SCANNING ELECTRON-MICROSCOPIC INSPECTION OF UNCOATED CAF2 SINGLE-CRYSTALS

Cleaved and mechanically polished surfaces of CaF2 single crystals in the uncoated stale are investigated by means of secondary (SE) and backscattered (BE) electron imaging in the scanning electron microscope with respect to their strongly different charge-up properties. There is a relationship between the density of preparation-induced defects and the amount of surface charge detectable by characteristic image disturbances. Different electrical contacting techniques of the crystals are tested to obtain imaging free of charge. For the cleavage face the relatively low electrical resistance of the bulk material of Q approximate to 10(13) Ohm cm controls the imaging conditions rather than the electron trapping by cleavage-induced surface defects. On mechanically polished surfaces already during the first slow scan with E(p) < 5 keV an equipotential surface is formed leading to a pronounced electron mirror effect detectable by SE and BE. However, also in this case Imaging of selected crystal areas free of disturbances succeeds if they are located within an electrical deceleration field.