Local structure of Fe impurity atoms in ZnO: bulk versus surface

TL;DR

This study investigates how Fe impurities incorporate into ZnO at bulk and surface levels, revealing different valence states and defect structures, and assessing their magnetic properties through experimental and computational methods.

Contribution

It provides new insights into the valence states and defect configurations of Fe in ZnO, highlighting differences between bulk and surface doping, and evaluates magnetic ordering tendencies.

Findings

Fe in bulk ZnO is isovalent (Fe2+), while near the surface it can be heterovalent (Fe3+)

Fe substitution with interstitial oxygen is energetically favorable with excess oxygen

Ferromagnetism is unlikely in Fe-doped ZnO without secondary phases or co-doping

Abstract

By studying Fe-doped ZnO pellets and thin films with various x-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fe-doping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe2+ -> Zn2+). In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe3+ -> Zn2+). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favourable. Furthermore, it is found that ferromagnetic states are energetically unfavourable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e. ZnFe2O4), or codoping with Cu.