Study of irradiation-induced change in properties of semiconductors by using positrons

The correlation between positron trapping and lattice defects was studied in semiconductors. The increase of the lifetime of positrons was observed with an increase of annealing temperature in Si irradiated by 3 MeV electrons. This fact indicated the formation of vacancy clusters from point defects. The trapping rate and lifetime of positrons associated with monovacancies and vacancy clusters were studied in SixGe1-x mixed crystals irradiated by electrons. No trapping was observed at x<0.2, and a constant trapping rate was shown between x = 0.2 and 0.4. The trapping rate increased at the concentration above x>0.6. This result agreed well with the transition of the band structure from Ge (x<0.2) to Si band (x>0.2) predicted by the band theory. The difference of lifetime by irradiation between electrons or α particles, and protons was found in annealing behavior of Si, and was attributed to the interaction between hydrogens and vacancies. The measurement by infrared absorption supported this result. Er ions were implanted into SiO2 layer formed on Si substrate, and the correlation between the emission of light and lattice defects was studied. The influence of the annihilation of positrons due to Er ion implantation was measured from Doppler broadening parameter. (Y. Kazumata)