Transition from elastic to plastic strain release in core−shell nanowires revealed by in-plane x-ray diffraction

We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding InxGa$_{1-x}$As shell in core–shell nanowire heterostructures grown on Si(111) substrates. The dimensions of the core and shell are kept constant whereas the indium concentration inside the shell is varied. Measuring the $22\bar{4}$ and $2\bar{2}0$ in-plane Bragg reflections normal to the nanowire side edges and side facets, we observe a transition from elastic to plastic strain release for a shell indium content x > 0.5. Above the onset of plastic strain relaxation, indium rich mounds and an indium poor coherent shell grow simultaneously around the GaAs core. Mound formation was observed for indium contents x = 0.5 and 0.6 by scanning electron microscopy. Considering both the measured radial reflections and the axial 111 Bragg reflection, the 3D strain variation was extracted separately for the core and the InxGa$_{1-x}$As shell.