The impact of nitrogen on the defect aggregation in silicon

A nitrogen-doped floating zone (FZ) crystal with a linear increase of the axial nitrogen content from zero to more than 1 x 10(15)at/cm(3) was grown. It was found that the defect-free zone increases with increasing nitrogen content, until it extends over the entire crystal volume. The inner COP region shrinks, until it disappears in the center of the crystal at 0.8 x 10(14) at/cm(3), while the inner boundary of the outer A-swirl region is shifted towards the crystal rim, until the A-swirl vanishes at 1.35 x 10(14) at/cm(3). It is shown that the dominant reaction paths for the suppression of vacancy and Si interstitial aggregation in silicon proceed via N-2+V reversible arrowN(2)V and N2V+I=N-2, respectively. The shift of the boundaries of the COP- and the A-swirl region as a function of the nitrogen concentration can be used to directly measure the radial variation of the vacancy and Si interstitial concentrations, respectively, just after V-I recombination is completed. The measured values are in excellent agreement with the theoretical calculations, if the incorporation of substitutional and single interstitial nitrogen at the growth interface is assumed with a ratio of ca. 1:7. No agreement between experimental and theoretical data is found when the stoichiometry of the above reactions is changed. Thus, N2V2 complexes are not likely to take part in the suppression of point defect aggregation. In nitrogen-doped Czochralski (CZ) crystals, it is proposed that the high oxygen content favors the formation of NO complexes at high temperatures. Thus, no N-2 is available for the reaction with vacancies. At lower temperatures, the equilibrium of the reaction 2NO reversible arrowN(2)+2O(i) shifts to the right-hand side and N2V complexes can form again. Depending on the temperature at which an appreciable N-2 concentration builds up, the aggregation temperature of vacancies and, hence, the density/size distribution of voids varies. The observed enhancement of the oxygen precipitation due to nitrogen doping is not only attributed to a higher free vacancy concentration, but also to the effective removal of Si interstitials emitted by the growing precipitates through the reaction N2V+I reversible arrowN(2). (C) 2001 Elsevier Science B.V. All rights reserved.