|M.Sc Student||Shasha Michal|
|Subject||Interaction of Activated Nitrogen with Polycrystalline|
|Department||Department of Chemistry||Supervisor||Professor Alon Hoffman|
|Full Thesis text|
Incorporation of nitrogen into polycrystalline diamond with low density of surface defects would be greatly beneficial since it could enhance the true behavior of surface functional groups. In this work, studies related to nitrogen incorporation onto polycrystalline diamond surface by two different methods are being presented. The first method is low energy N2 implantation and the second is RF N2 plasma exposure. Bonding configuration, concentration and thermal stability of nitrogen are analyzed using x-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) by performing stepwise in-situ annealing to 1000 °C.
Ion implantation method is a well-known process and has been extensively investigated for many years. In this work, it is shown that N2 implantation leads to the formation of surface defects in the near surface region that did not result in the retrieval of the characteristic optical phonon overtone of diamond upon annealing. On the other hand, RF nitridation process did not result in high density of surface defects and the characteristic optical phonon overtone of diamond was partially retrieved upon annealing to 1000 °C. The amount of nitrogen incorporated into the diamond surface was higher in the RF nitridation process (15.1 at.%), compared to the N2 implantation (7.1 at.%).
Bonding configuration upon RF nitridation most likely occurs in two different layers: (i) on the top surface and (ii) in the sub-surface region. XPS measurements revealed a symmetric broad N 1s peak, which indicates different bonding configurations with at least three different contributions. The peak at 398.5 eV is mostly associated to C=N conjugated bonds and C−NH2. The peak located at 399.8 eV is mostly due to C≡N and >N−. The peak at 401.9 eV is possibly associated to an interband transition loss of the N 1s peak and may also be due to N‒N bond in the diamond matrix. HREELS analysis discovered the formation of N−H species and their stability is found to be less than 700 °C. In addition, it was found that C=N and >N− species are stable above 1000 °C.
For further understanding RF nitridation and nitrogen implantation experiments were performed at different sample temperatures and analyzed using XPS and HREELS. Furthermore, RF nitrogen exposure and N2 implantation were performed on HOPG surface to compare a graphitic surface and diamond surface that went through a procedure that may cause it to relax into a graphitic character. Electron energy loss spectra (EELS) of the nitride diamond surface were also recorded and it showed that the RF nitridation did not result in high density of surface defects, whereas the N2 implantation leads to the formation of graphitic carbon in the near surface region.
HREEL spectra of nitride diamond surfaces displayed new components, which were mostly associated to various carbon/nitrogen/hydrogen bonds. To assign the peak position, similar HREEL studies were carried out on diamond samples deposited using different isotopic gas mixtures of CH4 and H2. These measurements led to the possibility of determining the new components into specific nitrogen bonds on the diamond surface.