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The non-steady-state radiation enhanced diffusion of Ti in MgO(100) prebombarded with 7 keV Ar[sup +] and Cl[sup +] 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[sub 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 purports that during the thermal relaxation stage of the energetic collision cascade, the initially identical defect structures induced by Ar[sup +] and Cl[sup +] implantation undergo different chemical reaction processes, leading to different Mg vacancy cluster distributions. Statistically, Mg vacancy clusters in the Cl[sup +] prebombarded MgO are larger in size than those in Ar[sup +] 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[sup +] prebombarded MgO is greater than that in Ar[sup +] prebombarded MgO due to the complete release of monovacancies stored in the vacancy clusters. This trend is just opposite of the case of short-time diffusion as in the previous case. This prediction was conformed by using prolonged diffusion experiments. [ABSTRACT FROM AUTHOR]

The effect of postbombardment annealing, or postannealing, has been investigated for Ti diffusion in ion prebombarded MgO(100). MgO postannealing was performed in ultrahigh vacuum after ion prebombardment of MgO and prior to Ti evaporation and diffusion. For postannealing temperatures of T= 800, 900, and 1000 °C, Ti diffusion in 7 keV Ar[sup +] prebombarded MgO exhibits the expected exponential decays with postannealing time and more rapid decays with higher temperatures. Cationic diffusion in the ion prebombarded sample proceeds in a nonsteady state condition of extra defects known as nonsteady state radiation enhanced diffusion (NSRED). Thermal dissociation of small vacancy clusters is responsible for NSRED. From the data of T ≤ 1000 °C, the derived dissociation energy of the vacancy clusters is only ∼10% of the normal value. For some specific postannealing times at T = 1100 °C, the subsequent Ti diffusion was more enhanced than that without postannealing, a phenomenon which is contrary to the usual understanding of annealing effects. This same effect was also observed for Ti diffusion in 7 keV C1[sup +] prebombarded MgO postannealed at 1100 °C. Previous work indicates that the small vacancy clusters for NSRED are mainly Mg divacancies. We propose a divacancy creation mechanism that is due to the thermal dissociation of larger vacancy clusters during postannealing. The dissociation energy ratio of the effective large vacancy clusters (LVC) to that of the divacancies (DIV) is E[sub LVC]/E[sub DIV] ∼ 1.4. The monovacancy coalescence was found to be unimportant for divacancy creation. A model based on this mechanism explains the unusual diffusion enhancement observed. [ABSTRACT FROM AUTHOR]