The chemical state of nitrogen, aluminum, oxygen and zinc in Al-N co-doped p-type ZnO thin films was investigated by X-ray photoelectron spectroscopy (XPS). N-1s peak were detected in both the two p-type ZnO thin films, showing two components. The higher binding energy peak may be due to the Al-N-O-H species, and the lower one perhaps derive from the (NH2)(-) cluster for the ammonia introduction. These two peaks both contribute to the p-type behavior in the ZnO films. A symmetry 74.4 eV binding energy in Al-2p3/2 photoelectron peaks revealed an Al-N bonding state, a key factor to the co-doping method. (C) 2008 Elsevier B.V. All rights reserved.