Float-zone crystals, having an axial N content which varied linearly from zero to more than 1015/cm3, were prepared. It was found that the defect-free zone increased, with increasing N content, until it extended over the entire crystal volume. The inner COP region shrank until it disappeared in the center of the crystal at 8 x 1013/cm3. Meanwhile, the inner boundary of the outer A-swirl region shifted towards the crystal rim until the A-swirl vanished at 1.35 x 1014/cm3. It was shown that the predominant reaction paths for the suppression of vacancy and Si interstitial aggregation proceeded via: N2 + V ↔ N2V and N2V + I ↔ N2, respectively. The shifts in the boundaries of the COP- and A-swirl regions, as a function of N concentration, could be used to measure directly the radial variations of the vacancy and Si interstitial concentrations, respectively; just after V-I recombination was complete. The measured values were in excellent agreement, with theoretical calculations, when the incorporation of substitutional and single interstitial N atoms at the growth interface was assumed to occur in the ratio of about 1:7. No agreement between experiment and theory was found when the stoichiometry of the above reactions was changed. Thus, N2V2 complexes were unlikely to take part in the suppression of point defect aggregation. In the case of N-doped Czochralski crystals, it was proposed that a high O content favoured the formation of NO complexes at high temperatures. Thus, no N2 was available for reaction with vacancies. At lower temperatures, the equilibrium of the reaction, 2NO ↔ N2 + 2Oi, shifted to the right-hand side and N2V complexes could again form. Depending upon the temperature at which an appreciable N2 concentration built up, the vacancy aggregation temperature, and therefore the void density and size distributions varied. An observed enhancement of O precipitation due to N doping was attributed to a higher free vacancy concentration, and to the effective removal of Si interstitials which were emitted by growing precipitates according to the reaction: N2V + I ↔ N2.

The Impact of Nitrogen on Defect Aggregation in Silicon. W.von Ammon, R.Hölzl, J.Virbulis, E.Dornberger, R.Schmolke, D.Gräf: Journal of Crystal Growth, 2001, 226[1], 19-30