Influence of phosphorus doping and post-growth annealing on electrical and optical properties of ZnO/c-sapphire thin films grown by sputtering

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 (∼102 Ω cm) at room temperature. Rapid thermal annealing (RTA) reduces the resistivity by several orders of magnitude. The core levels of Zn3s and Zn2p3/2, 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.