Enhanced phonon-drag by nanoscale design of homoepitaxial \hbox{$\beta$-Ga$_2$O$_3$}

TL;DR

This study demonstrates that nanoscale design of homoepitaxial -GaOb enhances phonon drag effects in thermoelectric devices, with significant increases in thermopower achieved by controlling film thickness and electron-phonon interactions.

Contribution

It reveals how geometric control at the nanoscale can significantly enhance phonon drag in -GaOb thin films, advancing thermoelectric material design.

Findings

Phonon drag thermopower increased from -0.4 mV/K to -3 mV/K at 100 K.

Crossover from 3D to quasi-2D electron-phonon interaction below 75 nm thickness.

Phonon-phonon to electron-phonon relaxation time ratio is ten times larger than in bulk.

Abstract

Phonon drag may be harnessed for thermoelectric generators and devices. Here, we demonstrate the geometric control of the phonon-drag contribution to the thermopower. In nanometer-thin electrically conducting $\beta$-Ga$_2$O$_3$ films homoepitaxially-grown on insulating substrates it is enhanced from -0,4 mV/K to up to -3 mV/K at 100 K by choice of the film thickness. Analysis of the temperature-dependent Seebeck coefficients reveal that a crossover from three-dimensional to quasi-two-dimensional electron-phonon interaction occurs for film thicknesses below 75~nm. The ratio of phonon-phonon to electron-phonon relaxation times in these confined structures is $10$ times larger than that of bulk. Generally the phonon drag can be tuned depending on the relations between the phonon-drag interaction length $\lambda_\text{PD}$, the phonon mean free path $\lambda$ and the film thickness $d$. Phonon drag can be enhanced for $\lambda_\text{PD}\gg\lambda>d$.