The flexibility and quasi-one-dimensional nature of nanowires offer wide-ranging possibilities for novel heterostructure design and strain engineering. In this work, we realize arrays of extremely and controllably bent nanowires comprising lattice-mismatched and highly asymmetric core-shell heterostructures. Strain sharing across the nanowire heterostructures is sufficient to bend vertical nanowires over backward to contact either neighboring nanowires or the substrate itself, presenting new possibilities for designing nanowire networks and interconnects. Photoluminescence spectroscopy on bent-nanowire heterostructures reveals that spatially varying strain fields induce charge carrier drift toward the tensile-strained outside of the nanowires, and that the polarization response of absorbed and emitted light is controlled by the bending direction. This unconventional strain field is employed for light emission by placing an active region of quantum dots at the outer side of a bent nanowire to exploit the carrier drift and tensile strain. These results demonstrate how bending in nanoheterostructures opens up new degrees of freedom for strain and device engineering.

Nanoscale substrates such as nanowires allow heterostructure design to venture well beyond the narrow lattice mismatch range restricting planar heterostructures, owing to misfit strain relaxing at the free surfaces and partitioning throughout the entire nanostructure. In this work, we uncover a novel strain relaxation process in GaAs/InGaAs core-shell nanowires that is a direct result of the nanofaceted nature of these nanostructures. Above a critical lattice mismatch, plastically relaxed mounds form at the edges of the nanowire sidewall facets. The relaxed mounds and a coherent shell grow simultaneously from the beginning of the deposition with higher lattice mismatches increasingly favoring incoherent mound growth. This is in stark contrast to Stranski-Krastanov growth, where above a critical thickness coherent layer growth no longer occurs. This study highlights how understanding strain relaxation in lattice mismatched nanofaceted heterostructures is essential for designing devices based on these nanostructures.

For the Ga-assisted growth of GaAs nanowires on Si(111) substrates by molecular beam epitaxy, growth temperature, As flux, and Ga flux have been systematically varied across the entire window of growth conditions that result in the formation of nanowires. A range of GaAs structures was observed, progressing from pure Ga droplets under negligible As flux through horizontal nanowires, tilted nanowires, vertical nanowires, and nanowires without droplets to crystallites as the As flux was increased. Quantitative analysis of the resulting sample morphology was performed in terms of nanowire number and volume density, number yield and volume yield of vertical nanowires, diameter, length, as well as the number and volume density of parasitic growth. The result is a growth map that comprehensively describes all nanowire and parasitic growth morphologies and hence enables growth of nanowire samples in a predictive manner. Further analysis indicates the combination of global Ga flux and growth temperature determines the total density of all objects, whereas the global As/Ga flux ratio independently determines the resultant sample morphology. Several dependencies observed here imply that all objects present on the substrate surface, i.e. both nanowires and parasitic structures, originate from Ga droplets.

We present a novel two-step approach for the selective area growth (SAG) of GaAs nanowires (NWs) by molecular beam epitaxy which has enabled a detailed exploration of the NW diameter evolution. In the first step, the growth parameters are optimized for the nucleation of vertically-oriented NWs. In the second step, the growth parameters are chosen to optimize the NW shape, allowing NWs with a thin diameter (45 nm) and an untapered morphology to be realized. This result is in contrast to the commonly observed thick, inversely tapered shape of SAG NWs. We quantify the flux dependence of radial vapour-solid (VS) growth and build a model that takes into account diffusion on the NW sidewalls to explain the observed VS growth rates. Combining this model for the radial VS growth with an existing model for the droplet dynamics at the NW top, we achieve full understanding of the diameter of NWs over their entire length and the evolution of the diameter and tapering during growth. We conclude that only the combination of droplet dynamics and VS growth results in an untapered morphology. This result enables NW shape engineering and has important implications for doping of NWs.

Herein, the charge carrier dynamics in GaAs/(In,Ga)As/(Al,Ga)As core–shell nanowires with different outer shell structures are studied. Localization of charge carriers in the minima of potential fluctuations is shown to govern the recombination processes at low temperatures. At higher temperatures, thermionic emission of carriers from the (In,Ga)As shell quantum well leads to a decrease of the luminescence efficiency for a sample with a GaAs outer shell. This effect can be reduced by introducing an AlAs barrier shell. However, the interfaces to this barrier act as an additional source for nonradiative recombination. These results will help in achieving high luminescence intensities at room temperature of light‐emitting devices based on nanowire shell quantum wells.

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates.arXiv:1708.02454v1 [cond-mat.mtrl-sci]

Surface energies play a dominant role in the self-assembly of three dimensional (3D) nanostructures. In this letter, we show that using surfactants to modify surface energies can provide a means to externally control nanostructure self-assembly, enabling the synthesis of novel hierarchical nanostructures. We explore Bi as a surfactant in the growth of InAs on the {110} sidewall facets of GaAs nanowires. The presence of surface Bi induces the formation of InAs 3D islands by a process resembling the StranskiKrastanov mechanism, which does not occur in the absence of Bi on these surfaces. The InAs 3D islands nucleate at the corners of the {110} facets above a critical shell thickness and then elongate along 110 directions in the plane of the nanowire sidewalls. Exploiting this growth mechanism, we realize a series of novel hierarchical nanostructures, ranging from InAs quantum dots on single {110} nanowire facets to zig-zag shaped nanorings completely encircling nanowire cores. Photoluminescence spectroscopy and cathodoluminescence spectral line scans reveal that small surfactant-induced InAs 3D islands behave as optically active quantum dots. This work illustrates how surfactants can provide an unprecedented level of external control over nanostructure self-assembly. KEYWORDS: nanowire, quantum dot, bismuth, surfactant, GaAs, semiconductor 2The bottom-up self-assembly of semiconductor nanostructures is directed by energy minimization. As a result, many aspects of nanostructure self-assembly are intrinsic and therefore difficult to control externally. For example, nanostructures such as nanowires (NWs) and quantum dots (QDs) spontaneously form low-energy facets during their synthesis. In the case of GaAs NWs grown by the Ga-assisted vapor-liquid-solid (VLS) mode, the sidewall facets are of {110} orientation. 1 This poses challenges for the realization of advanced hierarchical structures, such as QDs embedded within NWs, 2,3 since two-dimensional (2D) layer growth is always favored on GaAs{110} surfaces and three-dimensional (3D) islands do not form by the StranskiKrastanov (SK) mechanism. 2,4-6 The SK growth of InAs 3D islands on these surfaces has been observed after covering the facets with a thin AlAs layer, but, this is undesirable for most applications. 2,7 In contrast, GaAs NWs synthesized by Au-catalyzed growth typically exhibit {112} sidewall facets and the SK mechanism does occur on these surfaces, 6,8 however, the presence of Au can be detrimental to NW optoelectronic properties. 9 We recently reported that the presence of surface Bi can induce the self-assembly of InAs 3D islands directly on planar GaAs(110) surfaces. 10 The Bi surfactant was shown to modify the surface energies, reducing the energetic cost of 3D island formation. Furthermore, in contrast to more common surface-segregating elements like Sb and Te, which have been shown to reduce adatom diffusion and consequently inhibit the formation of 3D islands, 11,12 Bi has been found to increase adatom diffusion. 13 This makes Bi of particular i...

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