Within the last few years indium nitride (InN) gained substantial interest due to its controversially discussed apparent band gap and its predicted highest maximum electron mobility among several III–V compounds, including Al(Ga)N and GaAs. The band gap of epitaxial InN has been recently reported to be around 0.7 eV rather than the previously accepted value of 1.9 eV obtained from polycrystalline films. Thus, InN could be a promising material for applications in infrared opto-electronics or high-speed electronics. However, the structural quality of the InN epilayers is still inferior to GaN and needs to be improved. Also, the role of many contaminants in InN and their effect on the epilayer's conductivity and/or luminescence properties is still under investigation. This work describes recent studies of InN growth by molecular beam epitaxy on sapphire (0 0 0 1) substrates. The effect of buffer layer variations including a prior substrate nitridation step is discussed. Structural properties (X-ray diffraction, AFM and TEM images) and chemical profiles (SIMS) will be correlated to Hall data and Photo-luminescence spectra. The role of oxygen and hydrogen as possible donors in InN will be discussed. r 2004 Published by Elsevier B.V.