We studied the growth mechanism of GaAs nanowires in selective-area metalorganic vapor phase epitaxy (MOVPE) by investigating the dependences on substrate orientations and growth conditions. The nanowire structures were formed only on GaAs (I I l)B substrate under high temperature (750 degrees C) and low arsine partial pressure conditions. Structures selectively grown on substrates with various orientations always exhibited specific low-index facets such as { 110}, {111}A, and {111}B. It was also found that the appearance of these facets depended strongly on the growth conditions. Furthermore, we have observed a considerable lateral growth on the sidewalls of the nanowires when the growth temperature was lowered and arsine partial pressure was increased, indicating that the growth mode could be changed by the growth conditions. These results demonstrate that the growth mechanism of GaAs nanowires by SA-MOVPE is neither catalyst nor oxide assisted but by the formation of facets during growth. (c) 2006 Elsevier B.V. All rights reserved.