Solid-state interactions between Zr thin films and SiC

Abstract

Silicon carbide (SiC) has various applications in different technological fields as a structural material and a semiconductor. SiC possesses superior properties such as wide energy band-gap, chemical inertness, high hardness, high electrical field breakdown strength and high thermal conductivity. It acts as the main diffusion barrier of fission products in coated particle fuels such as the pebble bed modular reactor (PBMR) and is used in metal-ceramic composites for high temperature applications. Since SiC is also a large band gap semiconductor, it is envisaged to be used in high power, high temperature and high frequency electronic applications. Zirconium is a high yield fission product in nuclear fuel applications such as TRISO and M3 fuels. It is used as the metal matrix in M3 fuels which have TRISO particles scattered within Zr and also as the metal in ceramic composites. This metal also has attractive properties for electronic applications. In these applications the Zr/SiC interface is brought to high temperatures either during fabrication or applications stages. Under these conditions solid-state reactions between Zr and SiC are bound to occur. Previous investigations on the solid-state reactions between Zr and SiC have involved bulk Zr metal in contact with SiC. These experiments were performed at high temperatures and the reaction products reported by various authors differ. This work presents results of investigations of the solid-state reactions between Zr thin films (133 nm) and single crystalline 6H-SiC substrate. These samples were annealed between temperatures of 600 and 1000 °C for durations of 30, 60 and 120 minutes under high vacuum conditions. Samples with standard thickness of Zr thin films were prepared by sputter deposition technique on SiC substrates. The films were characterized by standard techniques, such as X-ray diffraction (XRD), secondary electron microscopy (SEM), atomic force microscopy (AFM) and Rutherford back scattering spectrometry (RBS). RBS analysis was performed to determine the deposited layer composition and thickness as well as the thickness of the reaction zone. Surface morphologies were examined using SEM, and surface roughness was examined using AFM. XRD was used to identify the phases present in the reaction zone. The RBS results indicate that the as-deposited spectra fit well with those annealed at 600 °C, thus showing there were no reactions taking place. The XRD results revealed that sputter deposited Zr thin film was polycrystalline. At annealing temperatures of 700 °C and above, Zr reacted with the SiC substrate and formed a mixed layer of Zr carbide (ZrC) and Zr silicides namely; ZrSi, Zr2Si and Zr5Si3(Cx). Annealing at 850 °C for 240 minutes revealed that all the deposited Zr had completely reacted. The interface reaction followed the parabolic growth law therefore indicating diffusion controlled reaction kinetics. The activation energy for the diffusion process obtained was 1.52 eV in the relatively narrow temperature range 700 to 850 °C. SEM and AFM images revealed that the as-deposited Zr thin films had a granular surface with an average granule diameter of 26.61 nm. The average granule diameter was observed to be lower than the as-deposited value after annealing. However after longer annealing durations and at higher temperatures the granule diameters were observed to generally increase. The average roughness Ra and root mean square roughness Rrms values of the as-deposited sample were 1.31 and 1.65 nm respectively. These values were observed to reduce after annealing at 700 °C for 30 minutes and increased with increasing annealing durations. The effective heat of formation model was tested and applied to the Zr/SiC system. The results obtained in this study were found to fit well with the model. The EHF model predicted ZrC and Zr2Si as the initial phases to form at the Zr/SiC interface and this was confirmed experimentally.