Crack-free aluminum nitride (AlN) epilayers were grown on sapphire using growth-interrupt technique by radio-frequency assisted molecular beam epitaxy. In-situ reflectance spectroscopy was introduced for real-time monitoring of the growth of AlN epilayers. X-ray diffraction and atomic force microscopy measurements reveal that the threading dislocation density decreases considerably by using the growth-interrupt technique. Raman spectroscopy is used to characterize the residual stress of AIN epilayers. The optical transmittance and absorption spectra of AlN epilayers show a high transmittance and a sharp absorption edge. (C) 2009 Elsevier B.V. All rights reserved.