Byung Jae Kim

Large-area transparent conductive few-layer graphene electrode in GaN-based ultra-violet light-emitting diodes

We report on the development of a large-area few-layer graphene (FLG)—based transparent conductive electrode as a current spreading layer for GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Large-area FLG was deposited on Cu using the chemical vapor deposition (CVD) method and subsequently transferred to the surface of the UV LED. UV light at a peak of 372 nm was emitted through the FLG-based transparent conductive electrode. The current spreading effects of FLG were clearly evident in both the optical images of electroluminescence (EL) and current-voltage (I-V) characteristics. Degradation of the FLG-based transparent conductive electrode could be induced by high power operation. Our results indicate that a large-area FLG-based electrode on GaN offers excellent current spreading and ultra-violet transparency properties when compared to the standard optoelectronic indium tin oxide (ITO) contact layer.

Three-Dimensional Multilayered Nanostructures with Controlled Orientation of Microdomains from Cross-Linkable Block Copolymers

Three-dimensional (3D) nanostructures were obtained by the directed formation of multilayer block copolymer (BCP) thin films. The initial step in this strategy involves the assembly and cross-linking of cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) BCP, in which 1.5 mol % of reactive azido (-N(3)) groups were randomly incorporated along the styrene backbone. Significantly, assembly of thin films of lamellar-forming BCPs on top of the underlying cross-linked cylindrical layer exhibited perpendicular orientations of microdomains between lamellae and cylinder layers. From the theoretical calculation of free energy in the multilayers, it was found that the nematic interactions between polymer chains at the interface play a critical role in the perpendicular orientation of lamellae on the cross-linked cylinder layers. Removal of the PMMA domains then affords nonsymmetrical nanostructures which illustrate the promise of this strategy for the design of well-defined 3D nanotemplates. It was also demonstrated that this structure can be effectively used to enhance the light extraction efficiency of GaN light-emitting diodes. Furthermore, we anticipate that such 3D nanotemplates can be applied to various areas, including advanced BCP nanolithography and responsive surface coating.

Transparent conductive graphene electrode in GaN-based ultra-violet light emitting diodes

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

Buried graphene electrodes on GaN-based ultra-violet light-emitting diodes

We report that the oxidation of graphene-based highly transparent conductive layers to AlGaN/GaN/AlGaN ultra-violet (UV) light-emitting diodes (LEDs) was suppressed by the use of SiNX passivation layers. Although graphene is considered to be an ideal candidate as the transparent conductive layer to UV-LEDs, oxidation of these layers at high operating temperatures has been an issue. The oxidation is initiated at the un-saturated carbon atoms at the edges of the graphene and reduces the UV light intensity and degrades the current-voltage (I-V) characteristics. The oxidation also can occur at defects, including vacancies. However, GaN-based UV-LEDs deposited with SiNX by plasma-enhanced chemical vapor deposition showed minimal degradation of light output intensity and I-V characteristics because the graphene-based UV transparent conductive layers were shielded from the oxygen molecules. This is a simple and effective approach for maintaining the advantages of graphene conducting layers as electrodes on UV-LEDs.

Three-dimensional graphene foam-based transparent conductive electrodes in GaN-based blue light-emitting diodes

We demonstrated three-dimensional (3D) graphene foam-based transparent conductive electrodes in GaN-based blue light-emitting diodes (LEDs). A 3D graphene foam structure grown on 3D Cu foam using a chemical vapor deposition method was transferred onto a p-GaN layer of blue LEDs. Optical and electrical performances were greatly enhanced by employing 3D graphene foam as transparent conductive electrodes in blue LED devices, which were analyzed by electroluminescence measurements, micro-Raman spectroscopy, and light intensity-current-voltage testing. The forward operating voltage and the light output power at an injection current of 100 mA of the GaN-based blue LEDs with a graphene foam-based transparent conductive electrode were improved by ∼26% and ∼14%, respectively. The robustness, high transmittance, and outstanding conductivity of 3D graphene foam show great potentials for advanced transparent conductive electrodes in optoelectronic devices.

Fabrication of GaAs subwavelength structure (SWS) for solar cell applications

We developed a novel GaAs subwavelength structure (SWS) as an antireflective layer for solar cell applications. The GaAs SWS patterns were fabricated by a combination of nanosphere lithography (NSL) and reactive ion etching (RIE). The shape and height of the GaAs SWS were controlled by the diameter of the SiO2 nanospheres and the etching time. Various GaAs SWS were characterized by the reflectance spectra. The average reflectance of the polished GaAs substrate from 200nm to 800nm was 35.1%. However, the average reflectance of the tapered GaAs SWS was reduced to 0.6% due to scattering and moth-eye effects.

GaN-based ultraviolet light-emitting diodes with AuCl 3 -doped graphene electrodes

We demonstrate AuCl3-doped graphene transparent conductive electrodes integrated in GaN-based ultraviolet (UV) light-emitting diodes (LEDs) with an emission peak of 363 nm. AuCl3 doping was accomplished by dipping the graphene electrodes in 5, 10 and 20 mM concentrations of AuCl3 solutions. The effects of AuCl3 doping on graphene electrodes were investigated by current-voltage characteristics, sheet resistance, scanning electron microscope, optical transmittance, micro-Raman scattering and electroluminescence images. The optical transmittance was decreased with increasing the AuCl3 concentrations. However, the forward currents of UV LEDs with p-doped (5, 10 and 20 mM of AuCl3 solutions) graphene transparent conductive electrodes at a forward bias of 8 V were increased by ~48, 63 and 73%, respectively, which can be attributed to the reduction of sheet resistance and the increase of work function of the graphene. The performance of UV LEDs was drastically improved by AuCl3 doping of graphene transparent conductive electrodes.

GaN-Based Light-Emitting Diode With Three-Dimensional Silver Reflectors

We present a simple and robust method to fabricate three-dimensional Ag reflectors on GaN light-emitting diodes (LEDs) using SiO 2 nanospheres as the template. First, the hexagonal arrays of SiO2 nanosphere monolayer were spun-cast on a benzocyclobutene (BCB) layer, which was prepared on a sapphire surface. Then, the bottom half of the SiO2 nanospheres were embedded into the BCB layer after heating, resulting in arrays of ldquonano-lensesrdquo that were in the shape of convex hemispheres. The concave-shaped hemisphere arrays were produced by etching the SiO2 nanospheres with an HF solution. Ag was deposited onto both patterns, concave and convex hemispheres, resulting in the formation of three-dimensional Ag reflectors. From the electroluminescence measurements, these Ag reflectors, which contained either concave or convex hemisphere patterns, were found to enhance the light output of GaN LEDs by as much as 29%-33%.

Large-area suspended graphene on GaN nanopillars

The authors have demonstrated large-area suspended graphene on GaN nanopillars predefined by nanosphere lithography and inductively coupled plasma etching. The graphene was successfully suspended over large areas without ripples and corrugations. Scanning electron microscopy, atomic force microscopy and micro-Raman spectroscopy were used to characterize the properties of the suspended graphene on nanopillars. The thermal properties of the suspended and supported graphene were investigated by varying the underlying GaN nanopilllar geometries from flat-top to sharp-cone morphologies and heating the resulting structures via irradiation with laser powers of 1.53 mW, 8.03 mW, and 16.19 mW. The heat transfer was effective even when the contact area between the suspended graphene and the supporting substrate was small, due to the high thermal conductivities of graphene and GaN. The extremely high thermal conductivity of the graphene can improve the thermal management in GaN-based high power electronic and optoel...

Enhancement of the Light-Extraction Efficiency of GaN-Based Light Emitting Diodes Using Graded-Refractive-Index Layer by SiO2 Nanosphere Lithography

A benzocyclobutene (BCB)-based graded-refractive-index (GRIN) layer was deposited on gallium nitride (GaN)-based blue light-emitting diodes (LEDs) to enhance the light-extraction efficiency. The GRIN layer, which was composed of both a BCB thin film and a porous structure, was fabricated by nanospheres lithography using SiO 2 nanospheres. The refractive index of the porous BCB layer was intentionally controlled to reduce the total internal reflection of GaN-based LEDs. The refractive indexes of the BCB layer and porous BCB layer were 1.58 and 1.2, respectively. Consequently, the photoluminescence intensity was enhanced 1.6 times after employing the GRIN BCB layer on the GaN layer, and the electroluminescence intensity at a 10 mA injection current was increased by 22% after employing the GRIN BCB layer on an indium tin oxide layer.