Zinc-blende Nanowires Research Articles

Phase Control Growth of InAs Nanowires by Using Bi Surfactant

To realize practical applications of nanowire-based devices, it is critical, yet challenging, to control crystal structure growth of III-V semiconductor nanowires. Here, we demonstrate that controlled wurtzite and zincblende phases of InAs nanowires can be fabricated using bismuth (Bi) as a surfactant. For this purpose, catalyst free selective area epitaxial growth of InAs nanowires was performed using molecular beam epitaxy (MBE). During the growth, Bi was used which may act as a wetting agent influencing the surface energy at growth plane ends, promoting wurtzite crystal phase growth. For a demonstration, wurtzite and zincblende InAs nanowires were obtained with and without using Bi-flux. Photoluminescence spectroscopy (PL) analysis of the nanowires indicates a strong correlation between wurtzite phase and the Bi-flux. It is observed that the bandgap energy of wurtzite and zincblende nanowires are ∼0.50 eV and ∼0.42 eV, respectively, and agree well with theoretical estimated bandgap of corresponding InAs crystal phases. A blue shift in PL emission peak energy was found with decreasing nanowire diameter. The controlled wurtzite and zincblende crystal phase and its associated heterostructure growth of InAs nanowires on Si may open up new opportunities in bandgap engineering and related device applications integrated on Si. Furthermore, this work also illustrates that Bi as a surfactant could play a dynamic role in the growth mechanism of III-V compound semiconductors.

Work function of polytypic gallium phosphide nanowires

In this work we investigate the work function of gallium phosphide nanowires by the means of frequency-modulated Kelvin probe force microscopy. Polytypic wurtzite/zinc blende nanowires were synthesized via self-catalytic molecular beam epitaxy. Mixed crystal phase was achieved by controlling the catalytic droplet contact angle and confirmed via transmission electron microscopy and Raman spectroscopy. Kelvin probe study showed a contrast between the work function of (110) zinc blende and (1120) wurtzite gallium phosphide: ϕZB = 4.28 eV and ϕWZ = 4.2 eV. Also, it was shown that sub-monolayer arsenic shell increases the work function up to 4.75 eV. Thus, two mechanisms for work function adjustment in the range 4.2-4.75 eV are shown. The results are important for optimization of Schottky barriers in nanowire-based devices.

Analytical description of nanowires. I. Regular cross sections for zincblende and diamond structures. Corrigendum

In the paper by König & Smith [Acta Cryst. (2019), B75, 788–802], several equations had minor errors in their coefficients defining characteristic lengths and area of zinc-blende nanowire cross sections, thereby deviating from the exact analytical solution by 1.4 ± 0.8 %. A fully corrected version of the paper is provided.

Atomistic investigation on the mechanical properties and failure behavior of zinc-blende cadmium selenide (CdSe) nanowire

Understanding the mechanical properties of cadmium selenium (CdSe) nanowire has received intense research interest due to its versatile application in semiconductor industries. We investigated the tensile mechanical properties and fracture mechanism of zinc-blende CdSe nanowire (NW) through molecular dynamics simulations by considering the effects of temperature variation, NW size, different loading directions, vacancy defects, and strain rates. We found that both ultimate tensile strength (UTS) and Young’s modulus (YM) linearly decrease with increasing temperature but increase with nanowire cross-sectional area. The [111] -directed CdSe NW shows the largest UTS, YM, and fracture toughness whereas, the lowest values are obtained for [100] direction. The largest failure strain is exhibited by [110] direction. The UTS is found more sensitive in the presence of Cd vacancy, while the YM is more sensitive to the removal of Se atoms. We noticed that at 100 K, the [100] -directed CdSe NW fails along {111} cleavage plane, however, at 600 K, both {111} and {100} cleavage planes activate and cause fracture at a lower strain value. Finally, both the fracture strength and strain increase with the increment in strain rates. The mechanical properties and fracture characteristics of the CdSe NW elucidated in this study will be a guide to design and fabricate CdSe-based optoelectronic and electronic devices.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.

High performance transistors and photodetectors based on self-catalyzed zinc-blende InP nanowires

InP nanowires are an important material for nanoscale electronic and optical devices. However, the crystal phase mixing and stacking faults severely degrade the device's performance. Here, we demonstrate high performance field-effect transistors and photodetectors based on high-quality InP nanowires. The ⟨110⟩-oriented InP nanowires, which are formed by spontaneous kinking from the original ⟨111⟩ nanowire roots, exhibit a stacking-faults-free zinc blende crystal structure. Based on the high-quality nanowire, the field-effect transistor exhibits a high electron mobility of 1438 cm2 V−1 s−1, much higher than that with stacking faults. A high responsivity of 5495 A/W is obtained from the photodetector, among the highest values reported for InP nanowire photodetectors. Moreover, the response/recovery time of the device is only 0.6/2.2 ms, 2–3 orders of magnitude shorter than other InP nanowire photodetectors. The high crystal quality and excellent electrical and optical properties make the zinc blende ⟨110⟩ InP nanowire a promising candidate for high performance electronic and optoelectronic devices.

Simultaneous Growth of Pure Wurtzite and Zinc Blende Nanowires.

The opportunity to engineer III-V nanowires in wurtzite and zinc blende crystal structure allows for exploring properties not conventionally available in the bulk form as well as opening the opportunity for use of additional degrees of freedom in device fabrication. However, the fundamental understanding of the nature of polytypism in III-V nanowire growth is still lacking key ingredients to be able to connect the results of modeling and experiments. Here we show InP nanowires of both pure wurtzite and pure zinc blende grown simultaneously on the same InP [100]-oriented substrate. We find wurtzite nanowires to grow along [Formula: see text] and zinc blende counterparts along [Formula: see text]. Further, we discuss the nucleation, growth, and polytypism of our nanowires against the background of existing theory. Our results demonstrate, first, that the crystal growth conditions for wurtzite and zinc blende nanowire growth are not mutually exclusive and, second, that the interface energies predominantly determine the crystal structure of the nanowires.

Electron Tomography Reveals the Droplet Covered Surface Structure of Nanowires Grown by Aerotaxy.

For the purpose of functionalizing III-V semiconductor nanowires using n-doping, Sn-doped GaAs zincblende nanowires are produced, using the growth method of Aerotaxy. The growth conditions used are such that Ga droplets, formed on the nanowire surface, increase in number and concentrations when the Sn-precursor concentration is increased. Droplet-covered wires grown with varying Sn concentrations are analyzed by transmission electron microscopy and electron tomography, which together establish the positioning of the droplets to be preferentially on {-111}B facets. These facets have the same polarity as the main wire growth direction, [-1-1-1]B. This means that the generated Ga particles can form nucleation sites for possible nanowire branch growth. The concept of azimuthal mapping is introduced as a useful tool for nanowire surface visualization and evaluation. It is demonstrated here that electron tomography is useful in revealing both the surface and internal morphologies of the nanowires, opening up for applications in the analysis of more structurally complicated systems like radially asymmetrical nanowires. The analysis also gives a further understanding of the limits of the dopants which can be used for Aerotaxy nanowires.

Spin-orbit coupling effects in zinc-blende InSb and wurtzite InAs nanowires: Realistic calculations with multiband k·p method

A systematic numerical investigation of spin-orbit fields in the conduction bands of III-V semiconductor nanowires is performed. Zinc-blende InSb nanowires are considered along [001], [011], and [111] directions, while wurtzite InAs nanowires are studied along [0001] and [10$\overline{1}$0] or [11$\overline{2}$0] directions. Realistic multiband $\vec{k} \cdot \vec{p}\,$ Hamiltonians are solved by using plane-wave expansions of real-space parameters. In all cases the linear and cubic spin-orbit coupling parameters are extracted for nanowire widths from 30 to 100 nm. Typical spin-orbit energies are on the $\mu$eV scale, except for InAs wurtzite nanowires grown along [10$\overline{1}$0] or [11$\overline{2}$0], in which the spin-orbit energy is about meV, largely independent of the wire diameter. Significant spin-orbit coupling is obtained by applying a transverse electric field, causing the Rashba effect. For an electric field of about 4 mV/nm the obtained spin-orbit energies are about 1 meV for both materials in all investigated growth directions. The most favorable system, in which the spin-orbit effects are maximal, are InAs WZ nanowires grown along [1010] or [11$\overline{2}$0], since here spin-orbit energies are giant (meV) already in the absence of electric field. The least favorable are InAs WZ nanowires grown along [0001], since here even the electric field does not increase the spin-orbit energies beyond 0.1 meV. The presented results should be useful for investigations of optical orientation, spin transport, weak localization, and superconducting proximity effects in semiconductor nanowires.

The electronic and optical properties of Cs adsorbed GaAs nanowires via first-principles study

In this study, we investigate the Cs adsorption mechanism on (110) surface of zinc-blende GaAs nanowire. The adsorption energy, work function, dipole moment, geometric structure, Mulliken charge distribution, charge transfer index, band structures, density of state and optical properties of Cs adsorption structures are calculated utilizing first-principles method based on density function theory. Total-energy calculations show that all the adsorption energies are negative, indicating that Cs adsorption process is exothermic and Cs covered GaAs nanowires are stable. The work function of nanowire surface has an obvious decrease after Cs adsorption. Besides, the ionization of nanowire surface is enhanced as well. More importantly, Cs adsorption contributes to a lower side shift of bands near Fermi level, and the corresponding band gap disappears. Additionally, the absorption peak and energy loss function after Cs adsorption are far higher than those before adsorption, implying better light absorption characteristic of nanowire surface after Cs adsorption. These theoretical calculations can directly guide the Cs activation experiment for negative electron affinity GaAs nanowire, and also lay a foundation for the further study of Cs/O co-adsorption on the nanowire surface.

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, InAs1-x Sbx 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 xv 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.

CdS nanowires formed by chemical synthesis using conjugated single-stranded DNA molecules

CdS nanowires were successfully grown by chemical synthesis using two conjugated single-stranded (ss) DNA molecules, poly G (30) and poly C (30), as templates. During the early stage of the synthesis with the DNA molecules, the Cd 2+ interacts with Poly G and Poly C and produces the (Cd 2+)–Poly GC complex. As the growth proceeds, it results in nanowires. The structural analysis by grazing angle x-ray diffraction and transmission electron microscopy confirmed the zinc-blende CdS nanowires with the growth direction of <220>. Although the nanowires are well surface-passivated with the DNA molecules, the photoluminescence quenching was caused by the electron transfer from the nanowires to the DNA molecules. The quenching can be used to detect and label the DNAs.

Modeling the electronic properties of GaAs polytype nanostructures: Impact of strain on the conduction band character

We study the electronic properties of GaAs nanowires composed of both the zincblende and wurtzite modifications using a ten-band k.p model. In the wurtzite phase, two energetically close conduction bands are of importance for the confinement and the energy levels of the electron ground state. These bands form two intersecting potential landscapes for electrons in zincblende/wurtzite nanostructures. The energy difference between the two bands depends sensitively on strain, such that even small strains can reverse the energy ordering of the two bands. This reversal may already be induced by the non-negligible lattice mismatch between the two crystal phases in polytype GaAs nanostructures, a fact that was ignored in previous studies of these structures. We present a systematic study of the influence of intrinsic and extrinsic strain on the electron ground state for both purely zincblende and wurtzite nanowires as well as for polytype superlattices. The coexistence of the two conduction bands and their opposite strain dependence results in complex electronic and optical properties of GaAs polytype nanostructures. In particular, both the energy and the polarization of the lowest intersubband transition depends on the relative fraction of the two crystal phases in the nanowire.

Addressing the electronic properties of III–V nanowires by photoluminescence excitation spectroscopy**This article belongs to the special issue: Emerging Leaders, which features invited work from the best early-career researchers working within the scope of J. Phys. D. This project is part of the Journal of Physics’ series 50th anniversary celebrations in 2017. Marta De Luca was selected by the Editorial Board

Semiconductor nanowires (NWs) have been attracting an increasing interest in the scientific community. This is due to their peculiar filamentary shape and nanoscale diameter, which renders them versatile and cost-effective components of novel technological devices and also makes them an ideal platform for the investigation of a variety of fascinating physical effects. Absorption spectroscopy is a powerful and non-destructive technique able to provide information on the physical properties of the NWs. However, standard absorption spectroscopy is hard to perform in NWs, because of their small volume and the presence of opaque substrates. Here, we demonstrate that absorption can be successfully replaced by photoluminescence excitation (PLE). First, the use of polarization-resolved PLE to address the complex and highly-debated electronic band structure of wurtzite GaAs and InP NWs is shown. Then, PLE is used as a statistically-relevant method to localize the presence of separate wurtzite and zincblende NWs in the same InP sample. Finally, a variety of resonant exotic effects in the density of states of InxGa1−xAs/GaAs core/shell NWs are highlighted by high-resolution PLE.

Electronic structure of Mn-doped InAs nanowires in the magnetic field

An eight-band k→·p→ effective-mass Hamiltonian for narrow-gap zinc-blende semiconductor nanowires in the magnetic field is derived, and then the electronic structures of Mn-doped InAs nanowires are calculated via the derived Hamiltonian. Through the calculation, it is found that the valence band structures of Mn-doped InAs nanowires can be changed greatly when R increases from 3 nm to 10 nm, and the energy interval between two neighbouring valence subbands will become narrower as the increase of the radius R. Meanwhile, we find the ground hole state of Mn-doped InAs nanowires is h0−1/2 in the magnetic field, and the valence band structures will present strong band-crossings because of the behaviors of two special hole states h0−3/2 and h03/2. Additionally, the split energies of the degenerate hole states will saturate at a certain temperature when the temperature T increases to 300 K, while the split energies can up to dozens of meV linearly when the concentration of manganese ions x increases to 0.02. However, the variations of T and x has little influences on the split energies of the degenerate electron states. Finally, we also study the radial probability densities of the electron states and the hole states in Mn-doped InAs nanowires. Surprisingly, a few of peaks will arise in the radial probability densities of the electron states, while there are only one or two peaks in the radial probability densities of the hole states.

First-principles study of size-, surface- and mechanical strain-dependent electronic properties of wurtzite and zinc-blende InSb nanowires

Using first-principle calculations with density functional theory, we investigated the modification of electronic properties in zinc-blende (ZB) and wurtzite (WZ) InSb nanowires (NWs) grown along the [111] and [0001] directions for different size, different surface coverage and different mechanical strain. The results show that before the surface passivation, ZBNWs and WZNWs exhibit the metallic character and the semiconductor character, respectively. WZNWs show a crossover from a direct to an indirect as diameter decreases. After the surface passivation, both ZBNWs and WZNWs are found to be direct-gap character. The electronic band structure shows a significant response to changes in surface passivation with pseudo hydrogen and halogen. The band structure with mechanical strain is strongly dependent on the crystal orientation and the NW diameter. In ZBNWs, compressive strain induces the indirect band gap character, whereas tensile strain can not form it. WZNWs have various strain dependence in that both compressive and tensile strain make InSb show a direct band gap character. A brief analysis of these results is given.

Band structures and spatial carrier confinement in GaAs/GaP core-shell nanowires: Core/shell composition and size effects

The core/shell composition and size effects on band structures and spatial confinement of carriers in zinc-blende and wurtzite GaAs/GaP core-shell nanowires are investigated by using first-principles calculations. Our results demonstrate that surface GaP shell layer in the core-shell nanowires suppresses surface states of GaAs core, resulting in an intrinsic semiconductor characteristic. The results of electronic structures suggest that band gap, band-structure characteristic, and spatial carrier confinement of core-shell nanowires strongly depend on their size, core/shell composition, and crystal phase. Unlike the direct band gap in wurtzite core-shell nanowires, there is an indirect-to-direct band-gap transition in zinc-blende core-shell nanowires with increasing composition x. The direct band-gap characteristic in zinc-blende core-shell nanowires can be maintained in a larger composition range (0.12 < x ≤ 1). The spatial charge-density distribution of band-edge states indicate that the smaller-sized GaAs/GaP core-shell nanowires (d < 5 nm) have a pseudo-type I band alignment that creates a stronger confinement for hole carriers but there is a type-I band alignment in larger-sized nanowires where both electrons and holes are localized in the GaAs core. The findings provide a theoretical guidance for engineering electronic structures of GaAs/GaP nanowires and their applications in electronic and optoelectronic devices.

The Native Material Limit of Electron and Hole Mobilities in Semiconductor Nanowires.

Piezoelectric surface acoustic waves are employed to induce radio frequency spatiotemporal dynamics of photogenerated electrons and holes in the GaAs core of individual GaAs/AlGaAs core/shell semiconductor nanowires. Comparison of the time-dependent interband optical recombination to numerical simulations allow to determine the charge carrier transport mobilities of electrons, μe = 500–250+500 cm2/(V s), holes, μh = 50–30+50 cm2/(V s) and their ratio μe:μh = (20 ± 5):1. Our method probes carrier transport at low carrier density. Thus, the obtained values represent the native material limit of these nanowires, determined by their structural properties. We show that for near-pristine nanowires, individual twin defects do not significantly affect electrical transport, in strong contrast to polytypic nanowires. In the acoustoelectrically modulated emission, we observe unambiguous signatures of (i) hole localization within long wurtzite-rich segments and (ii) electrons in zinc blende regions being reflected at the interface to a wurtzite-rich region. The experimentally observed periodic emission bursts are faithfully reproduced by advanced numerical simulations which include static band edge discontinuities between a single wurtzite segment in an otherwise pure zinc blende nanowire. Otherwise using the same input parameters as for near-pristine zinc blende nanowires, we can deduce from our simulations a minimum conduction band offset of ΔEC ≈ 20 meV at the interface between the zinc blende part and the wurtzite-rich region. These results furthermore confirm that a single wurtzite segment with sufficiently large band offsets efficiently traps holes and blocks electron transport.

Size- and structure-dependence of thermal and mechanical behaviors of single-crystalline and polytypic superlattice ZnS nanowires

Molecular dynamics (MD) simulations are carried out to study the thermal and mechanical behaviors of single-crystalline wurtzite (WZ), zinc-blende (ZB), and polytypic superlattice ZnS nanowires containing alternating WZ and ZB regions with thicknesses between 1.85 nm and 29.62 nm under tensile loading. The wires analyzed have diameters between 1.77 nm and 5.05 nm. The Green-Kubo method is used to calculate the thermal conductivity of the wires at different deformed states. A non-equilibrium MD approach is used to analyze the thermal transport behavior at the interfaces between different structural regions in the superlattice nanowires (SLNWs). The Young's modulus and thermal conductivity of ZB nanowires are approximately 2%–12% and 23%–35% lower than those of WZ nanowires, respectively. The lower initial residual compressive stress due to higher irregularity of surface atoms causes the Young's modulus of ZB nanowires to be lower. The dependence of the thermal conductivity on structure comes from differences in phonon group velocities associated with the different wires. The thermal conductivity of polytypic superlattice nanowires is up to 55% lower than that of single-crystalline nanowires, primarily because of phonon scattering at the interfaces and the resulting lower effective phonon mean free paths for each structural region. As the periodic lengths (1.85–29.62 nm) and specimen lengths (14.81–59.24 nm) of SLNWs decrease, these effects become more pronounced, causing the thermal conductivity to further decrease by up to 30%.

Crystal Phase and Facet Effects on the Structural Stability and Electronic Properties of GaP Nanowires

The control of electronic properties of GaP nanowires is of particular importance for their applications in nanoelectronics and optoelectronics. However, a fundamental understanding is still lacking of atomic and electronic properties of GaP nanowires due to the diversity of the crystal phase in the fabricated nanowires. Here we reveal the crucial role of the crystal phase and nanowire facets in the structural and electronic properties of zinc-blende, wurtzite, and polytypic GaP nanowires by using the first-principles calculations. The stability mechanism of GaP nanowires depends on the competition between the crystal phase and facet effects: the former dominates the stability of larger-sized nanowires, but the stability of ultrathin nanowires is mainly determined by the latter. This mechanism can be applied to explain a large amount of experimental observations about the formation of stacking faults and twin defects during the growth of III–V semiconductor nanowires. Likewise, electronic properties of Ga...