First-Principles Computational Exploration of Thermoelectric Properties of Bulk-GaN and Monolayer-GaN

We report the electronic and thermoelectric properties of electron- and hole-doped bulk-GaN and monolayer (ML)-GaN crystal structures using density functional theory and Boltzmann transport equations. In addition, we have studied the tuning of the band gap from the bulk to the ML which is expected to improve the thermoelectric properties of the ML-GaN. The negative formation energies show the thermodynamic stability of both bulk-GaN and ML-GaN. At the same time, there is no negative frequency in the phonon spectrum, which shows the dynamic stability of ML-GaN. The GGA (HSE)-calculated electronic band gap is 1.64 eV (3.25 eV), and 2.09 eV (3.85 eV) for the bulk-GaN and ML-GaN, respectively. Using the Phono3py code, it was found that, at room temperature, the kappa(l) of ML-GaN along the [1 0 0] and [0 1 0] directions is 72.877 W/m-K and 20.984 W/m-K, respectively, which is much smaller than the kappa(l) for bulk-GaN. Therefore, the figure-of-merit (ZT) predicted value is as high as 0.78 (0.98) for the electron- (hole-) doped bulk-GaN at room temperature. Furthermore, we established that reducing the dimensionality caused an increase (decrease) of the electronic conductivity (thermal conductivity). As a result, ZT significantly increased to 0.81 (1.19) for the electron- (hole-) doped ML-GaN at 800 K. The ZT > 1 for a material indicates high applicability in thermoelectric device applications.

Automated summary

In this study, the electronic and thermoelectric properties of electron- and hole-doped bulk-GaN and ML-GaN crystal structures were investigated using density functional theory and Boltzmann transport equations. The results showed that ML-GaN has good thermodynamic and dynamic stability, and reducing dimensionality can enhance the electronic conductivity and reduce the thermal conductivity, thereby significantly improving the thermoelectric performance of the material.