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

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 suppresses 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.

Characterization of the piezoresistance in highly doped p-type 3C-SiC at cryogenic temperatures.

This paper reports on the piezoresistive effect in p-type 3C-SiC thin film mechanical sensing at cryogenic conditions. Nanothin 3C-SiC films with a carrier concentration of 2 × 1019 cm-3 were epitaxially grown on a Si substrate using the LPCVD process, followed by photolithography and UV laser engraving processes to form SiC-on-Si pressure sensors. The magnitude of the piezoresistive effect was measured by monitoring the change of the SiC conductance subjected to pressurizing/depressurizing cycles at different temperatures. Experimental results showed a relatively stable piezoresistive effect in the highly doped 3C-SiC film with the gauge factor slightly increased by 20% at 150 K with respect to that at room temperature. The data was also in good agreement with theoretical analysis obtained based on the charge transfer phenomenon. This finding demonstrates the potential of 3C-SiC for MEMS sensors used in a large range of temperatures from cryogenic to high temperatures.