We have studied the growth of InN thin films and InAlN/InN heterostructures on a low-temperature grown GaN (LT-GaN) layer on a sapphire substrate by plasma-assisted molecular-beam epitaxy (PAMBE). Cross-sectional transmission electron microscopy revealed that the InN film stood on the LT-GaN layer like a bridge, indicating that the InN film was almost free standing. PL and optical absorption measurements of the InN film showed that the intrinsic band gap of hexagonal InN is less than 0.7 eV. We successfully grew coherent InAlN layers with an A] content up to 0.15 on the InN layer. This demonstrated that single-crystal InAlN/InN heterostructures can be grown by PAMBE. (C) 2002 Elsevier Science B.V. All rights reserved.