|M.Sc Student||Elfimchev Sergey|
|Subject||Photo-Thermal Electron Emission from Polycrystalline Diamond|
|Department||Department of Energy||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
Thermionic electron emission (TEE) forms the basis of both electron sources for a variety of applications and a direct energy conversion process that may be compact and scalable. Diamond is considered to be one of the most promising materials for electron emission devices due to its outstanding properties. Diamond saturated with hydrogen atoms has stable negative electron affinity (NEA). While diamond is a wide band-gap insulator, nitrogen atoms can produce doping level ~1.7eV below the conduction band minimum. Thus, the preparation of nitrogen doped diamond with well-defined hydrogen-terminated surfaces is a pre-requisite for high electron emission.
Hydrogen termination is essential for effective electron emission. In this work the impact of substrate temperature during hydrogenation on TEE properties was studied from polycrystalline diamond films. The investigated samples were deposited by hot filament chemical vapor deposition (HF CVD) method on silicon substrate. Unexpected results were obtained in this research. Significant TEE was observed at low temperature range immediately after hydrogenation. This low temperature TEE was found to display a broad maximum at 300 °C. Annealing at 700 °C results in irreversible changes in surface conditioning, and drastic reduction of TEE yield at low temperatures. We associate these effects with irreversible thermal induced physicochemical changes of the hydrogen bonding configuration adsorbed on the polycrystalline diamond surface resulting in changes in its surface electronic structure.
Another purpose of this research was to investigate the effect of light and temperature on electron emission from polycrystalline diamond films. Well pronounced synergetic effect of light and heat on electron emission from polycrystalline diamond films was observed and explained. The synergetic effect of photo-thermal electron emission (PTEE) may be rationalized as a combined three-step process: 1) photon excitation of electrons to the conduction band 2) electron-phonon scattering and recombination processes; 3) thermal excitation from near conduction band trap levels into vacuum.
The influence of surface morphology on TEE and PTEE was also examined. Electron emission was measured and compared from polycrystalline diamond samples with different morphologies and average grain size. It was found that the role of grain boundaries as electron source is irrelevant, while the surface morphology of polycrystalline diamond film can play an essential role on TEE properties. It was shown that there is an optimum average grain size for TEE around 180nm. The TEE intensities degrade significantly for lower grains most likely due to a high amount of recombination sites. Moreover, it was suggested that increasing of the average grain size resulted in reduction of effective surface for electron emission, which reveals to decrease of TEE yields.
In this work electron emission properties were investigated from nitrogen doped polycrystalline diamond films. Polycrystalline diamond films were doped with nitrogen by adding ammonia gas into growing chamber during HF CVD process. Electron emission measurements from nitrogen doped films show a most pronounced improvement of PEE and TEE. In this work the lowest threshold ever published for TEE around 200°C was measured from nitrogen doped diamond film.