In situ TEM Study of Electron Beam Stimulated Organization of Three-dimensional Void Superlattice in CaF2

Abstract

CaF2 is widely adopted as deep-UV window material and thin film optical coating. It has been known that ordered defect superlattices may form in electron irradiated CaF2.[1] However, the nature of the defects on the superlattice point (i.e., Ca colloids or voids) has not been certain and the self-organization mechanism has not been fully understood. In this study, single crystal CaF2 were irradiated under 200 keV electron beam at room temperature with in situ TEM observation of the dynamic process of defect ordering. The superlattice reached steady state after an electron dose of 1×1021e−/cm2 with void radius about 5 nm. Videos recorded during the in-situ observation reveal the dynamic self-organization process of the void superlattice. Coalescence was prevalent at the initial stages. Migration and preferential growth were dominant at the final stages of the superlattice formation. At a dose higher than 3×1021e−/cm2 the superlattice structure was destroyed. These critical doses of void superlattice formation and deformation seem to be independent of dose rate. The anisotropic diffusivity of the migrating “molecules” may explain the superlattice formation.