Damage evolution in LiNbO3 due to electronic energy deposition below the threshold for direct amorphous track formation

Ionization-induced defect formation and amorphization were investigated in x- and z-cut LiNbO3 crystals irradiated with Si and O ions at energies yielding an electronic energy deposition Se close to the threshold value for melting. The evolution of radiation damage was analyzed using Rutherford Backscattering Spectrometry in channeling configuration. Damage peaks occur which are caused by the electronic energy loss of the incoming ions. From the difference in the evolution of these peaks observed along the x- and z-directions, the contribution of point defects/clusters and amorphous regions to the total amount of damage was separated. The analysis of the fluence dependences in the framework of a common defect accumulation and amorphization model revealed cross sections of the damage evolution processes involved. The cross sections for the point defect production were compared quantitatively with those calculated on the basis of the exciton decay model in combination with the analytical thermal spike concept. Very good agreement between measured and calculated cross sections was obtained when assuming a certain threshold value nvthresh of the relative concentration of primary defects for the formation of stable point defects/defect clusters remaining after irradiation. The occurrence of nvthresh provides a threshold value of the electronic energy deposition Se ≈ 2.5 keV nm−1 below which no defects are detected and which very well agrees with the threshold value for the defect formation in the subthreshold irradiation regime reported in the literature. The small values of the cross sections for direct amorphization within a single impact indicate that continuous amorphous ion tracks are hardly formed, but that at best, few amorphous pockets may be created along the trajectories of individual ions, which grow with ongoing irradiation. A comparison of the experimental data with inelastic thermal spike calculations shows that the formation of amorphous pockets starts when the electronic energy loss just reaches the threshold value for melting. The steep increase of the defect concentration up to complete damage within a narrow ion fluence range is mainly determined by the stimulated growth of defect clusters and amorphous regions. The corresponding parameters do not correlate with Se but with the energy density deposited per ion and unit volume, which is inversely proportional to the ion velocity. In summary, the investigations provide a consistent explanation