Modeling the optical degradation kinetics of UV-C LEDs

Ultraviolet (UV) light-emitting diodes (LEDs) with respect to the conventional mercury gas-discharge lamps have different advantages, such as smaller size, lower energy consumption and tunable emission wavelength [1]. However, the limited lifetime [2], [3] of these devices limits their penetration in the market, requiring further investigation on the mechanisms responsible for degradation [4]. To this aim, in this study we investigate the reliability of AlGaN-based UV-C LEDs with an emission wavelength of 265 nm grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire. These samples were first submitted to constant current ageing test at 150 mA, while monitoring their electro-optical characteristics at exponentially-spaced intervals. From the optical trends during the ageing different trends can be observed: a) a strong degradation of the optical emission is evident especially at low measuring current levels, reaching the loss of 80% of the initial optical power; b) at high measurement current levels the degradation is lower, and preceded by a recovery phase, which allows to reach up to 15% more than the initial optical emission. In order to clarify the physical mechanisms responsible for this optical kinetics, the devices were subsequently investigated by means of numerical simulation reproducing the structure using the Sentaurus Synopsys suite. In particular, we identified two main processes that have a crucial impact on the optical behavior: an increase in the defect density within the QW and an increase in the interface defect concentration between the last barrier layer (LB) and the electron blocking layer (EBL). The first contributes to increase the nonradiative SRH recombination and therefore has a greater impact under low injection conditions [5], justifying the optical degradation observed mainly at low measuring current levels. The second process instead consists of an increase in negative charged defects at the EBL/LB interface. These defects favor better injection of holes in the QW leading to an increase in the optical emission observed in the early phase of the stress and at high measuring current levels. The implementation of both identified mechanisms leads to a good reproduction of the experimental optical kinetics for different current levels and allows one to estimate both the trends and the densities of the defects responsible for the degradation.