UC Santa Barbara materials professor Jim Speck is one of only eleven people — from an initial  group of 170 applicants, of whom 27 were invited to submit full proposals — to have received a prestigious 2024 Vannevar Bush Faculty Fellowship (VBFF) from the U.S. Department of Defense. The awards, up to $3 million each, allow recipients to “explore the frontiers of knowledge and advance transformative, fundamental research at their respective universities,” according to a DoD press release.

“The Vannever Bush Faculty Fellowship is…a beacon for tenured faculty embarking on groundbreaking ‘blue sky' research,” said Dr. Bindu Nair, director of the Basic Research Office in the Office of the Under Secretary of Defense for Research and Engineering. “Through this fellowship, DoD empowers some of the nation's most talented researchers to pursue ambitious ideas that defy conventional boundaries. The outcomes of VBFF-funded research have transformed entire disciplines, birthed novel fields, and challenged established theories and perspectives.”

Speck’s proposal, titled “Beyond the Band Minima: High Energy Electron Dispersion, Physics, and Technology,” is an outgrowth of work he has been pursuing in collaboration with UCSB materials science colleagues as well as industry members of the UCSB Solid State Lighting and Energy Electronics Center (SSLEEC) for nearly fifteen years. 

Around 2010, Speck (left) began long-term research with fellow UCSB materials professors Claude Weisbuch, like Speck, an experimentalist, and Chris van de Walle, a computational theoretician. (Van de Walle received a VBFF in 2022, and fellow UCSB materials professors Tresa Pollock, Susanne Stemmer, and Christopher Palmstrom earned VBFFs in 2017, 2016, and 2015, respectively). Speck, Weisbuch, and Van de Walle were collaborating to try to understand a phenomenon that limits the performance of gallium-nitride (GaN) light-emitting diodes (LEDs), a material and a device in which UCSB is a world leader. GaN is the semiconductor material that UCSB materials professor Shuji Nakamura used to develop the blue LED, which led to a world revolution in lighting and his receiving a Nobel Prize in 2014.

At the time, Speck recalls, “We were trying to understand not why [comparatively highly efficient] gallium nitride LEDs are efficient, but rather, what physical processes there are that can make them inefficient.”

That loss of efficiency, known as current “droop,” occurs in LEDs as voltage is increased beyond a certain threshold. Normally, an LED emits light as electrons and holes combine in a quantum well, and light is emitted. Sometimes, however, instead of an electron recombining with a hole to make a photon, Speck explains, two electrons recombine with a hole to make a “hot” electron, in a process aptly called non-radiative, because it does not emit light, only heat, and is, therefore, an element of efficiency loss.
   
Speck and Weisbuch began designing experiments to measure hot electrons arising from that non-radiative recombination process, called Auger recombination. “We worked hard to design experiments that would enable us to extract the very-high-energy Auger electrons out of the semiconductor,” Speck recalls, adding that their desire for a better understanding of the Auger process was driven by the fact that “if we look at the science of the semiconductor and the way light-emitting diodes work, there are no processes that should generate what we call hot carriers.” Releasing those hot electrons allowed them to then measure the particles and their energy in vacuum in a spectrometer, in an experiment referred to as electro-emission spectroscopy (EES).

In 2013, Speck and Weisbuch published an important paper describing research in their labs that had yielded the world’s first direct measurement of hot electrons. Thanks to work that Speck, Weisbuch, and Van de Walle have done together since then, Speck says, “All aspects of the technique have gone forward by leaps and bounds. It is still a very active area of our research, and is the foundation for the Vannevar Bush fellowship.” 

In terms of the “blue sky” nature of the science the DoD seeks in awarding VBFFs, Speck says, “We were the first and still are the only group in the world to make these measurements related to Auger recombination. And it wasn’t that we had the smoking gun. We actually had the bullet from the process. The Auger hot electrons that we measure are the ones that physically were involved in the non-radiative recombination process.”

Speck explains another anomaly, in addition to having identified nothing in an LED (which runs on very little energy) that should lead to formation of hot electrons. “One of the big surprises in the electron emission spectroscopy was that the energy of the electrons that come out doesn’t correspond to what theory says it should be,” he says. “We had a big discrepancy between our experimental results and theory.” 

That discrepancy was a big motivation for the VBFF proposal. In the program, Speck will develop techniques to directly measure the electronic structure of semiconductors — namely, the normally empty states, which are referred to as the conduction band. Existing techniques such as angle-resolved photoemission spectroscopy (ARPES) measure occupied states, which are referred to as the valence bands in semiconductors. Thus, the first major goal of the VBFF research is to develop the techniques and instrumentation, the latter of which may involve modifying extremely sophisticated existing instruments, to measure conduction-band structure. 

“Direct measurement of the conduction-band structure would profoundly impact many problems in semiconductors,” Speck explains. “For example, in high-voltage devices used for radio-frequency electronics and power electronics — such as those used to change voltage or to change AC to DC or vice versa —electrons can be accelerated to high energies; that is the area of high-electric-field transport. Understanding the physical processes in high-field transport depends on having knowledge about the conduction structure at energies far in excess of those usually considered for conventional semiconductor devices.

“You can think of the band structure as being like a map of the energy of the semiconductor,” he continues. “To really understand what's happening, you need the map and currently, part of the map is not on such firm ground, experimentally. Other parts are on firmer ground. This project is really about closing that knowledge gap by conducting experiments to see what processes are occurring, what is really happening. So, there's real discovery taking place as we look for physical processes we haven’t identified previously.”

Other goals of the VBFF research are to measure hot-carrier distributions in high field transport, and to continue investigating non-radiative processes in semiconductors. “There is huge interest in this work in the solid-state community,” Speck notes, “and more so in the condensed-matter physics community in measuring the conduction-band structure, whether in semiconductors, metals, or exotic materials, such as the topological insulators being pursued for use in quantum applications.”