Light emitting diodes (LEDs) in the UVB (280 nm – 315 nm) spectral range are of particular interest for applications such as plant growth lighting or phototherapy. In fact, LEDs offer numerous advantages compared to conventional ultraviolet light sources such as a tunable emission wavelength, a small form factor, and a minimal environmental impact. State-of-the-art devices utilize p-GaN and low aluminum mole fraction p-AlGaN layers to enable good ohmic contacts and low series resistances. However, these layers are also not transparent to UVB light thus limiting the light extraction efficiency (LEE). The exploitation of UV-transparent p-AlGaN layers together with high reflective metal contacts may significantly increase the LEE. In this paper, the output power of LEDs emitting at 310 nm with a UV-transparent and absorbing Mg-doped AlGaN superlattice is compared. A three-fold increase of the output power was observed for LEDs with UV-transparent p-AlGaN layers. To investigate these findings, LEDs with low reflective Ni/Au and high reflective Al contacts are fabricated and characterized. Together with ray tracing simulations and detailed measurements of the metal reflectivities, we were able to determine the LEE and the internal quantum efficiency (IQE). According to on-wafer measurements, the external quantum efficiency (EQE) increases from 0.3% for an absorbing p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice with Ni/Au contacts to 0.9% for a UV-transparent p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice with Al contacts. This 3× enhancement of the EQE can be partially ascribed to an improved LEE (from 4.5% to 7.5%) in combination with a 1.8× increase of the IQE when using a p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice instead of a p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice.
Martin Guttmann, Martin Hermann, Johannes Enslin, Sarina Graupeter, Luca Sulmoni, Christian Kuhn, Tim Wernicke, and Michael Kneissl, "Improved light extraction and quantum efficiencies for UVB LEDs with UV-transparent p-AlGaN superlattices (Conference Presentation)," Proc. SPIE 10104, Gallium Nitride Materials and Devices XII, 101041S (Presented at SPIE OPTO: February 02, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2250573.5391659258001.
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