Moldel for chemical effects in non-steady-state radiatio enhanced diffusion of Ti in MgO

摘要

The non-steady-state radiation enhanced diffusion of Ti in MgO(100) prebombarded with 7 keV Ar~+ and Cl~+ has been studied and the data have been first interpreted quantitatively in terms of a diffusion-reaction mechanism. Secondary ion mass spectroscopy was used for depth profiling and mass spectral measurements. The results revealed an unrealistic stoichiometry for formation of a volatile TiCl_x compound, indicating the failure of the hypothesis of volatile species formation during diffusion. A modified Kapinos-Platonov model [Rad. Eff. 103, 45 (1987)] was developed which provides a consistent interpretation of the data. The model purpors that during the thermal relaxatin stage of the energetic collision cascade, the initially identical defect structures induced by Ar~+ and Cl~+ implantation undergo different chemical reaction processes, leading to different Mg vacancy cluster distributions. Statistically, Mg vacancy clusters in the Cl~+ prebombarded MgO are larger in size than those in Ar~+ prebombarded MgO. The difference in these defect structures is the reason for the difference in the subsequent Ti diffusion in MgO as we observed previously [J. Chem. Phys. 115, 446 (2001)]. From this model it is derived that after diffusion for an extended time period, Ti diffusion in Cl~+ prebombarded MgO is greater than that in Ar~+ prebombarded MgO due to the complete release of onovacancies stored in the vacancy clusters. This trend is just opposite of the case of short-time diffusion as in the previous case. This trend is just opposite of the case fo short-time diffusion as in the previous case. This prediction was conformed by using prolonged diffusion experiemnts.