Ultraviolet AlGaN-based Microcavities

Doktorsavhandling, 2025

The demand for compact, efficient, and low-cost light emitters in the ultraviolet (UV) has been increasing due to important applications in sterilization, sensing, and optical communication. However, the wall plug efficiency of light-emitting diodes (LEDs) emitting below 320 nm, i.e., in the UVB (280-320 nm) or the UVC (<280 nm), is very low, typically below 10%, and the first laser diodes at these short wavelengths have just been demonstrated. Therefore, new types of device concepts in the UV are an interesting path to explore.

Microcavity emitters can provide a more directional light emission pattern and higher spectral purity, which is of great interest for many applications. Such light-emitting devices are embedded between two mirrors forming an optical cavity. The most suitable UV mirrors for microcavities are highly reflective dielectric SiO2/HfO2 distributed Bragg reflectors (DBRs) due to their ability to provide reflectivities above 99%. However, cavities with all-dielectric DBRs require substrate removal techniques to access both surfaces of the device for the deposition of the dielectric DBRs.

Here, we employed electrochemical etching as a substrate removal technique for the fabrication of microcavity emitters, leading to the first demonstration of UVB resonant-cavity LEDs (RCLEDs) and optically pumped UVC VCSELs with precise cavity length control. The UVB RCLEDs have a tunnel junction with a top n-doped current spreading layer which allows the ohmic contact to be placed in the periphery of the device outside of the DBR, which is not possible with a conventional UV LED design. The fabrication of UVC VCSELs requires electrochemical etching of higher Al-containing layers, which was only possible by photo-assisted electrochemical etching. These two device demonstrations have resulted in light emitters with higher directional emission and improved spectral purity, opening up a new design space for UV emitters.