This paper describes studies of the polarity of InN layers grown by radio-frequency plasma-assisted molecular-beam epitaxy. The InN layers in this study were grown on a variety of substrates: (0 0 0 1) sapphire substrates; (0 0 0 1) Ga-face and (0 0 0 (1) over bar) N-face free-standing GaN substrates, and (0 0 0 1) Si-face and (0 0 0 (1) over bar) C-face 6H-SiC substrates. It was found that the InN layers grown on the sapphire substrates had N-polarity. Furthermore, the growth temperature for successful growth of InN was strongly dependent on the substrate polarity: InN could be grown at 550degreesC on N-face GaN and C-face SiC substrates, while InN growth was only realized at a decreased temperature of 450degreesC on Ga-face GaN and Si-face SiC substrates. The polarity of the InN layers grown on the SiC substrates was further examined by etching the InN surface using KOH solution. The InN layers grown on Si-face and C-face SiC substrates had In-polarity and N-polarity, respectively. As a result, InN with N-polarity can be grown at higher temperature than InN with In-polarity, this can be explained by the different bonding configuration of the surface N atoms with the underlying In atoms for the two growth processes with In-polarity and N-polarity. (C) 2004 Elsevier B.V. All rights reserved.