InN nanocolumns were grown on (0 0 0 1) sapphire substrates by radio-frequency plasma-assisted molecular-beam epitaxy, simply with substrate nitridation, InN nucleation, and high V/III growth conditions. Here, the InN nucleation was conducted by forming In droplets on the nitridated substrates and subsequently nitriding these In droplets. We investigated the growth evolution of InN nanocolumns and the growth-temperature dependence of the morphology. X-ray rocking curves (XRC) for the (0 0 0 2) reflection of the samples basically consisted of two components: a broad peak and a very sharp one. We propose that the broad peak came from the lower part of relatively short nanocolumns and the near-interface region of relatively long nanocolumns. In contrast, the sharp peak came from the (strain-free) upper part of relatively short nanocolumns and the high-crystal-quality region apart from the interface in relatively long InN nanocolumns. The relative intensity of the latter to the former increased with growth time. The shape of the nanocolumns varied with growth temperature: nanocolumns grown at 380 and 420 degrees C had a taper-like appearance, but the top broadened with increasing growth temperature, becoming broader at the top than the base at 470 and 500 degrees C. When we grew InN nanocolumns under the same conditions, but without surface nitridation, the subsequent columns were not all along a c-axis. If In droplets were not used, then columns did not form. Thus, both the surface nitridation and In droplets were needed to form c-axis aligned nanocolumns. (c) 2007 Elsevier B.V. All rights reserved.