UC Berkeley

Light emitting diodes are robust and high efficiency light sources that have the potential to replace all lighting applications in the future. Some hindrances to ubiquitous adoption of LEDs though are cost per lumen and lack of high quality materials for green emission or tunable emission. Innovations are needed to continue improving overall efficiency of light emitting diodes as well as finding suitable materials to achieve complete visible tunability. Novel light emitting diodes are made from nanoscale materials to explore potential advantages over conventional thin film approaches. These atomic-scale structures have unique electrical and optical properties that could potentially lead to increased efficiencies.

Three platforms for improved light emitting diodes were designed, fabricated, and characterized. The first consisted of an n-type ZnO vertical nanowire array grown epitaxially from a p-type GaN thin film. The resulting device showed an increase of 13% in light output in the vertical direction as compared to a thin film LED due to waveguiding of light in the vertically oriented nanowires. The second device took advantage of the ability to synthesize InxGa1-xN materials in nanowire form with x greater than 30%, which would otherwise be unstable in thin film form due to phase segregation. Nanowire arrays were grown on top of conventional InGaN QW LEDs. Resulting light emission was a combination of transmitted QW blue electroluminescence and color converted photoluminescence from the array. Colors achieved ranged from blue, to blue-green, and red-orange. The final platform explored enhancement of electroluminescence by metal coating of nanopillar LEDs. The metal layer is proposed to confine light along the nanopillar and enhance radiative emission due to surface plasmons. Preliminary results suggest some extent of enhanced electroluminescence. However, fidelity of these results needs to be further explored due to concerns of light leakage through cracks in the metal layer.

Nanowire based light emitting diodes were fabricated and shown to offer advantages over thin film LEDs due to waveguiding of light in the nanowire, full-spectrum tunability, and surface plasmon electroluminescent enhancement.