Thermal transport properties of monolayer hexagonal group-III nitrides: A comparative first principles investigation

• Phonons are gradually becoming softer from h -BN to h -InN with enlarged frequency gap. • The varied thermal conductivities of different h -MN monolayers were illuminated. • Lone-pair electrons lead to the significant phonon anharmonicity of h -MN monolayers. Monolayer hexagonal group-III nitrides have drawn increasing attention because of their great application potential in electronic and energy devices, which are inevitably involved with thermal transport. Hence, in this work, we performed a comparative study of the thermal transport properties of h -MN monolayers by integrating the Boltzmann transport equation and the Wigner transport equation with first principles calculations. The results show that the phonons are gradually becoming softer from h -BN to h -InN with a significant phonon frequency gap appearing in h -AlN, h -GaN, and h -InN, which originates from the varied bonding strength and atomic mass. Then, we highlighted that h -InN exhibits an ultra-low thermal conductivity of 8.5–9.4 W/mK at 300 K, which is in sharp contrast to that of h -BN despite their similar planar structures. Meanwhile, with the order from h -BN, h -AlN, h -GaN to h -InN, the contributions of acoustic branches to thermal conductivity significantly decrease, while the contributions of optical branches increase. Further comparative analysis on heat capacities, group velocities, phonon lifetime, and phonon anharmonicity were employed to illuminate the underlying variation mechanism of the thermal conductivity of h -MN monolayers. Last but not least, an electronic level insight was proposed that the unpaired lone-pair valence electrons and significant polarized In-N and Ga-N bonds will lead to their increased phonon anharmonicities and lower thermal conductivities than that of h -BN. We believe this work will provide a fundamental guideline for the rational design of monolayer group-III nitrides and related devices in terms of thermal transport.