Degradation by sidewall recombination centers in GaN blue micro-LEDs at diameters<30 µm

A matrix of blue GaN/InGaN multi-quantum-well (MQW) micro-Light-Emitting Diodes (micro-LEDs) with diode dimensions ranging from 2-100 μm was prepared by masked dry etching and characterized by Photoluminescence (PL), Microcathodoluminescence (MCL), capacitance-voltage profiling in the dark and under monochromatic illumination, current-voltage measurements, admittance spectra, Deep Level Transient Spectroscopy with electrical (DLTS) and optical (ODLTS) injection. The changes observed in the PL, MCL spectra are due to the formation of deep hole traps at E v +0.75 eV and electron traps at E c -1 eV in the sidewalls of the micro-LEDs. The former give rise to the red defect band peaked near 600 nm and contribute to the increase of tunneling and leakage current with decreasing the diode diameter. The latter are prominent centers of nonradiative recombination. • DLTS spectra measured with strong injection of holes into the MQW region demonstrate an increase of the density of deep acceptors at E v +0.7 eV and E v +0.75 eV as the diode diameter decreases from 100 to 30 µm. • These deep acceptors are well documented and have been credibly attributed to either carbon on nitrogen site C N acceptors or to gallium vacancy complexes with shallow donors, V Ga -D. • Experiments with LED degradation during operation or after electron irradiation point to these centers as being responsible for the enhanced tunneling and increased leakage current. Experiments with nano-LEDs produced with dry etching favor the attribution of the deep acceptors to V Ga -D defects rather than to C N • This mechanism explains the very strong decrease in external quantum efficiency of micro-LEDs as the diameter falls below ~30 µm, since then the effect of trap states in the sidewalls becomes dominant. • The reported observation could form a basis for improvement of EQE of GaN-based micro-LEDs.