The interplay between Mn valence and the optical response of ZnMnO thin films

We show in this paper that the valence state of Mn is determinant for the optical response of Zn1−xMnxO thin films. In addition, thermal treatment is an efficient way to tailor these properties. This investigation examines the valence change of manganese in Zn1−xMnxO thin films at concentrations up to x = 0.7 after annealing by correlating reference samples: Mn3O4 and ZnO. The thin films were grown by spray pyrolysis using an aqueous-acetate route on glass substrates. A phase transition from Mn3O4 (hausmannite) to $$\alpha$$ -Mn2O3 (bixbyite) occurs after annealing, while Zn1−iMnxO keeps the wurtzite phase. X-ray photoelectron spectroscopy indicates that as-grown Mn oxide sample exhibits both Mn2+ and Mn3+ species with a predominance of Mn2+ while annealed Mn oxide presents Mn3+ and Mn4+. The presence of Mn2+ in as-grown Mn-doped ZnO is also confirmed by electron paramagnetic resonance. Scanning electron microscopy and optical absorption measurements show that the annealing on ZnMnO has the same influence as solely Mn oxide. The occurrence of Mn segregation is attributed to the predominance of Mn3+in the annealed system.