Hyoungwon Park

The fabrication of a patterned ZnO nanorod array for high brightness LEDs

In this study, a patterned ZnO nanorod array was formed on the ITO layer of GaN-based light-emitting diodes (LEDs), to increase the light extraction efficiency of the LED. The bi-layer imprinted resin pattern was used for selective growth of the ZnO nanorod array on the ITO layer. Compared to conventional LEDs grown on patterned sapphire substrate (PSS), the deposition of the blanket ZnO layer on the ITO layer increased the light extraction efficiency of the LED by about 10%. Further growth of the ZnO nanorod layer on the blanket ZnO layer increased the light extraction efficiency of the LED by about 23%. In the case that a patterned ZnO nanorod layer was formed on a blanket ZnO layer, the light extraction efficiency increased by about 34%. These enhancements of the device were caused by modulation of the refractive-index in ZnO layers and the surface roughening effects because of the unique design of the pattern, which was nanostructure-in-nanopattern, resulting in the formation of many escape cones on the LED surface.

Fabrication of SiNx-based photonic crystals on GaN-based LED devices with patterned sapphire substrate by nanoimprint lithography

SiNx-based photonic crystal (PhC) patterns were fabricated on the ITO electrode layer of a GaN-based light-emitting diode (LED) device on a patterned sapphire substrate (PSS) by a UV nanoimprint lithography process in order to improve the light extraction of the device. A three-dimensional finite-difference time-domain simulation confirmed that the light extraction of a GaN LED structure on a PSS is enhanced when SiNx PhC patterns are formed on the ITO top layer. From the I-V characteristics, the electrical properties of patterned LED devices with SiNx-based PhC were not degraded compared to the unpatterned LED device, since plasma etching of the p-GaN or the ITO layers was not involved in the patterning process. Additionally, the patterned LED devices with SiNx-based PhCs showed 19%-increased electroluminescence intensity compared with the unpatterned LED device at 445 nm wavelength when a 20 mA current is driven.

Enhancement of the photon extraction of green and blue LEDs by patterning the indium tin oxide top layer

The indium tin oxide (ITO) transparent electrode layer on green and blue light-emitting diodes (LEDs) was patterned with various-sized periodic hole arrays, size ranging from 300 nm to 380 nm, using thermal nanoimprint lithography and inductively coupled plasma (ICP) etching processes. The imprinted resin was used as a mask layer and etch resistance of the imprinted resin was adjusted in order to control the tapered and enlarged etch profile of the ITO layer, since the tapered etch profile can improve the light extraction efficiency of the LED by prominent scatterings. Photoluminescence intensity from InGaN multi-quantum wells for the green LED structure showed that up to 4.6 times stronger emission was exhibited with the patterned ITO electrode, compared to the identical sample with an un-patterned blanket ITO electrode layer. An electroluminescence (EL) intensity of a blue LED sample witha patterned ITO electrode layer was increased up to 23% compared to that of the identical sample with an un-patterned blanket ITO electrode layer.

Improvement of photon extraction efficiency of GaN-based LED using micro and nano complex polymer structures

A micro- and nanoscale complex structure made of a high refractive index polymer (n = 2.08) was formed on the ITO electrode layer of an edge-emitting type GaN blue light-emitting diode (LED), in order to improve the photon extraction efficiency by suppressing total internal reflection of photons. The nanoimprint lithography process was used to form the micro- and nanoscale complex structures, using a polymer resin with dispersed TiO2 nano-particles as an imprint resin. Plasma processing, such as reactive ion etching, was used to form the micro- and nano-scale complex structure; thus, plasma-induced damage to the LED device can be avoided. Due to the high refractive index polymeric micro- and nanostructure on the ITO layer, the electroluminescence emission was increased up to 20%, compared to an identical LED that was grown on a patterned sapphire substrate to improve photon extraction efficiency.

Formation of TiO2 Nano Pattern on GaN-Based Light-Emitting Diodes for Light Extraction Efficiency

A TiO2 nano-structure was formed on the indium-tin-oxide electrode of a GaN-based light-emitting diode (LED) in order to enhance the light extraction efficiency. The UV bi-layer imprinting and lift-off processes were used to form the TiO2 nano-structure without any plasma etching process, which can lead to degradation of the electrical properties of the device. As a result, the light output power of the LED on the patterned sapphire substrate (PSS) with the TiO2 nano-structure was enhanced up to 12% compared to identical LED formed on the PSS without TiO2 nano-structure. No electrical degradation was observed for the patterned LED device.

Two inch large area patterning on a vertical light-emitting diode by nano-imprinting technology

A vertical light-emitting diode (LED) with a chip size of 500 × 500 µm2 was fabricated by the laser lift-off (LLO) process of an InGaN-based blue LED wafer. After the LLO process, photonic crystal patterns by UV nano-imprint lithography were formed on the n-GaN top layer of the vertical LED over the entire area with a diameter of 2 inches. As the result of n-GaN patterning, light output power of the vertical LED with photonic crystals was increased by up to 44% compared to that of the vertical LED without a photonic crystal at a driving current of 1000 mA.

An accurate SPICE simulation of CMOS MMICs using fully software-based parameter extraction

An efficient and accurate simulation technique of CMOS MMICs using fully software-based parameter extraction is presented. The CMOS MMIC is divided into several blocks consisting of the board interconnection, the chip package block, and the internal MMIC circuit block. The electrical parameters of the board and the package are extracted purely from simulations. The impedance matching of the RF port has been achieved simply by using the simulated input impedance of the chip. The pure SPICE simulation of a whole L-band CMOS MMIC can reproduce the measured result within a reasonable error range.