Electrical and optical properties of phosphorus. P. doped ZnO thin films, which were grown on c-plane sapphire substrates, are studied using the Hall-effect measurement, secondary-ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), and optical absorption spectroscopy. An increase in P doping concentration converts ZnO thin films from insulating to n-type semiconducting with large resistivity (similar to 10(2) Omega cm) at room temperature. Rapid thermal annealing (RTA) reduces the resistivity by several orders of magnitude. The core levels of Zn3s and Zn2p(3/2), probed by XPS using C1s as reference, are shifted monotonically to larger energies with increasing P doping concentration. An analysis of the optical absorption spectra near the band edge of ZnO reveals an exciton absorption peak at 3.37 eV, which is independent of P doping. RTA also induces a large-energy shift for the XPS core levels; however, the optical absorption spectra show an apparent blue-shift upon annealing. The correlations between the electrical and optical properties are related to an existence of a near-surface electron accumulation layer as well as donors' activation by RTA. (C) 2011 Elsevier B.V. All rights reserved.