Characterization of Surface Defect Structures

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Calcium fluoride(fluorite), which is thermally stable and electrically insulating., is well known to desorb florine atoms as a result of electron bombardment in ultrahigh vacuum environment. The purpose of this research is to clarify the structure of surface defects created by electron bombardment on a calcium fluoride crystal.After electron bombardment on a calcium fluoride crystal under vacuum of 10^<-8> Torr with 90% of H_2O, the surface defects immediately reacted with residual H_20 to yield calcium oxide phase. Chemicalstate-discriminated X-rgy photoelectron diffraction measurements showed that the calcium oxide phase was in epitaxial relationship with the substrate, in which the 111 axis was common between the substrate and. the overlayer, and the orientation-was 180ﾟ rotated with each other.After electron bombardment under 10^<-9> Torr, surface defects were relatively stable. X-ray photoelectron spectroscopic measurements showed that fluorine atoms in two or three atomic layers from the 2 surface decreased in number by electron bombardment of about 1 mC/cm^2. Intensity of a specific peak at definite angle in an X-ray photoelectron diffraction pattern for Fls decreased after electron bombardment. In order to relate the change in an X-ray photoelectron diffraction pattern with surface defect structure, theoretical calculation based on single scattering scheme was performed. Experimental results were best interpreted by the model in which a part of fluorine atoms in the third atomic layer selectively desorbed.