Mechanism of low-energy electron stimulated desorption of O− from hydrogenated and hydrogen-free diamond surfaces exposed to activated oxygen

Abstract

Articles you may be interested inClarification of oxygen bonding on diamond surfaces by low energy electron stimulated desorption and high resolution electron energy loss spectroscopy Decay of secondary electron emission and charging of hydrogenated and hydrogen-free diamond film surfaces induced by low energy electronsIn this work we report on a study of the mechanism of O Ϫ electron stimulated desorption ͑ESD͒ from hydrogenated and hydrogen-free polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4 -22 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes: ͑i͒ total O Ϫ and H Ϫ yields as a function of incident electron energy and ͑ii͒ kinetic-energy distribution ͑KED͒ of O Ϫ desorbed from the hydrogen-free diamond surface. The discussed ESD mechanism is referred to the information obtained from x-ray photoelectron spectroscopy, near-edge x-ray absorption fine structure, and core level H ϩ photodesorption measurements which reveal formation of CvO and C-O-C bonds on the hydrogen-free diamond surface, and CvO and C-O-H bonds on the hydrogenated one. Based on the maximum kinetic-energy value of O Ϫ and the ESD threshold measured for hydrogen-free surface, all low-energy ͑5-10 eV͒ O Ϫ ions are attributed to desorption by the dissociative electron attachment ͑DEA͒ to C-O-C, while DEA to CvO occurs at the incident electron energy higher than ϳ10 eV. O Ϫ ESD from the hydrogenated diamond surface exposed to thermally activated oxygen is a more complicated process. Its threshold is substantially higher than for hydrogen-free diamond, and the line shape of the ESD yield curve is very similar to that of chemisorbed CO molecules. Several reaction pathways leading to production of O Ϫ by DEA are discussed for this sample. At incident electron energies higher than ϳ15 eV, O Ϫ ESD proceeds also via dipolar dissociation processes for both hydrogenated and hydrogen-free diamond surfaces.

Clarification of oxygen bonding on diamond surfaces by low energy electron stimulated desorption and high resolution electron energy loss spectroscopy

“…The O Ϫ ESD yield as a function of incident electron energy showed a strong resonance peak at 8.5 eV with two smaller peaks at 12 and 16 eV, and a monotonic increase toward higher incident energies. 22 The onset of O Ϫ desorption was detected at ϳ5 eV. It should be noted that in our HREELS measurements we used primary electron energy of 4.5 eV, below the threshold for desorption of both O Ϫ and H Ϫ .…”

“…Electronic mail: choffman@tx.technion.ac.il those of the O Ϫ ESD measurements. 22 This will allow us to clarify the uncertainties in the explanation of previously reported HREEL data of oxidized diamond surfaces, as mentioned above.…”

“…It can be concluded that in situ hydrogenation at the conditions applied in our work is less effective than ex situ MW hydrogen plasma treatment which produces the fully hydrogen-terminated diamond surface, although previous measurements performed by techniques less surface sensitive than HREELS indicated full coverage of the surface in situ exposed to activated hydrogen gas. 22 To clarify the uncertainties remained after analysis of the HREEL spectra, we correlate between primary electron energy losses due to vibrational excitations and direct interactions of these low energy primary electrons with surface adsorbates inducing ion desorption. Let us first consider the mechanism of the low energy ESD of O Ϫ from diamond surfaces.…”

“…2͑b͒. 22,28 Let us consider first the hydrogen-free diamond surface. The O Ϫ ESD yield as a function of incident electron energy showed a strong resonance peak at 8.5 eV with two smaller peaks at 12 and 16 eV, and a monotonic increase toward higher incident energies.…”

“…21,22 It was found that as with gas phase molecules, ion production from adsorbates by electron impact in the 0-30 eV range results essentially from two processes: namely, molecular fragmentation into a stable anion and a neutral species, which occurs via dissociative electron attachment ͑DEA͒, and fragmentation into an ion pair, that is, dipolar dissociation ͑DD͒. DEA processes display a resonance dependence on incident electron energy, and in most cases, these occur at energies from a few eV up to ϳ15 eV.…”