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US researchers use graphene to make flexible blue LEDs on plastic

Epitaxial graphene grown on SiC used as a template for the growth of GaN

Despite having many superior electrical properties, graphene has not had a significant impact in electronics because it has no band gap. Now researchers affiliated to the IBM T. J. Watson Research Center in New York, have discovered a new way of using it.

Instead of focusing on its electrical properties, they have exploited graphene's crystalline nature and the fact it does not form strong bonds with other materials. In this way, they have made fully functional flexible III-nitride blue LEDs on plastic.

In the work recently reported in Nature Communications, the team claim three major breakthroughs: large-scale epitaxial growth of 'single crystalline' GaN on graphene ([0001] orientation); exfoliation of entire single crystalline GaN film from the graphene template and dry-transfer onto a Si substrate; and multiple growth/transfer of GaN by the reuse of a single graphene template.

Above: Schematic of a method for growing/transferring single-crystalline thin films on/from epitaxial graphene. (a) Graphitisation of a SiC substrate to form epitaxial graphene. (b) Epitaxial growth of GaN on graphene. (c) Deposition of a stressor layer (Ni). (d) Release of GaN from the substrate with a handling tape. (e) Transfer of the released GaN/Ni/tape stack on a host substrate. (f) Removal of the tape and Ni by thermal release and wet etching, leaving a GaN film on the host substrate.

There have been efforts to grow 3D semiconductors directly on graphene or graphite, but polycrystalline clusters have been obtained because of the suppressed nucleation on graphene. In this study, the team employed epitaxial graphene grown on SiC as a template for the growth of GaN.

"With a careful control of growth kinetics and energetics, we achieved high quality single-crystalline GaN films on graphene. Remarkably, the surface roughness and a defect density of the GaN on graphene were comparable to those of GaN grown on conventional substrates, sapphire or SiC," report Jeehwan Kim and Can Bayram who led the research.

Subsequent exfoliation, they say, will allow a transfer of high quality GaN films on any substrates of interest.

For example, the team has shown the integration of GaN devices on a (100) Si substrate that can offer unprecedented flexibility in device design and fabrication for advanced photonic device applications. Moreover, it has demonstrated this technique enables the multiple GaN growth/transfer in a large scale with a reusable graphene/SiC substrate.

Considering the requirement of the expensive substrates for the conventional GaN growth methods, the technique of the multiple growth/transfer is also a significant progress toward extremely low-cost wafer-scale fabrication of GaN devices, claim the researchers.

To prove this concept, the team has grown p-GaN/InGaN-GaN multi-quantum wells/n- GaN on graphene/SiC substrates that have been previously used multiple-times. A fully functional vertical blue LED was obtained by growing LED stacks on reused graphene/SiC substrates followed by transfer onto plastic tapes.

According to the Kim and Bayram, the concept of semiconductor growth and transfer by using the graphene template could be generally applied to many other 3D and 2D materials. "In addition, our results pave the way where graphene-based technology is a reality and thin film single crystalline III-Nitride devices are available in near future," they say.

'Principle of direct van der Waals epitaxy of single-crystalline films on epitaxial graphene' by J. Kim & C. Bayram et al. Nat. Commun. 5, 4836 (11 Sep. 2014) doi:10.1038/ncomms5836

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