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

4 Scopus citations

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 - Nov 2014

Keywords

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

ASJC Scopus subject areas

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

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