Light extraction efficiency and internal quantum efficiency of fully UVC-transparent AlGaN based LEDs

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. (paper)