Pure Zinc Blende Structure Research Articles

(Invited) Stacking Faults in 3C-SiC: From the Crystal Structure, to the Electrical Behavior

Many attractive mechanical and electrical properties make silicon carbide (SiC) a material of interest in both electronic devices and sensors for nowadays technology (sustainable energies, hybrid vehicles, low power loss inverters). Silicon carbide exists in nature in several crystalline structures, called polytypes, differentiated by the stacking sequence of the tetrahedrially bonded Si-C bilayers. The only possible cubic structure, 3C-SiC, is obtained when the bilayer stacking is of the kind ABCABC..., resulting in a pure zinc-blende structure. 3C-SiC growth on Si is allowed within chemical vapour deposition (CVD) reactors, which would ensure a very high purity of the resulting product. The interface between 3C-SiC and Si is the origin of a high density of planar and volume defects, such as microtwins, anti-phase boundaries and stacking faults in the epilayer and voids in Si underneath the hetero-interface. Most of these defects, considered killers for devices, however, reduce their density, and some are totally annihilated, when very thick (several tens of microns) 3C-SiC epitaxial films are realized. The main defects that remain also after the growth of several tens of microns are the Stacking Faults (SFs). Three main SFs have been observed in 3C-SiC: the intrinsic one where only one plane is shifted with respect to the lattice position and two extrinsic SFs where two or three layers are shifted. In these last cases we observe locally the same crystallographic structure of the hexagonal polytypes: 4H-SiC and 6H-SiC. In the last period a large investigation has been done on these defects to see the crystallographic structure of these defects, the interactions between different SFs, the influence of the SFs on the internal stress of the material, the effects of several growth parameters (temperature, doping, growth rate, ...) on the density and evolution of these defects. Finally also several local electrical measurements by C-AFM has been done to understand the effect of these defects on the electrical behaviour of the devices.In Fig. 1 a three different HRTEM images of three different SFs are reported. It is possible to observe the different crystallographic structure of these defects. In Fig. 2 the interaction between these defects is reported. It is possible to observe that the SFs can annihilate or generate or close. The growth parameters have a large influence on the evolution of these defects. In Fig. 3 can be observed that, decreasing the growth rate and the growth temperature, the SFs density decreases. In Fig. 4 it is reported the effect of the doping on the SFs. Increasing the nitrogen concentration the SFs density decreases and the average length increases.At the conference more details on the SFs structure, the effect of these defects on the stress and on the electrical characteristics of the devices will be given. Figure 1

Selective Growth of Stacking Fault Free ⟨100⟩ Nanowires on a Polycrystalline Substrate for Energy Conversion Application.

Cubic semiconductor nanowires grown along ⟨100⟩ directions have been reported to be promising for optoelectronics and energy conversion applications, owing to their pure zinc-blende structure without any stacking fault. But, until date, only limited success has been achieved in growing ⟨100⟩ oriented nanowires. Here we report the selective growth of stacking fault free ⟨100⟩ nanowires on a commercial transparent conductive polycrystalline fluorine-doped SnO2 (FTO) glass substrate via a simple and cost-effective chemical vapor deposition (CVD) method. By means of crystallographic analysis and density functional theory calculation, we prove that the orientation relationship between the Au catalyst and the FTO substrate play a vital role in inducing the selective growth of ⟨100⟩ nanowires, which opens a new pathway for controlling the growth directions of nanowires via the elaborate selection of the catalyst and substrate couples during the vapor-solid-liquid (VLS) growth process. The ZnSe nanowires grown on the FTO substrate are further applied as a photoanode in photoelectrochemical (PEC) water splitting. It exhibits a higher photocurrent than the ZnSe nanowires do without preferential orientations on a Sn-doped In2O3 (ITO) glass substrate, which we believe to be correlated with the smooth transport of charge carriers in ZnSe ⟨100⟩ nanowires with pure zinc-blende structures, in distinct contrast with the severe electron scattering happened at the stacking faults in ZnSe nanowires on the ITO substrate, as well as the efficient charge transfer across the intensively interacting nanowire-substrate interfaces.

Crystal Structure Control of ZnxCd1−xS Alloyed Nanocrystals and Structural Dependent Fluorescence Properties

Crystal structure control is so important to the fluorescence properties that each element should be considered carefully. In conventional synthesis of ZnxCd[Formula: see text]S alloyed nanocrystals (NCs), most of studies focus on ligand–surface interaction on the formation of either zinc blende or wurtzite ZnxCd[Formula: see text]S nanocrystals, instead of the reactant source. In this work, mixed crystal phase was found easily in ZnxCd[Formula: see text]S alloyed NCs when reaction proceeded at high Zn/Cd source ratio condition. Therefore, we regulate the Zn/Cd ratio to obtain relative pure zinc-blende structure, and study the influence of structure change on the fluorescence properties. Further, we have proposed a two-step ZnS coating method to acquire ZnxCd[Formula: see text]S/ZnS NCs with separated crystal-phase between core and shell. Compared with maximum QY of 81% for ZnxCd[Formula: see text]S/ZnS NCs synthesized by conventional one-step coating method, the performance of the optimized NCs has significantly improved with maximum QY of 93%.

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.

Influence of Fe2+ and Ni2+ contents on the optical and electrical properties of ZnS quantum dots

The work reports a well controlled synthesis of un-doped and Fe2+ and Ni2+ doped ZnS quantum dots (QDs) based highly luminescent of pure cubic zinc blende structure. The samples were characterized for structural, morphological, optical and electrical parameters and great care has been made in correlating all these properties. The optimum concentrations of Zn1−xFexS and Zn1−xNixS dopants were found to be x = 0.2 for superior optical and electrical properties. Incorporation of Fe2+ or Ni2+ into ZnS lattice have generated acceptor level as an isolated state just above the valence band of ZnS, and hence electron requiring less excitation energy for transferring from the acceptor level to the conduction band. Moreover, it was found that the PL emission intensity of Fe2+ doped ZnS QDs is higher than that of Ni2+ doped. The results indicated that the active luminescent centers created by dopants and localized surface plasmon resonance energy transfer of ZnS QDs significantly increased with Fe2+ ions into the ZnS lattice. Over all in light of the results obtained during the study, the doped samples can have potential for function in vast variety of applications.

GaAsP Nanowires Grown by Aerotaxy.

We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III-V semiconductor based nanowires. By varying the PH3/AsH3 ratio and growth temperature, size selected GaAs1-xPx nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ x ≤ 0.43) were grown, which is the energy range needed for creating tandem III-V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III-V semiconductor based ternary nanowires.

Twin-Induced InSb Nanosails: A Convenient High Mobility Quantum System.

Ultra narrow bandgap III-V semiconductor nanomaterials provide a unique platform for realizing advanced nanoelectronics, thermoelectrics, infrared photodetection, and quantum transport physics. In this work we employ molecular beam epitaxy to synthesize novel nanosheet-like InSb nanostructures exhibiting superior electronic performance. Through careful morphological and crystallographic characterization we show how this unique geometry is the result of a single twinning event in an otherwise pure zinc blende structure. Four-terminal electrical measurements performed in both the Hall and van der Pauw configurations reveal a room temperature electron mobility greater than 12,000 cm(2)·V(-1)·s(-1). Quantized conductance in a quantum point contact processed with a split-gate configuration is also demonstrated. We thus introduce InSb "nanosails" as a versatile and convenient platform for realizing new device and physics experiments with a strong interplay between electronic and spin degrees of freedom.

Defect-Free Self-Catalyzed GaAs/GaAsP Nanowire Quantum Dots Grown on Silicon Substrate.

The III-V nanowire quantum dots (NWQDs) monolithically grown on silicon substrates, combining the advantages of both one- and zero-dimensional materials, represent one of the most promising technologies for integrating advanced III-V photonic technologies on a silicon microelectronics platform. However, there are great challenges in the fabrication of high-quality III-V NWQDs by a bottom-up approach, that is, growth by the vapor-liquid-solid method, because of the potential contamination caused by external metal catalysts and the various types of interfacial defects introduced by self-catalyzed growth. Here, we report the defect-free self-catalyzed III-V NWQDs, GaAs quantum dots in GaAsP nanowires, on a silicon substrate with pure zinc blende structure for the first time. Well-resolved excitonic emission is observed with a narrow line width. These results pave the way toward on-chip III-V quantum information and photonic devices on silicon platform.

Controllable growth and optical properties of InP and InP/InAs nanostructures on the sidewalls of GaAs nanowires

The growth and optical properties of InP and InP/InAs nanostructures on GaAs nanowires are investigated. InP quantum well and quantum dots (QDs) are formed on the sidewalls of GaAs nanowires successively with increasing the deposition time of InP. The GaAs/InP nanowire heterostructure exhibits a type-II band alignment. The wavelength of the InP quantum well is in the range of 857–892 nm at 77 K, which means that the quantum well is nearly fully strained. The InP quantum dot, which has a bow-shaped cross section, exhibits dislocation-free pure zinc blende structure. Stranski-Krastanow InAs quantum dots are subsequently formed on the GaAs/InP nanowire core-shell structure. The InAs quantum dots are distributed over the middle part of the nanowire, indicating that the In atoms contributing to the quantum dots mainly come from the vapor rather than the substrate. The longest emission wavelength obtained from the InAs QDs is 1039 nm at 77 K. The linewidth is as narrow as 46.3 meV, which is much narrower than those on planar InP substrates and wurtzite InP nanowires, suggesting high-crystal-quality, phase-purity, and size-uniformity of quantum dots.

Zn-doping of GaAs nanowires grown by Aerotaxy

Nanowires were grown by means of a novel aerosol-based method called Aerotaxy. Here an aerosol of Au catalyst nanoparticles in N2 is mixed with MOVPE precursors in a flow-through reactor at atmospheric pressure, whereby nanowires are produced continuously in high concentrations. We demonstrate the possibility of in situ doping of the NWs and the realization of well-controlled p-type GaAs nanowires using this Aerotaxy method. By controlling the cracking and concentration of the precursors, p-doped GaAs nanowires could be grown exhibiting a wide range of Zn doping levels. DEZn was used as the dopant source and the injected DEZn/TMGa ratio was varied from 0.1% to 3.4%. The morphology, the crystalline structure and the composition of the nanowires were studied using SEM, TEM and XEDS. The nanowires were grown straight without any significant tapering and this ideal morphology could be maintained up to an injected DEZn/TMGa ratio of 3.4%. The nanowires typically grew in the 〈111〉 direction with a pure zincblende structure, but by increasing the DEZn flow the number of twinning defects increased which we ascribe to Zn incorporation. Elemental analysis shows a high Zn content in the catalyst particle and also a gradient in the Zn content along the nanowire. The samples were analyzed optically using photoluminescence (PL). From the result we estimated the free hole concentration induced by Zn acceptors to be 1×1020cm−3 for DEZn/TMGa ratio of 3.4%. To our knowledge this is the first report on in situ doping of GaAs nanowires grown by Aerotaxy.

Palladium assisted hetroepitaxial growth of an InAs nanowire by molecular beam epitaxy

The palladium (Pd) assisted epitaxial growth of technologically important InAs nanowires grown on GaAs{111}B substrates using molecular beam epitaxy is reported. The grown free-standing InAs nanowires adapted a vapor–liquid–solid growth mechanism. The impacts of the catalyst particle density, growth temperature and input V/III precursor ratio have been investigated to identify better growth conditions for getting high-density non-<111>-orientated InAs nanowires. We assert here that two kind of nanowires are observed, one having a pure zinc-blende crystalline structure free of stacking faults, and the other with a defect-free wurtzite crystalline structure. However, few of them have defect imperfections too. The L- and Y-shaped nanowires confirm similar surface free energies for possible <110> growth directions. These unusual growth directions are attributed to the effect of the catalyst material as well as the surface-induced strain at the interface between the grown nanowires with substrates. The structural features of the grown nanowires are studied by employing scanning and transmission electron microscopic techniques. The obtained TEM results confirm that the nanowire catalyst interface is not a straightforward zinc-blende structured nanowire. Energy dispersive x-ray (EDX) analysis reveals that the tip of the grown nanowires with the chemical composition of Pd and In have a nearly 50:50 ratio, while the nanowire body did not have any catalyst traces other than the composition of InAs for both type of nanowires. The obtained high angle annular dark field (HAADF) TEM image for both types of nanowires along with the intensity profile provided evidence for cubic as well as hexagonal facets.

A new selenium precursor for the aqueous synthesis of luminescent CdSe quantum dots

Herein, we report a single-step synthesis of stable, luminescent aqueous colloids of mercaptopropionic acid capped cadmium selenide nanoparticles of controllable size. The reaction was carried out in open-atmosphere with one-pot by using selenium dioxide to replace selenium or its other hazardous, expensive and unstable precursors. Mercaptopropionic acid acts as both selenium reducer and capping molecule for these quantum dots. This new precursor of selenium has been successfully implemented to develop a new aqueous method to obtain fluorescent nanoparticles. The diffraction patterns and other morphological characterization indicate that as synthesized quantum dots have a pure cubic zinc blende structure with spherical shape. Three different colors: White, yellow, orange were achieved by tuning the reaction parameters. Small particle size in the range 1.6–2.9 nm was achieved. The absorption spectra clearly show the quantum confinement effect due to particle size. The quantum yield of these colloids is 5%, 24% and 2.6% for 1.6, 2.7 and 2.9 nm particles, respectively. The main advantage of the present protocol is that it is low cost, greener, water soluble precursors; the capping molecule itself acts as anion reducing agent and is easily up-scalable for larger quantity synthesis. Further, the technique yields good fluorescent nanoparticles.

Growth of InAs quantum dots on Si-based GaAs nanowires by controlling the surface adatom diffusion

InAs/GaAs dots-on-nanowire (NW) hybrid heterostructures are grown on Si substrate by metal organic chemical vapor deposition. The formation of InAs quantum dots (QDs) is intimately associated with the NW density as well as the substrate surface properties, both of which strongly affect the surface adatom diffusion. InAs QDs are realized with a short deposition time by reducing the NW density. The QDs exhibit specific facets and pure zinc blende structure, residing on a wetting layer of several nanometers. Photoluminescence emission from the QDs is observed at room temperature, with a linewidth of 186meV. The results are promising for future integration of III–V NW hybrid devices, especially photovoltaic cells on Si.

Growth of zinc blende GaAs/AlGaAs heterostructure nanowires on Si substrate by using AlGaAs buffer layers

Vertical GaAs/AlGaAs heterostructure nanowires (NWs) with pure zinc blende structure were grown on Si (111) substrates by using AlGaAs/GaAs buffer layers. Axial and radial heterostructure NWs with different facets can be realized by tuning the growth temperature, so quantum wells with a triangular cross section enclosed by two {112} and one {110} facets can be obtained. Intensity of the room temperature photoluminescence can be greatly enhanced by passivating the surface states of GaAs NWs via growth of higher band-gap AlGaAs shells. The relevant growth mechanism is discussed.

Doping studies of Tb (terbium) and Cu (copper) on CdSe nanorods

We have explored the structural and optical properties of the rare earth (Tb) and transition elements (Cu) co-doped CdSe nanorods. The pure and 5% Tb, Cu co-doped CdSe nanorods were synthesized by using solvothermal synthesis process. From transmission electron microscopy (TEM) analysis, it was observed that the average diameter of the Tb, Cu co-doped CdSe nanorods is about 25 nm with varied lengths in nanometer range. X-ray diffraction (XRD) studies show the wurtzite (hexagonal) structure with lattice parameters a = 4.214 Å and c = 6.904 Å for pure CdSe nanorods and zinc blende structure for co-doped nanorods. The UV–Visible (UV–Vis) studies show Tb, Cu co-doped nanorods showed blue shift with respect to the bulk value. The band gap of the co-doped CdSe nanorods was found to be more than un-doped CdSe nanorods. PL studies of co-doped CdSe nanorods showed slightly enhanced photoluminescence. The thermal stability of the co-doped CdSe nanorods was enhanced as compared to the un-doped counterparts.

GaAs and InAs Nanowires for Ballistic Transport

Tailoring of GaAs and InAs nanowires (NWs) to be suited for measurements of ballistic transport is discussed in this paper. Methods used to avoid imperfections most harmful for the transport properties are described. We consider the imperfections, which frequently occur in III-V NWs: occasional stacking faults, unintentional impurities (like gold atoms originating from the catalyst in the vapor-liquid-solid growth method) and imperfections associated with the NW side facets. Foremost important is obtaining GaAs and InAs NWs, in which either a pure wurtzite or pure zinc-blende structure is enforced, i.e., overcoming the inherent tendency of the two structures to intermix in III-V NWs. Next follows elimination, or at least minimization of the number of incorporated impurities. In InAs NWs, this has been achieved by using low-growth temperature combined with a low-growth rate. Finally, embedding the NWs in an in situ grown shell has provided a robust way for passivation of the surface states and keeping the electrons away from any impurities adhered to the surface.

Growth of pure zinc blende p-type GaAs nanowires by metal-organic chemical vapor deposition

Vertical p-type gallium arsenide (GaAs) nanowires with pure zinc blende structure were grown on GaAs (111)B substrate by metal-organic chemical vapor deposition via a Au-catalyst vapor-liquid-solid mechanism. The p-type doping was investigated by additional diethyl zinc (DEZn). In the high II/III ratio range (II/III>9.1%), there exists a critical length beyond which kinking takes place. Two possible reasons are discussed. Zn occurrence in the nanowires was verified by energy dispersive X-ray (EDX) analysis. Corresponding to II/III = 0.2%, the doping concentrations about 8 × 1018 cm−3.

Self-catalyzed GaAs nanowire growth on Si-treated GaAs(100) substrates

Self-catalyzed GaAs nanowire growth was obtained by molecular beam epitaxy on GaAs(001) substrates after predeposition of subnanometer-thick Si layers. Two substrate preparation methods are presented, the first based on the epitaxial growth of Si on GaAs and subsequent exposure to atmosphere, and the second on the direct deposition of Si on epiready GaAs substrates. X-ray photoemission spectroscopy shows that both methods result in a thin Si oxide layer that promotes the growth of GaAs nanowires aligned along the 〈111〉 direction. High densities of nanowires were obtained at substrate temperatures between 620 and 680 °C. Systematic electron microscopy studies indicate that nanowire growth is associated with the formation of Ga nanoparticles on the substrate surface, which act as a catalyst in the vapor-liquid-solid growth mechanism frame. The majority of the nanowires have a pure zinc-blende structure, and their photoluminescence is dominated by a photoluminescence peak 3 to 5 meV in width and centered at 1.516 to 1517 eV.

基于气液固模式生长的无缺陷闪锌矿结构GaAs/AlGaAs轴向异质纳米线

Pure zinc blende structure GaAs/AlGaAs axial heterostructure nanowires (NWs) are grown by metal organic chemical vapor deposition on GaAs(111) B substrates using Au-catalyzed vapor-liquid-solid mechanism. Al adatom enhances the influence of diameters on NWs growth rate. NWs are grown mainly through the contributions from the direct impingement of the precursors onto the alloy droplets and not so much from adatom diffusion. The results indicate that the droplet acts as a catalyst rather than an adatom collector.