Efficiency improvement analysis of nano-patterned sapphire substrates and semi-transparent superlattice contact layer in UVC light-emitting diodes

UVC light-emitting diodes (LEDs, lambda =275nm) with different types of contact layers and sapphire substrates were demonstrated on high-quality AlN templates. For LEDs on flat sapphire substrates (FSSs), replacing the absorbing p-GaN contact with p-AlGaN short-period superlattices (p-SPSLs) strongly enhanced the emission along the substrate normal. The integrated external quantum efficiency (EQE) increased from 2.4% to 3.9% under I=350mA. For LEDs with a p-SPSL contact, replacing the FSS with nano-patterned sapphire substrates slightly deteriorated the quality of epitaxy, but the overall EQE is still enhanced to 4.4% under I>350mA without lens encapsulation. According to the far-field intensity measurement, the light extraction is better improved along the high emission angle to the substrate normal. The interplay among substrates, dipole polarization, and EQE enhancement factors was further analyzed and discussed in the context.