Anomalous thermal conductivity anisotropy in bulk hexagonal AlN induced by structural phase transition

Hexagonal aluminum nitride (h-AlN), an ultrawide bandgap semiconductor with exceptional thermal conductivity, holds great promise for applications in power electronics and optoelectronics. Prior studies have shown that AlN undergoes a transition from the wurtzite structure (wz-AlN) to hexagonal magnesium oxide (h-MgO) structure under pressure. However, a thorough understanding of the thermal transport properties of these two hexagonal phases has yet to be established. Herein, we look into this issue by combining the first-principles calculations with phonon Boltzmann transport theory. With the inclusion of four-phonon scattering and phonon renormalization, we predict that the thermal conductivity (κ) of wz-AlN is 291 and 268 W/mK for the in-plane and out-of-plane directions at room temperature, respectively, showing good agreement with experimental measurements. In contrast, the h-MgO phase exhibits a significantly lower thermal conductivity, with the in-plane value of 50 W/mK and the out-of-plane value of 93 W/mK. Through further analysis of modal phonon transport and chemical bonding, we attribute the lower κ of h-MgO-AlN to its stronger anharmonicity resulting from the weaker Al–N bonding. More importantly, we uncover that the h-MgO phase exhibits unusual κ anisotropy with the out-of-plane κ being almost twice as high as the in-plane value, in stark contrast to the common behavior observed in wurtzite crystals. This anomaly is found to arise from the phase transition-induced strengthening of interlayer bonding. This work provides a fundamental understanding of the thermal transport behavior of hexagonal bulk AlN during the structural phase transition.