Frenkel defect interactions at surfaces of irradiated alkali halides studied by non-contact atomic-force microscopy

Irradiation of alkali halide crystal leads to production of Frenkel defects in the crystal bulk. Subsequent diffusion and interactions of these defects with the surface results in desorption processes at the surface. We have studied surface topography of electron bombarded alkali halide crystals (KBr, NaCl) and the desorption fluxes. It is found that the desorption proceeds in a layer-by-layer mode by growth and linking of pits of monoatomic depth, which results in modulation of surface step density. Electron-stimulated desorption fluxes are correlated with the surface step density. Based on these results it is concluded that H-centers decay at the (1 0 0) surface plane with the emission of halogen atoms, F*-centers recombine with the terrace edge and initiate emission of alkali atoms, and F-centers accumulate in the sub-surface region. Above certain temperature (∼450 K for KBr, 370 K for NaCl) the desorption proceeds in more complex, multilayer mode as a result of combination of the above described Frenkel defect mediated mechanism with simultaneous thermal restructuring of the surface.