Spectral components, initial improvement, and degradation of 265 nm UV-C LEDs

In this work, we analyze AlGaN single quantum well (SQW) UV-C LEDs with varying well thickness (1.4, 3, 6 and 9 nm), stressed at a constant DC current (100 mA, 100 A/cm2) at 25 °C. We analyzed the emission spectrum, and identified the origin of several parasitic components by means of current-dependent power spectral density measurements combined with TCAD simulations. A shoulder in the spectrum around 249 nm can be explained by band-to-band recombination at the interface between the interlayer and the p-side quantum barrier, whereas an emission peak near 308 nm is likely due to defect-assisted radiative recombination in the AlGaN quantum well or due to backdiffused Mg in the p-side quantum barrier. An additional peak at 337 nm is ascribed to band-to-band recombination at the interface between the p-GaN contact layer and the electron blocking layer. Two additional emission bands centered at 380 nm and 506 nm, respectively, can be observed, which are attributed to defect-assisted radiative processes within the p-side quantum barrier or p-GaN contact layer. Ageing tests showed both a continuous optical power decay and an initial improvement (recovery) in the optical performance of the device, which was more pronounced for LEDs with thicker AlGaN quantum wells. The degradation kinetic was analytically modeled by considering an increase in the Shockley-Read-Hall (SRH) recombination coefficient, caused by an increase in concentration of non-radiative defects, and a decrease in injection efficiency, originating from a higher concentration of electrically-active defects. The initial improvement was explained by formation of negative charge at the interface between the interlayer and the p-side quantum barrier.