Photoelectron- and Thermionic- Emission Microscopy of Barium/Scandium Thin Films on Tungsten
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Photoelectron- and Thermionic- Emission Microscopy of Barium/Scandium Thin Films on Tungsten Joel M. Vaughn, and Martin Kordesch Physics and Astronomy, Ohio University, Clippinger Labs RM 251B, Athens, OH, 45701 ABSTRACT Commercial scandium oxide doped thermionic cathodes have demonstrated current densities over 100 A/cm2. In order to understand the effect of Sc- and Ba- oxides on the emissivity of these cathodes we have imaged thin films of scandium oxide and barium oxide on tungsten foils using photoelectron emission microscopy and thermionic emission microscopy. Arrays of 100 um x 100 um squares of scandium and 25 um x 25 um squares of barium, 200 nm thick, were sputter deposited onto 50 um thick sheets of tungsten foil. Imaging squares of different sizes gives an unequivocal identification of each material and a completely consistent comparison of each material and cathode structure under identical conditions in one image. The metal squares oxidize in air before imaging. Each sample was heated in situ in a Bauer-Telieps style LEEM/PEEM used primarily in the ThEEM mode. The barium oxide squares emit below 875 K, and diffuse over the scandium below 875 K. Thermionic emission from scandium oxide squares is observed at temperatures significantly larger than 875 K. Failure of the barium oxide film cathode is through barium desorption. AES spectra show that the Sc does not desorb. While the origin of reduced emission temperature is commonly believed to be a result of a low work function monolayer of Ba and Sc oxides, in our study, the benefits of a combined Ba/Sc cathode are present in a thick (multi-layer), layered structure of barium oxide on top of a thick scandium oxide layer. INTRODUCTION In Photo-Electron Emission Microscopy (PEEM), image contrast is derived from work function differences at a given incident photon energy [1]. In practice, short-wavelength mercury arc lamps are used in the laboratory, producing light with maximum photon energies about 5 eV. Using electrons emitted due to the photoelectric effect for imaging (typically less than 1 eV kinetic energy), areas of the sample with work function less than the incident photon energy will emit electrons, and therefore be “light” in the image, areas with work function greater than the incident photon energy will be “dark” in the image. The relative image intensity in PEEM can also show variations due to the integrated electron yield at the illumination wavelength, once the photoelectron threshold has been exceeded. Because the photo-emitted electrons are very low in kinetic energy, image contrast corresponding to sensitivities of 0.01 monolayer of adsorbate can be routinely achieved. Heating the sample can re-distribute an adsorbate through diffusion, or initiate a chemical reaction of the adsorbate with the surface, or cause desorption. All of these processes can be imaged in situ in real time in PEEM. The PEEM / ThEEM microscope can also operate in a thermionic mode (ThEEM). The image contrast in ThEEM comes from spatial variations in the th
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