Electron irradiation damage in silicon containing high concentrations of boron

Abstract

Silicon containing high concentrations of up to 10²⁰ atom cm^-3 of boron alone, or doubly doped with boron and a group V donor impurity, have been irradiated by 1.5 or 2 MeV electrons at 300K. Infrared local mode absorption measurements have shown that this treatment can lead to the removal of a very large fraction of boron from unperturbed substitutional sites, and there is correlated growth of other local mode bands. Certain bands have been identified with a defect that appears to involve two boron atoms, while other bands are thought to arise from boron in interstitial sites. Annealing studies, involving both optical and electrical measurements, are consistent with this latter interpretation. The initial removal rate of boron from normal sites is very high and reaches a limiting value of about 3 atom cm^-3 per electron cm^-2, when the impurity concentration exceeds 10¹⁹ cm^-3. It is concluded that recombination of primary vacancies and intrinsic interstitials is almost completely inhibited in these samples. In the doubly doped crystals, it is suggested that the group V donors trap the vacancies, while boron traps the silicon interstitials. It is inferred that large vacancy clusters form in samples containing only boron impurities. These results show that the large changes observed in the conductivity of degenerate p type material may be interpreted as due to the removal of the acceptor impurities.