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

Vincenc Nemanič, Marko Žumer, Janez Kovač, Franz A M Koeck, Robert Nemanich

Research output: Contribution to journalArticle

3 Citations (Scopus)

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/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.

Original languageEnglish (US)
Pages (from-to)151-156
Number of pages6
JournalDiamond and Related Materials
Volume50
DOIs
StatePublished - 2014

Fingerprint

thermionic emission
Electron emission
Diamond films
diamond films
electron emission
Hydrogen
Nitrogen
nitrogen
hydrogen
Current density
Temperature
current density
thermionics
energy distribution
electron energy
ionization gages
Thermionic emission
Core levels
Electrons
deterioration

Keywords

  • Hydrogen exposure
  • Nitrogen doped diamond films
  • Thermionic electron emission
  • Thermionic electron energy distribution

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Materials Chemistry
  • Electrical and Electronic Engineering
  • Mechanical Engineering
  • Physics and Astronomy(all)
  • Chemistry(all)

Cite this

In situ reactivation of low-temperature thermionic electron emission from nitrogen doped diamond films by hydrogen exposure. / Nemanič, Vincenc; Žumer, Marko; Kovač, Janez; Koeck, Franz A M; Nemanich, Robert.

In: Diamond and Related Materials, Vol. 50, 2014, p. 151-156.

Research output: Contribution to journalArticle

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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/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.",
keywords = "Hydrogen exposure, Nitrogen doped diamond films, Thermionic electron emission, Thermionic electron energy distribution",
author = "Vincenc Nemanič and Marko Žumer and Janez Kovač and Koeck, {Franz A M} and Robert Nemanich",
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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

PY - 2014

Y1 - 2014

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.

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KW - Thermionic electron energy distribution

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