New approach to development of ultraviolet light-emitting diodes

A rapidly growing industrial UV-LED market has emerged with the introduction of UV curing techniques. The resulting light emitters are useful for disinfection of air, water and surfaces, since UV light will kill bacteria and viruses.

Aluminum nitride buffer layers are needed to make aluminum gallium nitride (AlGaN)-based UV emitters, but the resulting LEDs suffer from low efficiencies. In a recent paper, the investigators present a technique for increasing the efficiency of AlGaN UV LEDs through improved methods for growing the aluminum-based thin films.

LED efficiency can be improved by reducing threading dislocation density (TDD), but methods developed for gallium nitride on sapphire substrates, the industry standard for blue LEDs, have not worked well for aluminum nitride (AlN)/sapphire systems. In addition to reducing TDD, tensile stress in the film must be eliminated to avoid cracking. The authors grew AlN layers on a silicon carbide (SiC) substrate, rather than sapphire. By controlling the AlN nucleation process through both initial growth conditions and pre-treatment of the SiC substrate, they were able to produce crack-free films using metal organic chemical vapor deposition (MOCVD).

The study used two types of SiC wafers. One was atomically flat, while the second was rough with well-defined steps. The investigators also explored the use of pre-treatment with ammonia gas under high temperature and pressure and found the combination of atomically flat substrate and pre-treatment produced high quality films. Close inspection of the wafer after pre-treatment suggested a change in surface morphology had occurred.

The authors concluded that MOCVD is a viable method for growing high-quality AlN/SiC UV-LED layers. They expect that high-efficiency UV-LED devices will soon be made possible with further investigation and development of material improvements.

Source: “Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition,” by Christian J. Zollner, Abdullah Almogbel, Yifan Yao, Burhan K. SaifAddin, Feng Wu, Michael Iza, Steven P. DenBaars, James S. Speck, and Shuji Nakamura, Applied Physics Letters (2019) The article can be accessed at https://doi.org/10.1063/1.5123623 .