Abstract
Copper 2p 3/2 x-ray photoemission spectroscopy (XPS) measurements and near-edge x-ray absorption spectroscopy (NEXAFS) studies of copper-doped ZnO thin films were carried out in order to understand the role of the dopants in increasing the film resistivity. c-Axis-oriented ZnO films (thickness 120 nm) were deposited by r.f. sputtering on Cr-coated SiO 2 /Si substrates and subsequently implanted with metallic copper (0.5 at.%). After implantation, XPS results indicated that the dopant consists of a mixture of Cu + and Cu 0 . The c-axis film resistivity was found to decrease to 7 x 10 7 Ω. cm (prior to implantation : 5 x10 10 Ω.cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cu 2+ ) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2 x 10 11 Ω.cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu 2+ . The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cu 2+ ions to Cu + , which demonstrated that the observed decrease in film resistivity from 2 x 10 11 -10 7 Ω. cm is due to the Cu + oxidation state.
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