Lattice distortion and electronic structure of magnesium-doped nickel oxide epitaxial thin films

Abstract

Magnesium-doped nickel oxide (${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$) thin films are transparent over a wide ultraviolet-visible spectral range and over a wide Mg content range. However, the influence of the Mg dopant on the structure and properties of NiO films is poorly understood. In this work, the lattice distortion and the electronic structure of ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ ($0\ensuremath{\le}x\ensuremath{\le}0.52$) thin films deposited on ultrasmooth sapphire substrates were investigated using synchrotron x rays. Films with higher Mg content had lower values of Debye temperature and atomic order parameter. The nearest Ni-O distance and the in-plane nearest Ni-Ni distance both expanded with increasing Mg content. The Ni $2p$ core-level spectra and the valence band spectra of the ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ thin films showed complex multiplet structures that were caused by the strong electron correlation in the Ni $3d$ states, where the spectral features are strongly dependent on both the distortion of the ${\mathrm{NiO}}_{6}$ octahedra and the Mg content. We found that the electronic structures are mainly a result of hybridization of Ni $3d$ and O $2p$ in the ${\mathrm{NiO}}_{6}$ octahedra and the reduction of the Zhang-Rice bound state following Mg doping. Finally, the flexibility of the band gap tuning in ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ thin films is explained.