An extraordinarily low-energy threshold of less than 60 keV for ion track formation in silicon

Abstract

The impingement of a C₆₀ cluster ion upon a solid can realize the temporospatially correlated injection of sixty C atoms to the solid at the same time and place with a molecular dimension of 0.7 nm in diameter. This could result in ion track formation that differs from that of conventional monatomic ion irradiation. Although no ion tracks have been observed in Si even under high-energy 3.6-GeV monatomic U ion irradiation, certain tracks have been found in Si under low-energy 1-MeV C₆₀ ion irradiation. Here, we investigated track formation under an irradiation of less than 1 MeV: (i) With a decrease in energy, the diameters and lengths of the tracks decreased; however, the length decreased more steeply than the diameter. (ii) Although the tracks were fuzzily perceived down to 60-keV irradiation, no tracks were observed under 30-keV irradiation, except for an extended damage zone. Furthermore, we observed (iii) track formation below the electronic stopping threshold, and (iv) track length extension due to “the acceleration effect” of cluster-ion irradiation at low energies. (v) Finally, the approximated linearity between the track volume and C₆₀ energy is discussed.