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.
The monolithic integration of red, green and blue (RGB) GaN-based light-emitting diodes (LEDs) directly on a single chip is critically important for smart lighting and full color display applications. In this work, RGB InGaN/GaN dot-in-a-wire LED arrays were laterally arranged on a Si wafer using a three-step SiO<sub>x</sub>-mask selective area growth (SAG) technique, and on a sapphire wafer using a Ti-mask SAG technique. Tunable emission across the entire visible spectral range (~ 450 nm to 700 nm) can be readily achieved on a single Si wafer by varying the sizes and/or compositions of the dots. By separately biasing lateral-arranged multi-color LED subpixels, the correlated color temperature (CCT) values of such a ~ 0.016 mm<sup>2</sup> pixel can be varied from ~ 1900 K to 6800 K. The RGB pixel size can be further reduced by using the Ti-mask SAG technique on sapphire wafer. Full-color InGaN/GaN nanowire arrays with sizes of 2.8 × 2.8 μm<sup>2</sup> have been monolithically fabricated into the same pixel.