Understanding and engineering phonon transport in GaN/AlN superlattices (SLs) is critical for the thermal management of GaN high-electron-mobility transistors (HEMTs). While most studies suggest that interfacial roughness reduces the thermal conductivity of SLs, a few have reported its unexpected enhancement, yet the underlying mechanism, particularly concerning phonon coherence, remains elusive. In this study, we address this gap by developing ametric—the concentration degree of energy (CDE) in $k$-space—to directly quantify the degree of phonon coherence from atomic displacements obtained through lattice dynamics. Our simulations reveal that interfacial roughness can enhance the thermal conductivity of SLs, provided that the inherent periodicity of the SL is not disrupted. A mode-resolved coherence analysis based on the CDE metric reveals that while increased roughness leads to a rise in incoherent modes, it simultaneously enables more modes to propagate. Meanwhile, the preservation of periodicity is the decisive factor that significantly enhances phonon coherence. This insight successfully explains why SLs with moderate, periodicity-preserving roughness can achieve superior phonon coherence and higher thermal conductivity than ideal SLs. The coherence analysis framework is applicable to general SL systems.

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• Lattice thermal conductivity
• Roughness
• Transport phenomena
• Superlattices
• Molecular dynamics