Here we investigate the growth of InAs nanowires on < 111 > Si substrates masked by SiOx using metal-organic chemical vapor deposition. We study < 111 > (axial) and < 1-10 > (radial) growth of InAs NWs by varying growth duration, temperature, group-III molar flows, V/III ratio, mask material, mask opening size, and inter-wire distance. We find that growth takes place without an In droplet and the process evolves through three successive phases: nucleation of an InAs cluster, followed by two distinct nanowire growth phases. These two growth phases have different axial and radial growth rates, which originate in a transition from having In supply dominated by the open Si area in the first phase towards an In supply from the vapor/oxide mask in the second growth phase. The linear relation found between nanowire length and diameter vs. time in the last growth phase indicates that < 111 > growth is not surface diffusion limited as is usually the case for catalyzed growth. A high yield of vertical nanowires is obtained if group-III flow is above and V/III ratio below threshold values, in addition to having an arsenic-terminated Si surface. Furthermore, we observe that < 111 > and < 1-10 > growth is surface kinetically limited below 520 degrees C and 540 degrees C, respectively, with activation energies of 20 and 6.5 kcal/mol. This difference in activation energies limits the selectivity of the < 111 > to < 1-10 > growth to 25:1 under optimized conditions, which must be considered when fabricating axially modulated structures. However, we find that by placing wires in large arrays it is possible to completely stop the < 1-10 > growth rate in favor of the < 111 > growth rate. (C) 2012 Elsevier B.V. All rights reserved.