Small-angle neutron scattering from oxygen precipitates in silicon annealed at low temperatures

SANS has been used to measure the diffusion coefficient of oxygen in silicon in the temperature range 500-550 deg C. The results show that the oxygen diffusion is enhanced by a factor of ∼10 or less above the normal diffusion line, D (cm2s-1) = 0.13exp(-2.53eV/kT). A diffusion line of D(cm2s-1) = 6x10-5exp(-1.86eV/kT) fits the new results together with previous SANS data. The results suggest that this is an effective diffusion coefficient between normal interstitial oxygen diffusion and possibly the diffusion of oxygen dimers. This work is consistent with more recent work by other authors. SANS has been used to study the spatial dependence of precipitates across the diameter of 8-inch crystals grown in the 1980's and 1990's. The results show that Czochralski silicon grown today no longer suffers from the inhomogeneous precipitate growth across the crystal diameter observed in the 1980's. Temperature dependent scattering observed at Q∼0.2A-1 was found to be due to inelastic processes using time-of-flight measurements on SANS. This inelastic scattering can cloud the elastic scattering from weakly scattering precipitates. The time-of-flight chopper on SANS was used to separate the elastic and inelastic events on samples annealed at 500 and 525 deg C. The results showed that the Guinier radius of the precipitates was of the order of ∼10A and suggested oxygen diffusion close to normal oxygen diffusion in silicon at 525 deg C. An improvement in background levels should enable better measurements on silicon annealed at lower temperatures using this new technique. An energy analysis of the inelastic scattering showed that these were due to Umklapp processes where neutrons were gaining energy from phonons, at Q∼0.4A-1 and energy transfer of ∼13meV. (author)