Negative-tone block copolymer lithography by in situ surface chemical modification.

Negative-tone block copolymer (BCP) lithography based on in situ surface chemical modification is introduced as a highly efficient, versatile self-assembled nanopatterning. BCP blends films consisting of end-functionalized low molecular weight poly(styrene-ran-methyl methacrylate) and polystyrene-block-Poly(methyl methacylate) can produce surface vertical BCP nanodomains on various substrates without prior surface chemical treatment. Simple oxygen plasma treatment is employed to activate surface functional group formation at various substrates, where the end-functionalized polymers can be covalently bonded during the thermal annealing of BCP thin films. The covalently bonded brush layer mediates neutral interfacial condition for vertical BCP nanodomain alignment. This straightforward approach for high aspect ratio, vertical self-assembled nanodomain formation facilitates single step, site-specific BCP nanopatterning widely useful for various substrates. Moreover, this approach is compatible with directed self-assembly approaches to produce device oriented laterally ordered nanopatterns.

Enhancement of light extraction efficiency of OLEDs using Si3N4-based optical scattering layer.

An optical scattering layer, consisting of a Si3N4 nano-pillar array and a spin-coated hydrogen silsesquioxane (HSQ) planarization layer, was introduced to an organic light-emitting diode (OLED) substrate to increase the out-coupling efficiency. After plasma enhanced chemical vapor deposition (PECVD) of the Si3N4 layer, the nano-pillar array was created using nanoimprint lithography and reactive ion etching. As the Si3N4 pillar array has a refractive index of 2.0, photons generated in the organic layer are scattered by the Si3N4 structures and thus have a higher chance of being emitted from the device. The spin-coated HSQ planarization layer produces a flat substrate, which is essential for depositing a uniform organic material layer and assuring the electric conductivity of the transparent conducting oxide (TCO) layer. In this study, Si3N4 nano-structures with a height of 100 or 300 nm were used to enhance the out-coupling efficiency of the OLED devices. Although the electrical conductivity of the TCO layer deposited on the light scattering layer was slightly degraded, the OLED devices formed with the light scattering layer exhibited a higher luminous power at given electrical power. Consequently, the use of a planarized 300-nm-thick Si3N4 layer increased the external quantum efficiency of the OLED device by 50% at 10,000 cd/m2 compared to the reference OLED device fabricated on a flat glass substrate.

Improved AlGaInP vertical emitting light-emitting diodes using direct printing.

In this study, we fabricated a high-brightness AlGaInP light-emitting diode (LED) using the direct printing technique and dry etching. In general, wet etching is used for surface roughening to improve the light extraction of AlGaInP red LEDs. However, a structure fabricated by wet etching has limited height and shows a tiled cone shape after the etching process due to the AlGaInP crystal structure. These limitations reduce the light extraction of the LED. As a result, we fabricated a perfectly cone-shaped pattern with high aspect ratio using direct printing by etching to maximize the LED light extraction efficiency. Compared to the red LED with a wet-etched structure, the patterning enhanced the light output power by 12% without electrical degradation. This enhanced light output power was maintained even after the packaging process.

Fabrication of 3D nano-structures using reverse imprint lithography.

In spite of the fact that the fabrication process of three-dimensional nano-structures is complicated and expensive, it can be applied to a range of devices to increase their efficiency and sensitivity. Simple and inexpensive fabrication of three-dimensional nano-structures is necessary. In this study, reverse imprint lithography (RIL) with UV-curable benzylmethacrylate, methacryloxypropyl terminated poly-dimethylsiloxane (M-PDMS) resin and ZnO-nano-particle-dispersed resin was used to fabricate three-dimensional nano-structures.UV-curable resins were placed between a silicon stamp and a PVA transfer template, followed by a UV curing process. Then, the silicon stamp was detached and a 2D pattern layer was transferred to the substrate using diluted UV-curable glue. Consequently, three-dimensional nano-structures were formed by stacking the two-dimensional nano-patterned layers. RIL was applied to a light-emitting diode (LED) to evaluate the optical effects of a nano-patterned layer. As a result, the light extraction of the patterned LED was increased by about 12% compared to an unpatterned LED.

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.

Forming the graded-refractive-index antireflection layers on light-emitting diodes to enhance the light extraction.

Distributed antireflection (AR) layers with different composition ratios of ITO and SiO(2) formed on an ITO electrode of GaN-based LEDs provide substantial enhancement in light-extraction efficiency. By using the coradio frequency magnetron sputtering deposition, four 50 nm thick AR layers with graduated refractive indices were fabricated. The effect of the AR layers on enhancing the efficiency of the LED device was analyzed by electroluminescence (EL) and I-V measurements. As a result, the EL intensity of the LED device grown on the patterned sapphire substrate with AR layers was increased by up to 13% compared to the conventional patterned sapphire substrate-applied LED device without AR layers at a drive current of 20 mA. The AR layers on top of the LED device gradually changed the refractive indices between ITO (n=2.1) and air (n=1.0), which minimized the total internal reflection of generated light. And no degradation in the electrical characteristic of the LEDs was observed according to the I-V measurements.

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.

Scattering Optical Elements: Stand-Alone Optical Elements Exploiting Multiple Light Scattering.

Optical design and fabrication techniques are crucial for making optical elements. From conventional lenses to diffractive optical elements and to recent metasurfaces, various types of optical elements have been proposed to manipulate light where optical materials are fabricated into desired structures. Here, we propose a scattering optical element (SOE) that exploits multiple light scattering and wavefront shaping. Instead of fabricating optical materials, the SOE consists of a disordered medium and a photopolymer-based wavefront recorder, with shapes impinging on light on demand. With the proposed stand-alone SOEs, we experimentally demonstrate control of various properties of light, including intensity, polarization, spectral frequency, and near field. Due to the tremendous freedom brought about by disordered media, the proposed approach will provide unexplored routes to manipulate arbitrary optical fields in stand-alone optical elements.

In Situ Nanolithography with Sub-10 nm Resolution Realized by Thermally Assisted Spin-Casting of a Self-Assembling Polymer.

In situ nanolithography is realized based on warm spin-casting of block copolymer solutions. This advancement is based on Si-containing block copolymers with an appropriate thermodynamic driving force for spontaneous phase-separation combined with the thermal assistance provided by slight temperature elevations during the spin-casting. Sub-10 nm half-pitch nanoscale patterns are produced within 30 s without a separate annealing process.

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.