Chemical tuning of band alignments for Cu/HfO2 formula interfaces
First-principles density functional theory is used to explore the possibility of tuning effective work function of high dielectric/metal gate stacks. Using HfOmath formula/Cu as an example, we demonstrate that the effective work function of the stack can be tuned in a range of ~ 3.2 eV by incorporating a metallic monolayer at the interface. The calculations reveal that the interfacial charge transfer from the metallic monolayer to the oxide plays a key role in tuning the effective work function. The interfacial charge transfer not only depends on the ionic bonding between the O atoms and the metal monolayer, but it can also be affected by the metallic bonding between the gate Cu atoms and the metal monolayer. The weaker the metallic bonding, the stronger the interfacial dipole moment, and the greater the effective work function modulation. A linear correlation between the interfacial charge transfer and the valence band offset is observed which can be rationalized by an analogy to a parallel plate capacitor. We also find a correlation between interfacial atomic rearrangements and the interfacial charge transfer. The general chemical tuning trends established in this study could potentially provide a valuable guide for designing novel gate materials in conjunction with high-k dielectrics.