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The radiation hardness of GaN-based devices is a critical metric for applications in extreme environments. This study investigates the structural changes in GaN and AlN induced by swift heavy ion (SHI) irradiation, characteristic of space radiation environments. A multilayered GaN/AlN structure is exposed to 950 MeV Au ions at fluences of 1 × 10 12 and 8 × 10 12 ions/cm2. Subsequent post-irradiation characterization, including transmission electron microscopy and energy-dispersive x-ray spectroscopy, reveal no apparent amorphization across the entire sample. Notably, significant nanometer-sized cavities are observed in both GaN and AlN. The cavities in GaN exhibit an increase in number density and diameter with increasing SHI irradiation, with the average diameter progressing from 1.80 to 2.10 nm. In contrast, cavities in AlN appear considerably smaller. Molecular dynamics simulations, coupled with the inelastic thermal spike model, reproduce the presence of cavities in GaN and no cavities in the AlN structure. This difference is attributed to the faster heat dissipation and stronger bonding in AlN. Considering the overlapping of ion impacts at high fluences, simulations confirm the enlargement of cavity size in GaN. These findings contribute to a mechanistic understanding of the contrast in ion–matter interactions and induced microstructures between AlN and GaN under extreme ionizing radiation conditions. This disparity could potentially impact electronic performance through the formation of defect traps and interfacial strain fields. [ABSTRACT FROM AUTHOR]