The transport property of sputter-deposited ZnO films is examined by physical and chemical analyses. The goal is to elucidate the fundamental mechanism of carrier (electron) generation. This study evaluates crystallinity, chemical bonding states, chemical stoichiometry, and carrier concentration in order to elucidate the electron generation mechanisms of undoped ZnO film. It is found that the lateral resistance of films annealed under N-2 ambient is much lower than those annealed under O-2 ambient but roughly independently of the ambient gas used during deposition. XPS analysis reveals that, for similar grain size, the lower the specific oxygen-related spectrum component is, the higher the electron concentration is. The chemical role of oxygen atoms in undoped ZnO film is also examined and verified by using an oxygen isotope as the ambient gas during the ZnO film deposition. This relation, discovered by studying the impact of the specific oxygen-related spectrum component on electron concentration, is also elucidated by analyzing the behavior of depth profiles of oxygen isotope (O-18) in undoped ZnO film after the annealing process. (C) 2016 The Electrochemical Society. All rights reserved.