Little-understood physics in gallium-nitride LEDs garners UCSB’s Jim Speck a prestigious Vannevar Bush award

UC Santa Barbara materials professor Jim Speck is one of only 11 people — from an initial group of 170 applicants  — to receive a prestigious 2024 Vannevar Bush Faculty Fellowship (VBFF).

Speck’s proposal, “Beyond the Band Minima: High Energy Electron Dispersion, Physics, and Technology,” is an outgrowth of work he has been pursuing for some 15 years in collaboration with UCSB materials science colleagues and with industry members of the UCSB Solid State Lighting and Energy Electronics Center (SSLEEC).

Around 2010, Speck began long-term research with fellow UCSB materials professors Claude Weisbuch, an experimentalist, and Chris van de Walle, a computational theoretician, to explore 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.

“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,” Speck said.

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 explained, two electrons recombine with a hole to make a “hot” electron, in a process aptly called non-radiative, because it emits only heat, not light, and is therefore an element of efficiency loss.