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Korean Team Makes Flexible LEDs With GaN Micro-rods On Graphene

LEDs operate reliably after 1000 bending cycles 

A team of resarchers from Seoul National University in Korea have reported growing GaN micro-rods and coaxial quantum-well heterostructures on graphene films for applications in flexible optical devices.

Graphene films were grown on copper foil by means of chemical vapour deposition, and then used as the substrates for the growth of the GaN micro-rods, which were subsequently transferred onto SiO2/silicon substrates.

Highly silicon-doped, n-type GaN micro-rods were grown on the graphene films using MOCVD. The growth and vertical alignment of the GaN micro-rods, which is a critical factor for the fabrication of high-performance LEDs, were characterised using electron microscopy and X-ray diffraction.

According to the researchers the GaN micro-rods exhibited promising photoluminescence characteristics for optoelectronic device applications, including room-temperature stimulated emission.

To fabricate flexible LEDs, InxGa1-xN/GaN multiple quantum wells and a p-type GaN layer were deposited coaxially on the GaN micro-rods, and transferred onto Ag-coated polymer substrates using lift-off. Ti/Au and Ni/Au metal layers were formed to provide electrical contacts to the n-type and p-type GaN regions, respectively.

Above: GaN micro-rod LEDs fabricated on graphene films. (a) An FE-SEM image of coaxial GaN micro-rod LEDs on graphene. Scanning TEM images of (b) the top and (c) the sidewall of the MQW layers on the micro-rod LED. (d) A schematic illustration of the fabrication process for vertical structure micro-rod LEDs. (e) Magnified optical images of light emission from the LED. (f) The power-dependent EL spectra at room temperature.

According to the paper in the open access journal APL Materials, the micro-rod LEDs exhibited intense emission of visible light, even after transfer onto the flexible polymer substrate, and reliable operation was achieved following 1000 bending cycles. 

The researchers believe that  transferable fabrication route to achieve flexible inorganic semiconductor/graphene heterostructures on a polymer substrate has numerous potential applications in flexible optoelectronic devices, and provides a method of integrating organic substrates and inorganic semiconductor materials without the thermal budget restrictions that are typical of polymer substrates.

"˜Growth and characterisations of GaN micro-rods on graphene films for flexible light emitting diodes' by Kunook Chung et al, appears in APL Mat. 2, 092512 (2014); doi.org/10.1063/1.4894780



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