Determination of indium content of GaAs/(In,Ga)As/(GaAs) core-shell(-shell) nanowires by x-ray diffraction and nano x-ray fluorescence

Abstract

We present two complementary approaches to investigate the In content in GaAs/(In,Ga)As/(GaAs) core-shell-(shell) nanowire (NW) heterostructures using synchrotron radiation. The key advantage of our methodology is that NWs are characterized in their as-grown configuration, i.e., perpendicularly standing on a substrate. First, we determine the mean In content of the (In,Ga)As shell by high-resolution x-ray diffraction (XRD) from NW ensembles. In particular, we disentangle the influence of In content and shell thickness on XRD by measuring and analyzing two reflections with diffraction vector parallel and perpendicular to the growth axis, respectively. Second, we study the In distribution within individual NWs by nano x-ray fluorescence. Both the NW (111) basal plane, that is parallel to the surface of the substrate, and the {10-1} sidewall plane were scanned with an incident nanobeam of 50 nm width. We investigate three samples with different nominal In content of the (In,Ga)As shell. In all samples, the average In content of the shell determined by XRD is in good agreement with the nominal value. For a nominal In content of 15%, the In distribution is fairly uniform between all six sidewall facets. In contrast, in NWs with nominally 25% In content, different sidewall facets of the same NW exhibit different In contents. This effect is attributed to shadowing during growth by molecular beam epitaxy. At the same time, along the NW axis the In distribution is still fairly homogeneous. In NWs with 60% nominal In content and no outer GaAs shell, the In content varies significantly both between different sidewall facets and along the NW axis. This fluctuation is explained by the formation of (In,Ga)As mounds that grow simultaneously with a thinner (In,Ga)As shell. The methodology presented here may be applied also to other core-shell NWs with a ternary shell and paves the way to correlating NW structure with functional properties that depend on the as-grown configuration of the NWs.