Interactions of hydrogen with amorphous hafnium oxide

Abstract

We used density functional theory (DFT) calculations to study the interaction of hydrogen with amorphous hafnia ($\mathrm{a}\text{\ensuremath{-}}{\mathrm{HfO}}_{2}$) using a hybrid exchange-correlation functional. Injection of atomic hydrogen, its diffusion towards electrodes, and ionization can be seen as key processes underlying charge instability of high-permittivity amorphous hafnia layers in many applications. Hydrogen in many wide band gap crystalline oxides exhibits negative-U behavior (+1 and $\ensuremath{-}1$ charged states are thermodynamically more stable than the neutral state) . Our results show that in $\mathrm{a}\text{\ensuremath{-}}{\mathrm{HfO}}_{2}$ hydrogen is also negative-U, with charged states being the most thermodynamically stable at all Fermi level positions. However, metastable atomic hydrogen can share an electron with intrinsic electron trapping precursor sites [Phys. Rev. B 94, 020103 (2016).] forming a $[{e}_{tr}^{\ensuremath{-}}+\mathrm{O}--\mathrm{H}]$ center, which is lower in energy on average by about 0.2 eV. These electron trapping sites can affect both the dynamics and thermodynamics of the interaction of hydrogen with $\mathrm{a}\text{\ensuremath{-}}{\mathrm{HfO}}_{2}$ and the electrical behavior of amorphous hafnia films in CMOS devices.