Twofold origin of strain-induced bending in core–shell nanowires: the GaP/InGaP case

Abstract

Nanowires have emerged as a promising platform for the development of novel and high-quality heterostructures at large lattice misfit, inaccessible in a thin film configuration. However, despite core-shell nanowires allowing a very efficient elastic release of the misfit strain, the growth of highly uniform arrays of nanowire heterostructures still represents a challenge, for example due to a strain-induced bending morphology. Here we investigate the bending of wurtzite GaP/In Ga P core-shell nanowires using transmission electron microscopy and energy dispersive x-ray spectroscopy, both in terms of geometric and compositional asymmetry with respect to the longitudinal axis. We compare the experimental data with finite element method simulations in three dimensions, showing that both asymmetries are responsible for the actual bending. Such findings are valid for all lattice-mismatched core-shell nanowire heterostructures based on ternary alloys. Our work provides a quantitative understanding of the bending effect in general while also suggesting a strategy to minimise it.

Kinetic Control of Morphology and Composition in Ge/GeSn Core/Shell Nanowires

“…14,15 When the NW core diameter is smaller than the shell thickness, the core behaves as a compliant substrate, accommodating the lattice mismatch of the system and avoiding bending. 16,17 Moreover, the incorporation of Sn in Ge increases as the compressive strain is progressively relieved with increasing shell thickness, by a mechanism of compositional precursor (Ge/Sn ratio), as visible in Fig. 1b-d.…”

Strain engineering in Ge/GeSn core/shell nanowires

“…8,11,14,19 One-dimensional nanowires(NWs) provide additional degrees of freedom in tuning the effect of strain in the growth of lattice-mismatch heterostructures 24,25 when using a core/shell NW geometry. 26 The shell displays an increasing strain relaxation with thickness provided by the free surfaces at the sidewalls, while the elastic compliance of the NW core allows for enhanced strain relaxation in the shell, accommodating the lattice mismatch of the system and avoiding bending. 26,27 Recent studies on Ge/GeSn core/shell NWs 15,16,28,29 are mainly focused on small Ge core-sizes, where a low amount of residual strain is induced in the GeSn shell.In this Letter, we show how strain can be engineered by tuning the core and shell sizes and we explore high strain conditions focusing on large cores and high Sn contents.…”

“…26 The shell displays an increasing strain relaxation with thickness provided by the free surfaces at the sidewalls, while the elastic compliance of the NW core allows for enhanced strain relaxation in the shell, accommodating the lattice mismatch of the system and avoiding bending. 26,27 Recent studies on Ge/GeSn core/shell NWs 15,16,28,29 are mainly focused on small Ge core-sizes, where a low amount of residual strain is induced in the GeSn shell.In this Letter, we show how strain can be engineered by tuning the core and shell sizes and we explore high strain conditions focusing on large cores and high Sn contents. To this purpose, core diameters ranging from 50nm to 100nm are considered for the growth of the GeSn shell and the samples are analyzed using transmission electron microscopy(TEM) to assess the crystal quality 3 and the Sn incorporation.…”

Strain-Mediated Bending of InP Nanowires through the Growth of an Asymmetric InAs Shell

“…Despite several previous observations of unintentionally bent NWs [20,21,22,23,24], only the recent work by Lewis et al showed deliberate bent growth and its analysis [19]. Controllable strain-mediated bending requires directionality; therefore, bending in vapor-phase epitaxy systems, where a (relatively) uniform gaseous phase surrounds the NWs, is random [21,23,25]. Similarly, bent NWs grown in molecular beam epitaxy (MBE) systems where substrate is rotated show random directionality [24,26].…”

“…Similarly, bent NWs grown in molecular beam epitaxy (MBE) systems where substrate is rotated show random directionality [24,26]. The role of geometry, chemical composition, and the interplay between strain relaxation and surface energy in the growth of bent NWs have been examined [25,27]. Alternatively, molecular beam epitaxy system without rotation are directional and induce an inherent asymmetry directly related to bending [19,20].…”