Enhancement of current injection efficiency of AlGaN-based deep-ultraviolet light-emitting diodes by controlling strain relaxation

The external quantum efficiency (EQE) in electrically injected AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) is severely limited by their poor current injection efficiency (CIE). We report improvement in the CIE via controlling the relaxation of strains in the p-AlGaN hole injection layer (HIL) and the electron blocking layer (EBL). Simulation results show that an unrelaxed strain in the HIL associated with a relaxed strain in EBL can significantly enhance CIE. Deeper analysis indicates that high hole concentrations can be generated at HIL/EBL interface by strain-induced piezoelectric fields, which can then provide abundant numbers of holes for injection into quantum wells. Two sub-280 nm DUV-LEDs were fabricated with specific designs for different strain relaxations in the p-AlGaN HIL by changing the HIL thickness from 200 to 20 nm. The strain difference was identified using Raman spectroscopy. Electroluminescence measurements demonstrated much higher EQE in the strained-HIL DUV-LEDs. By separating the EQE contributions of three efficiencies, i.e. the CIE, the radiative recombination efficiency and the light extraction efficiency, we found that the EQE enhancement could mainly be attributed to the improved CIE, which agreed well with the simulation results. (paper)