Thin-film ultraviolet light-emitting diodes realized by electrochemical etching of AlGaN

Doctoral thesis, 2020

Ultraviolet (UV) light sources have a direct impact on everyone’s life. They are used to sterilize surfaces as well as for water purification. In addition, they are used in green houses to enhance health-promoting substances in plants, for phototherapy to treat skin diseases, for sensing and material curing. Today, most of these applications use mercury lamps that are fragile, bulky and toxic. AlGaN-based UV light-emitting diodes (LEDs) have the potential to solve all these issues, but their implementation has been limited due to their low electrical to optical power conversion efficiency (PCE) being below 10%. Blue-emitting GaN-based LEDs have already found their way into everyone’s home through general lighting. This was made possible by the tremendous performance improvements, reaching PCEs close to 90%. Unfortunately, the device concepts for achieving highly efficient GaN-based LEDs, such as the thin-film flip-chip (TFFC) design that can greatly improve light-extraction efficiency, are not easily transferred to AlGaN-based UV LEDs.

In this work, we demonstrate a new device platform to realize UV LEDs with a TFFC design based on electrochemical etching to remove the substrate. In the first part of this work, electrochemical (EC) etching of AlGaN layers with a high Al content up to 50% was demonstrated, which enabled the separation of epitaxial LED layers from their substrate while maintaining the high quality of the active region.
The second key technological step was the integration of EC etching in a standard UV LED fabrication process, which required protection schemes to prevent parasitic electrochemical etching of the LED structure and the development of a device design compatible with flip-chip bonding. Finally, this work was completed by the first demonstration of a TFFC UVB LED using electrochemical etching.