  We investigate the possibility that Hf defects exist in the Si channel of
HfO2 Si-based metal-oxide-semiconductor devices. We have studied, using ab
initio Density Functional Theory calculations, substitutional and interstitial
Hf impurities in c-Si, for various charge states. Our results indicate that 1)
the tetrahedral interstitial defect is energetically more favorable than the
substitutional, and 2) there are various stable charge states in the Si gap.
The possible presence of these charged impurities in the Si channel could lead
to a mobility reduction, due to coulombic scattering.
