Origins for Fermi level control in Metal/High-k/Si stacks with inserted dielectric layers

Ultrathin dielectric capping layers are a prominent route for tuning band offsets and threshold voltages of advanced devices. Recent results obtained for several materials systems are presented and discussed. Ta₂O ₅ is shown to be able to suppress the known dipole at Al ₂O₃-SiO₂ interfaces. In a different system, the diffusion of Al inside an Al₂O₃-capped HfO ₂-based device is studied in detail. The effect of the high temperature diffusion of Al to the HfO₂-SiO₂ interface is demonstrated, and presence of Al at that interface is correlated with the expected band offsets. La₂O₃ deposited in a similar way is known to have an opposite electrostatic effect. We show that La has a fundamentally different diffusion behavior under high temperatures, and that unlike Al it does not stop at the HfO₂-SiO₂ interface, but rather penetrates deep into the SiO₂.