|M.Sc Student||Koenka Israel Joel|
|Subject||A Study of the Grain Boundaries and Hydrogen in HF-CVD|
|Department||Department of Chemistry||Supervisors||Professor Alon Hoffman|
|Dr. Joan Adler|
|Full Thesis text|
The present work deals with the investigation of the Grain Boundaries (GB) and hydrogen in polycrystalline diamond films, grown by Chemical Vapour Deposition (CVD). Polycrystalline diamond films are composed of diamond grains, interconnected by grain boundaries. The composition of GB, which also holds most of the hydrogen in the material, varies between films grown with different methods and the specific nature of the various C-C and C-H chemical bonds in these films is often poorly understood. Characterization of these C/H structures is however an important step towards understanding the conduction mechanisms and controlling associated properties of diamond films.
In this work, using core-level Electron Energy Loss Spectroscopy (EELS) in the Transmission Electron Microscope (TEM), we have provided direct evidence for the preferential bonding of hydrogen in GB of CVD polycrystalline diamond films, as well as opening an opportunity for a detailed determination of the different bonding schemes by a careful examination of the C(1s) pre-edge absorption structure. Hydrogen bonding in the film was also investigated using Raman spectroscopy, with a special attention to trans-polyacetylene (t-PA) like bonds in the grain boundaries. A comparative study of hydrogenated and deuterated sub-micron diamond films (grown from H2/CH4 or D2/CD4 respectively) by scanning electron microscopy (SEM) and Raman spectroscopy showed that the latter films grow in a more orderly manner (both the grains and GB). Raman measurements of hydrogenated and deuterated films suggest that the t-PA chains in their grain boundaries are mostly shortly conjugated chains. It was also shown by secondary ion mass spectrometry (SIMS) depth profiling, that there are more H bonding sites in hydrogenated films than D bonding sites in deuterated films, yet the latter have a higher thermal stability. The presence of stressed SiC was also identified and its relaxation was observed after thermal treatments to high temperatures (10000C). Combined with an observed sharpening of the diamond optical phonon as a result of annealing, this suggests a partial diamond regrowth in the GB due to the thermal treatments.
We have also mapped in detail the different carbon phases in the film, using plasmonic EELS and Energy Filtered TEM (EF-TEM), and showed that graphite is present only in small inclusions in the film, and that amorphous carbon is present mostly in grain boundaries and edges.