Tae Hoon Seo

Graphene-silver nanowire hybrid structure as a transparent and current spreading electrode in ultraviolet light emitting diodes

We report a device that combines graphene film and Ag nanowires (AgNWs) as transparent and current spreading electrodes for ultra-violet (UV) light emitting diode (LED) with interesting characteristics for the potential use in the deep UV region. The current-voltage characteristics and electroluminescence (EL) performance show that graphene network on AgNWs well-operates as a transparent and current spreading electrode in UV LED devices. In addition, scanning electron microscopy and EL images exhibit that graphene film act as the protection layer of AgNWs layer as well as a transparent conducting network, by bridging AgNWs.

Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode

We report a device that combines indium tin oxide (ITO) nanodot nodes with two-dimensional chemically converted graphene (CCG) films to yield a GaN-based light emitting diode (LED) with interesting characteristics for transparent and current spreading electrodes for the potential use in the ultraviolet region. The current-voltage characteristics and electroluminescence output power performance showed that CCG network on ITO nanodot nodes operated as a transparent and current spreading electrode in LED devices.

Coupling of InGaN/GaN multiquantum-wells photoluminescence to surface plasmons in platinum nanocluster

We present the coupling of InGaN/GaN multiquantum-wells (MQWs)photoluminescence(PL) to surface plasmons (SPs) in platinum (Pt) nanoclusters (PNCs). To tune the extinction spectrum of Pt thin film through surface modification such as PNC, the thermal annealing method was employed. From conventional PL and time-resolved PL measurements, enhanced emission and faster luminescence decay time of the PNC-formed InGaN/GaN MQWs was observed with blueshifted emission behavior near the plasmon absorption band of PNC. A Purcell enhancement factor, which was calculated to describe the increase in spontaneous emission rate ( R se ) , revealed an approximate 2.2 times enhancement of R se at 425 nm. We believe that these phenomena result from efficient energy transfer in PNC-formed InGaN/GaN MQWs by SPs coupling.

Direct growth of GaN layer on carbon nanotube-graphene hybrid structure and its application for light emitting diodes.

We report the growth of high-quality GaN layer on single-walled carbon nanotubes (SWCNTs) and graphene hybrid structure (CGH) as intermediate layer between GaN and sapphire substrate by metal-organic chemical vapor deposition (MOCVD) and fabrication of light emitting diodes (LEDs) using them. The SWCNTs on graphene act as nucleation seeds, resulting in the formation of kink bonds along SWCNTs with the basal plane of the substrate. In the x-ray diffraction, Raman and photoluminescence spectra, high crystalline quality of GaN layer grown on CGH/sapphire was observed due to the reduced threading dislocation and efficient relaxation of residual compressive strain caused by lateral overgrowth process. When applied to the LED structure, the current-voltage characteristics and electroluminescence (EL) performance exhibit that blue LEDs fabricated on CGH/sapphire well-operate at high injection currents and uniformly emit over the whole emission area. We expect that CGH can be applied for the epitaxial growth of GaN on various substrates such as Si and MgO, which can be a great advantage in electrical and thermal properties of optical devices fabricated on them.

Graphene-GaN Schottky diodes

The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductors were investigated. Excellent rectifying behavior with a rectification ratio of ∼107 at ±2 V and a low reverse leakage current of 1.0 × 10−8 A/cm2 at −5 V were observed. The Schottky barrier heights, as determined by the thermionic emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.24 ± 0.13 eV, respectively. Despite the predicted low barrier height of ∼0.4 eV at the graphene-GaN interface, the formation of excellent rectifying characteristics with much larger barrier heights is attributed to the presence of a large number of surface states (1.2 × 1013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier height of 1.24 eV, the work function of graphene in the Au/graphene/GaN stack could be approximately estimated to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky rectifier in GaN semiconductors with n-type conductivity.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Thin Ni film on graphene current spreading layer for GaN-based blue and ultra-violet light-emitting diodes

We introduced a thin nickel (Ni) film onto graphene as a current spreading layer for GaN-based blue and ultraviolet (UV) light emitting diodes (LEDs). The thin Ni film was confirmed to improve the electrical properties of the graphene by reducing the sheet and contact resistances. The advantages of Ni on graphene were more remarkable in UV LEDs, in which the operation voltage was reduced from 13.2 V for graphene alone to 7.1 V. As a result, UV LEDs with Ni on graphene showed a uniform and reliable light emission, at ∼83% of electroluminescence of indium tin oxide.

Improving the graphene electrode performance in ultra-violet light emitting diode using silver nanowire networks

This paper reports a systematic study on the characteristics of silver nanowires (AgNWs) coated graphene and its application as a transparent current spreading electrode in ultra-violet light emitting diodes (UV-LEDs). The optimized values of optical transmittance and sheet resistance of AgNWs covered graphene were 87.7% at 375 nm and 50 ± 5 Ω/sq, respectively. Upon applying the AgNWs on graphene electrode, the UV-LED exhibited uniform bright light emission with a reduction in the forward voltage and about four-fold increase in the electroluminescence intensity. We attribute the observed performance improvements to a reduction in the sheet and contact resistances.

Efficiency Improvement in InGaN-Based Solar Cells by Indium Tin Oxide Nano Dots Covered with ITO Films

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

Enhanced Light Output Power of Near-Ultraviolet Light-Emitting Diodes with Au-Doped Graphene for Transparent and Current-Spreading Electrode

We report the implementation of Au-doped graphene film as a transparent and current-spreading electrode (TCSE) in GaN-based near-ultraviolet (UV) light-emitting diode (LED) to achieve good UV emission efficiency. The TCSE effects of Au-doped graphene film were clearly seen in both the electroluminescence (EL) and current–voltage (I–V) characteristics. The EL output power of 380-nm wavelength UV-LEDs with Au-doped graphene film was enhanced by about 20% at an injection current of 20 mA compared with that of conventional UV-LEDs. The increase of the light output power is attributed to the high UV transmittance of graphene, effective current spreading, and injection.