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.

Impact of Outer Shell Structure and Localization Effects on Charge Carrier Dynamics in GaAs/(In,Ga)As Nanowire Core–Shell Quantum Wells

“…Indeed, for similar samples from our lab differences in In content by a few percent have been observed to occur between both the different NW sidewall facets and different NWs for GaAs/(In,Ga)As core/shell NWs in a detailed transmission electron microscopy study . Additionally, X‐ray fluorescence measurements showed differences of the In content from different facets . The resulting potential fluctuations can explain differences in emission energy of several 10 meV.…”

“…Here, we expect that the presence of the AlAs shell next to the shell QW decreases its capture efficiency . Both samples show an additional shoulder at low energies, potentially indicating segments with strong alloy inhomogeneities or crystal phase quantum rings …”

X-ray diffraction reveals the amount of strain and homogeneity of extremely bent single nanowires

“…Around = 18 for NW1 and = 21 for NW2 a sudden change is detected in the RSM, which we associate with the top segment of the NW. This segment is likely to have a different chemical composition, since it might originate from axial wire growth during the shell growth, similarly found by AlHassan et al (2018). In Fig.…”

Beam damage of single semiconductor nanowires during X-ray nanobeam diffraction experiments

“…The development of X-ray optics and third-generation synchrotron radiation sources with high-brightness, nanofocused X-ray beams (Martínez-Criado et al, 2016;Leake et al, 2019) facilitates probing the structural parameters, spatial alloy distribution, crystal phases and strain distribution of single nanowires (NWs) by means of X-ray-based methods. These methods include nano-X-ray fluorescence (Al Hassan et al, 2018a), coherent Bragg ptychography (Hill et al, 2018) as well as coherent and non-coherent nanoprobe X-ray diffraction (nXRD) (Biermanns et al, 2013;Stankevič et al, 2015). For example, Al Hassan et al (2018b) demonstrated that the thicknesses of the core and shells within individual core-shell NWs, as well as the strain distribution, can be accessed in the as-grown geometry by recording reciprocal-space maps (RSMs) of in-plane Bragg reflections.…”