Oxygen Vacancy-Induced Monoclinic Dead Layers in Ferroelectric $Hf_xZr_{1-x}O_2$ With Metal Electrodes

Abstract: In this work, we analyze dead layer comprising non-polar monoclinic (m) phase in $Hf_xZr_{1-x}O_2$ (HZO)-based ferroelectric (FE) material using first principles analysis. We show that with widely used tungsten (W) metal electrode, the spatial distribution of the oxygen vacancy across the cross-section plays 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 - $HfO_2$ interface, when polarization points towards the metal and vacancy forms at trigonally bonded O atomic site, both interfacial relaxation and m- phase formation can lead to dead layers. For vacancies at other oxygen atomic sites and polarization direction, dead layer is formed due to sole interfacial relaxation with polar phase. We also establish the relative favorability of the m-phase dead layer for different Zr concentrations (x=1 and x = 0.5) and metal electrodes. According to our analysis, 50% Zr doped $HfO_2$ exhibits less probability of m-phase dead layer formation compared to pure $HfO_2$. Moreover, with electrodes consisting of noble metal (Pt, Pd, Os, Ru, Rh), m-phase dead layer formation is less likely. Therefore, for these metals, dead layer forms mainly due to the interfacial relaxation with polar phase.