Workfunction (WF) Simulations of Ta/HfO2, Ta2C/HfO2 and Ta2C/La2O3/HfO2 Capped High-k Stacks

Abstract

The electronic properties of tantalum based metal gates in contact with HfO2 are investigated through first-principles calculations. The (110) Ta surface has a p-type work function and is most likely to be formed at the interface with the dielectric. The influence of the transistor processing conditions is modeled by incorporating oxygen atoms in between the metal and the dielectric. Oxidizing the metal gate leads to an increase of the metal work function. To acquire either pMOS or nMOS work function values, the effective work function of the Ta (110)/HfO2 stack can be tuned by incorporating carbon or by inserting dielectric capping layers. The alloy of tantalum with carbon atoms reduces the metal work function which is also reported experimentally. Sandwiching a La2O3 capping layer between the gate and the dielectric leads to a work function reduction. These work function shifts are interpreted on the basis of the electronic density distribution at the interfacial region.