The effect of post-bombardment annealing, or post-annealing, was investigated for Ti diffusion in ion pre-bombarded MgO (100). The post-annealing was performed in ultra-high vacuum after ion pre-bombardment of MgO and prior to Ti evaporation and diffusion. For post-annealing temperatures of 800, 900 and 1000C, Ti diffusion in 7keV Ar+ pre-bombarded MgO exhibited the expected exponential decays with post-annealing time and more rapid decays with higher temperatures. Cationic diffusion in the ion pre-bombarded sample proceeds in a non-steady state condition of extra defects known as non steady-state radiation enhanced diffusion. Thermal dissociation of small vacancy clusters was responsible for non steady-state radiation enhanced diffusion. From the data for up to 1000C, the derived dissociation energy of the vacancy clusters was only ~10% of the normal value. For some specific post-annealing times at 1100C, the subsequent Ti diffusion was more enhanced than that without post-annealing, a phenomenon which was contrary to the usual understanding of annealing effects. This same effect was also observed for Ti diffusion in 7keV Cl+ pre-bombarded MgO post-annealed at 1100C. Previous work indicated that the small vacancy clusters for non steady-state radiation enhanced diffusion were mainly Mg divacancies. A divacancy creation mechanism was proposed that was due to the thermal dissociation of larger vacancy clusters during post-annealing. The dissociation energy ratio of the effective large vacancy clusters to that of the divacancies was about 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.

Effects of Post-Bombardment Annealing on Ti Diffusion in Ion Prebombarded MgO (100). M.Lu, C.Lupu, J.W.Rabalais: Journal of Applied Physics, 2002, 92[7], 3591-8