Crystal Structure Of Nanowires Research Articles

Large-Composition-Range Pure-Phase Homogeneous InAs1-xSbx Nanowires.

Narrow bandgap InAs1-xSbx nanowires show broad prospects for applications in wide spectrum infrared detectors, high-performance transistors, and quantum computation. Realizing such applications requires a fine control of the composition and crystal structure of nanowires. However, the fabrication of large-composition-range pure-phase homogeneous InAs1-xSbx nanowires remains a huge challenge. Here, we first report the growth of large-composition-range stemless InAs1-xSbx nanowires (0 ≤ x ≤ 0.63) on Si (111) substrates by molecular beam epitaxy. We find that pure-phase InAs1-xSbx nanowires can be successfully obtained by controlling the antimony content x, nanowire diameter, and nanowire growth direction. Detailed energy dispersive spectrum data show that the antimony is uniformly distributed along the axial and radial directions of InAs1-xSbx nanowires and no spontaneous core-shell nanostructures form in the nanowires. On the basis of field-effect measurements, we confirm that InAs1-xSbx nanowires exhibit good conductivity and their mobilities can reach 4200 cm2 V-1 s-1 at 7 K. Our work lays the foundation for the development of InAs1-xSbx nanowire optoelectronic, electronic, and quantum devices.

Improved effective equation for the Rashba spin-orbit coupling in semiconductor nanowires

Semiconductor Rashba nanowires are quasi-one dimensional systems that have large spin-orbit (SO) coupling arising from a broken inversion symmetry due to an external electric field. There exist parametrized multiband models that can describe accurately this effect. However, simplified single band models are highly desirable to study geometries of recent experimental interest, since they may allow to incorporate the effects of the low dimensionality and the nanowire electrostatic environment at a reduced computational cost. Commonly used conduction band approximations, valid for bulk materials, greatly underestimate the SO coupling in Zinc-blende crystal structures and overestimate it for Wurtzite ones when applied to finite cross-section wires, where confinement effects turn out to play an important role. We demonstrate here that an effective equation for the linear Rashba SO coupling of the semiconductor conduction band can reproduce the behavior of more sophisticated eight-band k$\cdot$p model calculations. This is achieved by adjusting a single effective parameter that depends on the nanowire crystal structure and its chemical composition. We further compare our results to the Rashba coupling extracted from magnetoconductance measurements in several experiments on InAs and InSb nanowires, finding excellent agreement. This approach may be relevant in systems where Rashba coupling is known to play a major role, such as in spintronic devices or Majorana nanowires.

Formation Mechanism of Twinning Superlattices in Doped GaAs Nanowires.

Recent investigations of III-V semiconductor nanowires have revealed periodic zinc-blende twins, known as twinning superlattices, that are often induced by a high-impurity dopant concentration. In the present study, the relationship between the nanowire morphology, crystal structure, and impurity dopant concentration (Te and Be) of twinning superlattices has been studied in GaAs nanowires grown by molecular beam epitaxy using the self-assisted (with a Ga droplet) vapor-liquid-solid process. The contact angle between the Ga droplet and the nanowire top facet decreased linearly with the dopant concentration, whereas the period of the twinning superlattices increased with the doping concentration and was proportional to the nanowire radius. Our model, which is based entirely on surface energetics, is able to explain a unified formation mechanism of twinning superlattices in doped semiconductor nanowires.

XXIV Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 10-13 марта 2020 г. Изменение кристаллической фазы в гетероструктурных Ga(As, P) нитевидных нанокристаллах под воздействием упругих напряжений

In this work, the influence of elastic strain on Ga(As,P) nanowire crystal structure was studied. The stable growth of nanowire in the metastable phase is explained by accounting the elastic strain in the nucleus of newly-forming nanowire layer. Within this approach, the crystal phase switching in GaP nanowire after Ga(AsP) insertion is explained.

8.0 MeV copper ion (Cu++) irradiation-induced effects on structural, electrical, optical and electrochemical properties of Co3O4-NiO-ZnO/GO nanowires

Co3O4-NiO-ZnO/GO nanowires were fabricated with the addition of graphene oxide (GO) obtained by a modified Hummer’s method. Nanocomposite powder of Co3O4-NiO-ZnO/GO was synthesized using the hydrothermal method. Thin films of nanowires powder were deposited on glass substrates by a drop-casting method. The effect of copper ion irradiation on the monolayer of nanostructured Co3O4-NiO-ZnO/GO nanowires was studied. Copper ion (Cu++) of energy 8.0 MeV and doses of 2.25 × 1015, 5.0 × 1015, 7.25 × 1015 and 1.0 × 1016 ions/cm2 were irradiated on the nanostructured thin films. The variations on the properties of the films were analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), Proton induced x-ray emission (PIXE), Rutherford back scattering (RBS), Two point probe, Diffuse reflectance spectroscopy (DRS), Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS). The results indicate that low doses of copper ion irradiation enhanced the properties of the nanowires while high doses induced structural defects and disorder to the Co3O4-NiO-ZnO/GO nanowires crystal structure, the level of defects increases with increasing of ion dose. Therefore, ions beam irradiation with low doses provides an encouraging technique for competent enhancement of properties of nanostructured Co3O4-NiO-ZnO/GO nanowires.

Crystal phase control in self-catalyzed InSb nanowires using basic growth parameter V/III ratio

In this work, the influence of V/III ratio on self-catalyzed InSb nanowire (NW) crystal structure is investigated using metal organic chemical vapor deposition. The effective balance of V/III ratio determines the change of InSb NW crystal structure from zincblende to wurtzite. Particularly, as group-V molar flow varies from 6.4 × 10−5 to 8.0 × 10−5 mol/min, wurtzite segment length of InSb NW increases from 16 ± 9 nm to 89 ± 9 nm, whereas zincblende crystal phase is observed for other molar flows. We interpret the observed phase transition to a lower surface energy of In-Sb alloy droplet than pure In-droplet, which makes triple phase line nucleation energetically favorable. This study gives insight on single growth parameter (V/III ratio) change to the InSb NW crystal control, such a technique is essential to selectively tune the crystal phase of single NW for various applications.

Impact of the Shadowing Effect on the Crystal Structure of Patterned Self-Catalyzed GaAs Nanowires.

The growth of regular arrays of uniform III-V semiconductor nanowires is a crucial step on the route toward their application-relevant large-scale integration onto the Si platform. To this end, not only does optimal vertical yield, length, and diameter uniformity have to be engineered, but also, control over the nanowire crystal structure has to be achieved. Depending on the particular application, nanowire arrays with varying area density are required for optimal device efficiency. However, the nanowire area density substantially influences the nanowire growth and presents an additional challenge for nanowire device engineering. We report on the simultaneous in situ X-ray investigation of regular GaAs nanowire arrays with different area density during self-catalyzed vapor-liquid-solid growth on Si by molecular-beam epitaxy. Our results give novel insight into selective-area growth and demonstrate that shadowing of the Ga flux, occurring in dense nanowire arrays, has a crucial impact on the evolution of nanowire crystal structure. We observe that the onset of Ga flux shadowing, dependent on array pitch and nanowire length, is accompanied by an increase of the wurtzite formation rate. Our results moreover reveal the paramount role of the secondary reflected Ga flux for VLS NW growth (specifically, that flux that is reflected directly into the liquid Ga droplet).

Lithography-free variation of the number density of self-catalyzed GaAs nanowires and its impact on polytypism

Abstract

The Features of GaAs Nanowire SEM Images

The detailed study of GaAs nanowires synthesized by molecular beam epitaxy performed by scanning electron microscopy allowed to reveal the presence of specific contrast in the images obtained. To understand the causes of the phenomenon the transmission electron microscopy of nanowire crystal structure was carried out. The results showed that it could be caused by the segments having polytypic crystal phase. It was also confirmed by the modelling of the electron beam scattering on such nanowire arrays.

The features of GaAs nanowire SEM images

AbstractThe detailed study of GaAs nanowires synthesized by molecular beam epitaxy performed by scanning electron microscopy allowed to reveal the presence of specific contrast in the images obtained. To understand the causes of the phenomenon the transmission electron microscopy of nanowire crystal structure was carried out. The results showed that it could be caused by the segments having polytypic crystal phase. It was also confirmed by the modelling of the electron beam scattering on such nanowire arrays.

(Invited) Welding of Nickel Nanowires By Ions Beam Irradiation: From Small to Large Scale

In order to replace conventional transparent conducting electrodes, metal nanowires (MNWs) networks have long been act as promising contenders due to their high mechanical flexibility. However, nanowire-nanowire contact resistance is main technical obstruction in terms of improvement in their performance. To reduce nanowire-nanowire contact resistance, welding of nanowires (NWs) is essential. In this work, MeV proton and argon ions beam irradiation induced nanoscale welding technique to fabricate X-, Y-, II- and T-shaped molecular junctions between nickel nanowires (Ni-NWs) is presented. Ni-NWs are irradiated by 2.75 MeV proton ions and 3.5 MeV argon ions at doses 1x1016ions/cm2 and 5x1015 respectively while temperature is kept constant. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and x-ray diffraction (XRD) results reveal that Ni-NWs are welded without losing stability in crystal structures of NWs. Afterward, a two-dimensional large scale random network of Ni-NWs is fabricated by 3 MeV proton ions beam irradiation induced nanoscale welding of Ni-NWs at crossing positions. Proton ion beam induced large scale network fabrication is confirmed by TEM and SEM techniques. Moreover, these networks are characterized optically using UV-VIS spectroscopy. It is observed that at a beam fluence of 5x1015 ions/cm2, perfect molecular junctions between Ni-NWs in X-, II-, and V-shapes are achieved and a large scale welded network of Ni-NWs is obtained without distorting the morphology of NWs. Moreover, structure of Ni-NWs remains stable under 3MeV proton ions beam irradiation and networks are found to be optically transparent. The results exhibit that fabrication of Ni-NWs network precede through three steps: i- High energy ions beam irradiation induced heat spikes lead to locally heat and fuse Ni-NWs, ii- Formation of molecular junctions on small scale, iii- Formation of large scale network Ions beam irradiation induced welding of Ni-NWs can improve performance of NWs networks by reducing contact resistance between NWs for application as transparent conducting electrodes. These high quality welded MNWs networks may potentially be applied as transparent conducting electrodes in optoelectronic devices.

Observation of self-assisted GaAs nanowire growth by molecular beam epitaxy on GaAs (1 1 1)B without SiO2 layer

GaAs nanowire growth was performed on bare GaAs (1 1 1)B substrates by molecular beam epitaxy technique. A small adjustment was made to the conventional self-assisted VLS growth by removing covered SiO2 layer and growing nanowires directly on the substrate surface in order to eliminate some issues that occur in the conventional self-assisted VLS growth. To study the growth mechanism, growth time was varied as a growth parameter. Nanowire crystal structure was investigated by XRD analysis. Evolution of surface morphology along with RHEED patterns were utilized to determine characteristics of the growth.

X-ray pole figure analysis of catalyst free InAs nanowires on Si substrate

We present crystallographic analysis of InAs nanowires by X-ray pole figure measurements and reciprocal space mapping in this report. The InAs nanowires have been grown on Si (111) substrates by a catalyst free approach using molecular beam epitaxy technique. The pole figure measurement of InAs nanowires for (111) and (220) reflections reveals important details such as the nanowire crystal structure, orientation, presence of twin defects and the epitaxial relation of the nanowires with the substrate. Angular relationship between several set of twin planes in each pole figure reflection is analyzed and the twin direction in each case is determined using a quantitative approach. Possibility of presence of mixed zinc blende /wurtzite phases in the nanowires is explored from an analysis of pole figure measurement for the (311) and (10−15) reflections and an asymmetric reciprocal space map (RSM) measurement.

Dislocation-free axial InAs-on-GaAs nanowires on silicon

We report on the growth of axial InAs-on-GaAs nanowire heterostructures on silicon by molecular beam epitaxy using 20 nm diameter Au catalysts. First, the growth parameters of the GaAs nanowire segment were optimized to achieve a pure wurtzite crystal structure. Then, we developed a two-step growth procedure to enhance the yield of vertical InAs-on-GaAs nanowires. We achieved 90% of straight InAs-on-GaAs nanowires by further optimizing the growth parameters. We investigated the composition change at the interface by energy dispersive x-ray spectroscopy and the nanowire crystal structure by transmission electron microscopy. The composition of the nominal InAs segment is found to be InxGa1−xAs with x = 0.85 and corresponds to 6% of lattice mismatch with GaAs. Strain mapping performed by the geometrical phase analysis of high-resolution images revealed a dislocation-free GaAs/In0.85Ga0.15As interface. In conclusion, we successfully fabricated highly mismatched heterostructures, confirming the prediction that axial GaAs/In0.85Ga0.15As interfaces are pseudomorphic in nanowires with a diameter smaller than 40 nm.

Polar Second-Harmonic Imaging to Resolve Pure and Mixed Crystal Phases along GaAs Nanowires.

In this work, we report an optical method for characterizing crystal phases along single-semiconductor III-V nanowires based on the measurement of polarization-dependent second-harmonic generation. This powerful imaging method is based on a per-pixel analysis of the second-harmonic-generated signal on the incoming excitation polarization. The dependence of the second-harmonic generation responses on the nonlinear second-order susceptibility tensor allows the distinguishing of areas of pure wurtzite, zinc blende, and mixed and rotational twins crystal structures in individual nanowires. With a far-field nonlinear optical microscope, we recorded the second-harmonic generation in GaAs nanowires and precisely determined their various crystal structures by analyzing the polar response for each pixel of the images. The predicted crystal phases in GaAs nanowire are confirmed with scanning transmission electron and high-resolution transmission electron measurements. The developed method of analyzing the nonlinear polar response of each pixel can be used for an investigation of nanowire crystal structure that is quick, sensitive to structural transitions, nondestructive, and on-the-spot. It can be applied for the crystal phase characterization of nanowires built into optoelectronic devices in which electron microscopy cannot be performed (for example, in lab-on-a-chip devices). Moreover, this method is not limited to GaAs nanowires but can be used for other nonlinear optical nanostructures.

Influence of Composition, pH, Annealing Temperature, Wave Form, and Frequency on Structure and Magnetic Properties of Binary Co1−x Al x and Ternary (Co0.97Al0.03)1−x Fe x Nanowire Alloys

Co1−x Al x and (Co0.97Al0.03)1−x Fe x nanowire alloys and elemental Co nanowire were prepared in anodic aluminum oxide templates by the electrochemical deposition method. The influence of composition, pH, annealing temperature, wave form, and frequency on structure and magnetic properties of Co/Al/Fe nanowires was studied. The changes in the saturation magnetization, coercivity, remanent squareness, and crystal structure of nanowires with changing of the above parameters were also investigated. The roomtemperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis change with the changing of the abovementioned parameters, which can be mainly attributed to the strengthening of anisotropy. X-ray diffraction observations demonstrated that the isolated nanowires have a crystalline structure and that their phases change with the annealing temperature. Magnetic measurements, on the other hand, showed that the coercivity reached a maximum value at x = 0.03 in the Co1−x Al x nanowires. Also, the coercivity of the Co1−x Al x nanowires was increased with increasing annealing temperature for all the compositions.

Demonstration of Hexagonal Phase Silicon Carbide Nanowire Arrays with Vertical Alignment

SiC nanowire based electronics hold promise for data collection in harsh environments wherein conventional semiconductor platforms would fail. However, the full adaptation of SiC nanowires as a material platform necessitates strict control of nanowire crystal structure and orientation for reliable performance. Toward such efforts, we report the growth of hexagonal phase SiC nanowire arrays grown with vertical alignment on commercially available single crystalline SiC substrates. The nanowire hexagonality, confirmed with Raman spectroscopy and atomic resolution microscopy, displays a polytypic distribution of predominantly 2H and 4H. Employing a theoretical growth model, the polytypic distribution of hexagonal phase nanowires is accurately predicted in the regime of high supersaturation. Additionally, the reduction of disorder-induced phonon density of states is achieved while maintaining nanowire morphology through a postgrowth anneal. The results of this work expand the repertoire of SiC nanowires by imp...

Effects of Polytypism on Optical Properties and Band Structure of Individual Ga(N)P Nanowires from Correlative Spatially Resolved Structural and Optical Studies.

III-V semiconductor nanowires (NWs) have gained significant interest as building blocks in novel nanoscale devices. The one-dimensional (1D) nanostructure architecture allows one to extend band structure engineering beyond quantum confinement effects by utilizing formation of different crystal phases that are thermodynamically unfavorable in bulk materials. It is therefore of crucial importance to understand the influence of variations in the NWs crystal structure on their fundamental physical properties. In this work we investigate effects of structural polytypism on the optical properties of gallium phosphide and GaP/GaNP core/shell NW structures by a correlative investigation on the structural and optical properties of individual NWs. The former is monitored by transmission electron microscopy, whereas the latter is studied via cathodoluminescence (CL) mapping. It is found that structural defects, such as rotational twins in zinc blende (ZB) GaNP, have detrimental effects on light emission intensity at low temperatures by promoting nonradiative recombination processes. On the other hand, formation of the wurtzite (WZ) phase does not notably affect the CL intensity neither in GaP nor in the GaNP alloy. This suggests that zone folding in WZ GaP does not enhance its radiative efficiency, consistent with theoretical predictions. We also show that the change in the lattice structure have negligible effects on the bandgap energies of the GaNP alloys, at least within the range of the investigated nitrogen compositions of <2%. Both WZ and ZB GaNP are found to have a significantly higher efficiency of radiative recombination as compared with that in parental GaP, promising for potential applications of GaNP NWs as efficient nanoscale light emitters within the desirable amber-red spectral range.

Sb-Induced Phase Control of InAsSb Nanowires Grown by Molecular Beam Epitaxy

For the first time, we report a complete control of crystal structure in InAs(1-x)Sb(x) NWs by tuning the antimony (Sb) composition. This claim is substantiated by high-resolution transmission electron microscopy combined with photoluminescence spectroscopy. The pure InAs nanowires generally show a mixture of wurtzite (WZ) and zinc-blende (ZB) phases, where addition of a small amount of Sb (∼2-4%) led to quasi-pure WZ InAsSb NWs, while further increase of Sb (∼10%) resulted in quasi-pure ZB InAsSb NWs. This phase transition is further evidenced by photoluminescence (PL) studies, where a dominant emission associated with the coexistence of WZ and ZB phases is present in the pure InAs NWs but absent in the PL spectrum of InAs0.96Sb0.04 NWs that instead shows a band-to-band emission. We also demonstrate that the Sb addition significantly reduces the stacking fault density in the NWs. This study provides new insights on the role of Sb addition for effective control of nanowire crystal structure.

Long exciton lifetimes in stacking-fault-free wurtzite GaAs nanowires

We present a combined photoluminescence and transmission electron microscopy study of single GaAs nanowires. Each wire was characterized both in microscopy and spectroscopy, allowing a direct correlation of the optical and the structural properties. By tuning the growth parameters, the nanowire crystal structure is optimized from a highly mixed zincblende–wurtzite structure to pure wurtzite. We find the latter one to be stacking-fault-free over nanowire lengths up to 4.1 μm. We observe the emission of purely wurtzite nanowires to occur only with polarization directions perpendicular to the wurtzite c-axis, as expected from the hexagonal unit cell symmetry. The free exciton recombination energy in the wurtzite structure is 1.518 eV at 5 K with a narrow linewidth of 4 meV. Most notably, these pure wurtzite nanowires display long carrier recombination lifetimes of up to 11.2 ns, exceeding reported lifetimes in bulk GaAs and state-of-the-art 2D GaAs/AlGaAs heterostructures.