Negative oxygen vacancies in HfO$_2$ as charge traps in high-k stacks
  We calculated the optical excitation and thermal ionization energies of oxygen vacancies in m-HfO$_2$ using atomic basis sets, a non-local density functional and periodic supercell. The thermal ionization energies of negatively charged V$^-$ and V$^{2-}$ centres are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are the likely candidates for intrinsic electron traps in the hafnum-based gate stack devices. 
J. L. Gavartin, D. Munoz Ramo, A. L. Shluger, G. Bersuker and B. H.   Lee