The role of Ga and Bi doping on the local structure of transparent zinc oxide thin films

Transparent undoped ZnO and additionally doped with Ga and Bi thin films were produced by magnetron sputtering. The thin films were comprehensively characterized by X-ray absorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission and scanning transmission electron (TEM, STEM) microscopy and Raman spectroscopy. All undoped and doped films crystallise in a ZnO phase with the hexagonal wurtzite crystal structure. The local structure of the thin films was investigated by temperature-dependent X-ray absorption spectroscopy at the Zn and Ga K-edges, as well as at the Bi L₃-edge. It was found that the doping of Ga³⁺ and Bi³⁺ ions in the ZnO wurtzite structure produces distinct effects on the thin film microstructure. The substitution of Zn²⁺ ions by smaller Ga³⁺ ions introduces a static disorder to the thin film structure, which is evidenced by an increase in the mean-square relative displacements σ²(Zn‒O) and σ²(Zn‒Zn). At the same time, large Bi³⁺ ions do not substitute zinc ions, but are likely located in the disordered environment at the ZnO grain boundaries. This conclusion was directly supported by energy-dispersive X-ray spectroscopy combined with TEM and STEM observations as well as by resonant and non-resonant μ-Raman experiments at room temperature, where the ZnO and ZnO:Bi spectra are similar, suggesting a lack of structural disorder in the wurtzite cell. On the other hand, the Raman disorder-activated phonon is pronounced for Ga-doping of the ZnO lattice, confirming the compositional disorder. Both XRD and XPS ruled out Ga₂O₃ phase in Ga-doped ZnO; conversely, Bi₂O₃ and a small amount of Bi‒metal phases are clearly discerned by XPS experiments, further suggesting that Bi is not incorporated in the ZnO wurtzite cell, but segregated to grain boundaries.