Silicon carbide (SiC) is both used as a diffusion barrier in Tristructural Isotropic (TRISO) coated nuclear fuel particles in nuclear reactors and as a wide band gap semiconductor in microelectronics applications. SiC has physical properties of high strength and hardness, high melting point and is chemically inert.
This thesis investigates the effectiveness of SiC as a barrier to ruthenium diffusion in TRISO coated nuclear fuel particles. In addition, the electrical performance and degradation mechanism of ruthenium-SiC based Schottky barrier diodes (SBDs), and their ability to operate at extremely high temperatures are also investigated.
In TRISO coated fuel particles, the high strength SiC layer acts as a containment layer to the high pressure generated in the kernel by the fission nuclear reaction. One element produced during the nuclear fission reaction is ruthenium (Ru). In this thesis, the Ru solid state reaction with SiC has been investigated by annealing a thin film of Ru on 4H-SiC and 6H-SiC in the air, argon and a vacuum. Analysis of the thin films after various annealing temperatures were performed by Rutherford Backscattering Spectrometry, X-Ray Diffraction analysis, Scanning electron microscopy, and Raman Spectroscopy. The study has shown that diffusion of Ru into SiC starts at a much higher annealing temperature in a vacuum, whereas, in air annealing, diffusion commences at a much lower temperature. The study has shown that SiC is not a perfect barrier to the diffusion of metallic fission products.
Due to its wide band gap property, electronic devices made from SiC can operate at very high temperatures, with very high power and fast switching times. Ru also happens to be a suitable material for making Schottky contacts with SiC. Due to its high melting temperature (2250oC), and chemical inertness, the Ru Schottky contact can function at very high operating temperatures. The electrical characteristics of Ru/SiC SBDs have been investigated by annealing the diodes in various environments namely air, argon and vacuum. The electrical characterisation of the SBDs after each annealing temperature was done by using current-voltage (IV) and capacitance-voltage (CV) characterisation techniques. This study has shown that Ru/4H-SiC SBDs will remain operational after annealing in vacuum up to temperatures of above 1000 oC and that the diodes degrade at a very low temperature of 400 oC when annealed in air. Ru/4H-SiC SBDs under argon annealing degrade at a temperature of 1000 oC. The degradation of the Ru/4H-SiC SBDs, when annealed in air, is explained by the fact that there is a possibility of formation of an oxide of Ru which is not conducting which leads to a high series resistance of the diode at the degradation temperature of 400 oC. In the case of argon annealing, the degradation of Ru/4H-SiC SBDs is due to the diffusion of Ru into 4H-SiC.
In a vacuum, the Ru/6H-SiC SBDs degrade at a temperature of 800 oC. The IV data show that Ru Schottky contact on 6H-SiC becomes ohmic at 900 oC. The formation of an ohmic contact is attributed to the formation of graphite flakes as evidenced by Raman analysis of Ru/6H-SiC thin films.
In atmospheric annealing environment, the Ru/6H-SiC SBDs degrade at a higher temperature (of 700 oC) than that of Ru/4H-SiC SBDs (400 oC).
Under argon annealing, there is very little difference in the degradation temperature between Ru-4H-SiC and Ru/6H-SiC SBDs. This study has shown that, in general, the SBDs degrade at higher temperatures when annealed in vacuum and argon while in air degradation takes place at a very much lower temperature.