Metal carbide-induced negative flatband voltage shift in TaCx and HfCx/HfO2 gate stacks

We systematically investigated the role of the top interface for TaCx and HfCx/HfO2 gate stacks on the effective work function (Φm,eff) shift by inserting a SiN layer at the gate/HfO2 top interface or HfO2/SiO2 bottom interface. We found that Φm,eff of the TaN gate electrode on HfO2 was larger than that on SiO2 because of the HfO2/SiO2-bottom-interface dipole. On the other hand, we found that Φm,eff values of the TaCx and HfCx gate electrodes on HfO2 agree with Φm,eff on SiO2. This is because the potential offset of the opposite direction with respect to the bottom interface dipole appears at the metal carbide/HfO2 interface. It is thus concluded that the top interface in the metal carbide/HfO2 gate stacks causes the negative Φm,eff shift.