Evolution of defects in silicon carbide implanted with helium ions

Abstract

Effects of accumulation of radiation damage in silicon carbide are important concerns for the use of silicon carbide in advanced nuclear energy systems. In the present work lattice damage in silicon carbide crystal (4H type) implanted with 100keV 4He+ ions was investigated with Rutherford backscattering spectrometry in channeling geometry (RBS/c) and positron beam Doppler broadening spectrometry (PBDB). Helium implantation was performed at the specimen temperature of 510K to avoid amorphization of the SiC crystal. Fluences of helium ions were selected to be in the range from 1×1016 to 3×1016ionscm−2, around the dose threshold for the formation of observable helium bubbles under transmission electron microscopes (TEM).The RBS/c measurements show distinctly different annealing behavior of displaced Si atoms at doses below or above the threshold for helium bubble formation. The RBS/c yield in the peak damage region of the specimen implanted to 3×1016He-ionscm−2 shows an increase on the subsequently thermal annealing above 873K, which is readily ascribed to the extra displacement of Si atoms due to helium bubble growth. The RBS/c yield in the specimen implanted to a lower ion fluence of 1.5×1016He-ionscm−2 decreases monotonously on annealing from ambient temperatures up to 1273K. The PBDB measurements supply evidence of clustering of vacancies at temperatures from 510 to 1173K, and dissociation of vacancy clusters above 1273K. The similarity of annealing behavior in PBDB profiles for helium implantation to 1×1016 and 3×1016ionscm−2 is ascribed to the saturation of trapping of positrons in vacancy type defects in the damaged layers in the specimens helium-implanted to the two dose levels.