|M.Sc Student||Reinharz Ortal|
|Subject||Influence of Surface Chemical Composition and Near-|
Surface Amorphization on the Thermionic Electron
Emission from Polycrystalline Diamond
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
Polycrystalline diamond (Poly-Di) films deposited using the chemical vapor deposition (CVD) method become of interest as an electrode material in application of thermionic energy converters. The interest in such application arises from the higher emission behavior in diamond as compared to other emitter materials which is attributed mainly to the negative electron affinity (NEA) which can occur in diamond. However, the thermionic electron emission (TEE) mechanism needs to be well understood before reproducible, reliable devices can be developed.
In the present work we report on the measurements of TEE in the temperature range of 500-650 °C by recording the energy spectra of the emitted electrons using a hemispherical electron energy analyzer under ultra-high vacuum (UHV) conditions. These measurements were performed for continuous thin un-doped Poly-Di films grown on Boron-doped Silicone substrates by hot-filament CVD method, exposed to various in situ surface-conditioning treatments and Ar ions bombardment with low energy Ar ion for high dose.
As-deposited Poly-Di surfaces were found to exhibit variations in their TEE properties depending on history and surface conditioning, thus indicating the need to control the diamond surface chemistry to obtain reproducible TEE properties. Hydrogen-terminated, bare, and oxygen-terminated in situ prepared surfaces, exhibit TEE intensities and peak onsets which are consistent with a physical picture in which TEE occurs from the bottom of the conduction band in the case of NEA and from the surface vacuum level in the case of positive electron affinity (PEA), respectively. As NEA and PEA are attributed to the electronic properties of the diamond grains themselves, the results presented in this work strongly suggest that the TEE process involves electron emission into the vacuum from the grain surfaces. It is suggested that electrons supply involves grain boundaries/defects states that may lead to the obtained TEE at moderate-low temperatures.
The structure modification by the ion irradiation of the near-surface region results in suppression of the TEE properties of Poly-Di film, which may be attributed to changing the near-surface conductivity, the electron mobility, and the surface escape probability. In addition, the TEE properties of such amorphized Poly-Di surface cannot be recovered by in situ hydrogenation, unlike the largely reproducible TEE properties of Poly-Di film, achieved by in situ hydrogenation reconditioning. These results indicate the important role of the inner subsurface atomic layers, which include the grain boundaries, diamond grains, and the diamond grain surfaces, on the TEE process from Poly-Di films.