Lattice preamorphization by ion irradiation: Fluence dependence of the electronic stopping power threshold for amorphization

A thermal-spike model has been applied to characterize the damage structure of the latent tracks generated by high-energy ion irradiations on $LiNbO3$ through electron excitation mechanisms. It applies to ions having electronic stopping powers both below and above the threshold value for lattice amorphization. The model allows to estimate the defect concentrations in the heavily damaged (preamorphized) regions that have not reached the threshold for amorphization. They include the halo and tail surrounding the core of a latent track. The existence of the preamorphized regions accounts for a synergy between successive irradiations and predicts a dependence of the amorphization threshold on previous irradiation fluence. The predicted dependence is in accordance with irradiation experiments using N $(4.53MeV)$⁠, O $(5.00MeV)$⁠, F $(5.13MeV)$⁠, and Si (5 and $7.5MeV$⁠). For electronic stopping powers above the threshold value the model describes the generation of homogeneous amorphous layers and predicts the propagation of the amorphization front with fluence. A theoretical expression, describing this propagation, has been obtained that is in reasonable agreement with silicon irradiation experiments at 5 and $7.5MeV$⁠. The accordance is improved by including in a simple phenomenological way the velocity effect on the threshold. At the highest fluences (or depths) a significant discrepancy appears that may be attributed to the contribution of the nuclear collision damage.