Influence of dose rate on bubble formation by high energy He implantation in silicon

He+ ions were implanted into a (111) epitaxial n-type silicon wafer at different dose rates (fluxes) ranging from 2.5x10(12) to 1.3x10(13) cm(-2) s(-1) while keeping the incident energy and dose constant (1.6 MeV, 2x10(16) cm(-2)). After implantation the samples were subjected to thermal annealing at 800 degreesC for 30 min. Cross section transmission electron microscopy (XTEM) was used to characterize the damage layer. Even in the as-implanted samples the TEM observations revealed the formation of a buried layer containing a dense array of small bubbles. After annealing, a large band of defects made up of bubbles and dislocations was observed in all samples. However, the characteristics of the damage layer found depended on the flux. For the lowest flux, only platelets and planar clusters of helium bubbles lying in the {001} planes were observed. Their nucleation is discussed in terms of the trap-mutation process. For higher fluxes a continuous band of bubbles with rows of prismatic punching related dislocation loops was observed. These dislocations can extend over several micrometers away from the buried layer and are emitted from clusters. For the highest flux these clusters were found to lie in the {100} or {110} planes. The plate-like structures are discussed in terms of the diluted system. The damage evolution with increasing dose rates is explained by taking into account the vacancy production. (C) 2001 American Institute of Physics.