Tim Wernicke is leading III-N epitaxy laboratory of the Experimental Nanophysics and Photonics group at the Technische Universität Berlin and his research focuses on the epitaxy of AlGaN-based UV-LEDs and lasers.
Åsa Haglund is a Professor at the Deptartment of Microtechnology and Nanoscience at Chalmers University of Technology and her research group focuses on visible and UV vertical-cavity surface-emitting lasers, thin-film UV light-emitting diodes.
Up to now,
if you wanted to work with ultraviolet (UV) light you needed bulky, fragile and expensive lasers or gas discharge lamps available only at distinct wavelengths. With LEDs and lasers based on the ultra-wide bandgap semiconductor AlGaN a new age of UV technology and applications is dawning. However, developing semiconductor-based UV light-emitters requires a holistic approach. Therefore, we have joined forces between material physicists at TU Berlin and device physicists at Chalmers, creating energy efficient UV light-emitting diodes (LEDs) and the world’s shortest emission wavelength from a vertical-cavity surface-emitting laser (VCSEL). Why should we care about UV light-emitters?
With the ongoing pandemic the need for disinfection is now even more important than ever. UV illumination has proven efficient in sterilizing SARS-CoV-2, but also other virus and bacteria, making it an important tool to combat future pandemics, but also to provide clean drinking water. UV illumination is also of great use for other applications such as promoting healthy substances in plants, dermatological phototherapy, curing of materials, gas sensing etc.
Why UV LEDs and lasers? What are the challenges?
Today, UV illumination is often provided by mercury-based UV lamps. By replacing them with semiconductor-based UV LEDs and lasers the light source will become environmentally friendly, have a small form factor (µm-sized instead of cm-m sized) and be tailorable in wavelength. However, the development of semiconductor-based UV light sources has been limited by material challenges such as high defect densities, low electrical conductivity, and low optical transparency. On top of this, device concepts used for high-performing visible and infrared emitters are not that easily implemented for UV devices. Therefore, UV LEDs suffer from a poor power conversion efficiency that is typically below 10% and VCSELs with an emission wavelength shorter than UVA did not existed a short while ago.
What has been achieved? What does the future hold?
Low defect density and highly conductive UV-transparent device structures have been demonstrated using short-period superlattices, and more recently tunnel junctions. To boost the light extraction efficiency in LEDs and enable VCSELs with highly reflective mirrors, we have developed a substrate removal technique based upon selective lateral electrochemical etching. When the substrate has been removed, the LED surface can be roughened, which increases the light extraction efficiency by about 25%. Instead of roughening the surface, we can form a laser cavity by depositing high-reflectivity dielectric mirrors on both sides. This resulted in the world’s shortest emission wavelength from a VCSEL at 310 nm. By combining advanced material growth and innovative device fabrication we have pushed these devices forward, but this is only the beginning. If you join this seminar, we will also reveal what at we think the future holds.