Defects in semiconductors after electron irradiation or in high-temperature thermal equilibrium, as studied by positron annihilation

Abstract

The authors report on positron lifetime studies which investigate vacancy-type defects in elemental and compound semiconductors after electron irradiation or in thermal equilibrium concentrations at high temperatures. In Si the upper limits of positron lifetimes in monovacancies (272 ps) and divacancies (285 ps) as well as the lifetime in phosphorus-vacancy complexes ( approximately 255 ps) are determined after low-temperature electron irradiation and annealing. In addition, the influence of doping on positron trapping and defect annealing is studied. In electron-irradiated GaAs the annealing between 200 K and 350 K, which is not observed after low-energy irradiation, is ascribed to divacancies or Ga vacancies present in defect complexes. In the annealing processes above 450 K As vacancies disappear. The detection of radiation-induced vacancies is substantially influenced by doping and by the temperature at which measurements are made. In as-grown GaSb a positron lifetime of 253 ps is found, which increases by about 12 ps on low-temperature electron irradiation. The annealing occurs in stages at 200 K and 350-500 K. In Si and Ge the temperature variation of the mean positron lifetime between ambient temperature and the melting points does not exceed a few picoseconds at most. This is in contrast to results in Si recently reported by Dannefaer and co-workers. The present high-temperature results on Si and Ge may be interpreted in terms of a thermal vacancy concentration too low to be detected by positrons. However, an insufficient interaction between positrons and high-temperature vacancies cannot be excluded.