Relaxation process of ion irradiation defects in IV-semiconductors (Si, Si 0.85Ge 0.15, Ge, and heavily P-doped (∼10 20 cm −3) n +-Si) was investigated. The IV-semiconductors were irradiated with 25 keV Ar + ions (dose: 1×10 13–1×10 16 cm −2, dose rate: 3×10 11–6×10 12 cm −2 s −1, temperature: 25–250 °C). Amorphicity was comprehensively evaluated by using spectroscopic ellipsometry. For samples irradiated at temperatures around 100 °C, dose rate dependence of amorphicity was remarkably observed. The Arrhenius plot of critical dose rate, at which defect generation and annihilation rates were balanced, showed that activation energies for defect relaxation process were 0.43, 0.51, 0.88, and 0.18 eV for Si, Si 0.85Ge 0.15, Ge, and n +-Si, respectively. These results suggest that defect relaxation process is governed by migration of V 0 in Si, Si 0.85Ge 0.15, and Ge, while V 2− in n +-Si. The migration energy of V 0 in Si 0.85Ge 0.15 obeys the Vegard's law.