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.