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This letter demonstrates a way for modifying the effective work function Phim of a molybdenum (Mo) gate electrode by interface dipole engineering in a metal gate/high-k gate stack. N-type Mo gate Phim (~4.2 eV) was achieved on a HfLaO gate dielectric even after 950-degC rapid thermal annealing (RTA) due to the presence of a La-induced interface dipole layer. By alloying with ~14%-19% of aluminum (Al), the effective Mo gate Phim on HfLaO significantly increased by ~0.6 eV after 950-degC RTA. The incorporation of Al into the HfLaO gate dielectric was evident after the high-temperature anneal. The Phim modulation was attributed to Al-induced interface dipole formation, of which has opposite polarity to the La-induced dipole, at the high-k/SiO2 interface. This novel concept of employing two opposing interface dipoles in the same metal gate/high-k stack for Phim tunability would provide insights for future gate stack interface engineering.