Efficiencies of Eu³⁺ ions and hydrogen atoms as donors in ZnO thin films

The donor efficiencies of Eu³⁺ ions and hydrogen atoms in ZnO crystalline films were investigated with reference to that of Ga³⁺ ions. It was found that Eu³⁺ ions acted as extrinsic donors in ZnO:Eu films, yielding a resistivity of 1.8 × 10⁻³ Ω cm at a doping level of 1 at. %. This value is comparable to one for intrinsic donors in undoped ZnO films. The conductivity was maintained as the deposition temperature was increased to 200 °C, and this is evidence for the contribution of extrinsic donors. Deposition of Ga-doped and Eu-doped ZnO films in an H₂O gas flow produced oxyhydrogenated ZnO:(Ga, H) and ZnO:(Eu, H) films in which the Ga³⁺ and Eu³⁺ donors were deactivated by oxidization. Nevertheless, hydrogen donors contributed to electrical conduction yielding a resistivity of 1 × 10⁻² Ω cm. Postannealing in an H₂ gas ambient alleviated the excessive oxidization of the films and thereby reactivated the donor action of Ga³⁺ and Eu³⁺ ions, causing the resistivity to recover to 10⁻³ Ω cm for ZnO:(Ga, H) and 10⁻² Ω cm for ZnO:(Eu, H). In contrast, vacuum annealing of ZnO:(Ga, H) and ZnO:(Eu, H) films increased resistivity through removal of hydrogen donors while not affecting the oxidized condition of the samples.