Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas

Abstract

In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with ~100 nm GaN thickness, which suppress the phonon mean free path. However, when the thickness is increased to ~1.2 μm, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to ~40 mW m -1 K -2 at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs.

The elphbolt ab initio solver for the coupled electron-phonon Boltzmann transport equations

“…In this case, the phonon drag effect leads to a large increase in mobility-at 300 (50) K, the drag gain of mobility is about a factor of 2.5 (50). Such high gains in mobility can, potentially, be exploited along with those previously identified in the thermopower 21,23,56 to boost the thermoelectric figure-ofmerit. Note that, in this case, the phonon drag effect is not fully captured by the partially decoupled solution, which predicts about half the value given by the dragged full solution at 50 K. At this temperature, the RTA and dragless full solutions undershoot the value of the dragged full solution by about a factor of 5.…”

Giant Thermoelectric Seebeck Coefficients in Tellurium Quantum Wires Formed Vertically in an Aluminum Oxide Layer by Electrical Breakdown

“…Therefore, EBD treated semiconductor Te QW energy filtering is much stronger than for metal Cu QWs due to the Te narrow band gap (∼0.35 eV) (Supporting Information, Figure S4). Hence S ⊥ Te QWs was much larger than for previously reported 1D Te NWs due to quantum confinement and energy filtering. , Figure g shows single-phase Te QWs Seebeck coefficient with respect to applied external magnetic field up to H = ± 400 Oe in the y direction (Figure f), indicating that phonon drag and magneto Seebeck effects are involved in EBD treated elemental Te QWs at 300 K . The lack of magnetic field effects indicates there is no significant influence from magneto Seebeck on V ⊥ for Te/AO/Te systems.…”

The elphbolt ab initio solver for the coupled electron-phonon Boltzmann transport equations

“…In this case, the phonon drag effect leads to a large increase in the mobility -at 300 (50) K, the drag gain of mobility is about a factor of 2.5 (50). Such high gains in the mobility can, potentially, be exploited along with those previously identified in the thermopower [21,23,57] to boost the thermoelectric figure-of-merit. Note that in this case the phonon drag effect is not fully captured by the partially decoupled solution which predicts about half the value given by the dragged full solution at 50 K. At this temperature, the RTA and dragless full solutions undershoot the value of the dragged full solution by about a factor of 5.…”