Origin of the efficiency drop in far-UVC LEDs emitting between 218 and 238 nm

The rapid drop in the external quantum efficiency (EQE) of light emitting diodes (LEDs) in the far-ultraviolet-C (far-UVC) spectral range with emission wavelengths between 218 and 238 nm toward shorter wavelengths has been investigated. AlGaN far-UVC LED heterostructures with different emission wavelengths were grown by metal-organic vapor phase epitaxy on AlN/sapphire templates by adjusting the Al mole fractions of the multiple quantum wells (MQW), the quantum well barriers, and the n-AlGaN current spreading layer. The EQE was measured over a large current density range and analyzed by an ABC model fit using the Titkov–Dai method. Measurements of the degree of polarization of light emission were used as the basis for a Monte Carlo ray-tracing simulation to determine the light extraction efficiency (LEE). Based on this, the carrier injection efficiency (CIE) into the AlGaN MQW active region, the radiative recombination efficiency, and the LEE for the different far-UVC LEDs were determined. The EQE exhibits a reduction with decreasing wavelength, aligning with the trend exhibited by all partial efficiencies. The most substantial contribution to the EQE decline was identified as the CIE decrease, attributable to increased electron overflow over the active region.