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

The mechanism of the track formation in crystalline silicon (c-Si) under C60 ion irradiation with the medium energies of 1-9 MeV is discussed. In this energy region, the track formation was not expected because the energy E was much lower than the threshold of Eth = 17 MeV extrapolated in the past literature. The track formation was observed under 3 MeV C60 irradiation but not under 200 MeV Xe ions, while both the irradiations have almost the same Se of 14 keV/nm but different nuclear stopping Sn. The involvement of Sn was strongly suggested for the track formation in c-Si at the sub-threshold Se. The inelastic thermal spike (i-TS) calculations cannot reproduce the high threshold of c-Si from the melting transition but the vaporization transition. The tracks formed by the melting transitions are assumed to quickly disappear by highly enhanced recrystallization in c-Si. Only the tracks formed by the vaporization survive. However, the Se dependence on the track radii formed under 1-9 MeV C60 irradiation was not reproduced by the vaporization transition but the melting one. Collisional processes due to high Sn value interrupt the efficient recrystallization in c-Si. Then the tracks formed by the melting transitions survive under 1-9 MeV C60 irradiation.