Spatial variations in O precipitation were studied in wafers which had been subjected to rapid thermal annealing in N at temperatures above 1150C, before carrying out a 2-step precipitation treatment. It was found that samples which had been subjected to high-temperature pre-annealing exhibited a consistent enhancement of O precipitation which depended upon the rapid thermal annealing time and temperature. Measurements of O precipitate density showed that such precipitation was not homogeneous within the wafer, but instead peaked near to the surfaces; leaving a minimum in the bulk. Heat treatment in N resulted in the formation of defects which acted as precursors for the nucleation of O precipitates. There was good evidence that these precursors were generated by thermal nitridation of the surface and by consequent vacancy injection into the bulk.

M.Pagani, R.J.Falster, G.R.Fisher, G.C.Ferrero, M.Olmo: Applied Physics Letters, 1997, 70[12], 1572-4