2023, Knauer, A., Kolbe, T., Hagedorn, S., Hoepfner, J., Guttmann, M., Cho, H.K., Rass, J., Ruschel, J., Einfeldt, S., Kneissl, M., Weyers, M.

High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.

2018, Munshi, A. Mazid, Kim, Dong-Chul, Heimdal, Carl Philip, Heilmann, Martin, Christiansen, Silke H., Vullum, Per Erik, van Helvoort, Antonius T. J., Weman, Helge

Wide-bandgap group III-nitride semiconductors are of special interest for applications in ultraviolet light emitting diodes, photodetectors, and lasers. However, epitaxial growth of high-quality III-nitride semiconductors on conventional single-crystalline substrates is challenging due to the lattice mismatch and differences in the thermal expansion coefficients. Recently, it has been shown that graphene, a two-dimensional material, can be used as a substrate for growing high-quality III-V semiconductors via quasi-van der Waals epitaxy and overcome the named challenges. Here, we report selective area growth of AlGaN nanopyramids on hole mask patterned single-layer graphene using metal-organic vapor phase epitaxy. The nanopyramid bases have a hexagonal shape with a very high nucleation yield. After subsequent AlGaN/GaN/AlGaN overgrowth on the six {10 (1) over bar1} semi-polar side facets of the nanopyramids, intense room-temperature cathodoluminescence emission is observed at 365 nm with whispering gallery-like modes. This work opens up a route for achieving III-nitride opto-electronic devices on graphene substrates in the ultraviolet region for future applications.