Electron-surface interaction and metallization of the CaF2(111)-surface studied by photothermal techniques

Abstract

Abstract The interaction of low energy electrons with the surface of alkaline-earth halides results in a variety of microscopical physical phenomena commonly described in terms of defect formation and diffusion, surface metallization and desorption of neutral- and charged particles. These processes are accompanied by local changes in the electronic and geometrical structure of bulk and surface and result in a variation of macroscopically measurable parameters like modulated optical reflectance and a deformation of the crystal lattice. In this paper it will be shown that photothermal analysis, that so far has mostly been used for the determination of optical and thermophysical properties of materials, is also capable of measuring defect related nonthermal phenomena apparent during electron irradiation of insulator surfaces. Experiments were performed with an intensity modulated electron beam of typically 1 μA at 1 keV focused into a spot of 1 mm2 on the (111)-surface of a polished CaF2 single crystal under ultra-high vacuum conditions. Measurements revealed that modulated reflectance is sensitive to changes in optical properties induced by electron irradiation induced defects at low electron dosages. At a dosage level where metallization starts, a dramatic change in the modulated reflectance signal was observed indicating changes in electronic structure due to metal clustering at the surface. The photothermal displacement technique has been utilized to monitor surface deformations induced by electron bombardment. It was found that results cannot be explained by a thermo-elastic expansion model that works well for metal surfaces. Therefore, this technique can be used for the measurement of nonthermal contributions to lattice expansion resulting from volume changes of created defects.