FORCE MICROSCOPY OF CLEAVED AND ELECTRON-IRRADIATED CAF2(111) SURFACES IN ULTRA-HIGH-VACUUM

We present scanning force micrographs of as-cleaved and electron-irrad iated CaF2(lll) surfaces taken in ultra-high vacuum at room temperatur e. Among the forces acting on the tip, the electrostatic force was fou nd to make an important contribution. This allows us to study the ioni c conductivity of the crystals. Freshly cleaved surfaces can be imaged in contact mode with high resolution exhibiting the hexagonal structu re of the (lll) surface. For electron-irradiated surfaces, noncontact mode is required for imaging radiation-induced stoichiometric changes. Strong adhesive forces between the tip and metal-enriched areas are f ound to be a severe obstacle for contact mode imaging. Weak irradiatio n with 850 eV electrons results in the formation of 10 nm-wide holes w ith surrounding elevations as the first stages of metallization. (C) 1 997 Elsevier Science B.V.