emitting diode (LED) array using the oblique-angle deposition approach. And, more importantly, we demonstrate highly efficient electrical carrier injection into the nanorods. As a result, we show that at a 20 mA dc current injection, the light output power density of our nanorod LED array is 3700 mW cm−2. More general, this contact scheme will pave the ways for making direct contacts to other kinds of nanoscale optoelectronic devices.

Reduction in the Efficiency-Droop Effect of InGaN Green Light-Emitting Diodes Using Gradual Quantum Wells

The effect of gradual indium gallium nitride (InGaN) quantum wells (QWs) on the suppression of efficiency-droop in green light-emitting diodes (LEDs) is numerically investigated. The presented scheme increases the internal quantum efficiency by 45.5% at I=20 mA and 55.7% at I=100 mA, indicating a considerable reduction of efficiency-droop. This improvement is attributable mainly to the use of the gradual InGaN QWs structure that significantly alleviates band bending in the valence band, improving the transport efficiency of injected holes above that of conventional LEDs. The radiative recombination is thus enhanced as the overlap between electron and hole wave functions is increased. Most importantly, the leakage of injected electrons to p-type region is correspondingly reduced, in turn suppressing the efficiency-droop in the LED.

Enhanced conversion efficiency of InGaN multiple quantum well solar cells grown on a patterned sapphire substrate

This study demonstrated the enhanced conversion efficiency of an indium gallium nitride (InGaN) multiple quantum well (MQW) solar cell fabricated on a patterned sapphire substrate (PSS). Compared to conventional solar cells grown on a planar sapphire substrate, threading dislocation defects were found to be reduced from 1.28 × 109 to 3.62 × 108 cm−2, leading to an increase in short-circuit current density (JSC = 1.09 mA·cm−2) of approximately 60%. In addition, the open-circuit voltage and fill factor (VOC = 2.05 V; FF = 51%) of the solar cells grown on PSS were nearly identical to those of conventional devices. The enhanced performance is primarily due to improvements in the crystalline quality of the epitaxial layers, reducing the trapping of photogenerated electrons and holes by nonradiative recombination centers in MQW, with a corresponding increase in the transport efficiency of the carriers outside the device.

Monitoring of wound healing process of human skin after fractional laser treatments with optical coherence tomography.

Fractional photothermolysis induced by non-ablative fractional lasers (NAFLs) or ablative fractional lasers (AFLs) can remodel the skin, regenerate collagen, and remove tumor tissue. However, fractional laser treatments may result in severe side effects, and multiple treatments are required to achieve the expected outcome. Thus, the treatment outcome and downtime after fractional laser treatments are key issues to determine the following treatment strategy. In this study, an optical coherence tomography (OCT) system was implemented for in vivo studies of wound healing after NAFL and AFL treatments. According to the OCT scanning results, the laser-induced photothermolysis including volatilization and coagulation could be morphologically identified. To continue monitoring the wound healing process, the treated regions were scanned with OCT at different time points, and the en-face images at various tissue depths were extracted from three-dimensional OCT images. Furthermore, to quantitatively evaluate the morphological changes at different tissue depths during wound healing, an algorithm was developed to distinguish the backscattering properties of untreated and treated tissues. The results showed that the coagulation damage induced by the NAFLs could be rapidly healed in 6 days. In contrast, the tissue volatilization induced by AFLs required a longer recovery time of 14 days. In conclusion, this study establishes the feasibility of this methodology as a means of clinically monitoring treatment outcomes and wound healing after fractional laser treatments.

High breakdown voltage in AlGaN/GaN HEMTs using AlGaN/GaN/AlGaN quantum-well electron-blocking layers.

In this paper, we numerically study an enhancement of breakdown voltage in AlGaN/GaN high-electron-mobility transistors (HEMTs) by using the AlGaN/GaN/AlGaN quantum-well (QW) electron-blocking layer (EBL) structure. This concept is based on the superior confinement of two-dimensional electron gases (2-DEGs) provided by the QW EBL, resulting in a significant improvement of breakdown voltage and a remarkable suppression of spilling electrons. The electron mobility of 2-DEG is hence enhanced as well. The dependence of thickness and composition of QW EBL on the device breakdown is also evaluated and discussed.

Slanted n-ZnO/p-GaN nanorod arrays light-emitting diodes grown by oblique-angle deposition

High-efficient ZnO-based nanorod array light-emitting diodes (LEDs) were grown by an oblique-angle deposition scheme. Due to the shadowing effect, the inclined ZnO vapor-flow was selectively deposited on the tip surfaces of pre-fabricated p-GaN nanorod arrays, resulting in the formation of nanosized heterojunctions. The LED architecture composed of the slanted n-ZnO film on p-GaN nanorod arrays exhibits a well-behaving current rectification of junction diode with low turn-on voltage of 4.7 V, and stably emits bluish-white luminescence with dominant peak of 390 nm under the operation of forward injection currents. In general, as the device fabrication does not involve passivation of using a polymer or sophisticated material growth techniques, the revealed scheme might be readily applied on other kinds of nanoscale optoelectronic devices.

Determination of Junction Temperature in InGaN and AlGaInP Light-Emitting Diodes

The junction temperature of a light-emitting diode (LED) directly and greatly affects its performances. Therefore, the reliable measurement and accurate estimation of the junction temperature of an LED is extremely important. This paper proposes an approach for directly determining the dependence of junction temperature on injected currents in InGaN and AlGaInP LEDs. Various important physical parameters that affect the junction temperature of an LED are also considered.

Enhanced external quantum efficiency in GaN-based vertical-type light-emitting diodes by localized surface plasmons

Enhancement of the external quantum efficiency of a GaN-based vertical-type light emitting diode (VLED) through the coupling of localized surface plasmon (LSP) resonance with the wave-guided mode light is studied. To achieve this experimentally, Ag nanoparticles (NPs), as the LSP resonant source, are drop-casted on the most top layer of waveguide channel, which is composed of hydrothermally synthesized ZnO nanorods capped on the top of GaN-based VLED. Enhanced light-output power and external quantum efficiency are observed, and the amount of enhancement remains steady with the increase of the injected currents. To understand the observations theoretically, the absorption spectra and the electric field distributions of the VLED with and without Ag NPs decorated on ZnO NRs are determined using the finite-difference time-domain (FDTD) method. The results prove that the observation of enhancement of the external quantum efficiency can be attributed to the creation of an extra escape channel for trapped light due to the coupling of the LSP with wave-guided mode light, by which the energy of wave-guided mode light can be transferred to the efficient light scattering center of the LSP.

Effect of Surface Texture and Backside Patterned Reflector on the AlGaInP Light-Emitting Diode: High Extraction of Waveguided Light

This paper describes a novel structure of an AlGaInP light-emitting diode (LED) to extract the waveguided light for high-brightness applications. Four devices are considered and compared. They are AlGaInP-sapphire LEDs with: 1) a planar Ag reflector (LED-A); 2) a patterned Ag reflector (LED-B); 3) a planar Ag reflector and surface-roughened features (LED-C); and 4) a patterned Ag reflector and surface-roughened features (LED-D). The patterned Ag reflector can be used to direct some of the waveguided light that bounces within the AlGaInP LED and sapphire substrate to the top escape cone of the LED surface. Additionally, the roughened features, which are randomly distributed on the LED surface, enable the waveguided light that is trapped inside the LED chip to be coupled efficiently into the air. As a result, the external quantum efficiencies of LED-A, LED-B, LED-C, and LED-D measured at I = 350 mA are η = 9.20%, 10.46%, 15.22%, and 16.75%, respectively.