High-Current Stress of UV-B (In)AlGaN-Based LEDs: Defect-Generation and Diffusion Processes

The aim of this paper is to investigate the degradation mechanisms of UV-B AlGaN-based light-emitting diodes (LEDs) submitted to constant current stress beyond the typical application conditions. We demonstrate the existence of two main degradation mechanisms that significantly impact the relative amplitude of the main quantum well (QW) peak at 310 nm and of a parasitic peak at ~340 nm related to the electron overflow toward the last quantum barrier before the p-side. These mechanisms are thoroughly investigated by means of electrical and optical measurements, and by photocurrent (PC) spectroscopy. We demonstrate that in the first 50 h of stress, degradation is caused by an increased defect density in the last quantum barrier and/or carrier escape from the QWs, which results in a decrease in the QW emission and in an increase in the parasitic peak at 340 nm. For longer stress times, degradation is dominated by a diffusion process, causing an increase in defect density and nonradiative recombination in the LED. This has a direct impact on both the QW peak and the parasitic peak, which show a significant decrease for long stress times. PC spectroscopy demonstrates that the defects responsible for degradation are most likely located next to mid-gap, thus acting as efficient nonradiative recombination centers.