Two types of near-UV light-emitting diodes (LEDs) with an InGaN/GaN single quantum well (QW) differing only in the presence or absence of an underlayer (UL) consisting of an InAlN/GaN superlattice (SL) were examined. The InAlN-based ULs were previously shown to dramatically improve internal quantum efficiency of near-UV LEDs, via a decrease in the density of deep traps responsible for nonradiative recombination in the QW region. The main differences between samples with and without UL were (a) a higher compensation of Mg acceptors in the p-GaN:Mg contact layer of the sample without UL, which correlates with the presence of traps with an activation energy of 0.06 eV in the QW region, (b) the presence of deep electron traps with levels 0.6 eV below the conduction band edge (Ec) (ET1) and at Ec 0.77 eV (ET2) in the n-GaN spacer underneath the QW, and the presence of hole traps (HT1) in the QW, 0.73 eV above the valence band edge in the sample without UL (no traps could be detected in the sample with UL), and (c) a high density of deep traps with optical ionization energy close to 1.5 eV for the LEDs without UL. Irradiation with 5 MeV electrons led to a strong decrease in the electroluminescence (EL) intensity in the LEDs without UL, while for the samples with UL, such irradiation had little effect on the EL signal at high driving current, although the level of driving currents necessary to have a measurable EL signal increased by about an order of magnitude. This is despite the 5 times higher starting EL signal of the sample with UL. Irradiation also led to the appearance in the LEDs with UL of the ET1 and HT1 deep traps, but with concentration much lower than without the UL, and to a considerable increase in the Mg compensation ratio.

No 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, E c -0.5 eV, E c -0.75 eV, and E c -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 E v +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 (E c -0.75 eV, E c -1.1 eV, E v +0.95 eV) and nonradiative recombination in the QWs via electron traps E c -0.5 eV and hole traps E v +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.

The electrical properties, electroluminescence (EL) power output and deep trap spectra were studied before and after 5 MeV electron irradiation of near-UV single-quantum-well (SQW) light emitting device (LED) structures differing by the presence or absence of InAlN superlattice underlayers (InAlN SL UL). The presence of the underlayer is found to remarkably increase the EL output power and the radiation tolerance of LEDs, which correlates with a much lower and more slowly changing density of deep traps in the QW region with radiation dose, and the higher lifetime of charge carriers, manifested by higher short-circuit current and open-circuit voltage in current-voltage characteristics under illumination. The observed phenomena are explained by the capture of native defects segregated at the growing surface by In atoms in the underlayer which traps them in the underlayer and prevents their penetration into the QW region.

No abstract