Chongmin Lee

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.

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.

Graphene as a diffusion barrier for Al and Ni/Au contacts on silicon

The insertion of chemically vapor deposited graphene layers between Al metallization and Si substrates and between Au and Ni metal layers on Si substrates is shown to provide a significant reduction in spiking and intermixing of the metal contacts and reaction with the Si, where the bilayer graphene was transferred to the samples after the Cu-foil was etched. The graphene prevents reaction between Al and Si up to the temperatures of 700 °C and the intermixing of Au and Ni up to the temperatures of at least 600 °C. The outstanding performance of the graphene as a metal diffusion barrier will be very useful to improve the stability of the metallizations at elevated temperatures.

Selective chemical etch of gallium nitride by phosphoric acid

The authors report on the direct comparison of the chemical etch characteristics on both Ga- and N-face gallium nitride (GaN) by phosphoric acid. First, Ga-face GaN was grown next to N-face GaN by a polarity inversion method in a metal-organic chemical vapor deposition reactor. Micro-photoluminescence, atomic force microscopy, scanning electron microscopy, and micro-Raman spectroscopy were used to analyze the etch characteristics of Ga- and N-face GaN before and after a H3PO4-based chemical etch. Ga-face was chemically stable in a phosphoric acid solution. However, the chemical etch continued proceeding on the N-face GaN due to the weak repulsive force to OH− ions. Dodecagonal nano-pyramids which dramatically enhanced the photoluminescence intensity were observed on N-face GaN after a H3PO4-based chemical etch.

ZnO-Based Cyclodextrin Sensor Using Immobilized Polydiacetylene Vesicles

We report that polydiacetylenes (PDAs) vesicles were successfully immobilized and chemisorbed on single crystal ZnO surfaces. Immobilized PDAs on ZnO were found to be sensitive to temperature and selectively sensitive to α- and γ-cyclodextrins. This approach is attractive for the on-chip integration of various types of sensors, ultraviolet light emitting diodes, and transparent electronics with PDAs.

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...