The influence of Ce doping on the structural and optoelectronic properties of RF-sputtered ZnO films

Abstract

Using X-ray diffraction, UV-Visible spectroscopy, XPS and photoluminescence (PL) measurements, the structural, optical and electronic properties of ZnO and Ce-doped ZnO thin films were investigated; the films were deposited on glass substrates by RF reactive-magnetron sputtering and post-annealed at $$300\,^{\circ }\hbox {C}$$ in an oxygen atmosphere. Under similar deposition conditions, both films crystallized into hexagonal wurzite lattice structures. The pure ZnO film exhibited a c-axis preferential orientation, whereas the Ce-doped exhibited an a-axis preferential orientation. The films display uniform textured surfaces with columnar-like microstructures. The UV-Vis spectra showed high transparencies of 90 % on average for both films. Band gaps of $$\hbox {E}_{g}=3.23\,\hbox {eV}$$ and $$\hbox {E}_{g}=3.27\,\hbox {eV}$$ for pure and Ce-doped film, respectively, were measured. The doped film spectrum was shifted to the blue as a result of the Burstein-Moss effect. The XPS spectra show that the VB edge of the doped film shifts toward lower binding energy, at $$\sim $$ 1.3 eV below $$\hbox {E}_{F}$$ , while the VB edge of the pure film is located at $$\sim $$ 2.0 eV below $$\hbox {E}_{F}$$ . Additionally, $$\hbox {Ce}^{3+}$$ and $$\hbox {Ce}^{4+}$$ ions coexist in the ZnO matrix in fractions of $$\sim $$ 70 and $$\sim $$ 30 %, respectively. The PL spectra show that both types of ions induce extra electron states that allow multiple emission peaks in the blue–green region.