Microscopic Origin of Temperature‐Dependent Anisotropic Heat Transport in Ultrawide‐Bandgap Rutile GeO ₂

ABSTRACT Ultrawide‐bandgap rutile is a promising semiconductor for power electronics, where efficient heat dissipation is essential to suppress self‐heating and ensure device reliability. However, the temperature dependence and microscopic origin of its anisotropic heat transport have remained experimentally unresolved. Here, temperature‐dependent time‐domain thermoreflectance measurements combined with first‐principles phonon‐transport calculations quantify the thermal conductivity of single‐crystal rutile from 80 to 350 K along [001] and [110]. At 295 K, the thermal conductivity reaches 47.5 W along [001] and 32.5 W along [110], corresponding to an anisotropy ratio of 1.46, in good agreement with theory. The thermal conductivity follows an approximate dependence rather than a simple law, indicating additional scattering beyond purely three‐phonon‐limited transport. Mode‐resolved analysis shows that the room‐temperature anisotropy originates from larger phonon group velocities along [001] and direction‐dependent phonon lifetimes, while depopulation of high‐frequency phonons upon cooling reduces the anisotropy. The temperature‐dependent thermal boundary conductance of Al/rutile interfaces further indicates predominantly elastic interfacial transport. These findings establish the microscopic basis of bulk and interfacial heat transport in rutile for ultrawide‐bandgap electronics.