Effect of H and Si impurities on device performance based on HfO ₂ gate oxide

Based on first-principles theoretical calculations, we investigate the hydrogenation effect on the defect properties of oxygen vacancies (V _o) in HfO₂. A defect complex of V_o and H behaves as a shallow donor, being in a positive charge state for a wide range of the Fermi levels, and this complex is very stable against its dissociation into V_o and H. We suggest that the V_o-H complex is responsible for the formation of positive fixed charges, which neutralize negative fixed charges during the post-annealing process of SiO_x/HfO₂ stack. We find that at p+poly-Si/HfO₂ gates, Si interstitials can be incorporated from the electrode into the gate oxide with a charge transfer to the electrode. The resulting interface dipole raises the Fermi level of poly-Si toward the pinning level, causing high flat band voltage shifts. In O-rich grown HfO₂ on Si substrates, the Si atoms substitute for the Hf sites, leading to the formation of Hf-silicate layers, while under O-poor conditions, they remain as interstitial defects, binding with the Hf atoms, and behave as a negative-U trap, which causes the threshold voltage instability.