Rutile GeO₂ is emerging as an ultrawide-bandgap semiconductor for power electronics, where self-heating limits performance. How efficiently it conducts heat — and why the conductivity differs along different crystal axes — determines whether the material is thermally viable.
This paper measures thermal conductivity of single-crystal rutile GeO₂ from 80 to 350 K along two crystal directions. At room temperature: 47.5 W/m·K along [001] versus 32.5 along [110], an anisotropy ratio of 1.46. But the temperature dependence doesn't follow the expected T⁻¹ law — it goes as T⁻¹·⁴, indicating scattering mechanisms beyond simple three-phonon processes.
Mode-resolved analysis reveals the microscopic origin: the anisotropy comes from both larger phonon group velocities along [001] and direction-dependent phonon lifetimes. Upon cooling, high-frequency phonons depopulate, suppressing their contribution and reducing the anisotropy. The anisotropy is temperature-dependent because it arises from a specific phonon subpopulation that freezes out at low temperatures. The material's thermal management strategy depends on which direction faces the heat source — and that dependence itself changes with operating temperature.