Influence of phosphorous and high temperature annealing on the nanostructures of 3C-SiC

Abstract

The integrity and property behaviour of the SiC layer of the TRISO coated particle for high temperature reactors (HTR) are very important. The SiC is the main barrier for gaseous and metallic fission product release. An investigation was proposed to determine the effect that different Si isotopes may have on the SiC crystal structure during the CVD manufacturing process and after transmutation due to irradiation. 30Si transmutes to phosphorous (31P) and other transmutation products during irradiation, which may affect the integrity of the SiC layer. This study describes the work done on unirradiated SiC, but the SiC samples were prepared with varying phosphorous levels to “simulate” the presence of P due to transmutation. Phosphorousdoped 3C-SiC layers were deposited on Si (100) substrates using the concept of the hot wall chemical vapour deposition with silane (SiH4) and propane (C3H8) as precursors diluted in hydrogen (H2). Propane was introduced in the reactor chamber prior to the silane addition during the heating-up cycle to prepare a thin layer of SiC on the silicon substrate. This was to ensure epitaxial growth of mono-crystalline layers once silane is added at the growth temperature. However by changing growth procedures polycrystalline layers were also obtained. Phosphorus doping of the layers was done during epitaxy using tertiary butyl phosphine (TBP) (C4H9PH2) as a donor source, the doping level was varied between 1.1x1015 and 1.2x1019 atom/cm3. The Pdoped SiC layers were characterized after high temperature annealing from 1600 ºC to 2100 ºC using five techniques namely X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The HRTEM micrograph of the decomposition of SiC at 2100 ºC are shown and discussed. Nanotubes were not identified during the TEM and HRTEM analysis although graphitic structures were identified. The preliminary conclusion reached is that the P-content at these experimental levels (1.1x1015 - 1.2x1019 atom/cm3) does not have a significant influence on the nanostructure of SiC at high temperatures without irradiation.