Metal-Free Graphene as Transparent Electrode for GaN-Based Light-Emitters (original) (raw)
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Journal of Applied Physics, 2014
This study elucidates the correlation among conductivity of graphene and interface aspects in GaN light-emitting diodes (LEDs). Using a multilayer graphene of low sheet resistance, it is demonstrated that graphene alone can make ohmic contact with p-GaN without necessitating additional interlayer. Large-area blue LED with relatively low contact resistance in the order of 10 À2 ohm-cm 2 and improved forward voltage of 3.2 6 0.1 V was realized irrespective of the use of the interlayer. The results from parallel evaluation experiments performed by varying the layer numbers of graphene with ultrathin NiO x interlayer revealed that the poor lateral conductivity of monolayer or few layer graphene can be well compensated by the interlayer. A combination of three layer graphene and NiO x offered device with enhanced electro-optical performance. But the Schottky barrier associated with the inadequate adhesion of transferred graphene dominates all the benefits and becomes a major bottleneck preventing the formation of low resistance stable ohmic contact. V C 2014 AIP Publishing LLC. [http://dx.
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.
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.
GaN‐Based Nanorods/Graphene Heterostructures for Optoelectronic Applications
physica status solidi (b), 2019
The insulating character of sapphire, meltback etching of Si, bulk and surface defects prevented the efficient integration of GaN nanostructures in optoelectronic devices. Here, it is demonstrated that graphene can simultaneously serve as an electrical bottom contact, a chemically inert buffer layer, and a superior lattice and thermal matched growth substrate. Vertically aligned, high crystal quality GaN nanorods (NRs) without bulk defects such as threading dislocations and with only a mild strain at the NRs’ base are grown by metal‐organic vapor‐phase epitaxy on defect‐free graphene using nanometer‐sized AlxGa1−xN nucleation islands. Here no influence of the supporting substrate on the GaN epitaxy is observed. However, at defects in graphene the effects of dangling bonds and the underlying substrate, presumably through nanoholes in graphene, on the properties of GaN NRs are visible. It is also shown that surface defects in InxGa1−xN/GaN NRs from planar films produced by etching of ...
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.
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.
One-step graphene coating of heteroepitaxial GaN films
Nanotechnology, 2012
Today, state-of-the-art III-Ns technology has been focused on the growth of c-plane nitrides by metal-organic chemical vapor deposition (MOCVD) using a conventional two-step growth process. Here we show that the use of graphene as a coating layer allows the one-step growth of heteroepitaxial GaN films on sapphire in a MOCVD reactor, simplifying the GaN growth process. It is found that the graphene coating improves the wetting between GaN and sapphire, and, with as little as ∼0.6 nm of graphene coating, the overgrown GaN layer on sapphire becomes continuous and flat. With increasing thickness of the graphene coating, the structural and optical properties of one-step grown GaN films gradually transition towards those of GaN films grown by a conventional two-step growth method. The InGaN/GaN multiple quantum well structure grown on a GaN/graphene/sapphire heterosystem shows a high internal quantum efficiency, allowing the use of one-step grown GaN films as 'pseudo-substrates' in optoelectronic devices. The introduction of graphene as a coating layer provides an atomic playground for metal adatoms and simplifies the III-Ns growth process, making it potentially very useful as a means to grow other heteroepitaxial films on arbitrary substrates with lattice and thermal mismatch.