Impact of Si impurities in HfO2: Threshold voltage problems in poly-Si/HfO2 gate stacks

Abstract

We perform first-principles theoretical calculations to study the atomic and the electronic structures of Si impurities in HfO2. We also examine the impact of Si impurities on the threshold voltage problems in ploy-Si/HfO2 gates and the growth morphology of HfO2 films on Si substrates. In the ploy-Si/HfO2 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 flat-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.