AlGaN-based ultraviolet-C (UVC) light emitting diodes (LEDs) with emission wavelengths below 240 nm (far-UVC) gained considerable interest in recent years due to their potential of skin-safe inactivation of viruses and bacteria. However, far-UVC LEDs suffer from low external quantum efficiency (EQE) and rapid output power degradation over time, limiting their implementation in applications. This thesis addresses these challenges by exploring the influence of the heterostructure design on the performance of far-UVC LEDs. Precise control of the Si-doping concentration is essential for n-AlGaN layers with Al mole fractions above 80 %. Hall-effect and capacitance-voltage measurements (CVM) on n-AlGaN layers indicated compensation effects of the Si donor. Additionally, the lower dislocation density provided by AlN bulk substrates further improved the n-type conductivity. However, parasitic absorption of the far-UVC emission stemming from point defects in the AlN bulk substrate was observed, highlighting the importance of highly transparent AlN bulk substrates. To investigate the rapid output power degradation of far-UVC LEDs, the charge carrier density in the multi-quantum well (MQW) was reduced by implementing a large number of quantum wells (QWs). As a result, a positive correlation between the output power and the lifetime of the LEDs was observed. Moreover, an increase in the vacancy-type point defect concentration with increasing Al mole fraction was found, which provides an explanation for the exponential decrease in EQE. Efficient Mg-doping of high Al mole fraction AlGaN layers remains a major challenge. This thesis demonstrates that linearly graded, distributed polarization doped (DPD) AlGaN is a promising alternative to conventional Mg-doped AlGaN. CVM revealed high net polarization charge densities in the DPD layers, in accordance with theoretical calculations. The work of this thesis led to the realization of flip-chip mounted far-UVC LEDs with emission wavelengths of 233 nm and 226 nm, EQEs of 0.7 % and 0.08 %, and L70 lifetimes of 13 h in both cases.
Marcel Pascal Schilling (Thu,) studied this question.