Pure Zinc Blende Research Articles

Metal-sown selective area growth of high crystalline quality InAsSb nanowires and networks by molecular-beam epitaxy

Scalable in-plane InAsSb nanowires and networks have attracted intense research interest in optoelectronics and quantum computation. However, the poor crystalline quality of InAsSb nanowires and networks limits the development of high-performance nanodevices. Here, we report the growth of high crystalline quality InAsSb nanowires and networks on patterned Ge substrates by molecular-beam epitaxy. We find that high crystalline quality InAsSb nanowires can be successfully achieved by the conventional selective area growth route. But continuous nanowires and networks cannot be obtained by this growth manner. To overcome this problem, a metal-sown selective area growth route is developed. By precisely tuning the growth parameters, the well-aligned InAsSb nanowires and networks have been successfully fabricated. It is determined that the morphologies of nanowires and networks are dependent on the local growth rate and the V/III ratio, and the V/III ratio has an obvious effect on the polarity of nanowires and networks. Detailed structural studies confirm that these well-faceted nanowires are pure zinc blende single crystals, and there is a strict epitaxial relationship between the nanowire and the substrate. The energy dispersive spectroscopy analyses indicate that the Sb content is evenly distributed along the in-plane direction and has an obvious gradient along the out-of-plane direction. The successful fabrication of high crystalline quality InAsSb nanowires and networks provides new opportunities for exploring potential optoelectronic applications.

Embedded high-quality ternary GaAs1−x Sb x quantum dots in GaAs nanowires by molecular-beam epitaxy

Semiconductor quantum dots are promising candidates for preparing high-performance single photon sources. A basic requirement for this application is realizing the controlled growth of high-quality semiconductor quantum dots. Here, we report the growth of embedded GaAs1−x Sb x quantum dots in GaAs nanowires by molecular-beam epitaxy. It is found that the size of the GaAs1−x Sb x quantum dot can be well-defined by the GaAs nanowire. Energy dispersive spectroscopy analyses show that the antimony content x can be up to 0.36 by tuning the growth temperature. All GaAs1−x Sb x quantum dots exhibit a pure zinc-blende phase. In addition, we have developed a new technology to grow GaAs passivation layers on the sidewalls of the GaAs1−x Sb x quantum dots. Different from the traditional growth process of the passivation layer, GaAs passivation layers can be grown simultaneously with the growth of the embedded GaAs1−x Sb x quantum dots. The spontaneous GaAs passivation layer shows a pure zinc-blende phase due to the strict epitaxial relationship between the quantum dot and the passivation layer. The successful fabrication of embedded high-quality GaAs1−x Sb x quantum dots lays the foundation for the realization of GaAs1−x Sb x -based single photon sources.

Zincblende InAsxP1-x/InP Quantum Dot Nanowires for Telecom Wavelength Emission.

InAsxP1-x quantum dots (QDs) in InP nanowires (NWs) have been realized as a platform for emission at telecom wavelengths. These QDs are typically grown in NWs with the wurtzite crystal phase, but in this case, ultrathin diameters are required to achieve defect-free heterostructures, making the structures less robust. In this work, we demonstrate the growth of pure zincblende InAsxP1-x QDs in InP NWs, which enabled an increase in NW diameters to about 45 nm, achieved by employing Au-assisted vapor liquid solid growth in a chemical beam epitaxy system. We studied the growth of InP/InAsxP1-x heterostructures with different compositions to control the straight growth along the ⟨100⟩ direction and to tune the emission wavelength. Interestingly, we found that the growth mechanism for pure InAs QDs is different compared to that for InAsxP1-x alloy QDs. This allowed us to optimize different growth protocols to achieve straight growth of the final QD NWs. We successfully obtained the growth of InAsxP1-x QDs with a composition in the range of x = 0.24-1.00. By means of microphotoluminescence measurements, we demonstrate the tunability of the emission in dependence of the InAsxP1-x QD composition and morphology, remarkably observing an emission at the telecom O-band for a 10 nm thick QD with 80% of As content.

Effective surface passivation of GaAs nanowire photodetectors by a thin ZnO capping.

The III-V nanowire (NW) structure is a good candidate for developing photodetectors. However, high-density surface states caused by the large surface-to-volume ratio severely limit their performance, which is difficult to solve in conventional ways. Here, a robust surface passivation method, using a thin layer of ZnO capping, is developed for promoting NW photodetector performance. 11 cycles of ZnO, grown on pure zinc blende high-quality GaAs NWs by atomic layer deposition, significantly alleviates the undesirable effect of the surface states, without noticeable degradation in NW morphology. An average 20-fold increase in micro-photoluminescence intensity is observed for passivated NWs, which leads to the development of detectors with high responsivity, specific detectivity, and optical gain of 9.46 × 105 A W-1, 3.93 × 1014 Jones, and 2.2 × 108 %, respectively, under low-intensity 532 nm illumination. Passivated NW detectors outperform their counterparts treated by conventional methods, so far as we know, which shows the potential and effectiveness of thin ZnO surface passivation on NW devices.

Cobalt-substituted ZnS QDs: a diluted magnetic semiconductor and efficient photocatalyst.

Recently, understanding the origin of induced magnetic characteristics in transition metal atom-doped QDs has been a major focus owing to their potential applications in the area of spintronic devices. A detailed experimental and theoretical investigation was conducted to understand the physical properties of Co-doped ZnS QDs containing different weight percentages of Co atoms [CoxZn1-xS (x = 0.00, 0.03, 0.06, and 0.09)], prepared using chemical co-precipitation techniques. X-ray diffraction studies proved that all the prepared QDs formed an extremely pure cubic zinc blende crystallographic phase free of contaminants. The validation of the quantum dot nature of all the samples was provided by the HRTEM images, BET studies, and blue shift in the absorption spectra. Both the obtained FTIR and PL spectra at room temperature also confirmed the phase purity of the prepared QDs. The observed weak ferromagnetic behavior of the doped samples was due to the presence of p-d hybridization between the 3d levels of Co2+ ions and 3p levels of S2- ions of the host ZnS QDs. Hysteresis loops that were obtained at room temperature validated this weak ferromagnetic nature. These obtained results were also supported theoretically using DFT calculations. FDTD simulations provided a detailed explanation for the observed blue shift in the absorption spectra originating from the quantum confinement effect of doped and undoped ZnS QDs. The dielectric properties of all the samples were examined properly, and it was also found that the grain boundaries contributed effectively to providing the dielectric response. The doped ZnS sample containing more Co dopants at low frequencies showed a progressive rise in polarisation loss. In addition, Co-doped ZnS QDs are efficient photocatalysts. A pH-dependent photodegradation test of ciprofloxacin (CIP) antibiotic was conducted using 9% Co-doped ZnS QDs. It was observed that 9% Co-doped ZnS nanocatalysts has sufficient capability to degrade CIP to around 94.7% in a solution of pH 10 within one hour. Therefore, besides showing photocatalytic effects, Co-doped ZnS QDs act as ideal dilute magnetic semiconductors (DMSs) and will undoubtedly become excellent candidates for the microelectronics industry because of their special ability to exhibit spin-dependent magneto-electro-optical properties that find use in spin-polarized light-emitting diodes, solid-state lasers, and spin-transistor devices.

Ultra-small ZnS quantum dots embedded in N-doped carbon matrix for high-performance Li-ion battery anode

We report zinc sulfide (ZnS) quantum dots composed of three different polymorphs (pure zinc blende, pure wurtzite, and poly-type (zinc blende/wurtzite)) uniformly embedded in an N-doped carbon matrix as a potential electrode material for anode applications in lithium-ion batteries (LIBs), for the first time. The ZnS polymorph quantum dots with cubic and hexagonal lattices anchored onto a N-doped carbon matrix ([email protected]) electrode is prepared by a polyol-assisted pyro-synthesis technique. A combined operando XRD and ex-situ XAS analyses revealed that the electrochemical reaction mechanism in [email protected] was reversible and comprised of an insertion reaction followed by a conversion-cum-alloying reaction. Due to the structural property of the composite and its unique electrochemical behavior, the [email protected] electrode registers a reversible specific capacity as high as 800 mAhg−1 with a high initial energy round-trip efficiency of 83%, and better cycling stability and power. The results from this study can help in the production of high-performance electrode materials with unique structural and morphological composition using an efficient pyro-synthetic reaction for rechargeable battery applications.

Structural and optical properties of Be-doped high-quality self-catalyzed GaAs nanowires

Crystal-phase control and crystalline quality improvement of GaAs nanowires (NWs) have been realized by dopant (Be) incorporation in GaAs NWs. We demonstrate the improvement of crystalline quality by X-ray diffraction (XRD) spectra combined with high-resolution transmission electron microscopy (HRTEM). The crystal-phase control from the wurtzite (WZ)/zinc blende (ZB) mixed phase to the pure ZB phase under the effect of Be doping is clearly revealed by Raman spectra combined with HRTEM. The photoluminescence (PL) revealed the free exciton and WZ/ZB type-II emission peaks of undoped GaAs NWs transform into Be impurity-related emission peak of Be-doped GaAs NWs.

Growth of zinc-blende GaN on muscovite mica by molecular beam epitaxy

The mechanisms of plasma-assisted molecular beam epitaxial growth of GaN on muscovite mica were investigated. Using a battery of techniques, including scanning and transmission electron microscopy, atomic force microscopy, cathodoluminescence, Raman spectroscopy and x-ray diffraction, it was possible to establish that, in spite of the lattice symmetry mismatch, GaN grows in epitaxial relationship with mica, with the [11–20] GaN direction parallel to [010] direction of mica. GaN layers could be easily detached from the substrate via the delamination of the upper layers of the mica itself, discarding the hypothesis of a van der Waals growth mode. Mixture of wurtzite (hexagonal) and zinc blende (ZB) (cubic) crystallographic phases was found in the GaN layers with ratios highly dependent on the growth conditions. Interestingly, almost pure ZB GaN epitaxial layers could be obtained at high growth temperature, suggesting the existence of a specific GaN nucleation mechanism on mica and opening a new way to the growth of the thermodynamically less stable ZB GaN phase.

Hole and Electron Effective Masses in Single InP Nanowires with a Wurtzite-Zincblende Homojunction.

The formation of wurtzite (WZ) phase in III-V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (me= 0.080 m0). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ7C ↔ Γ7uV with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be mlh = 0.26 m0, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs.

Droplet manipulation and horizontal growth of high-quality self-catalysed GaAsP nanowires

Abstract Self-catalyzed horizontal nanowires (NWs) can greatly simplify the CMOS integration processing compared with the regular vertical counterparts. However, self-catalyzed growth mode poses challenges in manipulating the droplets to produce single-crystalline horizontal NWs with a uniform diameter. Here, we demonstrated a novel method to manipulate the droplet through altering the droplet surface energy. Ga-droplet was successfully moved from top to sidewalls in GaAsP NWs by introducing Be and lowering the surface energy, and pinned at the tip despite the absence of planar defects. This can switch the growth direction, with a successful rate of 100 %, from vertical to horizontal through the assistance of few sparse twins. The produced NWs tend to be bounded by low energy facets, which leads to the self-catalysed growth of horizontal NWs with a greatly improved diameter uniformity along the axis. Besides, the lowered surface energy can effectively suppress the wurtzite nucleation, producing pure zinc blende single-crystalline horizontal NWs. This study establishes an essential step toward the efficient integration of NWs into CMOS compatible devices.

Calculation of Surface Properties of Cubic and Hexagonal Crystals through Molecular Statics Simulations

Surface property is an important factor that is widely considered in crystal growth and design. It is also found to play a critical role in changing the constitutive law seen in the classical elasticity theory for nanomaterials. Through molecular static simulations, this work presents the calculation of surface properties (surface energy density, surface stress and surface stiffness) of some typical cubic and hexagonal crystals: face-centered-cubic (FCC) pure metals (Cu, Ni, Pd and Ag), body-centered-cubic (BCC) pure metals (Mo and W), diamond Si, zincblende GaAs and GaN, hexagonal-close-packed (HCP) pure metals (Mg, Zr and Ti), and wurzite GaN. Sound agreements of the bulk and surface properties between this work and the literature are found. New results are first reported for the surface stiffness of BCC pure metals, surface stress and surface stiffness of HCP pure metals, Si, GaAs and GaN. Comparative studies of the surface properties are carried out to uncover trends in their behaviors. The results in this work could be helpful to the investigation of material properties and structure performances of crystals.

Stabilization of the Morphology and Crystal Phase in Ensembles of Self‐Catalyzed GaAs Nanowires

Recent experimental data reveal the two stable contact angles of gallium droplets promoting the vapor–liquid–solid growth of GaAs nanowires, around 90° and 130°, with the critical angle of developing the truncated growth interface being smaller, but very close to 130°. Herein, a model is presented for the time evolution of the nanowire radii and preferred crystal structures in large ensembles of self‐catalyzed GaAs nanowires. The model qualitatively explains earlier experimental observations and even contradictory results. It is shown that all nanowires stabilize to a stationary radius and pure zincblende phase within the optimal range of V/III flux ratios. Lower V/III flux ratios yield pure zincblende phase without any radius stabilization, whereas higher V/III ratios lead to a limited focusing of the radius, polytypism, or even pure wurtzite phase of GaAs nanowires.

Classification of the Morphologies and Related Crystal Phases of III–V Nanowires Based on the Surface Energy Analysis

Zincblende-wurtzite polytypism in III–V nanowires has been of great interest for a long time. However, full understanding and control over the crystal phase is still lacking. Here, we propose a model for the morphologies and related crystal phases of vapor–liquid–solid III–V nanowires by considering the minimum surface energies of different side wall facets as a function of the droplet contact angle. On the basis of the recent experimental data and earlier theoretical calculations of the surface energies, a simple classification of the possible growth modes is presented. It shows that the wurtzite phase forms in vertical nanowires at intermediate contact angles whereas the zincblende phase is predominant for vertical or inverse-tapered nanowires with a truncated top at larger contact angles. The small stable contact angle corresponds to faulted wurtzite–zincblende intermix. Pure zincblende phase is possible for even smaller contact angles only in the kinetic growth regimes. The wurtzite and zincblende dom...

Structural and optical properties of novel CdSe nanoparticles produced via a facile synthetic route: Studies on the effects of cadmium sources

We report on the successful synthesis of CdSe nanoparticles (NPs) via a facile aqueous approach. Investigation on the effects of various cadmium sources in the precursor solution on the CdSe NPs is discussed. The structural and morphological properties characterized by the X‐ray diffraction (XRD) and scanning electron microscope (SEM) displayed good features of the as‐prepared CdSe NPs. The XRD pattern displayed a pure zinc blende crystal structure for all samples, with the most crystalline sample observed for CdSe NPs prepared using anhydrous cadmium chloride. The estimated crystallite sizes were below 6 nm for all the CdSe NPs samples. Mixed shapes of spherical and nanorods of varying sizes were observed from the SEM images for the as‐prepared NPs prepared using different cadmium sources. The optical studies conducted by photo‐spectroscopy pointed out the CdSe NPs prepared using anhydrous cadmium chloride gave the best optical properties. The emission wavelengths were in the range 565 to 574 nm while the optical band gaps were in the range 2.94 to 3.23 eV for all the as‐prepared CdSe NPs samples. All the samples, however, displayed quantum confinement effects giving room for further fabrication and engineering to suit specific applications in the biological field. The obtained results demonstrated that aqueous phase synthetic route employed in this study could be successfully adopted for production of high‐quality CdSe NPs because of its facile and inexpensive nature.

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.

Growth of foreign-catalyst-free vertical InAs/InSb heterostructure nanowires on Si (1 1 1) substrate by MOCVD

In this study, growth of Au-free InAs/InSb vertical heterostructure (HS) nanowires (NWs) on highly lattice mismatched Si (1 1 1) substrate by metal organic chemical vapor deposition (MOCVD) is demonstrated. Careful selections of InSb growth parameters lead to In-rich growth regime such that direct impingement of growth precursors on top of grown InAs NW (self-assembled) stems result in axial InSb HSs nucleation on InAs stems. The observed indium (In) droplet on InSb HS NW tip shows that InSb HS growth followed a self-catalyzed growth mechanism. InSb HS axial growth rate and morphology are controlled by growth temperature and growth time. A small change in the growth parameters significantly affects In particle accumulation, particle size and its chemical composition, and thus affects the InSb nucleation on the stem. Especially, it is found that (i) growth time of less 10 min yields vertical InSb HS NW on the top of the InAs stem, (ii) as the growth time increases above 10 min, the growth turned to be InSb wrapped InAs/InSb core-shell HS due to coaxial lateral growth. High resolution transmission electron microscopy (HRTEM) study reveals that all the InSb HSs exhibit pure zinc-blende (ZB) crystal structure. The results presented here provide significant guidelines for external catalyst-free InAs/InSb HS NW growth using MOCVD on silicon (Si) substrate for various optoelectronics and electronics applications.

N-type doping and morphology of GaAs nanowires in Aerotaxy

Controlled doping in semiconductor nanowires modifies their electrical and optical properties, which are important for high efficiency optoelectronic devices. We have grown n-type (Sn) doped GaAs nanowires in Aerotaxy, a new continuous gas phase mass production technique. The morphology of Sn doped nanowires is found to be a strong function of dopant, tetraethyltin to trimethylgallium flow ratio, Au–Ga–Sn alloying, and nanowire growth temperatures. High temperature and high flow ratios result in low morphological quality nanowires and in parasitic growth on the wire base and surface. Alloying and growth temperatures of 400 °C and 530 °C, respectively, resulted in good morphological quality nanowires for a flow ratio of TESn to TMGa up to 2.25 × 10−3. The wires are pure zinc-blende for all investigated growth conditions, whereas nanowires grown by metal-organic vapor phase epitaxy with the same growth conditions are usually mainly Wurtzite. The growth rate of the doped wires is found to be dependent more on the TESn flow fraction than on alloying and nanowire growth temperatures. Our photoluminescence measurements, supported by four-point probe resistivity measurements, reveal that the carrier concentration in the doped wires varies only slightly (1–3) × 1019 cm−3 with TESn flow fraction and both alloying and growth temperatures, indicating that good morphological quality wires with high carrier density can be grown with low TESn flow. Carrier concentrations lower than 1019 cm−3 can be grown by further reducing the flow fraction of TESn, which may give better morphology wires.

High-Responsivity Photodetection by a Self-Catalyzed Phase-Pure p-GaAs Nanowire.

Defects are detrimental for optoelectronics devices, such as stacking faults can form carrier-transportation barriers, and foreign impurities (Au) with deep-energy levels can form carrier traps and nonradiative recombination centers. Here, self-catalyzed p-type GaAs nanowires (NWs) with a pure zinc blende (ZB) structure are first developed, and then a photodetector made from these NWs is fabricated. Due to the absence of stacking faults and suppression of large amount of defects with deep energy levels, the photodetector exhibits room-temperature high photoresponsivity of 1.45 × 105 A W-1 and excellent specific detectivity (D*) up to 1.48 × 1014 Jones for a low-intensity light signal of wavelength 632.8 nm, which outperforms previously reported NW-based photodetectors. These results demonstrate these self-catalyzed pure-ZB GaAs NWs to be promising candidates for optoelectronics applications.

Excitation energy dependence of the life time of orange emission from Mn-doped ZnS nanocrystals

Mn-doped ZnS nanocrystals were prepared by co-precipitation method. X-ray diffraction analysis indicates pure cubic zinc blende structure. We report for the first time experimental observation of the dependence of the decay time of Mn2+ emission from Mn-doped ZnS nanocrystals on excitation energy. Photon energy smaller than the ZnS bandgap induces faster decay time compared to the one equal to the bandgap energy. In addition, while the decay spectra of orange emission indicate both sub μs and ms, the time-resolved luminescence measurement suggests that the sub μs component belongs to the tail of the blue emission.

Effect of doped substrates on the growth of GaAs nanowires via metal organic chemical vapor deposition

Vertical GaAs nanowires were grown on different doped substrates via Metal Organic Chemical Vapor Deposition by catalyst assisted vapor-liquid-solid mechanism. It is found that both n and p type doped substrates affect catalyst distribution during the formation of alloy catalysts. The catalyst density decreases with an increase in the doping concentration of the substrates. In the growth of GaAs nanowires, the growth rate, which is mostly determined by the atoms diffusion from the pyrolysis of precursors on the surface of nanowires and substrates, is proportional to the catalyst densities. Moreover, the structures of as-grown nanowires are all pure zinc blende without any defects. These results will be valuable for the applications of nanowire-based optical and electrical devices.