Atomic force microscopy studies of alkali halide surfaces nanostructured by DIET

We report on surface topography modification of single crystal alkali halides due to creation of the excitonic states by keV electron irradiation. The DIET-structured surfaces have been studied with nanometer scale resolution by means of a dynamic (non-contact) atomic force microscopy (DFM) in UHV. The force microscopy studies reveal that randomly spread rectangular pits of monolayer depth in the topmost layer of the crystal are formed during irradiation. Growth and coalescence of the pits lead to almost layer-by-layer desorption mode. It is demonstrated that varying surface topography affects the yield of both the halogen and the alkali atom desorption component, as well as velocity spectrum of desorbing halogen atoms (thermal versus non-thermal ratio). We propose a model in which periodic changes of the surface topography with the increasing electron fluence (from initially flat to rough at about half monolayer desorbed, back to flat after a complete monolayer removal) are modulating the surface recombination probability for the excited F-centers. By controlling the population of traps in the bulk these surface processes are causing modulation of the diffusion range of mobile defects migrating from the bulk of the material towards its surface and the bulk recombination probabilities of F- and H-centres.