Epitaxial ZnO thin films have been grown on (0001) sapphire substrates by pulsed laser deposition. The structural and electrical transport properties of the films are measured as a function of the growth temperature and substrate cleaning procedure. The XRD measurements reveal that the cleaning procedure affects drastically the twin formation. Its origin could be attributed to a residual hydrocarbon layer or defects (oh- or O vacancies) at the sapphire surface. The electrical transport characteristics of the films are found to depend moderately on the existence of twinning. (C) 2009 Elsevier B.V. All rights reserved.