Performance evaluation of GaN light-emitting diodes using transferred graphene as current spreading layer (original) (raw)
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Optics Express, 2014
Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.
Journal of Physics D: Applied Physics, 2012
This paper describes a detailed systematic study based on the fabrication and performance of InGaN/GaN blue light-emitting diodes (LEDs) with multilayer graphene film as a current spreading electrode. Two facile approaches to improve the electrical coupling between graphene and p-GaN layer are demonstrated. Using chemical charge transfer doping, the work function ( ) of graphene is tuned over a wide range from 4.21 to 4.93 eV with substantial improvements in sheet resistance (R s ). Compared with pristine graphene, the chemically modified graphene on p-GaN yields several appealing characteristics such as low specific contact resistance (ρ c ) and minimized barrier height. In addition, insertion of a thin gold interlayer between graphene and p-GaN profoundly enhances the contact properties at the interface. Combining these two approaches in a single LED, the current spreading and thus the device forward voltage (V f ) are considerably improved comparable to that of an LED fabricated with an indium tin oxide electrode. The importance of pre-metal deposition oxygen plasma treatment and rapid thermal annealing in improving the contact characteristics is also addressed.
Applied Physics Letters, 2013
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.
Graphene as a Transparent Conductive Electrode in GaN-Based LEDs
Materials, 2022
Graphene combines high conductivity (sheet resistance down to a few hundred Ω/sq and even less) with high transparency (>90%) and thus exhibits a huge application potential as a transparent conductive electrode in gallium nitride (GaN)-based light-emitting diodes (LEDs), being an economical alternative to common indium-based solutions. Here, we present an overview of the state-of-the-art graphene-based transparent conductive electrodes in GaN-based LEDs. The focus is placed on the manufacturing progress and the resulting properties of the fabricated devices. Transferred as well as directly grown graphene layers are considered. We discuss the impact of graphene-based transparent conductive electrodes on current spreading and contact resistance, and reveal future challenges and perspectives on the use of graphene in GaN-based LEDs.
Japanese Journal of Applied Physics, 2011
We report the enhanced light output power of GaN-based light-emitting diode (LED) by using graphene film as a transparent conducting electrode. Monolayer graphene was synthesized on copper foil by using chemical vapor deposition method and directly transferred onto the GaN-LED as a top electrode. Compared to the conventional LEDs using indium tin oxide (ITO) layer for an electrode material, the light output power of LED with graphene electrode was improved by 25%. This was attributed excellent graphene characteristics of high electrical conductivity, high optical transmittance of nearly 97% over a wide range of infrared, visible, and ultraviolet region and large area uniformity with fewer defects.
Nanotechnology, 2010
This work demonstrates a large-scale batch fabrication of GaN light-emitting diodes (LEDs) with patterned multi-layer graphene (MLG) as transparent conducting electrodes. MLG films were synthesized using a chemical vapor deposition (CVD) technique on nickel films and showed typical CVD-synthesized MLG film properties, possessing a sheet resistance of ∼620 / with a transparency of more than 85% in the 400-800 nm wavelength range. The MLG was applied as the transparent conducting electrodes of GaN-based blue LEDs, and the light output performance was compared to that of conventional GaN LEDs with indium tin oxide electrodes. Our results present a potential development toward future practical application of graphene electrodes in optoelectronic devices.
Applied Physics Letters, 2011
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.
Metal-Free Graphene as Transparent Electrode for GaN-Based Light-Emitters
Japanese Journal of Applied Physics, 2013
Graphene contacts to p-GaN are considered as an alternative to indium-tin-oxide transparent electrodes in GaN based verticalcavity surface-emitting lasers (VCSELs). Contact properties were investigated on light-emitting diode and p-GaN test structures, where dielectric apertures were used to eliminate the influence of the metal pads used to bias the contacts. Using single layer graphene we were able to operate light emitting diodes with current densities of 300 A/cm 2. Addition of a second layer of graphene increased the maximum bias current to 1 kA/cm 2. However, the contacts are non-linear and cannot withstand high current densities for a long time. The results are promising but further investigation and improvement is needed for graphene to be a viable alternative to indium-tin-oxide for blue VCSELs.
GaN-based light-emitting diodes on graphene-coated flexible substrates
Optics Express, 2014
We demonstrate GaN-based thin light-emitting diodes (LEDs) on flexible polymer and paper substrates covered with chemical vapor deposited graphene as a transparent-conductive layer. Thin LEDs were fabricated by lifting the sapphire substrate off by Excimer laser heating, followed by transfer of the LEDs to the flexible substrates. These substrates were coated with tri-layer graphene by a wet transfer method. Optical and electrical properties of thin laser lift-offed LEDs on the flexible substrates were characterized under both relaxed and strained conditions. The graphene on paper substrates remained conducting when the graphene/paper structure was folded. The high transmittance, low sheet resistance and high failure strain of the graphene make it an ideal candidate as the transparent and conductive layer in flexible optoelectronics.