The electronic structure, formation energy, and thermal stability of nitrogen-vacancy related complexes in silicon have been investigated using density functional theory and semi-empirical Hartree-Fock calculations. The calculated energies of formation in the ground state showed that VN 2 was not stable, whereas V 2 N 2 when formed from VN 2 was the most stable, followed by N 2 and V 2 N 2 formed from a divacancy. The calculated free energy changes of the considered chemical reactions confirmed the low stability of VN 2 compared to V 2 N 2 . The latter can form during crystal growth from VN 2 , whereas reactions between N 2 and divacancy can also occur upon wafer heating. At low nitrogen concentration (ϳ5 ϫ 10 13 cm Ϫ3 ͒, only about 10% of vacancy concentration was converted into VN 2 , while at a high nitrogen concentration (ϳ10 16 cm Ϫ3 ͒ about 75% of vacancies are trapped by nitrogen. V 2 N 2 appeared to create a potential well of Ϫ2.4 eV for oxygen and about Ϫ0.3 eV for vacancies, suggesting that the stable V 2 N 2 is a nucleus for oxygen precipitation while it is a weak trapping center for vacancies.

Defect size and density distributions were obtained as a function of depth in nitrogen doped CZ silicon (N-CZ) following Hi–Lo–Hi and Lo–Hi annealing, using an oxygen precipitate profiler. The defects were also delineated by Wright etching and Nomarski optical microscopy on both cleaved and bevel polished samples. In addition to the enhanced precipitation and absence of voids previously reported for N-CZ Si, an unexpected mode of precipitation has been found near the annealed wafer surface, just above the traditional denuded zone. This oxynitride precipitate is discussed with regard to N-related complex interactions and point defect supersaturations/injection. High resolution transmission electron microscopy revealed that most precipitates have an octahedral shape with two distinct amorphous phases, which reflect a transition from an initial phase containing both N and O to one with primarily O, as verified with Z-contrast TEM and electron energy loss spectroscopy.

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