Helium has a decisive effect on the microstructure of silicon carbide materials after implantation and subsequent annealing. A dense population of bubbles and dislocation loops is already observed at relatively low displacement doses after annealing of helium-implanted ␣-SiC, while no visible damage appears after irradiation without helium implantation under otherwise equal conditions. The defects are separated from grain boundaries by defect-free zones of approximately 0.5 m width. The most intriguing features of the evolving microstructure are lenticular cavities ͑platelets͒, which transform to disk-shaped arrangements of bubbles with associated dislocation loops or even stacks of loops. The observed microstructural evolution and its dependence on implantation dose, annealing temperature, and time are quantitatively explained and discussed in terms of diffusion of interstitial He atoms and their clustering between adjacent lattice planes, thus forming nanocracks during implantation. The relaxation of the high gas pressure by matrix atom transfer from bubbles to loops during annealing and the coarsening of bubble-loop complexes are described by a coupled twocomponent Ostwald ripening process.

Implantation of 2450 at. ppm helium into silicon carbide at room temperature results in the formation of helium platelets with surprisingly uniform diameters about 9 nm, remaining constant upon annealing up to 1270 K. Estimation of the pressure in the platelets suggests the presence of solid helium even above ambient temperature. The narrow size distribution and the limitation of growth of the platelets is attributed to their trapping by circular dislocation dipoles forming close to their rim when they reach a critical size. Upon annealing to ഠ1500 K, the platelets disintegrate into disks of bubbles and, attached to them, interstitial-type dislocation loops appear. The total volumes of bubble and loop components in such complexes are found to be equal. This striking relation is attributed to the transfer of matrix atoms from the bubbles to the associated loops by dislocation core diffusion. [S0031-9007(99)08776-1]

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