Oxygen vacancy-induced monoclinic dead layers in ferroelectric HfO2 with metal electrodes

In this work, we analyze the dead layer comprising non-polar monoclinic (m) phase in HfO2-based ferroelectric material using first principles analysis. We show that with the widely used tungsten (W) metal electrode, the density and the spatial distribution of the oxygen vacancy across the cross section play a key role in dictating the favorability of m-phase formation at the metal–HfO2 interface. The energetics are also impacted by the polarization direction as well as the depth of oxygen vacancy, i.e., position along the thickness. At the metal–HfO2 interface with polarization pointing toward the metal, both interfacial relaxation and m-phase formation can lead to dead layers when (i) single vacancy forms at a trigonally bonded O atomic site or (ii) single and double vacancies form at trigonally and tetrahedrally bonded O atomic sites, respectively. For vacancies at other oxygen atomic sites and polarization direction, a dead layer is formed due to sole interfacial relaxation with the polar phase. Moreover, with electrodes consisting of a noble metal (Pt, Pd, Os, Ru, and Rh), a m-phase dead layer formation is less likely than W. Therefore, for these metals, a dead layer forms mainly due to the interfacial relaxation with the polar phase.