Si Dopants Enable Catalyst-Free n-Type GaAs Nanowire Growth by Modifying Facet Energies

Abstract

Abstract Body: Control over dopant concentration and distribution is essential to engineering nanowire-based devices. However, controllable n-type doping has remained challenging for GaAs, which is the foundation for many important III-V heterostructures. In particular, silicon behaves as a p-type dopant in GaAs nanowires grown by a vapor-liquid-solid (VLS) process in molecular beam epitaxy (MBE) conditions. Recently, it was shown that Si can act as a donor when incorporated in a catalyst free vapor-solid (VS) growth process,[1] highlighting the importance of understanding the growth and dopant incorporation mechanisms to controlling conductivity and carrier type. Here we present evidence that Si stabilizes the nanowire facets necessary for nanowire elongation, leading to uniform doping in a core region and enhanced doping at the nanowire surface. The growth conditions used here produce crystalline clusters of GaAs on Si (111) substrates in the absence of a Si flux. The introduction of Si leads to a high yield of uniform vertical nanowires (Figure 1(b)). We used atom probe tomography (APT) to measure the Si distribution in three dimensions and resolve variations in doping across the diameter of the nanowire. The reconstructions in Figure 2(a) and Figure 3(a) show a portion of the GaAs nanowire and the AlGaAs passivation layer (the entire diameter is not within the field of view). The dopant concentration increases from the nanowire center to the edge and peaks near the interface between GaAs core and AlGaAs shell (Figure 2(b)), indicating that different incorporation rates may be associated with distinct facets whose prevalence is influenced by the Si dopant itself. Additionally, Si-enriched regions and clusters appear at the intersection of {110} facets of some nanowires (Figure 3(b)), which maybe be linked to variations in surface faceting. The above observations suggest that Si alters the surface energy of GaAs and stabilizes the facets that promote nanowire growth via preferential incorporation. Si has previously been observed to induce twin-defect mediated growth in GaAs,[2] which may also be related to modification of surface energies. Surface etching to remove Si enriched layers provides a pathway to uniformly doped n-type GaAs nanowires that are free of catalyst impurities.