Single-crystalline ZnO films have been grown on a-plane sapphire in plasma assisted molecular beam epitaxy by introducing a high-temperature ZnO buffer layer. The residual electron concentration of the films can be lowered to 1.5 x 10(16) cm(-3), comparable with the best value ever reported for ZnO films grown on a rare and costly substrate of ScAlMgO4. A 3 x 3 reconstruction has been observed on the films grown in this route, which reveals that the films have very smooth surface. X-ray phi-scan spectrum of the films shows six peaks with 60 intervals, and two-dimensional X-ray diffraction datum indicates the single-crystalline nature of the films. Low temperature photoluminescence spectrum of the films shows a dominant free exciton emission and five phonon replicas, confirming the high quality of the films. (C) 2010 Elsevier B.V. All rights reserved.