A MODEL SYSTEM FOR SCANDATE CATHODES

In cathode-ray tubes, high current densities at relatively low operati ng temperatures can be achieved with dispenser cathodes containing sca ndium oxide. The surface structure and precise emission mechanism of t he scandate cathode are not known. The surface layers of such a cathod e consist of the four elements Sc, Ba, O, and W. We have studied Sc, B a, and O overlayers and various combinations of these elements deposit ed on the W(001) surface, under ultrahigh vacuum conditions. We have p erformed in situ measurements of the work function changes accompanyin g the surface treatments, and correlate these with the observed surfac e structures. High resolution medium-energy ion scattering was used to determine the compositional depth distributions and the geometrical s tructures at this model cathode surface. Auger electron spectroscopy p eak height ratios show that W(001), with all three overlayer elements present, can be considered representative for real scandate cathodes. The observed ultra-low work function of the W(001)/Sc,Ba,O model syste m explains the high current densities on real scandate cathodes.