Stacking III-Nitride Ultraviolet-B Light Emitters with High Efficiency via a Lattice-Engineered Architecture

AlGaN-based ultraviolet (UV) light emitters in the range of 280-320 nm (UVB band) show irreplaceable prospects in the field of medical care, but suffer from a low-efficiency issue known as the "UVB gap." This issue stems from the large lattice mismatch between low-Al-content AlGaN and AlN, which causes the AlGaN epilayer to endure a significant compressive stress during structural stacking, resulting in dislocation multiplication and surface roughening, and thus seriously deteriorating the device performance. Herein, a lattice-engineered architecture through surface pretreatment is proposed, by which dense and discrete nanocrystalline graphite masks, formed by the decomposition of metal organics, are introduced to bring about controllable epitaxial lateral overgrowth and consequent stress. The stress in AlN is then continuously modulated from a compressive to a tensile state of 2.51 GPa with a strain of 0.51%, making its in-plane lattice constant equivalent to that of freestanding Al_0.79Ga_0.21N. As such, high-quality full-Al-content AlGaN epitaxy, in particular with an Al content below 50%, is realized, which brings about a significant performance improvement in 310-nm UVB LEDs, with a maximum wall-plug efficiency achieving 4.88%. This study makes a major breakthrough in the stacking of AlGaN-based UVB light emitters and definitely speeds up their further applications.