Spatially resolved light emission provides insight into the degradation of UV LEDs

AlGaN-based ultraviolet light-emitting diodes (UV LEDs) with emission wavelengths in the UVB or UVC spectral range are promising devices, which are expected to replace discharge lamps in many applications. Wavelengths at about 310 nm, for example, can be used in the field of plant-growth irradiation. Recent studies show that this irradiation stimulates the production of secondary metabolites in plants. These substances are known to have health benefits as they can reduce the likelihood of developing a cardiovascular disease and cancer. In contrast, 230 nm radiation can inactivate viruses, fungi and bacteria on the human skin without harming it. For almost all applications, UV LEDs must have long lifetimes and their emission power must be stable over many thousands of hours. However, the emission power of UVB and UVC LEDs typically decreases during long-term operation. This can be attributed to degradation mechanisms that occur within the semiconductor layer structure of the LED. To make the LEDs suitable for application, these mechanisms must be eliminated or slowed down. For this reason, FBH is working to study and identify degradation mechanisms in UV LEDs. In collaboration with the Max-Born-Institut (MBI), FBH was able to find out how current density and radiative recombination efficiency are spatially distributed within a UVB LED chip during long-term operation [1].

In our investigations we combined electrical and optical excitation of the active region of a UVB LED chip while imaging its light emission by a UV camera (Figs. 1 and 2). This made it possible to distinguish between spatial variations in current density and in efficiency of radiative recombination of charge carriers. For the experiment we fabricated UVB LED chips with a dedicated small circular emission area (diameter 120 µm). The homogeneity of both spatial distributions was found to decrease during long-term operation. In the electroluminescence (EL) intensity distribution of the aged LED, areas are emerging from which no or less light is emitted (Fig. 1b). The corresponding photoluminescence (PL) intensity shows a complementary distribution (Fig. 2b). Thus, areas of high EL intensity correspond to areas of low radiative recombination efficiency and vice versa. We can conclude from the results that the distribution of the EL light emission in the active region is mainly determined by the distribution of the current density, and that the current density distribution changes during operation. Furthermore, the degradation is more prominent in the areas where the local current density increased throughout long-term operation.

Accordingly, maintaining a constant and homogeneous current density distribution in UVB LEDs is a key factor to improve their long-term stability. Furthermore, it is likely that these findings can also be transferred to UVC LEDs emitting around 230 nm. The next step will be to investigate targeted changes in the semiconductor heterostructure and chip design that are expected to affect the current density distribution. Our aim is to find an optimum that enables stable emission power over longer operating times.

This work was partially supported by the Federal Ministry of Education and Research (BMBF) through the Twenty20 initiative “Advanced UV for Life” under contract no. 03ZZ0130A.

Publication

 [1] J. Ruschel, J. W. Tomm, J. Glaab, T. Kolbe, A. Knauer, J. Rass, N. Lobo Ploch, T. A. Musengezi, S. Einfeldt, ”Spatially resoIved degradation effects in UVB LEDs stressed by constant current operation“, Appl. Phys. Lett. 122, 131103 (2023). - DOI: 10.1063/5.0141530