Understanding thermal transport in wide band gap (WBG), AlGaN/GaN heterostructures grown on Si can improve the reliability of emerging power electronic devices and enable new device architectures for sensing, controlling and harvesting thermal energy. Since the highly conductive, two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on built-in polarization fields (not doping) and is confined to few-nanometer thicknesses, its charge carriers exhibit much higher mobilities in comparison to their doped counterparts.1 This can lead to enhancements in the Seebeck coefficient that can be exploited for the realization of monolithically-integrated AlGaN/GaN thermoelectric-based sensors and energy harvesters. However, the impact of temperature and AlGaN/GaN film structure on the thermoelectric behavior of the 2DEG has yet to be fully mapped.
Here, we examine the Seebeck coefficient of suspended, gated AlGaN/GaN 2DEG heterostrucures over a wide temperature range (50 to 300 K) with varying GaN buffer layer thickness for the first time. This allows us to control the roughness of the AlGaN/GaN interface where the 2DEG forms. For rough interfaces, we observe a linear increase in Seebeck coefficient with temperature, typical of its “diffusive” nature. However, for pristine interfaces (RMS roughness ~ 1 nm), we observe a Seebeck coefficient enhancement in a broad temperature range (50 to 150 K). Such enhancements are usually attributed to phonon drag effects, where non-equilibrium phonons deliver excessive momenta to the electrons since phonon-phonon scattering is suppressed at low temperature.2 This scenario would lead to the Seebeck coefficient and thermal conductivity peaking at approximately the same temperature.3
Contrary to the expected behavior, experimental measurements of thermal conductivity in our samples show that the thermal conductivity peak is at least >50 K higher, thus the enhancement cannot be reconciled by phonon drag alone. Furthermore, the phonon drag effect is usually suppressed for samples with high carrier densities (> 1019 cm-3), which are present in our samples.2 Thus, we uncover that the observed effects can be attributed to a combination of phonon drag and energy filtering due to inelastic scattering of electrons in pristine AlGaN/GaN 2DEGs. These effects are inhibited for the rough samples due to dominant roughness scattering. Overall, the results are essential for understanding energy harvesting and sensing utilizing WBG materials, over a wide temperature range.
1 O. Ambacher et al., J. Appl. Phys. 87, 334 (2000).
2 J. Zhou et al., Proc. Natl. Acad. Sci. 112, 14777 (2015).
3 G. Wang et al., Phys. Rev. Lett. 111, 046803 (2013).