In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure

Abstract Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5 eV and thermionic electron emission (TE) has been detected at temperatures less than 500 °C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18 months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~ 5 × 10 − 5  A/cm 2 at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ~ 1.8 × 10 − 9  A/cm 2 at 500 °C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3 × 10 − 4  mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After 2 h of exposure with the sample at 500 °C, the surface exhibited a stable emission current density of ~ 2.3 × 10 − 6  A/cm 2 (an increase by a factor of ~ 1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5 × 10 − 8  mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400 °C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~ 200 meV, and the TE intensity increased by a factor of ~ 100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.