Graphene-Based Transparent Conductive Electrodes

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Mar 21 2022

In an article recently published in the open-access journal Materials, researchers discussed the utility of graphene as a transparent conductive electrode for the development of GaN-based light-emitting diodes (LEDs).

Study: Graphene as a Transparent Conductive Electrode in GaN-Based LEDs. Image Credit: Rost9/Shutterstock.com

Background

In the ultraviolet (UV) and visible spectral ranges, GaN-based LEDs have been extensively explored. Because the p-type cladding layers frequently employed in GaN-based LEDs have deep acceptors with high activation energy, the p-cladding layer is insufficiently conductive.

As a result, current crowding along the p-electrode margins occurs, potentially jeopardizing the LED devices' stability and dependability. A transparent current spreading layer (TCSL) with high optical transparency and low electrical sheet resistance is required for a more homogenous current distribution. Scientists sought to directly employ Ni/Au as a transparent and conductive electrode to overcome the current crowing problem and increase the transparency of Ni/Au thin films in the 400–750 nm spectrum region by quick thermal annealing (RTA).

Following that, many studies reported the use of transparent conductive oxides as TCSL, like indium tin oxide (ITO). ITO is not appropriate for use in flexible devices because of its mechanical brittleness. Graphene, a two-dimensional (2D) carbon material with a single atom thickness, has low sheet resistance (R_S), excellent transparency, and high heat conductivity. GaN-based LEDs with graphene as the TCSL have been extensively explored in the second decade of the 21^st century.

About the Study

In this study, the authors presented an overview of current graphene-based transparent conductive electrodes in GaN-based LEDs. The manufacturing process and the attributes of the created devices were emphasized. The effects of graphene-based transparent conductive electrodes on contact resistance and current spreading were explored. The opportunities and problems associated with the use of graphene in GaN-based LEDs were also discussed.

The researchers primarily focused on the investigations related to lowering contact resistance and maintaining the high transparency and low sheet resistance of graphene-based TCSL after incorporation into GaN-based LEDs. The multiple efforts to integrate graphene-based TCSL into GaN-based LEDs with multi-quantum wells (MQW) or single-quantum wells (SQW) were reviewed. A hybrid graphene structure that included graphene networked with ITO nanodots, a metal interlayer, nanoparticles, nanowires, and various graphene geometries was presented.

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The team illustrated the spreading's working mechanisms. They also discussed contact engineering work aimed at minimizing contact resistance, which was considered as one of the primary barriers to graphene-based TCSL integration in GaN-based LEDs. The two basic strategies for lowering the interface barrier between p-GaN and graphene, namely, p-doping graphene for work function adaptation and inserting a thin dielectric film, were demonstrated. Advanced device architectures of GaN-based LEDs with graphene as TCSL and 3D architecture were provided.

Observations

One of the studies reported that on a planar SiO₂ substrate, the R_S of three-layer graphene (3LG) was found to be 1000 Ω/sq, and only 300 Ω/sq on nanorods. Changing the substrate from SiO₂ to a triangle lattice air hole photonic crystal (PC) GaN-based LED resulted in a drop in R_S in two-layer graphene (2LG) from 300 Ω/sq to 107 Ω/sq.

Following the deposition of few-layer graphene (FLG) onto NiO_x/p-GaN LEDs, 300-s thermal annealing in N₂ reduced the transmission from over 95% at 450 nm to roughly 93.6% by RTA at 350°C. By lowering the Ni thickness from 2 nm to 1 nm, the transmission of graphene/NiO_x was enhanced from roughly 80% to over 90%.

As-grown graphene's sheet resistance and contact resistance were many orders of magnitude higher than traditional ITO in the early stages of growth, yet both materials' transparency in the visible spectral area was comparable. The introduction of damage during the fabrication process was attributed to the high sheet resistance in graphene films, whereas the high contact resistance could be due to the intrinsic large work function difference between graphene and p-GaN, as well as poor adhesion and graphene damage caused by the transfer process.

Hybrid architectures that network graphene with conductive components in the form of layers, nanoparticles, or nanowires were used to reduce sheet resistance. Work function engineering of graphene through p-doping and the addition of an ultrathin oxide layer such as NiO_x improved the contact resistance at the graphene/p-GaN interface. However, the creation of agglomerated metal nanoparticles in the case of metal doping limited the light transmission.

Conclusions

In conclusion, this review elucidated that the attributes of graphene-based TCSLs can be improved by changing the production processes or achieving graphene hybrid structures. Transfer and direct growth of graphene sheets on GaN-based LEDs can be both used in their production.

The authors emphasized that to avoid a large transmission loss when inserting an ultrathin oxide layer such as NiO_x between graphene and p-GaN, both the stoichiometry and thickness of NiO_x must be carefully managed. They also believe that systematic research is currently lacking, and a reasonable design strategy for graphene as a TCSL in GaN-based LEDs is necessary.

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