Influence of Copper Dopants on the Resistivity of ZnOFilms

Abstract

Copper 2p3/2 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 SiO2/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 Cu0. The c-axis film resistivity was found to decrease to 7×107 Ω · cm (prior to implantation: 5×1010 Ω · cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cu2+) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2×1011 Ω · cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu2+. The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cu2+ ions to Cu+, which demonstrated that the observed decrease in film resistivity from 2×1011–107 Ω · cm is due to the Cu+ oxidation state.