Thermal donor formation in silicon enhanced by high-energy helium irradiation

The enhanced thermal donor (TD) formation was investigated in the float-zone n-type silicon irradiated with 7 MeV helium ions at fluences from 5 × 109 to 1 × 1012 cm−2 and subsequently annealed up to 500 °C. Results show that radiation damage produced by helium ions remarkably enhances TD formation when annealing temperature exceeds 375 °C, i.e. when the majority of vacancy-related recombination centers anneals out. At low fluences (below 1 × 1011 cm−2), the profile of radiation enhanced TD follows well the distribution of primary damage–vacancies. The excess concentration of TD is proportional to helium fluence and peaks at 1 × 1014 cm−3 if annealing temperature reaches 475 °C. At higher fluences of helium, annealing temperature must be increased to stimulate formation of excess TDs. In this case, the concentration profile of TDs is more complex. Its growth starts from the irradiated surface and, with increasing temperature, it gradually extends up to the end-of-range of helium ions. Again, the excess TDs reach its maximum concentration of about 1.2 × 1014 cm−3 at 475 °C.