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 VN2 was not stable, whereas V2N2 when formed from VN2 was the most stable, followed by N-2 and V2N2 formed from a divacancy. The calculated free energy changes of the considered chemical reactions confirmed the low stability of VN2 compared to V2N2. The latter can form during crystal growth from VN2, whereas reactions between N-2 and divacancy can also occur upon wafer heating. At low nitrogen concentration (similar to5 x 10(13) cm(-3)), only about 10% of vacancy concentration was converted into VN2, while at a high nitrogen concentration (similar to10(16) cm(-3)) about 75% of vacancies are trapped by nitrogen. V2N2 appeared to create a potential well of -2.4 eV for oxygen and about -0.3 eV for vacancies, suggesting that the stable V2N2 is a nucleus for oxygen precipitation while it is a weak trapping center for vacancies. (C) 2003 The Electrochemical Society.