Nanoscale and Quantum Engineering of III-Nitride Heterostructures for High Efficiency UV-C and Far UV-C Optoelectronics

Ultraviolet (UV)-C and far UV-C optoelectronic devices, including LEDs and laser diodes, have attracted significant attention due to their ability to inactivate pathogens, which is essential for water purification, food preservation, and surface sterilization. While conventional UV-C devices are largely focused on 260-280 nm spectrum, recent studies suggested that shorter wavelengths, i.e., ~200-220 nm in the far UV-C, are not only more effective at sterilization but can limit the dangers of human exposure to UV radiation. To date, however, such devices exhibit extremely low efficiency, due to the materials imperfection, insufficient light extraction, and the difficulty in p-doping of Al-rich AlGaN materials. Our studies suggest that some of these critical issues can be fundamentally addressed through nanoscale and quantum engineering of III-nitride heterostructures. Due to the efficient strain relaxation, high quality III-nitride nanostructures can be epitaxially grown on foreign substrates without forming extensive defects/dislocations. Recent advances in selective area epitaxy have further shown that their structural and optical properties can be precisely controlled. Due to efficient strain relaxation, Al (or Ga)-substitutional Mg formation energy is significantly reduced in defect-free nanocrystals, compared to conventional epilayer structures. Significantly improved p-type conduction has been achieved in AlN and Al-rich AlGaN. Moreover, with the use of a new epitaxy process, that is, in situ tuning of the surface Fermi level during epitaxy, the incorporation of Mg-acceptors is significantly enhanced without the formation of extensive compensating defects. These advances have enabled the realization of tunnel junction (TJ) UV-C and far UV-C LEDs with unprecedented performance, including high external quantum efficiency (EQE) (>10%) and high electrical efficiency. Our recent studies further suggest that high luminescence emission efficiency in the deep UV can be realized by exploiting strong quantum confinement of charge carriers, through either the formation of quantum dot-like nanoclusters or monolayer quantum wells. We have further demonstrated that AlGaN-based quantum dot laser diodes can exhibit significantly reduced threshold current, due to the large gain and differential gain. Moreover, we show that both the transparency carrier density and electron overflow/leakage can be reduced by one to two orders of magnitude utilizing the special technique of p-type modulation doping in the quantum dot laser active region. These studies provide critical insight for the design and development of high efficiency UV-C and far UV-C LEDs and laser diodes that are relevant for a broad range of applications.