Chan Wook Baik

Graphene/carbon nanotube hybrid-based transparent 2D optical array.

Graphene/carbon nanotube (CNT) hybrid structures are fabricated for use as optical arrays. Vertically aligned CNTs are directly synthesized on a graphene/quartz substrate using plasma-enhanced chemical vapor deposition (PECVD). Graphene preserves the transparency and resistance during CNT growth. Highly aligned single-walled CNTs show a better performance for the diffraction intensity.

Optimization of electron beam focusing for gated carbon nanotube field emitter arrays

We fabricated gated field emitter arrays with a novel focusing structure of electron beams, where the focusing electrode concentrically surrounded each gate hole. Carbon nanotube emitters were screen printed inside an amorphous-Si concave well far below the gate. It was theoretically and experimentally verified that the concave well structure effectively focused the emitted electron beams to their designated phosphor pixels by modulating focusing gate voltages. For the vacuum packaged field emission displays with the pixel specification fitting high-definition televisions, color reproducibility of approximately 71% was achieved at the brightness of 400 cd/m/sup 2/.

Selective formation of GaN-based nanorod heterostructures on soda-lime glass substrates by a local heating method

We report on the fabrication of high-quality GaN on soda-lime glass substrates, heretofore precluded by both the intolerance of soda-lime glass to the high temperatures required for III-nitride growth and the lack of an epitaxial relationship with amorphous glass. The difficulties were circumvented by heteroepitaxial coating of GaN on ZnO nanorods via a local microheating method. Metal-organic chemical vapor deposition of ZnO nanorods and GaN layers using the microheater arrays produced high-quality GaN/ZnO coaxial nanorod heterostructures at only the desired regions on the soda-lime glass substrates. High-resolution transmission electron microscopy examination of the coaxial nanorod heterostructures indicated the formation of an abrupt, semicoherent interface. Photoluminescence and cathodoluminescence spectroscopy was also applied to confirm the high optical quality of the coaxial nanorod heterostructures. Mg-doped GaN/ZnO coaxial nanorod heterostructure arrays, whose GaN shell layers were grown with various different magnesocene flow rates, were further investigated by using photoluminescence spectroscopy for the p-type doping characteristics. The suggested method for fabrication of III-nitrides on glass substrates signifies potentials for low-cost and large-size optoelectronic device applications.

Perspectives on Nanotechnology for RF and Terahertz Electronics

Nanotechnology is key to the 21st Century, involving all aspects of nanoscale science and technology and generating a paradigm shift in diverse areas of physics, chemistry, electronics, materials, engineering, and even medicine and biology, as a result of its interdisciplinary nature. RF and terahertz electronics are among the fastest growing areas as a result of the discovery, fabrication, and investigation of nanomaterials, in particular carbon nanotubes, graphenes, and compound semiconductors. These advances in nanotechnology have led to the development of nano RF or terahertz devices, which are capable of transcending conventional devices in their compactness, efficiency, performance, and operating frequency. This paper sets out the evolution of nanotechnology in RF and terahertz electronics in which a challenge awaits for the microwave science and engineering research community.

GaN light-emitting diodes on glass substrates with enhanced electroluminescence

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates by controlling GaN crystal morphology, crystallinity, and device fabrication. Depending on the degree of epitaxy, we studied three different GaN morphologies; randomly oriented GaN polycrystals, nearly single-crystalline pyramid arrays, and fully single-crystalline pyramid arrays, which were fabricated by controlling the epitaxial relationship with the substrates. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. All the different GaN morphologies were further fabricated into LEDs to investigate their EL characteristics. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of about 2700 and 1150 cd m−2 at peak wavelengths of 537 and 480 nm, respectively.

ZnO nanostructures with controlled morphologies on a glass substrate

We report morphology-controlled selective growth of ZnO nanostructures on glass substrates by using catalyst-free metal-organic chemical vapor deposition. For the morphology-controlled selective growth, a microheating method using a series of microheaters was developed, which provided well-controlled local heating based on the microheater geometry and spatial arrangement. ZnO nanostructure morphology depended on the local growth temperature, so various nanostructure morphologies were obtained selectively at specific positions on glass substrates by using local microheating. The monolithic integration of nanostructures with different morphologies will have great potential for applications in multifunctional devices.

Selective Formation of Carbon Nanotubes and Its Application to Field-Emitter Arrays

At room-temperature ambient, carbon nanotubes (CNTs) were selectively synthesized on each bridge of the microheater array (MHA) formed on a glass substrate. Both multiwalled (MW) and single-walled CNTs could be formed by controlling the MHA temperature, which was confirmed from Raman spectroscopy. By incorporating the selectively grown MW CNTs into the lateral-gated field-emitter arrays, high current density of electron emission was observed with low device leakage.

Formation of 10-μm-level patterned organic thin film using microthermal evaporation

This article reports the fabrication of 10-μm-level stripe patterns on an organic layer using microthermal evaporation. During this process, an organic layer is coated on a glass substrate with a bridge-type microheater array, and then selectively evaporated and deposited to form a fine pattern onto another glass substrate, separated by a micron-scale gap from the microheater array. Using a unique numerical model for microthermal evaporation, thickness profiles of the pattern were calculated and matched to experimental data. Furthermore, high optical qualities of the transferred luminescent pattern layer were confirmed using microphotoluminescence spectroscopy. This microthermal evaporation method confirms the feasibility of fabricating fine organic patterns with scalability to larger sizes.

Electrically Driven Diffraction Grating Designed for Visible-Wavelength Region

We report on an electrically driven diffraction grating designed for visible light, where the refractive index of a liquid crystal (LC) was modulated periodically at an interval of 700 nm by applying an external dc bias to a metallic nanograting (NG). The LC-NG structure exhibited a maximum refractive index variation (Δn) of 0.088 and a diffraction efficiency (η) change of 0%-16% with a large diffraction angle of 64° for incident light of 633-nm wavelength. This approach, with the help of faster electronics, provides an opportunity of developing active holograms for real 3-D displays.

Prospect of GaN light-emitting diodes grown on glass substrates

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates. We found that GaN morphology affected the EL and achieved enhanced EL of GaN-LEDs on glass by identifying the optimal GaN morphology having both high crystallinity and compatibility for device fabrication. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of ~ 9100 cd/m2 at the peak wavelength of 495 nm. The EL color could be adjusted mainly by varying the quantum well temperature. In addition, new growth methods for achieving high GaN crystallinity at a low growth temperature (e.g. ~700°C) were briefly reviewed and attempted by adopting selective heating. We expect that performance of the GaN LEDs on glass can be much enhanced by enhancing GaN crystallinity and p-GaN coating, and evolvement of low-temperature growth of high-quality GaN might even customize ordinary glass as a substrate, which enables high-performance, low-cost lighting or display.