Sukho Yoon

Effects of As/P exchange reaction on the formation of InAs/InP quantum dots

InAs self-assembled quantum dots (SAQDs) were grown on InP at various temperatures and V/III ratios by metalorganic chemical vapor deposition. The density, size distribution, and shape of the InAs SAQDs changed significantly with temperature and V/III ratio. Careful analysis of the total volume of the dots grown at various conditions showed that the volume far exceeded the amount of deposition supplied from the gas-phase sources. The amount of excess InAs and the aspect ratio (height/lateral size) of the SAQD increased with temperature and V/III ratio, strongly suggesting that the As/P exchange reaction at the surface played an important role in the kinetics of SAQD formation. Insertion of a lattice-matched InGaAs buffer layer suppressed the excess InAs formation, and lowered the aspect ratio, confirming the effect of the As/P exchange reaction.

High-Reflectance and Thermally Stable AgCu Alloy p-Type Reflectors for GaN-Based Light-Emitting Diodes

We report on the formation of high-quality AgCu alloy p-type reflectors for GaN-based light-emitting diodes (LEDs). Compared with Ag contacts, the AgCu alloy reflectors produce lower specific contact resistance (7.5times10-5 Omegamiddotcm2), higher light reflectance (89.5% at 400 nm), and better thermal stability (absence of interfacial voids), when annealed at 400 degC in N2 : O2(=1:1) ambient. LEDs fabricated with the AgCu reflectors show light output power better than that of LEDs with the Ag reflectors. The ohmic mechanism for the AgCu alloy reflectors is explained in terms of the formation of Ag-Ga solid solution and the presence of Cu-oxide nano-particles at the contact/GaN interface

Measurements of current spreading length and design of GaN-based light emitting diodes

The authors report on the experimental method to measure the current spreading length in GaN-based light emitting diodes (LEDs) based on a one-dimensional current-distribution model neglecting vertical series resistance of the LEDs. It is clearly shown that the measured current spreading length is in good agreement with the calculated results, exhibiting a strong dependence on the injected current density (or forward bias voltage). LEDs fabricated with hybrid p-type reflectors by using the proposed design rule and measured current spreading lengths show enhancement of the output power by 10% as compared to LEDs made with standard reflectors.

Enhanced light extraction of GaN-based light-emitting diodes by using textured n-type GaN layers

The authors report on the enhancement of the light extraction efficiency of GaN-based light-emitting diodes (LEDs) via the texturing of n-type layers. Compared with standard LEDs, LED fabricated with the textured n-type layers produced a significant improvement in the output power, depending on the reflectivity of the n electrode, the etch-pit size, and the chip dimension. The textured LEDs were found to yield the output power enhancement as high as 54%. However, it was also found that the electrical property of the textured LEDs can be degraded when the size of the etch pits is too large, indicating that a well-controlled texturing process is required for the realization of high-efficiency LEDs.

Improvement of efficiency and electrical properties using intentionally formed V-shaped pits in InGaN/GaN multiple quantum well light-emitting diodes

We demonstrate a high efficiency and an improvement of the electrical properties in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) using intentionally formed V-shaped pits. Efficiency droop behaviors are measured and LEDs with V-shaped pits act like LEDs with a low dislocation density. The reverse voltage at −10 μA of LEDs with V-shaped pits shows −120 V, which is comparable to p-i-n rectifiers grown on a free-standing GaN, and reverse leakage current is decreased indicating electrical passivation of dislocation. A calculated diode ideality factor shows that electron tunneling at low forward voltage is suppressed in LEDs with V-shaped pits.

Alternating-current Light Emitting Diodes with a Diode Bridge Circuitry

Most solid-state light emitting devices operate under direct current (DC) condition now. We report the alternating current (AC) light emitting devices fabricated with a diode bridge circuitry which is also made of light emitting diodes (LEDs). The LED bridge circuitry which is flipped on a silicon submount is composed of 4 branches with 7 LED chips and participates as a light emitting component as well. The AC LED can be operated with radiant flux of 0.83 W at an electric power of 8.5 W. This concept could be applied to fabricate compact and economical AC LEDs for a solid-state illumination.

Consideration of the Actual Current-Spreading Length of GaN-Based Light-Emitting Diodes for High-Efficiency Design

Based on the proposed experimental method, the current spreading length of GaN-based light-emitting diodes (LEDs) was measured and analyzed for practical device design. In this study, Thompsons and Guos models, which are categorized according to vertical series resistance (in particular, p-type contact resistance), were used to extract device parameters. It was shown that the measured current spreading length strongly depends on the injected current density. For LEDs fabricated with low-resistance p-type contacts, this behavior could be explained in terms of the accelerated current crowding with higher current densities occurring as a result of the reduced voltage drop across the junction, which is in good agreement with Thompsons relation. However, for LEDs fabricated with high-resistance p-contacts, unlike Guos prediction, the measured current spreading length also showed a strong dependence on the injected current density. This was attributed to thermal heating at the p-contact, resulting in the reduction of the voltage drop across the p-contact and so junction voltage, which is also in agreement with Thompsons model. Based on the measured parameters and the design rule, efficient p-type reflectors, namely, hybrid reflectors were designed. Compared with conventional ones, LEDs fabricated with the hybrid reflectors exhibited better output power at a reasonable forward voltage, indicating that the proposed method is effective in understanding the actual current spreading and hence the practical design of high-efficiency LEDs.

Highly transparent and low-resistant ZnNi/indium tin oxide Ohmic contact on p-type GaN

The authors report the improvement of GaN light-emitting diodes (LEDs) by applying a ZnNi/indium tin oxide (ITO) (5nm∕380nm) electrode with high transparency and low resistance to p-GaN. The Pt/ITO (5nm∕380nm), Ni∕Au∕ITO (2.5nm∕5nm∕380nm), and Ni∕Au (2.5nm∕5nm) electrodes were prepared and annealed at 400, 500, and 600°C for 1min in air. The ZnNi/ITO contacts showed the lowest specific contact resistance of ∼1.27×10−4Ωcm2 and the highest transmittance of ∼90% at 460nm. LEDs fabricated with ZnNi/ITO p electrodes showed the best performance with a forward voltage of 3.28V and a typical brightness of 11. 7mcd at 20mA.

Electrical spin injection from room-temperature ferromagnetic (Ga, Mn)N in nitride-based spin-polarized light-emitting diodes

We present the electrical spin injection from room-temperature ferromagnetic (Ga, Mn)N in nitride-based spin-polarized light-emitting diodes. The electroluminescence spectra from the spin LED indicate the existence of the spin polarization via optical polarization of emitted light up to room temperature. This demonstrates that the spin injection from the (Ga, Mn)N layer into (In, Ga)N quantum wells was achieved persisting up to room temperature by comparing it with the magnetic field dependence of the Hall resistance, which is proportional to the out-of-plane magnetization. These results support that (Ga, Mn)N is an appropriate material for a spin injection source in room-temperature operating semiconductor spintronic devices.

Enhanced Optical Power of InGaN/GaN Light-Emitting Diode by AlGaN Interlayer and Electron Blocking Layer

We report on the effect of an AlGaN interlayer with a low Al composition, which is inserted between multiple quantum wells (MQWs) and an AlGaN electron blocking layer (EBL) in light-emitting diodes (LEDs). The band diagram shows that this interlayer reduces the hole barrier height between the last GaN barrier in MQWs and the AlGaN EBL to enhance the hole injection. The optical output power at 20 mA of LEDs with interlayer is increased by 20% more than that of LEDs without an interlayer. This improvement is attributed to the enhanced radiative recombination rate, which is due to a more uniform hole distribution and higher hole concentration in MQWs.