Abstract
We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding In x Ga1−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 and 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 In x Ga1−x As shell.
Highlights
Compared to planar heteroepitaxy, the formation of axial or radial heterostructures in the form of nanowires has opened up new horizons for the design of heterostructures in a more efficient and less costly way [1,2,3,4,5]
We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding InxGa1−xAs shell in core–shell nanowire heterostructures grown on Si(111) substrates
The formation of mounds is observed for indium concentrations of 50% and above by scanning electron microscopy (SEM). We extend on these findings by calculating the 3D strain of the GaAs core and InxGa1−xAs shell at the edges and side-facets, revealing a strain variation of the core which reaches its maximum for nanowires with 50% of indium in the InxGa1−xAs shell
Summary
The formation of axial or radial heterostructures in the form of nanowires has opened up new horizons for the design of heterostructures in a more efficient and less costly way [1,2,3,4,5]. The InxGa1−xAs shell of the nanowires in sample 1 is compressed as the red peak is centered at a higher Qz224 ̄ value compared to the unstrained position assuming the nominal In content, which is indicated by a vertical cut line. The red data points of samples 2–4 being centered at positive e2z2 ̄0 values (figure 5(d)) compared to e2z24 ̄ reveal that the 22 ̄0 lattice planes of the InxGa1−xAs shell undergo higher expansion compared to the 224 ̄ lattice planes resulting from the compression along [111]. For the nanowire model with 30% nominal indium concentration, the inner volume of the GaAs core undergoes a lattice expansion of 0.15% along [22 ̄0] and a maximum lattice compression of −0.20% at the core–shell interface, forming a strain variation of 0.35% This is in agreement with the strain variation deduced experimentally from the difference between the green and blue Gaussian peaks of 0.41%. This experimental value might be overestimated because of normalization of the GaAs peak (see above)
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