Theoretical predicted high-thermal-conductivity cubic Si₃N₄ and Ge₃N₄: promising substrate materials for high-power electronic devices

Ceramic substrates play key roles in power electronic device technology through dissipating heat, wherein both high thermal conductivity and mechanical strength are required. The increased power of new devices has led to the replacement of Al₂O₃ by high thermal conducting AlN and further β-Si₃N₄ based substrates. However, the low mechanical strength and/or anisotropic mechanical/thermal properties are still the bottlenecks for the practical applications of these materials in high power electronic devices. Herein, using a combination of density functional theory and modified Debye-Callaway model, two new promising substrate materials γ-Si₃N₄ and γ-Ge₃N₄ are predicted. Our results demonstrate for the first time that both compounds exhibit higher room temperature thermal conductivity but less anisotropy in expansion and heat conduction compared to β-Si₃N₄. The mechanism underpins the high RT κ is identified as relatively small anharmonicity, high phonon velocity and frequency. The suitability of these two nitrides as substrate materials was also discussed.