Radiation enhanced diffusion in MgO

Radiation enhanced diffusion and ion beam mixing of ¹⁸O, Ca, and Zn buried tracer layers in MgO, grown by molecular beam epitaxy, have been measured following irradiation with 2.0 MeV Kr⁺ and 1.0 MeV Ne⁺, He⁺, and H⁺ from 30 to 1500 °C. The ion beam mixing parameter varied between 1.0 and 5.0 Å⁵ eV^-1 for the different tracers at 30 °C and increased slowly with increasing temperature. These results are consistent with ballistic mixing. In the highest temperature range investigated, 1350-1500 °C, the radiation enhanced diffusion coefficient for ¹⁸O was proportional to the square root of the irradiation flux and displayed an apparent activation enthalpy of 1.2 eV. These dependencies on flux and temperature are indicative of recombination-limited kinetics, with the measured activation enthalpy representing one-half the migration enthalpy of anion vacancies. From 1150 to 1350 °C an activation enthalpy of 4.1 eV was obtained. The unexpectedly high value is attributed to the dissociation energy of small vacancy clusters. Measurements on the cation sublattice were limited to temperatures below 900 °C owing to the excessive thermal diffusion associated with extrinsic vacancies, which are present for trivalent impurity charge compensation.