TY - JOUR
T1 - In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure
AU - Nemanič, Vincenc
AU - Žumer, Marko
AU - Kovač, Janez
AU - Koeck, Franz A M
AU - Nemanich, Robert
N1 - Funding Information:
This work was supported by the Slovenian Research Agency ( P2-0056 , J2-4287 ). FAM Koeck and RJ Nemanich appreciate helpful discussions with Dr. Tianyin Sun, and they acknowledge the support of the US Office of Naval Research through grant number N00014-10-1-0540 .
Publisher Copyright:
© 2014 Elsevier B.V. All rights reserved.
PY - 2014/11
Y1 - 2014/11
N2 - 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/cm2 at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ∼1.8 × 10-9 A/cm2 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/cm2 (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 tomolecular 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.
AB - 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/cm2 at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ∼1.8 × 10-9 A/cm2 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/cm2 (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 tomolecular 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.
KW - Hydrogen exposure
KW - Nitrogen doped diamond films
KW - Thermionic electron emission
KW - Thermionic electron energy distribution
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U2 - 10.1016/j.diamond.2014.10.003
DO - 10.1016/j.diamond.2014.10.003
M3 - Article
AN - SCOPUS:84909590468
SN - 0925-9635
VL - 50
SP - 151
EP - 156
JO - Diamond and Related Materials
JF - Diamond and Related Materials
ER -