Heat-treatment-induced defects in low-resistivity silicon

Czochralski-grown silicon has been investigated with doping levels up to 5×1018 cm−3 of boron or phosphorus by positron annihilation. For the highly boron-doped samples vacancies could be created upon heat treatment in the 700–1000 °C range up to a concentration of 1017 vacancies cm−3. Reducing either the boron concentration or the interstitial oxygen concentration (by prolonged heat treatment at 1200 °C) reduced the amount of created vacancies. No vacancies were detected in highly phosphorus-doped materials. Combined positron-lifetime and Doppler-broadening measurements indicate that the vacancies are essentially of monovacancy nature and have impurities close by which contribute with high-momentum electrons. Isothermal heat treatments at 700, 750, 900, and 950 °C indicate a reaction of the form X+Y⇄Z, where Z contains a vacancy. The forward reaction has an activation enthalpy of 2.7 eV, while the back reaction has the value 3.0 eV. It is suggested that X corresponds to substitutional boron and Y to interstitial oxygen.