Despite a rapidly-growing demand for efficient man-made DUV light sources, widespread adoption of AlGaN-based DUV LEDs is currently obstructed by extremely poor extraction of DUV photons due to the intrinsic material properties of AlGaN including low hole concentration and poor light extraction efficiency (LEE). Conventional LEE-enhancing techniques used for GaInN-based visible LEDs turned out to be ineffective for DUV LEDs due to a strong absorption of DUV light by p-GaN contact layer, and predominant TM polarized anisotropic emission from Al-rich AlGaN multi-quantum well (MQW) active region grown on c-plane sapphire substrate. Therefore, a new LEE-enhancing approach addressing the unique intrinsic property of AlGaN DUV LEDs is strongly desired.
In this study, we present DUV LEDs having arrays of TC shaped active mesas coated with MgF2/Al reflectors on the inclined sidewalls to extract strong TM-polarized in-plane emission trough the sapphire substrate. Ray tracing simulations reveal that the TC DUV LEDs show an isotropic emission pattern and much enhanced light-output power in comparison with stripe-type DUV LEDs with the same MgF2/Al reflectors. Consistent with the ray tracing simulation results, the TC DUV LEDs show an isotropic emission pattern with 37.1% higher light-output power as well as lower operating voltage than the stripe-type DUV LEDs. Based on our results, we suggest strategies to design an optimized DUV LEDs for further enhancing the optical and electrical performances simultaneously. In addition, we propose a next generation DUV LED with an array of Al nanoparticles capable of enhancing IQE and LEE simultaneously by surface plasmon resonance coupling.
AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) are being developed for their numerous applications such as purification of air and water, sterilization in food processing, UV curing, medical-, and defense-related light sources. However, external quantum efficiency (EQE) of AlGaN-based DUV LEDs is very poor (<5% for 250nm) particularly due to low hole concentration and light extraction efficiency (LEE). Conventional LEE-enhancing techniques used for GaInN-based visible LEDs turned out to be ineffective for DUV LEDs due to difference in intrinsic material property between GaInN and AlGaN (Al<~30%). Unlike GaInN visible LEDs, DUV light from a high Al-content AlGaN active region is strongly transverse-magnetic (TM) polarized, that is, the electric field vector is parallel to the (0001) c-axis and shows strong sidewall emission through m- or a-plane due to crystal-field split-off hole band being top most valence band. Therefore, a new LEE-enhancing approach addressing the unique intrinsic property of AlGaN DUV LEDs is strongly desired.
In this study, an elegant approach based on a DUV LED having multiple mesa stripes whose inclined sidewalls are covered by a MgF2/Al omni-directional mirror to take advantage of the strongly anisotropic transverse-magnetic polarized emission pattern of AlGaN quantum wells is presented. The sidewall-emission-enhanced DUV LED breaks through the fundamental limitations caused by the intrinsic properties of AlGaN, thus shows a remarkable improvement in light extraction as well as operating voltage simultaneously. Furthermore, an analytic model is developed to understand and precisely estimate the extraction of DUV photons from AlGaN DUV LEDs, and hence to provide promising routes to maximize the power conversion efficiency.
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