Impact of Si impurities in HfO ₂: Threshold voltage problems in poly-Si/HfO ₂ gate stacks

We perform first-principles theoretical calculations to study the atomic and the electronic structures of Si impurities in HfO ₂. We also examine the impact of Si impurities on the threshold voltage problems in ploy-Si/HfO ₂ gates and the growth morphology of HfO ₂ films on Si substrates. In the ploy-Si/HfO ₂ interface region, Si atoms easily migrate from the poly Si and remain as ionized interstitials. A charge transfer from the oxide to poly Si induces interface dipoles, which may cause high fiat-band voltage shifts, especially for p+ poly Si electrodes. In addition, since interstitial Si atoms behave as negative-U traps, these defects are suggested to be the origin of the threshold voltage instability. Under O-rich growth conditions, the Si impurities favorably substitute at the Hf sites, resulting in the formation of Hf-silicate layers at the oxide/Si substrate interface. On the other hand, under O-poor growth conditions, the Si impurities tend to be interstitials, binding with the Hf atoms and forming a Hf-silicide structure.