Au ion irradiation induces ultralow thermal conductivity in GaN

Abstract

Gallium nitride (GaN) is widely considered as a crucial semiconductor for the nuclear industry and space explorations due to its superior radiation hardness. Despite extensive studies of the electronic and optical properties of irradiated GaN, the effects of particle irradiation on the thermal properties remain largely unexplored. Here, we begin with single-crystalline GaN and employ an accelerator equipped with heavy gold ions (Au2+) as the radiation source in order to imitate extreme environments and maximize lattice damages. Eight different irradiated samples are prepared with the fluence of Au2+ spanning four orders of magnitude from 1011 to 1015 cm−2. The thermal conductivity (κ) of the ion-affected regions is measured using the laser pump–probe technique of frequency-domain thermoreflectance. We find that κ decreased consistently and notably with increasing irradiation fluence and observe a transition from crystal to glass-like thermal transport. Remarkably, the room-temperature κ of the GaN sample with the highest Au2+ fluence of 1 × 1015 cm−2 reaches about 1 Wm−1 K−1, which is two orders of magnitude lower than the κ of pristine GaN and approaches the theoretical minimum. A Callaway-type model captures the phonon–point defect scattering in samples with relatively low ion fluences. At higher fluences, the increased defect types and densities, together with the formation of nitrogen bubbles, further suppress phonon transport. Our findings are instrumental in fundamentally understanding the impact of heavy-ion irradiation on thermal transport and may prove useful for the application of GaN-based devices in radiation-intense environments.