III-Nitride-based nanowire LEDs have shown high internal quantum efficiency and stable light emission over a wide current range to enable white phosphor-free white-light emission. The structures provide a unique advantage for flexible electronics where the out-of-plane three-dimensional nanowires are invariant to applied bending. Testing this concept, InGaN dot-in-wire light-emitting diodes on sapphire substrates were transferred onto flexible polyethylene terephthalate (PET) substrates using a bonding and laser-liftoff process.
In0.15Ga0.85N nanowire blue LEDs were grown on sapphire substrates having Ni/Au contacts applied to the top p-doped region and a lateral insulating polyimide layer applied between the nanowires. The nanowire structures were then bonded onto a transfer wafer and separated from its sapphire growth substrate by laser-liftoff (LLO) using a 266 nm KrF laser. A double-transfer technique, where the inverted LED structures were then transferred and bonded onto the PET with a silver-based adhesive that formed the final bottom contact. The LEDs before and after the transfer process did not show measurable degradation in the I-V and optical characteristics. The 425 nm luminescence peak was found to remain constant during applied mechanical strain on the flexible substrate suggesting the nanowire LEDs did not experience any plane-strain during bending. A constant 2.5 V turn-on voltage, and a forward current of 0.4 mA at 4 V was measured under concave and convex bending. Atomic force and scanning electron microscopy characterization will also be shown of the nanowire device before and after double transfer as well as numerical simulation of the mechanical motion of the nanowire structures during bending.
Mohsen Asad, Renjie Wang, Yong-Ho Ra, William S. Wong, and Zetian Mi, "Electrical and optical properties of flexible nanowire blue light-emitting diodes under mechanical bending (Conference Presentation)," Proc. SPIE 10104, Gallium Nitride Materials and Devices XII, 101040P (Presented at SPIE OPTO: January 31, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2254543.5391659236001.
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