Reaching the perfect epitaxy recipe for UV LEDs

March 15, 2017 // By Julien Happich
While today's AlGaN-based ultraviolet LEDs rely on planar structures, they face many unresolved material-related challenges including a lack of efficient p-type doping in high-Al-content AlGaN, high threading dislocations and defect densities in planar UV LEDs grown on foreign substrates, and low a light extraction efficiency (typically less than 10%).

In a recent paper published in the Nanoscale journal under the title "Self-planarized quantum-disks nanowires ultraviolet-B emitter utilizing pendeo-epitaxy", researchers from King Abdullah University of Science and Technology (KAUST) explain how they came to get the equivalent of a nearly defect-free planar structure by growing dislocation-free AlGaN-based nanowires nucleated so close together that they end up coalescing at their upper extremities.

Through the precise control of the initial GaN nucleation process on a clean silicon wafer, and following specific growth conditions using catalyst-free plasma assisted molecular beam epitaxy (PAMBE), they claim to have overcome many of the issues surrounding nanowire growth, obtaining high-density, spontaneously coalesced, dislocation-free AlGaN NWs with a uniform height, meaning they present a self-planarized top surface. About 30nm in diameter at their base, the inverse-tapered nanowires expanded as they grew to a diameter of roughly 70nm at the top of their 400nm length.

The nanowires were grown to integrate 15 pairs of AlxGa1-xN/AlyGa1-yN Qdisks occupying nearly half the volume of the entire NW, together with a thin p-type AlGaN electron-blocking layer. A p-type graded AlGaN layer (about 25nm thick) and a p-GaN layer (about 20nm) were grown as a top contacting layer.

Because of their high density (roughly 1x1010cm-2), their coalescence and their uniform height distribution over a large area, the nanowires reached a fill factor over 95% and Ni/Au layers could directly be deposited on top of the coalesced p-GaN layer without prior planarization.

(a) Top-view SEM image of the device structure, and tilted-angle SEM of the NWs in the inset; (b) STEM-HAADF image of the coalesced AlGaN NWs.