Disparities between the predicted and the measured effective work functions (EWFs) in advanced metal oxide semiconductor devices, or Fermi-level pinning (FLP), have gained significant attention when high-$k$ dielectrics began to emerge. Using a systematic approach for EWF extraction, combined with a comparison to unpinned SiO${}_2$ references, it was found that no intrinsic FLP exists in the model dielectric Al${}_2$O${}_3$. Extrinsic FLP was found with one of the metals investigated Ta, where a 0.4 eV increase in the EWF was observed in the electrical characteristics and confirmed by backside spectroscopy. The physical origins of the band offsets related to the EWF increase have been analyzed in detail. A 2-nm interfacial layer at the Ta-Al${}_2$O${}_3$ interface has been found and is suggested as the source of extrinsic FLP. Interfacial dipoles originating from Ta-O chemical bonds are considered as the mechanism responsible for the band offset. The results of this model system are then used to explain some of the peculiarities occurring at complex devices which are used in technological applications.

• Received 20 July 2011

DOI:https://doi.org/10.1103/PhysRevB.84.155317