Separate control of number density and size of ErAs nanoparticles by a modified diffusion length process using two flux conditions during molecular beam epitaxy

Abstract

We have developed a two-step process for growth of ErAs nanoparticles (NPs) on GaAs-(001) surfaces which enables independent control of the NP size and number density (ND). A key feature is the extension of the Er-atom diffusion length between the steps. We use a short duration high Er flux to produce a low diffusion length (LDL) deposition step, aimed at controlling the NP nuclei density. This is followed by a long duration low Er flux to produce a high diffusion length (HDL) deposition step chosen such that the Er-atom diffusion length prevents additional nucleation, and only allows growth of pre-existing NP nuclei. We observed that deposition under the HDL condition alone causes surface roughening, which is prevented when combined with the LDL condition. This indicates that pre-existing nuclei are sinks to Er adatoms deposited under HDL conditions. Although in the present study the diffusion length was controlled by flux change alone, many other factors such as substrate temperature, surfactants, flux-ratios, should also be equally effective. This two-step approach is of general applicability, providing a means for separate control of NP size and ND in many other materials involving composite nanostructures.