The growth of metal-free carbon nanotube (CNT) arrays on SiC surface was investigated systematically by using high temperature annealing of 6H-SiC(0 0 0 (1) over bar) crystals under various atmospheres, including inert, hydrogen-containing, and oxygen-containing gaseous environments. Carbon nanowall structure consisting of graphene sheets standing vertically on the substrate forms under the inert and hydrogen-containing atmospheres, while vertically aligned CNT arrays can be obtained in oxygen-containing atmospheres, such as H2O. The comparative studies reveal that oxygen-containing species play a critical role in the formation of CNTs on SiC. Transient SiO nanoclusters formed at the C/SiC interface are proposed to be the active sites for CNT growth on SiC. (C) 2010 Elsevier B.V. All rights reserved.