Anomalous positive flatband voltage shifts in metal gate stacks containing rare-earth oxide capping layers

It is shown that the well-known negative flatband voltage (V_FB) shift, induced by rare-earth oxide capping in metal gate stacks, can be completely reversed in the absence of the silicon overlayer. Using TaN metal gates and Gd₂O₃-doped dielectric, we measure a ∼350 mV negative shift with the Si overlayer present and a ∼110 mV positive shift with the Si overlayer removed. This effect is correlated to a positive change in the average electrostatic potential at the TaN/dielectric interface which originates from an interfacial dipole. The dipole is created by the replacement of interfacial oxygen atoms in the HfO₂ lattice with nitrogen atoms from TaN.