Diffusion and surface effects of SiC implanted with fission product elements

Abstract

The diffusion and surface effects of several fission product elements implanted in SiC have been investigated. SiC is used as the main barrier for fission products in modern high temperature gas cooled reactors. An understanding of the transport behaviour of the implanted ions and their interactions with SiC will shed some light into SiC‟s effectiveness in the retention of fission products. The diffusion behavior of cesium implanted into SiC was investigated by isochronal and isothermal annealing methods up to temperatures of 1500 ºC. Ion implantation was performed at three temperatures, which were room temperature, 350 and 600 ºC. The implantation fluences in all cases were in the order of 1016 ions per cm2. The implantation depth profiles before and after annealing were determined by Rutherford backscattering spectroscopy (RBS). The cesium diffusion results were compared with results from our previous studies on the diffusion behaviour of atoms of iodine, silver, strontium and xenon in SiC. Amorphisation of the SiC was observed for room temperature implantations but not for implantations at 350 ºC and at 600 ºC. A strong temperature dependence of irradiation induced diffusion was observed for the cesium implanted samples. For room temperature implanted samples, isochronal annealing cycles showed that almost 50% of the implanted cesium is lost after the first annealing cycle. This behavior was similar to previous results on the loss of strontium during the first annealing cycle and to some extent previous data on silver loss where also some loss was observed after the first annealing cycles. However this behavior was in sharp contrast to the diffusion behaviour of iodine and xenon where no losses were observed after similar annealing cycles. About 25 % of the cesium is lost in the case of the samples implanted at 350 ºC while no loss of implanted cesium is observed for samples annealed at 600 ºC. For new high temperature isochronal annealing studies of silver and iodine room temperature implanted samples, complete loss of the implanted silver was observed after annealing at 1500 ºC for 30 hours. In the case of the iodine implanted samples under the same conditions, a significant amount of iodine was retained. These results sho These results shoThese results shoThese results shoThese results shoThese results sho These results sho These results sho These results sho wed that silver wed that silverwed that silverwed that silver wed that silver wed that silver wed that silver diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster through the initially amo through the initially amothrough the initially amothrough the initially amo through the initially amo through the initially amo through the initially amothrough the initially amo through the initially amothrough the initially amothrough the initially amothrough the initially amothrough the initially amorphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. RBS results also showed that thermal decomposition of SiC at these temperatures due to the sublimation of silicon leaving a carbon layer on the surface appears to be more drastic in the case of the silver implanted samples as compared to the iodine implanted samples. Iodine appears to have some effect in slowing down the decomposition of SiC. This result was confirmed by Scanning Electron Microscopy (SEM) measurements. SEM also revealed that the surfaces of silver implanted samples contained more cavities than the iodine implanted samples after the annealing cycles. The thermal decomposition was also investigated by Raman spectroscopy. Raman spectroscopy was performed using two laser wavelength excitation regimes. These were the 514 nm laser (visible region) and the 244 nm laser (deep ultraviolet region, DUV). Raman spectra show that the surface region of SiC at 1600 ºC had completely decomposed and the top surface layer was now covered by a carbon layer. Raman measurements with laser in the visible region showed that the extent of decomposition at 1600 ºC was greater for the silver implanted samples than for the iodine implanted samples. When ions of both elements (iodine and silver) were co-implanted in the same wafer and annealed under the same conditions as the single element implanted samples, iodine was observed to assist in the retention and trapping of silver ions. This was observed via Secondary Ion Mass Spectrometry (SIMS), Heavy Ion Elastic Recoil Detection Analysis (HI-ERDA), and Atom Probe Tomography (APT).