A metal gate stack on a substrate comprises: an interfacial layer on the substrate; a high-κ metal oxide layer on the interfacial layer, the high-κ metal oxide layer comprising a dipole region adjacent to the interfacial layer, the dipole region comprising niobium (Nb); a high-κ metal oxide capping layer on the high-κ metal oxide layer; a positive metal-oxide-semiconductor (PMOS) work function material above the high-κ metal oxide capping layer; and a gate electrode above the PMOS work function material. The dipole region is formed by driving Nb species of a Nb-based film into the high-κ metal oxide layer to form a dipole region.