Soo-Kun Jeon

Air-voids embedded high efficiency InGaN-light emitting diode

To improve the light extraction efficiency of InGaN-light emitting diode (LED), inverted hexagonal cone shaped air voids with {10–11} GaN crystal planes were formed between a patterned sapphire substrate and GaN epitaxial layer using a H3PO4-based hot chemical etching method. The air-voids embedded LED showed 12% and 210% higher optical power than a patterned substrate LED and a flat substrate LED, respectively. A ray tracing simulation revealed that the light extraction through the top face of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.

The effect of the internal capacitance of InGaN-light emitting diode on the electrostatic discharge properties

The electrostatic discharge (ESD) properties of the InGaN-light emitting diode (LED) were investigated in terms of the internal capacitance of the InGaN-LED. The LEDs with higher internal capacitance were found to be more resistant to external ESD impulses. The internal capacitance of the InGaN-LED was controlled by the silicon doping level of the n-GaN layer bordering the active layer. The human body model ESD yield at −500 V was increased from 27% to 94% by increasing the internal capacitance. Moreover, the high ESD pass yield was maintained up to −7000 V.

InGaN-light emitting diode with high density truncated hexagonal pyramid shaped p-GaN hillocks on the emission surface

To increase the light extraction efficiency, high density truncated hexagonal pyramid shaped submicron p-GaN hillocks were formed on the emission surface of an InGaN∕GaN multiple quantum well light emitting dicode (LED) using an in situ silicon carbon nitride self-masking layer. The self-assembled hillock density was raised up to a low 109cm−2 using several nanometers of a Si0.4C0.6N1 self-masking layer. The self-assembled hillock LED resulted in the optical power improvement up to 80% with similar electrical properties as a normal LED. This device showed a higher electrostatic discharge pass yield at over 1000V reverse stress voltage.

Via-hole-based vertical GaN light emitting diodes

A vertical GaN-light emitting diode (LED) has been fabricated on a sapphire substrate with periodic via holes formed by a laser drilling technique. n-contact metal which was deposited on the backside of sapphire substrate was directly connected with an Ohmic metal of n-GaN layer through the via holes. The via-hole-based vertical GaN-LED demonstrated an optical power improvement of up to 12.5% with lower forward operating voltage compared with a conventional GaN-LED. In addition, this vertical LED showed just 0.8% and 1.5% variations of optical power and operation voltage at the 500h reliability test.

The effect of the last quantum barrier on the internal quantum efficiency of InGaN-light emitting diode

The effect of the last quantum barrier (LQB) on the internal quantum efficiency of GaN-light emitting diode (LED) was systematically investigated using a dual-wavelength GaN-LED design. Compared with a conventional GaN-LQB, a high indium contained In0.03Ga0.97N-LQB efficiently reduced the unintentional Mg impurity in the last quantum well and improved its photoluminescence and electroluminescence intensity up to 72% and 15%, respectively.

InGaN Light-Emitting Diode With Quasi-Quantum-Dot-Shaped Active Layer Using SiCN Interfacial Layer

A quasi-quantum-dot (QQD)-shaped InGaN-GaN multilple-quantum-well light-emitting diode (LED) was achieved using a silicon carbon nitride (SiCN) interfacial layer. QQDs with ~100-nm diameter and ~4-nm height were uniformly formed inside the InGaN active layer due to strain and affinity difference between the InGaN and SiCN layer. The surface morphology and structural properties of QQD-LED were measured with atomic force microscopy, secondary ion mass spectrometry, and X-ray diffraction. Device performance of QQD-LEDs were evaluated and compared with normal LEDs. The QQD-LED showed ~15% higher photoluminescence intensity and ~10% higher optical output power

Successively dry-wet etched InP microlens for bell shaped LED

A method of fabricating monolithic InP microlenses, using successive dry-wet etching, is proposed. Dry etching is performed with reactive ion beam etching using Ar/Cl/sub 2//H/sub 2/ gases. Wet etching with HBr/H/sub 3/PO/sub 4//(0.5M) K/sub 2/Cr/sub 2/O/sub 7/ is used for successive wet etching. In the other fabrication methods of microlenses, microlens structures are limited by the characteristics of photoresist, polymer or etchants. More suitable lens structure can be achieved by our proposed method. We have fabricated InGaAsP/InP surface emitting BS-LEDs with the monolithic InP microlenses by this method. The Bell Shaped LED with this microlens shows improved coupling efficiency to fiber.

High performance near-ultraviolet flip-chip light-emitting diodes with distributed Bragg reflector

We have fabricated the near-ultraviolet (NUV) flip-chip (FC) light-emitting diodes (LEDs) with the high external quantum efficiency (EQE) using distributed Bragg reflectors (DBRs) and compared with conventional FC-LED using silver (Ag) reflector. Reflectance of Ag is very high (90 ~ 95 %) at visible spectrum region, but sharply decrease at NUV region. Therefore we used DBR composed of two different materials which have high-index contrast, such as TiO2 and SiO2. However, to achieve high-performance NUV flip-chip LEDs, we used Ta2O5 instead of TiO2 that absorbs lights of NUV region. Thus, we have designed a DBR composed of twenty pairs of Ta2O5 and SiO2 using optical coating design software. The DBR designed by our group achieves a reflectance of ~99 % in the NUV region (350 ~ 500 nm), which is much better than Ag reflector. Optical power is higher than the Ag-LED up to 22 % @ 390 nm.

Crystallographically Defined Emitter Contact Technology for Self-Alignment Process in InP/InGaAs HBTs

An approach for fabricating a self-aligned InP/InGaAs heterojunction bipolar transistor (HBT) with InGaAs dummy emitter layer was demonstrated. The self-aligned emitter-base structure employs crystallographically defined emitter contact (CDC) technology to define a desired emitter-to-base contact spacing. An anisotropic wet etching of the InGaAs dummy emitter layer grown on the HBT layer structure leads to the crystallographically etched profiles to configure the shape of the emitter contact. InP/InGaAs HBTs demonstrated good current-voltage characteristics with a current gain more than 40 and a knee voltage of approximately 0.5 V up to a collector current density of 1 X 10 5 A/cm 2 , indicating the effectiveness of the new self-alignment technology.

High performance GaN based blue flip-chip light-emitting diodes

In this study, high performance nitride-based flip-chip (FC) light-emitting diodes (LEDs) using optimized distributed bragg reflector (DBR) were fabricated and compared with conventional FC-LED using silver (Ag) reflector. Most of FCLEDs are using the silver (Ag) as reflector due to its superior reflectance at visual spectrum region. However, A silver has detrimental problems such as electro-chemical migration and agglomerations, which resulting in reliability issues such as degradation of power drop, unstable operating voltage and leakage issues. Our DBR structure was designed to have 99% at whole visible spectrum range (400~750nm), which is higher reflectance than silver reflector (90~95%). Optical power is higher than higher than the Ag-LED up to 30% @ 500mA. As the current increases up to 1A, the gap slightly decreased. Reliability test results show stable optical power, operating voltage, and leakage maintenance.