Dose dependence of formation of nanoscale cavities in helium-implanted 4H-SiC

A cross-sectional transmission electron microscope investigation of dose dependence and annealing behavior of microstructures in helium-implanted silicon carbide is presented. Specimens of silicon carbide (4H–SiC) were mainly implanted with 30-keV-He ions at intermediate temperatures of 500 and 873 K to doses ranging from 5×10¹⁵ to 2×10¹⁷ cm⁻², and subsequently annealed at temperatures up to 1173 K. For comparison other specimens were implanted at 293 K up to 5×10¹⁵ cm⁻² and subsequently annealed. Three dose regimes of microstructural evolution were found, each of them exhibiting quite different annealing behavior. No cavities were found at the low dose range below 3.5×10¹⁶ cm⁻². At intermediate doses of (3.5–5)×10¹⁶ cm⁻², a low density of planar clusters of bubbles located in the basal plane was found in a well defined region around the damage peak on annealing above 973 K. After implantation to the high dose of 10¹⁷ cm⁻², a high density of bubbles in alignment in the basal plane was formed, and remained stable during the subsequent annealing at temperatures up to 1173 K. The difference in annealing behavior suggests that helium is trapped with different energies for the three dose regimes. There appears to be a threshold dose for the formation of the bubble clusters of approximately 5×10¹⁵ cm⁻² for implantation at room temperature. This increases to a value between 2×10¹⁶ and 5×10¹⁶ cm⁻² for implantation at 500 and 873 K, respectively. Such a temperature dependence is consistent with that of effective damage production, indicating a certain level of damage is necessary for the formation of bubble clusters. The formation of planar clusters of bubbles at the intermediate doses is ascribed to an Ostwald ripening (OR) process through the dissociation of the simple He/vacancy clusters and metastable bubbles and the formation of planar cavities of helium on annealing above 973 K.