InAs nanowire growth on oxide-masked 〈111〉 silicon

Abstract

Here we investigate the growth of InAs nanowires on 〈111〉 Si substrates masked by SiO x using metalorganic 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 °C and 540 °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. © 2012 Elsevier B.V. All rights reserved.