Effects of postbombardment annealing on Ti diffusion in ion prebombarded MgO(100)

Abstract

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+ 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 Cl+ 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 ELVC/EDIV∼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.