Can antimonide-based nanowires form wurtzite crystal structure?

Diameter Control of GaSb Nanowires Revealed by In Situ Environmental Transmission Electron Microscopy

Several nanowire properties are strongly dependent on their diameter, which is notoriously difficult to control for III-Sb nanowires compared with other III-V nanowires. Herein environmental transmission electron microscopy is utilized to study the growth of Au nanoparticle seeded GaSb nanowires in situ. In this study, the real time changes to morphology and nanoparticle composition as a result of precursor V/III ratio are investigated. For a wide range of the growth parameters, it is observed that decreasing the V/III ratio increases the nanoparticle volume through Ga accumulation in the nanoparticle. The increase in nanoparticle volume in turn forces the nanowire diameter to expand. The effect of the V/III ratio on diameter allows the engineering of diameter modulated nanowires, where the modulation persisted after the growth. Lastly, this study demonstrates the observed trends can be reproduced in a conventional ex situ system, highlighting the transferability and importance of the results obtained in situ.

Iuliacumite: A Novel Chemical Short-Range Order in a Two-Dimensional Wurtzite Single Monolayer InAs1-xSbx Shell on InAs Nanowires

A chemical short-range order is found in single monolayer InAs_1-xSb_x shells, which inherit a wurtzite structure from the underlying InAs nanowire, instead of crystallizing in the energetically preferred zincblende structure. The chemical order is characterized by an anticorrelation ordering vector in the ⟨112̅0⟩ direction and arises from strong Sb-Sb repulsive interactions along the atomic chains in the ⟨112̅0⟩ direction.

Composition modulation by twinning in InAsSb nanowires

We observe a composition modulated axial heterostructure in zincblende (ZB) InAs_0.90Sb_0.10 nanowires initiated by pseudo-periodic twin boundaries using scanning tunneling microscopy. The twin boundaries exhibit four planes with reduced Sb concentration due to a lower Sb incorporation during lateral overgrowth of a 4H wurtzite as compared to a ZB stacking sequence. We anticipate that this leads to compositional band offsets in addition to known structural band offsets present between 4H and ZB polytypes, changing the band alignment from type II to type I.

Epitaxially grown III-arsenide-antimonide nanowires for optoelectronic applications

Epitaxially grown ternary III-arsenide-antimonide (III-As-Sb) nanowires (NWs) are increasingly attracting attention due to their feasibility as a platform for the integration of largely lattice-mismatched antimonide-based heterostructures while preserving the high crystal quality. This and the inherent bandgap tuning flexibility of III-As-Sb in the near- and mid-infrared wavelength regions are important and auspicious premises for a variety of optoelectronic applications. In this review, we summarize the current understanding of the nucleation, morphology-change and crystal phase evolution of GaAsSb and InAsSb NWs and their characterization, especially in relation to Sb incorporation during growth. By linking these findings to the optical properties in such ternary NWs and their heterostructures, a brief account of the ongoing development of III-As-Sb NW-based photodetectors and light emitters is also given.

Recent advances in III-Sb nanowires: from synthesis to applications

The excellent properties of III-V semiconductors make them intriguing candidates for next-generation electronics and optoelectronics. Their nanowire (NW) counterparts further provide interesting geometry and a quantum confinement effect which benefits various applications. Among the many members of all the III-V semiconductors, III-antimonide NWs have attracted significant research interest due to their narrow, direct bandgap and high carrier mobility. However, due to the difficulty of NW fabrication, the development of III-antimonide NWs and their corresponding applications are always a step behind the other III-V semiconductors. Until recent years, because of advances in understanding and fabrication techniques, electronic and optoelectronic devices based on III-antimonide NWs with novel performance have been fabricated. In this review, we will focus on the development of the synthesis of III-antimonide NWs using different techniques and strategies for fine-tuning the crystal structure and composition as well as fabricating their corresponding heterostructures. With such development, the recent progress in the applications of III-antimonide NWs in electronics and optoelectronics is also surveyed. All these discussions provide valuable guidelines for the design of III-antimonide NWs for next-generation device utilization.

Sb Incorporation in Wurtzite and Zinc Blende InAs1-x Sbx Branches on InAs Template Nanowires

The physical properties of material largely depend on their crystal structure. Nanowire growth is an important method for attaining metastable crystal structures in III-V semiconductors, giving access to advantageous electronic and surface properties. Antimonides are an exception, as growing metastable wurtzite structure has proven to be challenging. As a result, the properties of these materials remain unknown. One promising means of accessing wurtzite antimonides is to use a wurtzite template to facilitate their growth. Here, a template technique using branched nanowire growth for realizing wurtzite antimonide material is demonstrated. On wurtzite InAs trunks, InAs_1-x Sb_x branch nanowires at different Sb vapor phase compositions are grown. For comparison, branches on zinc blende nanowire trunks are also grown under identical conditions. Studying the crystal structure and the material composition of the grown branches at different x_v shows that the Sb incorporation is higher in zinc blende than in wurtzite. Branches grown on wurtzite trunks are usually correlated with stacking defects in the trunk, leading to the emergence of a zinc blende segment of higher Sb content growing parallel to the wurtzite structure within a branch. However, the average amount of Sb incorporated within the branch is determined by the vapor phase composition.

Self-Catalyzed Growth of Vertical GaSb Nanowires on InAs Stems by Metal-Organic Chemical Vapor Deposition

We report the first self-catalyzed growth of high-quality GaSb nanowires on InAs stems using metal-organic chemical vapor deposition (MOCVD) on Si (111) substrates. To achieve the growth of vertical InAs/GaSb heterostructure nanowires, the two-step flow rates of the trimethylgallium (TMGa) and trimethylantimony (TMSb) are used. We first use relatively low TMGa and TMSb flow rates to preserve the Ga droplets on the thin InAs stems. Then, the flow rates of TMGa and TMSb are increased to enhance the axial growth rate. Because of the slower radial growth rate of GaSb at higher growth temperature, GaSb nanowires grown at 500 °C exhibit larger diameters than those grown at 520 °C. However, with respect to the axial growth, due to the Gibbs-Thomson effect and the reduction in the droplet supersaturation with increasing growth temperature, GaSb nanowires grown at 500 °C are longer than those grown at 520 °C. Detailed transmission electron microscopy (TEM) analyses reveal that the GaSb nanowires have a perfect zinc-blende (ZB) crystal structure. The growth method presented here may be suitable for other antimonide nanowire growth, and the axial InAs/GaSb heterostructure nanowires may have strong potential for use in the fabrication of novel nanowire-based devices and in the study of fundamental quantum physics.

Wurtzite ZnTe Nanotrees and Nanowires on Fluorine-Doped Tin Oxide Glass Substrates

ZnTe nanotrees and nanowires were grown on fluorine-doped tin oxide glass by physical vapor transport. Sn from a fluorine-doped tin oxide layer catalyzed the growth at a growth temperature of 320 °C. Both the stem and branch nanowires grew along ⟨0001⟩ in the rarely observed wurtzite structure. SnTe nanostructures were formed in the liquid catalyst and simultaneously ZnTe nanowire grew under Te-limited conditions, which made the formation of the wurtzite structure energetically favorable. Through polarization-dependent and power-dependent microphotoluminescence measurements from individual wurtzite nanowires at room temperature, we could determine the so far unknown fundamental bandgap of wurtzite ZnTe, which was 2.297 eV and thus 37 meV higher than that of zinc-blend ZnTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands is estimated to be 69 meV.

Surface effects of vapour-liquid-solid driven Bi surface droplets formed during molecular-beam-epitaxy of GaAsBi

Herein we investigate a (001)-oriented GaAs_1−xBi_x/GaAs structure possessing Bi surface droplets capable of catalysing the formation of nanostructures during Bi-rich growth, through the vapour-liquid-solid mechanism. Specifically, self-aligned “nanotracks” are found to exist trailing the Bi droplets on the sample surface. Through cross-sectional high-resolution transmission electron microscopy the nanotracks are revealed to in fact be elevated above surface by the formation of a subsurface planar nanowire, a structure initiated mid-way through the molecular-beam-epitaxy growth and embedded into the epilayer, via epitaxial overgrowth. Electron microscopy studies also yield the morphological, structural, and chemical properties of the nanostructures. Through a combination of Bi determination methods the compositional profile of the film is shown to be graded and inhomogeneous. Furthermore, the coherent and pure zincblende phase property of the film is detailed. Optical characterisation of features on the sample surface is carried out using polarised micro-Raman and micro-photoluminescence spectroscopies. The important light producing properties of the surface nanostructures are investigated through pump intensity-dependent micro-PL measurements, whereby relatively large local inhomogeneities are revealed to exist on the epitaxial surface for important optical parameters. We conclude that such surface effects must be considered when designing and fabricating optical devices based on GaAsBi alloys.

Demonstration of Sn-seeded GaSb homo- and GaAs-GaSb heterostructural nanowires

The particle-assisted epitaxial growth of antimonide-based nanowires has mainly been realized using gold as the seed material. However, the Au-seeded epitaxial growth of antimonide-based nanowires such as GaSb nanowires presents several challenges such as for example direct nucleation issues and crystal structure tuning. Therefore, it is of great importance to understand the role of seed material choice and properties in the growth behavior of antimonide-based nanowires to obtain a deeper understanding and a better control on their formation processes. In this report, we have investigated the epitaxial growth of GaSb and GaAs-GaSb nanowires using in situ-formed tin seeds by means of metalorganic vapor phase epitaxy technique. This comprehensive report covers the growth of in situ-formed tin seeds and Sn-seeded GaSb nanowires on both GaAs and GaSb (111)B substrates, as well as GaAs-GaSb nanowires on GaAs (111)B substrates. The growth behavior and structural properties of the obtained GaSb nanowires are further investigated and compared with the Au-seeded counterparts. The results provided by this study demonstrate that Sn is a promising seed material for the growth of GaSb nanowires.