Transient nanowire bending during the growth of core/shell heterostructures with large lattice mismatch

The elastic accommodation of lattice mismatch in core/shell nanowires presents unique features because both the shell and the core can be strained depending on their volume ratio. This allows the realization of heterostructure combinations with large lattice mismatch, as well as the extensive strain-mediated tailoring of their electronic properties [1]. Owing to the large stresses involved, any shell thickness asymmetry around the core may cause the bending of the nanowires, forming shapes like circular arcs, which can be a desired effect or not, depending on the targeted application.
Here, we monitor the asymmetric shell growth, the asymmetric strain, and the resulting bending during the growth of GaAs/In0.4Al0.6As core/shell nanowires [2]. We show how the profile of the elemental beams in a conventional MBE system accounts for the asymmetric growth rate of the shell around the core, in spite of the substrate rotation during growth. The resulting asymmetric stresses around the core cause the bending of the nanowires to the side of the thinner shell, which, in turn, enhances further the growth rate and stress asymmetry, and thus the bending itself, in a self-feeding circle. Nevertheless, after a critical minimum shell thickness on all core sides has been reached, the bending is reversed and the nanowires straighten up. Analyzing cross sections normal to the axis of such straightened up nanowires by STEM, we developed a method to reconstruct, for the first time, the evolution of the shell thickness on every core side and the corresponding bending angle during growth. Furthermore, geometric phase analysis revealed the spatial distribution of strain in the core and the shell with respect to the uneven shell thicknesses around the core. Our results shine a light on the growth of lattice-mismatched core/shell nanowires and could serve as a guide to design deliberate curved structures or, instead, strategies to suppress the bending.

Figure 1. (a) Side-view SEM image of as-grown core/shell nanowires on a Si substrate. Inset: HAADF-STEM image of a cross-sectional TEM lamella. (b) Evolution of the shell thickness on two opposite core sides (thinnest vs. thickest shell) during growth and the corresponding bending angle. (c) Schematic representation of the transient nanowire bending.

[1] L. Balaghi et al., Nature Commun 10, 2793 (2019); Nature Commun 12, 6642 (2021).
[2] D. Hilliard et al., in preparation.