Characterization of Secondary Electron Emission from Materials with Low or Negative Electron Affinity
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efficiency of the electron transport to the surface and by the emission probability at the surface/vacuum interface. The energy spread of the emitted electron distribution is determined by electron scattering processes in the bulk and at the surface, while the emission uniformity depends on the compositional uniformity of the bulk and surface as well as the surface morphology. In this study, we use secondary electron emission spectroscopy to investigate the emission characteristics of diamond films having different bulk properties (e.g. crystallinity, impurity concentration) and different adsorbate species and coverages. An incident electron beam is used to generate a low-energy electron distribution in the material, and these electrons are subsequently emitted into vacuum via a cold emission process. If X is small or negative, the electron emission process is not impeded by a surface energy barrier. Consequently, the measured electron intensity provides information about the efficiency of the transport and emission processes, while the measured EDCs reveal the electron energy distribution and provide information about the surface electronic properties. By combining secondary electron intensity and energy distribution measurements, the emission characteristics of different materials and surfaces can be assessed. EXPERIMENT Two different diamond samples were investigated in this study. The first sample was a 5.0 x 5.0 x 0.25 mm 3 (100) p-type, semiconducting, single-crystal diamond (natural type 2B) from Harris Diamond Corporation that was clear and transparent in appearance. The second sample was a microwave-plasma chemical-vapor-deposited (CVD) diamond film that had been lifted off of a Si growth substrate [7]. The 10.0 x 10.0 x 0.02 mm 3 free-standing polycrystalline diamond was boron doped with a resistivity of 155 KQ2-cm and was light blue in color. Although the resistivity of the C(100) diamond was not measured, the transparency of the sample indicated that the 125 Mat. Res. Soc. Symp. Proc. Vol. 509 ©1998 Materials Research Society
impurity concentration was lower than in the CVD diamond sample. The experiments were performed in a UHV system having a base pressure of - 5 x 10-11 Torr, and the diamond samples were mounted on tantalum foil sample holders designed to allow resistive heating and thermocouple temperature measurement. Measurements were taken from hydrogenated, cesiated, and thermally-cleaned (i.e. "bare") surfaces. The hydrogenation procedure involved exposure to atomic H produced by a hot filament [8] while the cesiation procedure involved evaporation from a Cs dispenser. Prior to hydrogen dosing or Cs deposition, the samples were cleaned by a thermal desorption procedure [8]. However, no special surface treatment was performed before inserting the samples into vacuum or prior to the initial heat treatment. We measured the yield and energy distribution of the secondary electrons using experimental techniques that have been described elsewhere [9]. The secondary electrons are defined to include a
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