Covering a graphene contact with a thin layer of nickel allows the performance of an ultraviolet LED built with this upper electrode to get close to that of a conventional device

Nickel is used twice in the fabrication of LEDs with novel contacts: first as mask (that is not removed perfectly), and again in combination with graphene, to lower sheet resistance

Grapheneis yet to fulfil its potential as a great electrode for making ultraviolet LEDs. But it has recently made great strides in that direction, thanks to the efforts of a partnership between two universities in South Korea: Gwangju Institute of Science and Technology and Chonbuk National University.

One of the most attractive features of graphene is its incredibly high transparency over a vast spectral range, but when it is used as a p-contact in ultraviolet LEDs, it leads to a very high turn-on voltage. The South Korean team has now partly addressed this, showing that this voltage can plummet from 13.2 V to 7.1 V by adding a thin nickel film onto this allotrope of carbon.

The tremendous transparency of graphene partly stems from its thinness – it is only 2-3 nm thick. “Graphene is grown on a metal foil, with compositing grains, so that it is a monolayer somewhere and could be a bilayer elsewhere,” explains corresponding author Dong-Seon Lee from Chonbuk National University. In comparison, a standard p-contact for ultraviolet LEDs, indium tin-oxide (ITO), is typically several hundred nanometres thick. Although this greater thickness hampers transparency, it aids sheet resistance, which is inversely proportional to film thickness. Graphene lags in terms of sheet resistance, but not by much, thanks to its mobility that is three orders of magnitude higher than ITO.

The researchers learnt how to enhance graphene-based ultraviolet LEDs by first studying the performance of three different blue-emitting devices: one had an ITO layer, another a graphene layer, and a third had a nickel film on top of the graphene. All three blue-emitting chips had the same heterostructure, and were formed by MOCVD growth on GaN-on-sapphire templates. To add a graphene layer, the team mixed this material with PMMA and deposited the result on the wafers, before warm acetone removed the acrylate.

To make graphene-only LEDs and those with the contact made from nickel and graphene, a 100 nm-thick layer of nickel was deposited onto the graphene-coated structures to act as a mesa-etching mask (see Figure). Etching with reactive ions and an inductively coupled plasma exposed the n-GaN, before the nickel mask was etched in hydrochloric acid.

Fabrication of the nickel-on-graphene LED involved deposition of a 3 nm-thick nickel layer. This device, and that with just the graphene contact, were then annealed in nitrogen at 500 °C.

The team has studied the transmittance of all three types of contact. For graphene, it exceeds 90 percent from 300 nm to 700 nm; while for ITO it is above 80 percent above 400 nm, but falls to 70 percent at 380 nm; and for nickel-on-graphene, it is about 75 percent for all wavelengths above 380 nm. Sheet resistance for the graphene sample is 1250 Ω/¨, while the addition of nickel drops this to 690 Ω/¨.In comparison, for ITO it is just 43 Ω/¨. Lee admits that this is an issue: “One of the most important things in graphene research must be [the development of] a growth technology enabling not only ultra-high transparency, but a comparable sheet resistance to ITO.”

The team has scrutinised the surface of the graphene-coated LED with a scanning electron microscope and found that the wet-etching process for removing the nickel mask is not perfect – it leaves traces of the material. These nano-islands that remained cover a very small fraction of the surface but are not believed to impact the optical transmission or sheet resistance of the graphene.
Driven at 20 mA, forward voltages for the ITO, nickel-on-graphene and graphene LEDs were 3.5 V, 4.8 V and 6.2 V, respectively. According to the team, graphene leads to a higher forward voltage than ITO, due to higher sheet and contact resistances. The addition of a thin nickel layer onto the graphene leads to the diffusion of these metal atoms, which have a lower workfuntion and reduce the Schottky barrier height.

The team went on to apply its nickel-on-graphene contacts to ultraviolet LEDs emitting at 380 nm. At 20 mA, this device produced 83 percent of the output of the ITO control. This shows that for ultraviolet LEDs, when the carbon allotrope is coated with nickel, it can get close to the performance of the more traditional ITO-based design – and further optimisation should enable this structure to surge ahead.

“Our next plan is to use the metal-combined-graphene-transparent as an electrode for a diverse range of optoelectronic devices, such as solar cells, detectors and transistors,” says Lee.

J.-P. Shim et. al. Appl. Phys. Lett.
102 151115 (2013)