Metal-Assisted Growth of III-V Nanowires By Molecular Beam Epitaxy

Abstract

<p> The mechanisms operating during the metal-assisted growth of III-V nanowires (NWs) by molecular beam epitaxy on (1 1 l)B substrates were investigated through a series of experiments aimed at determining the influence of growth conditions on the morphology and crystal structure. Using GaAs as the principal material system for these studies, it is shown that a good control of these two characteristics can be achieved via a tight control of the temperature, V /III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400°C and 500°C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1 nm^(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600°C, a V /III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07 nm/s, and suggest the axial growth is group V limited. Increasing the flux ratio favored uniform sidewall growth, thus making the process suitable for the fabrication of core-shell structures. Further observation of steps on the sidewall surface of strongly tapered NWs suggests that radial growth of the shell proceeds in a layer-by-layer fashion, with the edge progressing in a step-flow mode toward the tip. </p> <p> From the experimental considerations, an analytical description of the growth is proposed, based on a simple material conservation model. Direct impingement of growth species on the particle, coupled to their diffusion from the sidewall and the substrate surface, are considered in the derivation of expressions for the time evolution of both axial and radial growths. Factors that take into account the nonunity probability of inclusion of group III adatoms in the axially growing crystal are introduced. Moreover, a step-mediated growth is included to describe the axial evolution of the shell. </p>