Carbon is a typical impurity in thin films fabricated by metal-organic chemical-vapor deposition (MOCVD). The role of carbon in undoped and, nitrogen-doped ZnO thin films was studied experimentally and theoretically to understand the possible compensation effects. ZnO thin films are fabricated by low-pressure MOCVD using diethylzinc, nitric oxide (for nitrogen-doped films), or oxygen precursors (for undoped films). Compared with sputtering-fabricated ZnO film, the carbon concentration in the MOCVD-fabricated ZnO film is very high. Furthermore, the MOCVD-fabricated ZnO:N film has an even higher carbon concentration than the undoped ZnO. Considering the signal observed previously by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, it is possible that the incorporated carbon has formed complexes with doped nitrogen. The first-principles calculations predict that the formation energy for carbon interstitial (C-i) is relatively high. However, due to the large binding energy between C-i and No (nitrogen substitute on oxygen site), the formation energy of the carbon-nitrogen (CN) defect complex is lower than that of the C-i. As a result, with nitrogen doping, the carbon-impurity concentration would be high. In the insulator or p-type ZnO films, the CN defect complexes have 1+ charge state. Therefore, the existence of carbon in the ZnO:N film could be another possible passivation factor to the nitrogen acceptor, in addition to hydrogen. (c) 2006 American Vacuum Society.