Mechanism of ion track formation in silicon by much lower energy deposition than the formation threshold

Mechanism of the ion track formation in crystalline silicon (c-Si) is discussed, particularly under 1-9 MeV C-60 ion irradiation. In this energy region, the track formation was not expected because the energy E was much lower than the threshold of E (th) = 17 MeV determined by extrapolation from higher energy data in the past literature. The track formation is different between irradiations of C-60 ions and of monoatomic ions: The tracks were observed under 3 MeV C-60 ion irradiation but not under 200 MeV Xe ions, while both the irradiations have the same electronic stopping (S (e)) of 14 keV nm(-1) but much higher nuclear stopping (S (n)) for the former ions. The involvement of S (n) is suggested for the C-60 ions. While the inelastic thermal spike (i-TS) calculations predict that the high energy monoatomic ion irradiation forms the tracks, the tracks have never been experimentally detected, suggesting quick annihilation of the tracks by highly enhanced recrystallization in c-Si. Exceptions are C-60 ions of 1-9 MeV, where the track radii are well reproduced by the i-TS theory with assuming the melting transition. Collisional damage induced by the high S (n) from C-60 ions obstructs the recrystallization in c-Si. Then the tracks formed by the melting transition survive against the recrystallization. This is a new type of the synergy effect between S (e) and S (n), different from the already-known mechanisms, i.e., the pre-damage effect and the unified thermal spike. While c-Si was believed as a radiation-hard material in the S (e) regime with high S (e) threshold, this study suggests that c-Si has a low S (e) threshold but with efficient recrystallization.