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

Stemless InSb nanowire networks and nanoflakes grown on InP

Among the experimental realization of fault-tolerant topological circuits are interconnecting nanowires with minimal disorder. Out-of-plane indium antimonide (InSb) nanowire networks formed by merging are potential candidates. Yet, their growth requires a foreign material stem usually made of InP-InAs. This stem imposes limitations, which include restricting the size of the nanowire network, inducing disorder through grain boundaries and impurity incorporation. Here, we omit the stem allowing for the growth of stemless InSb nanowire networks on an InP substrate. To enable the growth without the stem, we show that a preconditioning step using arsine (AsH3) is required before InSb growth. High-yield of stemless nanowire growth is achieved by patterning the substrate with a selective-area mask with nanohole cavities, containing restricted gold droplets from which nanowires originate. Interestingly, these nanowires are bent, posing challenges for the synthesis of interconnecting nanowire networks due to merging failure. We attribute this bending to the non-homogeneous incorporation of arsenic impurities in the InSb nanowires and the interposed lattice-mismatch. By tuning the growth parameters, we can mitigate the bending, yielding large and single crystalline InSb nanowire networks and nanoflakes. The improved size and crystal quality of these nanostructures broaden the potential of this technique for fabricating advanced quantum devices.

Reconstruction of three-dimensional strain field in an asymmetrical curved core-shell hetero-nanowire

Crystal orientation and strain mapping of an individual curved and asymmetrical core-shell hetero-nanowire (NW) is performed based on transmission electron microscopy. It relies on a comprehensive analysis of scanning nanobeam electron diffraction data obtained for 1.3 nm electron probe size. The proposed approach also handles the problem of appearing twinning defects on diffraction patterns and allows for the investigation of materials with high defect densities. Based on the experimental maps and their comparison with finite element simulations, the entire core-shell geometry including full three-dimensional strain distribution within the curved core-shell NW are obtained. Our approach represents, therefore, a low-dose quasi-tomography of the strain field within a nanoobject using only a single zone axis diffraction experiment. Our approach is applicable also for electron beam-sensitive materials for which performing conventional tomography is a difficult task.

Exploiting flux shadowing for strain and bending engineering in core-shell nanowires

Here we report on the non-uniform shell growth of In_xGa_1-xAs on the GaAs nanowire (NW) core by molecular beam epitaxy (MBE). The growth was realized on pre-patterned silicon substrates with the pitch size (p) ranging from 0.1 μm to 10 μm. Considering the preferable bending direction with respect to the MBE cells as well as the layout of the substrate pattern, we were able to modify the strain distribution along the NW growth axis and the subsequent bending profile. For NW arrays with a high number density, the obtained bending profile of the NWs is composed of straight (barely-strained) and bent (strained) segments with different lengths which depend on the pitch size. A precise control of the bent and straight NW segment length provides a method to design NW based devices with length selective strain distribution.

Intrinsic Magnetic (EuIn)As Nanowire Shells with a Unique Crystal Structure

In the pursuit of magneto-electronic systems nonstoichiometric magnetic elements commonly introduce disorder and enhance magnetic scattering. We demonstrate the growth of (EuIn)As shells, with a unique crystal structure comprised of a dense net of Eu inversion planes, over InAs and InAs_1-xSb_x core nanowires. This is imaged with atomic and elemental resolution which reveal a prismatic configuration of the Eu planes. The results are supported by molecular dynamics simulations. Local magnetic and susceptibility mappings show magnetic response in all nanowires, while a subset bearing a DC signal points to ferromagnetic order. These provide a mechanism for enhancing Zeeman responses, operational at zero applied magnetic field. Such properties suggest that the obtained structures can serve as a preferred platform for time-reversal symmetry broken one-dimensional states including intrinsic topological superconductivity.

In situx-ray analysis of misfit strain and curvature of bent polytypic GaAs-InxGa1-xAs core-shell nanowires

Misfit strain in core-shell nanowires can be elastically released by nanowire bending in case of asymmetric shell growth around the nanowire core. In this work, we investigate the bending of GaAs nanowires during the asymmetric overgrowth by an In_xGa_1-xAs shell caused by avoiding substrate rotation. We observe that the nanowire bending direction depends on the nature of the substrate's oxide layer, demonstrated by Si substrates covered by native and thermal oxide layers. Further, we follow the bending evolution by time-resolvedin situx-ray diffraction measurements during the deposition of the asymmetric shell. The XRD measurements give insight into the temporal development of the strain as well as the bending evolution in the core-shell nanowire.

In situpassivation of GaxIn(1-x)P nanowires using radial AlyIn(1-y)P shells grown by MOVPE

Ga_xIn_(1-x)P nanowires with suitable bandgap (1.35-2.26 eV) ranging from the visible to near-infrared wavelength have great potential in optoelectronic applications. Due to the large surface-to-volume ratio of nanowires, the surface states become a pronounced factor affecting device performance. In this work, we performed a systematic study of Ga_xIn_(1-x)P nanowires' surface passivation, utilizing Al_yIn_(1-y)P shells grownin situby using a metal-organic vapor phase epitaxy system. Time-resolved photoinduced luminescence and time-resolved THz spectroscopy measurements were performed to study the nanowires' carrier recombination processes. Compared to the bare Ga_0.41In_0.59P nanowires without shells, the hole and electron lifetime of the nanowires with the Al_0.36In_0.64P shells are found to be larger by 40 and 1.1 times, respectively, demonstrating effective surface passivation of trap states. When shells with higher Al composition were grown, both lifetimes of free holes and electrons decreased prominently. We attribute the acceleration of PL decay to an increase in the trap states' density due to the formation of defects, including the polycrystalline and oxidized amorphous areas in these samples. Furthermore, in a separate set of samples, we varied the shell thickness. We observed that a certain shell thickness of approximately ∼20 nm is needed for efficient passivation of Ga_0.31In_0.69P nanowires. The photoconductivity of the sample with a shell thickness of 23 nm decays 10 times slower compared with that of the bare core nanowires. We concluded that both the hole and electron trapping and the overall charge recombination in Ga_xIn_(1-x)P nanowires can be substantially passivated through growing an Al_yIn_(1-y)P shell with appropriate Al composition and thickness. Therefore, we have developed an effectivein situsurface passivation of Ga_xIn_(1-x)P nanowires by use of Al_yIn_(1-y)P shells, paving the way to high-performance Ga_xIn_(1-x)P nanowires optoelectronic devices.

Assembling your nanowire: an overview of composition tuning in ternary III-V nanowires

The ability to grow defect-free nanowires in lattice-mismatched material systems and to design their properties has made them ideal candidates for applications in fields as diverse as nanophotonics, nanoelectronics and medicine. After studying nanostructures consisting of elemental and binary compound semiconductors, scientists turned their attention to more complex systems-ternary nanowires. Composition control is key in these nanostructures since it enables bandgap engineering. The use of different combinations of compounds and different growth methods has resulted in numerous investigations. The aim of this review is to present a survey of the material systems studied to date, and to give a brief overview of the issues tackled and the progress achieved in nanowire composition tuning. We focus on ternary III _x III_1-x V nanowires (AlGaAs, AlGaP, AlInP, InGaAs, GaInP and InGaSb) and IIIV _x V_1-x nanowires (InAsP, InAsSb, InPSb, GaAsP, GaAsSb and GaSbP).

Anomalous Photoelectrical Properties through Strain Engineering Based on a Single Bent InAsSb Nanowire

In this study, electrical and photoresponse properties of bent InAsSb nanowires (NWs) were investigated to explore the impact of bending strain on the photoelectrical properties. The corresponding morphological and structural observations demonstrate the phase segregation and strain in the core-shell zinc-blende-structured InAsSb NWs. It is found that the device made of bent InAsSb individual NW presents the switch from negative photoconductivity (NPC) and positive photoconductivity (PPC). The transformation between NPC and PPC can be achieved by not only gate voltage but also bias voltage, indicating the potential in the pervasive computing of bent InAsSb NWs. This work combines the semiconductor properties, light excitation, and piezoelectric effect of the InAsSb NWs, providing new ideas for next-generation photoelectrical nanodevices.

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

The growth of Sn-rich group-IV semiconductors at the nanoscale can enrich the understanding of the fundamental properties of metastable GeSn alloys. Here, we demonstrate the effect of the growth conditions on the morphology and composition of Ge/GeSn core/shell nanowires by correlating the experimental observations with a theoretical interpretation based on a multiscale approach. We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires changes from hexagonal to dodecagonal upon increasing the supply of the Sn precursor. This transformation strongly influences the Sn distribution as a higher Sn content is measured under the {112} growth front. Ab initio DFT calculations provide an atomic-scale explanation by showing that Sn incorporation is favored at the {112} surfaces, where the Ge bonds are tensile-strained. A phase-field continuum model was developed to reproduce the morphological transformation and the Sn distribution within the wire, shedding light on the complex growth mechanism and unveiling the relation between segregation and faceting. The tunability of the photoluminescence emission with the change in composition and morphology of the GeSn shell highlights the potential of the core/shell nanowire system for optoelectronic devices operating at mid-infrared wavelengths.

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

Controlling nanomaterial shape beyond its basic dimensionality is a concurrent challenge tackled by several growth and processing avenues. One of these is strain engineering of nanowires, implemented through the growth of asymmetrical heterostructures. Here, we report metal-organic molecular beam epitaxy of bent InP/InAs core/shell nanowires brought by precursor flow directionality in the growth chamber. We observe the increase of bending with decreased core diameter. We further analyze the composition of a single nanowire and show through supporting finite element simulations that strain accommodation following the lattice mismatch between InP and InAs dominates nanowire bending. The simulations show the interplay between material composition, shell thickness, and tapering in determining the bending. The simulation results are in good agreement with the experimental bending curvature, reproducing the radius of 4.3 µm (±10%), for the 2.3 µm long nanowire. The InP core of the bent heterostructure was found to be compressed at about 2%. This report provides evidence of shape control and strain engineering in nanostructures, specifically through the exchange of group-V materials in III-V nanowire growth.

Molecular beam epitaxial growth of GaAsSb/GaAsSbN/GaAlAs core-multishell nanowires for near-infrared applications

We report on the bandgap engineering of the GaAsSb/GaAsSbN heterostructured nanowires (NWs) in the core-shell architecture using the unique properties of dilute nitride material system for near-infrared photodetection. A high density of vertical GaAsSb/GaAsSb(N)/GaAlAs core-multishell configured NWs with well faceted, smooth surface morphology has been grown on Si (111) substrates using Ga-assisted molecular beam epitaxy. A low Sb content GaAsSb core has been shown to enable the coherently strained growth of dilute nitride shell with higher Sb content in GaAsSbN shell NWs. A systematic study of N and V/III beam equivalent pressure ratios is carried out to achieve the large band-gap reduction, while successfully incorporating higher Sb content in the dilute nitride shells (GaAs_1-x Sb _x N; x = 0.27). The incorporation of N acts to relieve strain and provide a smooth surface morphology as well as redshift the 4K photoluminescence (PL) peak energy by ∼160 meV in comparison to a non-nitride shell. The selected area diffraction pattern confirms zinc-blende structure in all the NWs and did not show any noticeable planar defects in dilute nitride NWs. We successfully, thus demonstrate GaAsSb/GaAsSbN/GaAlAs core-shell NWs by engineering the lattice strain of nitride shell with the non-nitride ternary core, for extending the 4K photoemission up to 1.43 μm.

Widely tunable GaAs bandgap via strain engineering in core/shell nanowires with large lattice mismatch

The realisation of photonic devices for different energy ranges demands materials with different bandgaps, sometimes even within the same device. The optimal solution in terms of integration, device performance and device economics would be a simple material system with widely tunable bandgap and compatible with the mainstream silicon technology. Here, we show that gallium arsenide nanowires grown epitaxially on silicon substrates exhibit a sizeable reduction of their bandgap by up to 40% when overgrown with lattice-mismatched indium gallium arsenide or indium aluminium arsenide shells. Specifically, we demonstrate that the gallium arsenide core sustains unusually large tensile strain with hydrostatic character and its magnitude can be engineered via the composition and the thickness of the shell. The resulted bandgap reduction renders gallium arsenide nanowires suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, with the additional possibility of monolithic integration in silicon-CMOS chips.