Carrier scattering in high-κ/metal gate stacks

A significant degradation of the mobility has been repeatedly observed at low inversion density in high-κ/metal gate metal-oxide-semiconductor field-effect transistors. However, the scattering mechanisms responsible for this degradation are still debated. It is often assumed that the mobility is limited by remote charges (RCS) at the interface between SiO₂ and HfO₂. However, the amount of charges needed to reproduce the experimental mobilities is usually very high (a few 10¹³cm⁻²), and does not seem to be consistent with the measured threshold voltages. Scattering by localized dipoles hardly solves these discrepancies. Here, we investigate the contribution from three alternative mechanisms in a non-equilibrium Green's functions framework: (i) scattering by band offset fluctuations at the SiO₂/HfO₂ interface, (ii) scattering by dielectric constant fluctuations in SiO₂ and HfO₂, and (iii) scattering by workfunction fluctuations in a granular metal gate. None of these mechanisms significantly shifts the threshold voltage. We show that mechanisms (i) and (iii) efficiently scatter the carriers at low inversion densities. This reduces the amount of RCS charges needed to reproduce the experimental data. RCS and these mechanisms show different dependences on the thickness of the HfO₂ layer, which might help to identify the dominant contributions.