The refractive index and optical absorption of wurzite InN epilayers grown on Si(l 11) substrates with a beta-Si3N4/AlN(0001) double-buffer by nitrogen-plasma-assisted molecular-beam epitaxy were studied by employing spectroscopic ellipsometry (SE). The crystalline quality of the InN epilayers were investigated by cross-sectional transmission electron microscopy, X-ray diffraction, and scanning electron microscopy. SE results analyzed by the Adachi's model for the dielectric function show that the optical absorption edge of InN varies in the range of 0.76 - 0.83 eV depending on the carrier concentration, which in turn can be adjusted by the thickness of the AlN buffer layer. (c) 2005 Elsevier B.V. All rights reserved.