Light extraction efficiency and internal quantum efficiency of fully UVC-transparent AlGaN based LEDs
Martin Guttmann, Anna Susilo, Luca Sulmoni, Norman Susilo, Eviathar Ziffer, Tim Wernicke, Michael Kneissl
Abstract
Abstract The light extraction efficiency (LEE), external quantum efficiency (EQE), and current–voltage characteristics of deep ultraviolet light emitting diodes (DUV-LEDs) with different aluminum mole fractions in the p-AlGaN layers have been investigated. Optimizing the p-AlGaN layer composition requires a tradeoff between reducing the absorption losses and limiting the increases in the p-contact resistance and operation voltage. AlGaN multiple quantum well LEDs emitting around 263 nm with different AlGaN:Mg short period super lattices (p-SPSL) ranging from x = 33% (UV-absorbing) to x = 68% (UV-transparent) average aluminum mole fraction have been explored. DUV-LEDs with different p-contact metals and UV-reflectivities have been characterized by electroluminescence measurements and analyzed by ray-tracing simulations. The comparison shows an increased operating voltage and a five-fold increase of the on-wafer EQE with a maximum value of 3.0% for DUV-LEDs with UV-transparent p-SPSL ( x = 68%) and UV-reflective indium contacts in comparison to LEDs with a UV-absorbing p-SPSL ( x = 33%). Ray-tracing simulations show that the increase in EQE can be partially ascribed to a 2.5-fold improved LEE in combination with a two-fold increase in internal quantum efficiency.