Defect States Induced in GaN-Based Green Light Emitting Diodes by Electron Irradiation

Abstract

The spatial distribution of deep traps in electron irradiated green multi-quantum-well (MQW) GaN/InGaN light emitting diodes was determined by deep level transient spectroscopy with electrical and optical injection. Four major electron traps with levels near Ec-0.2 eV, Ec-0.5 eV, Ec-0.75 eV, and Ec-1.1 eV were observed. The concentration of all electron traps increased with fluence of 6 MeV electrons, correlating with a decrease of the external quantum efficiency (EQE) of LEDs. The concentration of hole traps at Ev+0.45 eV also increased with irradiation. The observed EQE changes are partly attributed to trapping of electrons and holes by electron traps in the GaN barriers (Ec-0.75 eV, Ec-1.1 eV, Ev+0.95 eV) and nonradiative recombination in the QWs via electron traps Ec-0.5 eV and hole traps Ev+0.45 eV. Even traps with levels far from midgap can effectively participate in nonradiative recombination by forming close pairs similar to donor-acceptor pairs if their density is high. This can occur in In-rich fluctuation regions in the QWs, even as the densities averaged over the entire area of QWs as measured by deep level spectroscopy remain relatively low.