  Using ab initio density functional total energy and molecular dynamics
simulations, we study the effects of various forms of nitrogen post deposition
anneal (PDA) on the electric properties of hafnia in the context of its
application as a gate dielectric in field effect transistors (FET). We consider
the atomic structure and energetics of nitrogen containing defects which can be
formed during the PDA in various N-based ambients: N2, N+2, N, NH3, NO, N2O. We
analyse the role of such defects in fixed charge accumulation, electron
trapping and in the growth of the interface SiOx layer. We find that nitrogen
anneal of the oxides leads to an effective immobilization of native defects
such as oxygen vacancies and interstitial oxygen ions, which may inhibit growth
of silica layer. Nitrogen in any form effectively incorporates into the
pre-existing oxygen vacancies and, therefore may decrease the concentration of
shallow electron traps. However, nitrogen in any form is unlikely to
significantly reduce the fixed charge in the dielectric.
