Paper EN-ThP2
Reactivation of Thermionic Electron Emission from Nitrogen Doped Diamond Films by Atomic Hydrogen Exposure

Thursday, November 3, 2011, 6:00 pm, Room East Exhibit Hall

Thermionic electron emission from nitrogen doped hydrogen terminated diamond film has been evaluated 18 months after it had been deposited on a 25 mm diameter molybdenum substrate by microwave plasma assisted chemical vapour deposition. A hydrogen enriched surface layer, enabling a negative electron affinity, was formed during the film deposition procedure. The initial thermionic emission current density of the aged film obtained at 500 °C and 0.2 V/micrometer electric field was stable but relatively low compared to the value measured on a freshly deposited film. The value at a base pressure of 2x10^-7 mbar was ~ 1.4x10^-9A/cm². Such a low current density is presumed to be a consequence of oxidation, surface adsorption of contaminats and hydrogen depletion from the surface layer, all processes which proceed during ambient atmosphere exposure since the manufacturing date. Reactivation of the surface was achieved by introducing atomic hydrogen, generated by the hot filament cathode of the ionization gauge. A stable current density of ~ 2x10^-6 A/cm² (an increase factor of ~1400) was achieved after exposure to hydrogen at the base pressure of 2.2x10^-4 mbar for 2 hours again at 500 °C and 0.2 V/micrometer. The atomic hydrogen exposure rate was estimated by subsequent measurement of the change in optical transmittance of the tungsten phosphate glass plates. The increase in emission with hydrogen was ascribed to two effects: 1) reactivation of the surface, and 2) molecular assisted emission.

Research at ASU is supported through the Office of Naval Research and at JSI through the Slovenian Research Agency (grant BI-US/09-12/021).