Ki Yeon Yang

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.

Direct indium tin oxide patterning using thermal nanoimprint lithography for highly efficient optoelectronic devices

Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) material on account of its high conductivity and transmittance in a visible range. In order to improve the efficiency of optoelectronic devices using TCO materials, the transmittance and conductivity of the TCO layers need to be improved. Recently, various techniques, which use nanostructures on the surface to improve the transmittance, have attracted considerable attention. In this study, a direct ITO nanopatterning technique using thermal nanoimprint lithography was presented. An ITO nanoparticle solution is used as an imprint resin and postimprint annealing process was followed. The optical transmittance of the ITO glass was increased by forming periodic ITO dot patterns on the ITO glass. UV-visible near infrared spectra showed that the transmittance at 485nm of the ITO glass with the directly patterned ITO layer was 5% higher than that of ordinary ITO glass.

A Fabrication Method of the Silica-Based Moth-Eye Structures and Its Application to the Organic Light-Emitting Diodes

For the application of moth-eye patterns to practical optical devices, a novel nano-patterning technique, which can fabricate transparent and robust material based nano patterns through a simple process, needs to be developed, because the moth-eye structures are usually exposed to the outer environment. In this study, silica based moth-eye structures were directly fabricated on a glass substrate by the direct hydrogen silsesquioxane (HSQ) printing and annealing technique. The transmittance of the glass substrate was enhanced by 3–4% in the visible range by the silica moth-eye structures and it was confirmed that the light output efficiency of the green organiclight emitting diode (OLED) was enhanced by 7–9% using the encapsulation glass with the silica moth-eye patterns.

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.

Electrical characterization of Ge-Sb-Te phase change nano-pillars using conductive atomic force microscopy

The electrical characteristic of phase change material was studied in nano-scale using nanoimprint lithography and a conducting atomic force microscopy measurement system. Nanoimprint lithography was used to fabricate the nano-scale phase change material pattern. A Pt-coated AFM tip was used as a top electrode to measure the electrical characteristics of the GST nano-pillar. The GST nano-pillar, which is 200 nm in diameter, was amorphized by 2 V and 5 ns reset pulse and was then brought back to the crystalline phase by applying 1.3 V and 150 ns set pulse. Using this measurement system, the GST nano-pillar was switched between the amorphous and crystalline phases more than five times. The results of the reset and the set current measurement with the GST nano-pillar sizes show that the reset and the set currents also decreased with the decrease of the GST pillar size.

Novel fabrication technique for nanoscale hydrogen silsesquioxane structures using a direct printing technique

Hydrogen silsesquioxane (HSQ), a type of spin-on glass, is an attractive material for nanobio, semiconductor and photoelectronic applications because it can be transformed into SiO2 by a simple annealing process. Studies on the fabrication of nanosized structures are necessary for the simple and easy production of HSQ nanostructures because of their unique characteristics. In this study, a fabrication method was developed for HSQ nanostructures as small as 50 nm using a direct printing technique with a poly (dimethylsiloxane) (PDMS) mold. Using this technique, HSQ nanostructures can be fabricated on a curved substrate

Direct Indium Tin Oxide Nanoparticle Printing Technique for Improvement of Light Extraction Efficiency of GaN-Based LEDs

To enhance the light extraction efficiency of the GaN-based light emitting diode (LED), indium tin oxide (ITO) nanoparticle photonic crystal patterns are fabricated on the surface of the GaN-based blue LED device using the direct printing technique of the ITO nanoparticles. According to electroluminescence (EL) measurements, the EL intensity of the GaN-based blue LED with photonic crystal patterns is 28% higher than an identical LED without photonic crystal patterns. Printing the ITO nanoparticles eliminates the need for a plasma etching process of the ITO layer so that the current-voltage characteristics do not degrade.

Formation of TiO2 nanopattern using reverse imprinting and sol-gel method

TiO2 and its nanopattern fabrication have been studied intensively because of its wide band gap and photocatalystic nature. TiO2 nanopatterns can be made by conventional patterning techniques, consisting of deposition, photolithography, and etching processes. However, these processes include complicated and expensive process steps, such as photolithography. Therefore, a simpler and more economic process is needed. In this work, TiO2 nanopatterns were fabricated using reverse-imprint lithography and the sol-gel method. Ethanol based TiO2 sol was prepared using tetrabuthylorthotitanate (C16H36O4Ti) and diethanolamine (C4H11NO2). TiO2 sol was then coated on the surface of the replicated polymer mold of hard-polydimethylsiloxane and polydimethylsiloxane by spin coating and transferred to the substrate by the reverse imprinting process at 200°C. A postimprint annealing process was subsequently carried out to form the TiO2 polycrystalline phase. The x-ray diffraction and x-ray photoelectron spectroscopy results...

Low temperature fabrication of residue-free polymer patterns on flexible polymer substrate

Low temperature imprinting on various substrates is essential for applying nanoimprint lithography (NIL) to the patterning process of large-area substrates with low thermal resistance. In addition, the imprinting resist must be distributed uniformly over a large area with near-zero residue imprinting to reduce the level of pattern damage during the residue removal process. In this study, thermal imprinting with a poly(benzyl methacrylate) (PBMA) resist, which can be coated uniformly over a large-area substrate by spin-coating and imprinted at low temperatures, was used to form micro- to nano-sized patterns. The PBMA patterns with a near-zero residual layer could be formed on the substrate using the partial-filling method. Using this imprinting technique, nano-sized PBMA patterns with a minimized residual layer were transferred to a Si wafer and poly(ethylene terephthalate) (PET) film with dimensions 50 ×50 mm2. Metal patterns, as small as 70 nm, were fabricated on the substrate using this imprinting and lift-off process.

Fabrication of 100nm Sized Patterns on a Non-Planar Substrate by Using Nanoimprinting Lithography

A faithful pattern transferring onto a non-planar substrate was demonstrated by nano-imprinting technique. Uniform pressing of a flexible template onto a substrate was important for the faithful pattern transferring. Both the UV-based and thermal imprinting techniques were used to transfer patterns of 200nm sized features to the non-planar substrates such as outer wall of rod and inner surface of cylinder and it could be used for nano-devices such as lab on a chip.