MSE Seminar Series

Date: Thursday, September 18, 2025
Time: 4:00pm
Location: Kimball B11

About the speaker

Leo Schowalter was employed by the GE Global Research Center after receiving his Ph.D. in Physics from the University of Illinois in 1981.  He was a professor in the Rensselaer Polytechnic Institute (RPI) Physics Department from 1987 and became Department Chair in 1997, the same year that he co-founded Crystal IS with another former GE colleague. Crystal IS commercialized single-crystal AlN substrates and UVC LEDs based on pseudomorphic AlGaN on these substrates for disinfection and instrumentation applications.  Crystal IS was acquired by Asahi Kasei in 2011 and is now a wholly owned subsidiary.

Starting in 2017, Dr. Schowalter was appointed the Asahi Kasei Innovative Devices Industry-Academia Collaborative Chair at Nagoya U. This team, led by Prof. Amano and with Asahi Kasei and Crystal IS support, announced the world’s first UVC laser diode in 2019 and, in early 2020, announced the best performance for far UVC LEDs in the 230nm to 240nm wavelength range. Dr. Schowalter continues the collaboration with Prof. Amano as a Visiting Professor but has also now joined the Florida-based startup company Visium (formally Lit Thinking) which is developing cost-effective far UVC for human-safe disinfection of shared spaces.  Most recently, he has established the Visium Ultralabs in the Cornell Tech Park to further push the AlGaN ultrawide bandgap semiconductors into the far-UVC.

Abstract

The development of AlGaN ultrawide bandgap (UWBG) semiconductors on native AlN substrates for UVC and far UVC device applications

Ultrawide bandgap semiconductors (bandgaps greater than 4.5 eV) have attracted recent attention for new applications in high power switches (EV and smart grid) and high-power rf.  UWBG semiconductors are also being used to make novel light emitters at wavelengths shorter than 280nm. However, the UWBG semiconductors still present big challenges in growth and doping.  One of the best examples are the pseudomorphic AlGaN alloys gown on high quality AlN substrates where 2-inch diameter is now widely available and 100mm are being sampled. These single-crystal AlN substrates have enabled growth of high quality Al1-xGaxN alloys that are pseudomorphically strained to match the lattice of the underlying AlN substrate. The low extended defect density has made it possible to take advantage of a new kind of conductivity (distributed polarization doping) made possible in materials that have a spontaneous polarization.  Most significantly, p-type Al1-xGaxN has now been demonstrated in Al1-xGaxN without the use of impurity doping.  This breakthrough solved the critical problem of unacceptable resistivities in the doped layers with increasing band gap (particularly for p-type dopants).

Pseudomorphic growth and distributed polarization doping have made the achievement of new devices possible, such as the UVC laser diode which we first demonstrated in 2019 as part of a Asahi Kasei/Nagoya University joint effort.  These laser diodes open new possibilities for optical instruments which will benefit from very compact sources of radiation which are coherent and nearly monochromatic. It is also possible to precisely direct laser radiation in ways that simply are not possible for LEDs.  As this technology develops, it may even be possible to achieve higher wall plug efficiencies than are achievable with UVC LEDs due to improved photon extraction efficiencies. However, the lifetime of these diodes (which operate at much higher current densities than LEDs) is still an important issue that needs to be addressed.

Finally, I would like to introduce our new UWBG semiconductor effort at the Visium UltraLabs in the Cornell Tech Park to enable far UVC (shorter than 235nm) LEDs.