Decrease in the injection efficiency and generation of midgap states in UV-C LEDs: a model based on rate equations

The aim of this paper is the definition of a model for the degradation of an AlGaN-based ultraviolet (UV) Light Emitting Diode (LED) with a nominal wavelength of 285 nm (UV-C). These devices are widely used in disinfection, sterilization, water purification, medical devices, in plant lighting and as insect traps; moreover, UV antiviral treatments are being developed recently, under the push of the current COVID-19 emergency. We analyzed the behavior of the devices during a constant current stress at the current of 250 mA, through electrical (I-V), optical (L-I) and spectral (PSD) measurements and steady state photocapacitance (SSPC) analysis. By investigating the optical measurements, we found out the presence of two different degradation mechanisms, one before 1000 min of stress and one after 1000 min of stress. We ascribed the first one to a decrease in the injection efficiency and we modeled it with a system of three differential rate equations to describe the dynamics of the de-hydrogenation of gallium vacancies, that lead to a defects generation. On the other hand, the second degradation mechanism is well correlated to the generation of midgap defects (Ec-2.15 eV), as detected from the SSPC analysis, that indicates the generation of non-radiative centers induced by the stress.