a, c, Ge (red) and Si (cyan) is shown to form a shell around the hexagonal Ga (green) and the As (blue) core. a, A three-dimensional volume reconstruction of part of a hex-GaAs/Ge core/shell nanowire with thicknesses of 35 nm/80 nm. For clarity, only a slab of 50 nm thickness of the entire 1.4-µm-long analysis is shown. c, A three-dimensional volume reconstruction of part of a hex-GaAs/Si 0.25 Ge 0.75 core/shell nanowire with thicknesses of 35 nm/46 nm. For clarity, only a slab of 40 nm thickness of the entire 1.1-µm-long analysis is shown. b, d, A plot of the atomic species concentration in the Ge (Si 0.25 Ge 0.75 ) shell in the rectangles in a (white) and c (yellow) as a function of the radial distance across the core/shell structure. Every data point in the plot represents a 2-nm slice taken along the entire length of the nanowire analyses excluding the cubic top part of the nanowire. Constant incorporation of As at a level of approximately 400 parts per million (b) (200 parts per million (d)) is observed in the entire shell while the Ga concentration quickly drops to a value close to the noise level of ~10 parts per million. e, A radial profile of the SiGe core/shell structure from the APT measurement integrated over a 1.0-µm length of the structure showing a Ge content of around 0.75 as shown in d. On the dotted rectangular volume of e, we carry out a nearest-neighbour analysis for Si atoms as previously used to evaluate random alloys of GeSn83,84. The nearest-neighbour analysis evaluates the distances between each pair of Si atoms and its first (to fourth) neighbours (NN1 to NN4). f, In b and d and f, the atomic concentration of the individual elements is plotted in the volume of the shaded regions indicated in a, c and e. A plot comparing the nearest-neighbour analysis on the measurement data to a randomized dataset. This gives us no indication of Si clustering and has been established as a reliable way to evaluate random alloys46.