As per the Minamata Convention on Mercury, regulation on mercury use will be stricter from the year of 2020, and safe AlGaNbased ultraviolet (UV) light sources are urgently needed for killing SARS-CoV-2 (corona virus). AlGaN-based ultraviolet-B (UVB) light-emitting diodes (LEDs) and UVB laser diodes (LDs) have the potential to replace toxic mercury UV lamps. Previously, the internal-quantum-efficiency (η int ) was enhanced from 47 to 54% in AlGaN UVB multiquantum wells (MQWs). However, some nonlinear behaviors in both light output power (L) and external quantum efficiency (η ext ) in the 310 nm band UVB LEDs were observed, and later on, such nonlinearities were overcome by reducing the thicknesses of quantum well barriers (T QWB ) in MQWs. After relaxing the n-AlGaN electron injection layer up to 50% underneath the MQWs and using a highly reflective Ni/Al p-electrode, L and η ext of the 310 nm band UVB LED were greatly improved from 12 mW and 2.3% to record values of 29 mW and 4.7%, respectively. Similarly, for the 294 nm band UVB LED, η ext and L values, respectively, were also remarkably improved up to 6.5% and 32 mW at room temperature under bare wafer conditions using a better carrier confinement scheme in the MQWs as well as using a moderately Mgdoped p-type multiquantum-barrier electron-blocking layer (p-MQB EBL). The moderately doped p-MQB EBL was used to achieve better hole transport to enhance the hole injection toward the MQWs as well as to block the high-energy electron from overshooting. Possible explanations and recommendations for the improvements in the performances of 294−310 nm UVB LEDs are broadly discussed. Most importantly, such controllable multi-UVB-wavelength emitters may extend nitride-based LEDs to previously inaccessible areas, for example, electrically pumped AlGaN-based UVB LDs.

The interest in semipolar orientations has been increasing because the reduced piezoelectric field can improve the performance of nitride-based optoelectronic devices. However, the crystalline quality of semipolar AlN on m-plane sapphire is still not good enough to realize light emitters with sufficiently high efficiency. We performed high-temperature annealing on AlN on m-plane sapphire to improve the crystalline quality. For (10-1-3) and (11-22) AlN on m-plane sapphire, the crystalline quality improved as the annealing temperature was increased up to 1700 °C, whereas beyond 1750 °C the AlN layer started to deteriorate and desorb. The crystalline quality was further improved by additional growth of AlN. In addition, X-ray rocking curve measurements and transmission electron microscopy confirmed that the density of stacking faults was reduced after the additional growth of AlN.