Diffusion of ion implanted iodine in 6H-SiC

Abstract

The diffusion of iodine implanted 6H-SiC has been investigated using Rutherford backscattering Spectrometry (RBS). SiC is used as the main barrier in the modern high temperature gas cooled reactors. An understanding of the transport behaviour of iodine in 6H-SiC will shed some light into SiC’s effectiveness in the retention of fission products. Room temperature iodine implantations were performed to a fluence of 1 x 1016 iodine ions per cm2. Iodine depth profiles were determined by Rutherford backscattering spectroscopy. Previous studies indicate that iodine diffusion cannot be detected for temperatures below 1000oC. Isochronal annealing experiments for 5 hours cycles were then performed starting from 1000oC. 5 hour annealing cycles at 1200oC and 1300oC were also performed. These relatively short annealing cycles were used to study the evolution of the iodine depth profiles and to determine the temperature where the first noticeable broadening of the iodine implanted profile is observed. Broadening of the iodine profile was initially observed at temperatures around 1200oC. Isothermal annealing cycles were then performed at this temperature of 1200oC where the broadening was first observed. The annealing experiments were performed for a total annealing time of 60 hours. Isothermal annealing experiments were also performed at a lower temperature of 1100oC also for a total annealing time of 60 hours. The experimental data was analyzed by fitting the iodine depth profiles to a Gaussian function using the GENPLOT program. Diffusion coefficients were obtained by comparing the full widths at half maximum, (FWHM) of the iodine profiles before and after annealing. Results for 1100oC show that the diffusion coefficient is below 10-21 m2 s-1 . Solving the diffusion equation numerically for this diffusion coefficient shows that the iodine does not traverse more than 2 ìm through SiC after annealing for one year. Intact SiC coatings which have a thickness of 35 ìm in the fuel particles will therefore be able to prevent iodine release from the particle at this temperature. Results for 1200oC show that the diffusion coefficient is in the order of 10-20 m2 s-1. The results also reveal that iodine loss through the front surface is relatively low with only 15% of iodine loss observed after annealing for a total time of 60 hours. Further indications of SiC ability to retain iodine are observed when an iodine peak is still present after annealing for 30 hours at a temperature of 1500oC. Radiation damaged produced during ion implantation was investigated by Rutherford backscattering in conjunction with channeling. The results of the RBS/channeling reveal that the room temperature implantations produce an amorphous layer from the surface up to a depth of 260 nm. Some epitaxial regrowth is observed starting at approximately 1000oC.