Work Function Engineering Of Pt/HfO₂Gate Stack:First-principles Calculations

Work function is a crucial factor in improving the high-k/metal gate stack. Forplanar bulk the metal-oxide-semiconductor (MOS) devices, the metal gate candidatesmust have “band edge” work function of Si substrate, i.e.,4.1eV and5.1eV forn-MOS and for p-MOS, respectively. Although the vacuum work function (Φ) ofmany metals seems to comply with the requirement mentioned above, the real workfunction of metals in the final device will always be shifted from the Φ when thesemetals contact with high-k dielectrics, because of interface effects such as mismatchstrain, interface dipole, gate dielectrics bulk defects, fermi level pinning etc. Therefore,effective work function (Φeff), a work function metric which combines Φ with theseinterface effects, should be introduced, and its modulating in high-k/metal gate stackis a crucial and challenging task in metal-oxide-semiconductor field effect transistors.In this paper, we’ve chosen Pt electrode and hetero-structure Pt/HfO2stackrespectively, to investigate systematically the engineering of work function inhigh-k/metal gate stack by using first principle calculations, including modulation ofthe work function by strain engineering and interface doping effect, the details are asfollows:1. Firstly, we’ve investigated the work function on the Pt surface under lateralstrain states along different angles using first-principles calculations. We found thatthe Φ versus strain is strongly anisotropic. The compressive strain state can increaseΦ, but the tensile strain state can decrease Φ, while the biaxial strain states can lead tothe maximum change in Φ. The above modifications of Φ can be related to the effectof the strain states on the bulk electronic structure and surface dipole, and themechanism responsible for such change can be explained by analyzing the changes inthe relative cell volume, α. Finally, we propose a canonical strain-Φ relationship thatcan be utilized in designing improved high-k/metal gate MOSFETs.2. Secondly, we’ve applied first principles calculations to systematically studythe effect of metal elements incorporation on Φeffof the Pt/HfO2interface, our resultsindicate that Φeffis closely related to the atomic dopants with different positions(configurations), concentrations and species (electronegativity). Based on establishedand optimized the Pt/HfO2interface model, we obtained the interface effective workfunction by analyzing interface additional dipole moment which is attributed to interface charge transfer. The further doping of metal elements at the interface givesrise to the redistribution of charge at the interface, and changes the value of Φeff.Firstly, the Φeffdecreases when Al atoms substitute Hf in the HfO2side. Conversely,the Φeffincreases when the Al atoms substitute O or Pt atoms, or become the interfaceinterstitial atoms. Moreover, the change of Φeffas well as interface formation energy,strongly depends on the doping concentrations. Finally, we’ve examined the differentchemical composition, e.g., Al, Ni, Ti, Ta, etc. substitute, and reached a roughlyuniform rule, i.e., Φeffincreases with the decreasing trend of the electronegativity ofdoping elements. The introduction of atomic dopants mentioned above will be aneffective method to modify the interface effective work function.