High-quality ZnO films were grown on Si(l 00) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 mu m were grown at different growth temperatures of 350, 450 and 550 degrees C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 mu m at 550 degrees C. A ZnO buffer layer with a growth temperature of 450 degrees C (450 degrees C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 degrees C-buffer sample (6.7 nm), the 550 degrees C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (002) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 degrees C-buffer sample was the narrowest as 0.209 degrees, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 degrees C. Consistent with these results, the 450 degrees C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm(-1) was found in Raman scattering spectra for all films with buffers, which is corresponding to the E-2 mode. (c) 2005 Elsevier B.V. All rights reserved.