A Vannevar Bush Fellowship for Jim Speck
The UCSB materials professor will study a key loss mechanism in gallium-nitride LEDs
UC Santa Barbara materials professor Jim Speck received a 2024 Vannevar Bush Faculty Fellowship (VBFF) from U.S. Department of Defense (DoD), which will allow him to further investigate loss mechanisms in gallium nitride (GaN) LEDs. GaN, of course, is the semiconductor material that Shuji Nakamura, before becoming Speck’s colleague in the UCSB Materials Department, used to invent the blue LED, which enabled the white LED and a world revolution in lighting and would earn Nakamura the Nobel Prize in 2014.
Speck received the highly prestigious Vannevar Bush Faculty Fellowship — only eleven were awarded in 2024 — to pursue high-risk, high-reward research, the kind, the DoD said in a release last summer, that has “transformed entire disciplines, birthed novel fields, and challenged established theories and perspectives.” The five-year, $3-million award will enable Speck to build on research he has pursued for nearly fifteen years related to little-understood physics behind current droop, an important loss mechanism in GaN LEDs.
Normally, an LED emits light as electrons and holes combine in a quantum well, rise to a higher energy level, and then emit light as the extra energy is released. Sometimes, however, rather than an electron recombining with a hole to make a photon, 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 will work closely with UCSB colleagues, including materials professors Chris Van de Walle, an expert in modeling loss mechanisms in GaN semiconductors and LEDs, and Claude Weisbuch, with whom Speck designed and ran experiments that enabled them to become the first to measure hot electrons arising from non-radiative recombination, known as 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 explains, adding that their desire for a better understanding of the Auger process was driven by a curious fact: “If we look at the science of the semiconductor and the way light-emitting diodes work, there are no processes that should generate hot carriers.”
Releasing those hot electrons allowed him and Weisbuch to measure the particles and their energy in vacuum in a spectrometer, and in 2013, they published an important paper describing their research to achieve 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.”
Much remains to be understood, however, about the complex physics behind Auger recombination and the resulting current droop. Speck’s VBFF will allow him and his UCSB colleagues to spend the next five years diving deep into that knowledge frontier.
