Origin of the efficiency drop in far-UVC light emitting diodes

In recent years, nitride-based far-UVC light emitting diodes (LEDs) with emission wavelengths shorter than 240 nm have attracted increasing attention. They are useful light sources for applications such as gas sensing and skin-safe disinfection – particularly against multi-drug-resistant germs. Currently, such applications rely on bulky, expensive light sources based on gas discharge lamps and optical filters. Therefore, there is a high demand for small, compact, powerful, and reliable LEDs emitting below 240 nm. However, up to now all LEDs with wavelengths below 240 nm achieve wall plug efficiencies of only a few percent or less, and the efficiency decreases drastically at shorter wavelengths. E.g., Fig. 1 shows the on-wafer emission power at 50 mA for LEDs with different wavelengths.

To investigate the origin of the efficiency drop in far-UVC LEDs, we have grown LEDs with emission wavelengths between 218 nm and 242 nm by metalorganic vapor phase epitaxy (MOVPE) on AlN/sapphire templates. To realize the different emission wavelengths, we adjusted the Al mole fractions of the AlGaN multiple quantum wells, the quantum well barriers, and the n-AlGaN current spreading.

The external quantum efficiency (EQE) is defined as the product of the carrier injection efficiency (CIE), the radiative recombination efficiency (RRE), and the light extraction efficiency (LEE). To pinpoint the origin of the efficiency drop with emission wavelength, we measured the EQE as a function of the current and analyzed it by an ABC model fit using the Titkov-Dai method. This approach allowed us to determine the REE and the product of CIE and LEE at the EQE maximum. We used measurements of the degree of polarization of the emitted light as the basis for Monte Carlo raytracing simulation to determine the LEE. Combining the results of both methods, we calculated the CIE into the quantum wells for the different far-UVC LEDs at the EQE maximum.

The results in Fig. 2 show that the drop in CIE is the most significant parameter responsible for the drop in emission power and EQE, which decreased by a factor of 11 from 234 nm ((22 ± 3) %) to 223 nm ((2 ± 1) %). The RRE at the EQE maximum decreased from (60 ± 5) % for 234 nm LEDs to (22 ± 5) % for 223 nm LEDs, corresponding to a factor of 2.7. Finally, the LEE was found to be the highest with 3.2 % for 238 nm LEDs, decreasing by a factor of 2.6 for 223 nm LEDs. Device simulation revealed that the drop in CIE can be attributed to an increased electron leakage current into the p-type layers. This leakage occurs because the bandgap energy offset between the quantum well barriers and the electron blocking layer becomes too small with decreasing emission wavelengths. Therefore, the development of advanced designs of the quantum well barriers and/or the electron blocking layer are routes to improve the EQE of LEDs emitting below 230 nm. 

Based on the optimizations of our far-UVC LEDs, we fabricated 1 mm x 1 mm LED chips mounted on planar AlN ceramic submounts, which, operated at 200 mA and 20 °C, emit 0.2 mW, 2.1 mW, 4.2 mW, and 10.7 mW at 222 nm, 226 nm, 229 nm, and 233 nm, respectively (Fig. 3). These devices show peak EQE of 0.02 %, 0.3 %, 0.5 %, and 1.1 %, respectively [1].

This work was partially supported by the European Union through IBB within the Pro FIT project “UV-Multi” (Contract No. 10203290).

Publication

[1] T. Kolbe et al., Advances in the epitaxial growth of far-ultraviolet C light emitting diodes, invited talk GR6-1, 14th International Conference on Nitride Semiconductors.