Hot-electron effects in InAs nanowire Josephson junctions

The controlled tailoring of the energy distribution in an electron system opens the way to interesting new physics and device concepts, as demonstrated by research on metallic nanodevices during recent years. Here we investigate how Josephson coupling in a superconductor-InAs nanowire junction can be tuned by means of hot-electron injection and we show that a complete suppression of superconductive effects can be achieved using a power as low as 100 pW. Nanowires offer a novel design freedom as they allow axial and radial heterostructures to be defined as well as control over doping profiles, which can be crucial in the development of devices—such as nanorefrigerators—where precisely controlled and predictable energy barriers are mandatory. Our work provides estimates for unknown key thermal and electrical parameters, such as the electron-phonon coupling, in our InAs nanostructures.      

内嵌InAs量子点的场效应晶体管特性研究

研究了内嵌InAs量子点的异质结场效应晶体管在室温和低温下的电学特性,获得了量子点影响下器件的输出特性曲线。在室温下,通过分别测试在近红外光照和量子点充电条件下器件的Ⅰ-Ⅴ特性,证明了量子点通过类似纳米悬浮栅的作用,对邻近沟道的二维电子气施加影响。在低温下观察到器件漏电流出现负微分电导现象。这一现象可由2DEG和量子点之间的共振隧穿来解释。这些结果提供了一种新的操作传统场效应晶体管的方法,并有望制成新型量子点存储器。     

InAs nanowire superconducting tunnel junctions: Quasiparticle spectroscopy,thermometry, and nanorefrigeration

We demonstrate an original method based on controlled oxidation for creating high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires (NWs).We show clean tunnel characteristics with a current suppression by ＞4 orders of magnitude for a junction bias well below the Al gap of A0 ≈ 200 μeV.The experimental data agree well with the BardeenCooper-Schrieffer theoretical expectations for a superconducting tunnel junction.The studied devices employ small-scale tunnel contacts functioning as thermometers as well as larger electrodes that provide proof-of-principle active cooling of the electron distribution in the NWs.A peak refrigeration of approximately δT =10 mK is achieved at a bath temperature of Tbath ≈ 250-350 mK for our prototype devices.This method introduces important perspectives for the investigation of the thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.     

Electrical properties of electron-gun-evaporated InAs thin films

Measurements of the resistivity and the Hall coefficient versus the temperature (77–300 K) and the magnetic field (0–11 kG) were performed on InAs thin films (100–1500 Å thick) obtained by vacuum evaporation from an electron gun and condensation onto glass substrates at room temperature. Analysis of the electrical results reveals the typical behaviour of disordered structures. The conductivity versus temperature curves exhibit an anomalous sharp variation which appears to separate two temperature ranges characterized by different conductivity slopes. Observation of the samples in a transmission electron microscope reveals the polycrystalline structure of the films. Electron spectroscopy for chemical analysis shows the sample to be non-stoichiometric with an excess of indium in the inner layers and the presence of indium and arsenic oxides at the surface of the films. Numerical analysis, performed with a multiparametric best-fit procedure, shows that the conductivity conforms to the percolation conductivity model but with an exponential temperature dependence, in accordance in the high temperature region with the more recent hypotheses regarding disordered structures.     

Field dependent transport properties in InAs nanowire field effect transistors

We present detailed studies of the field dependent transport properties of InAs nanowire field-effect transistors. Transconductance dependence on both vertical and lateral fields is discussed. Velocity-field plots are constructed from a large set of output and transfer curves that show negative differential conductance behavior and marked mobility degradation at high injection fields. Two dimensional electrothermal simulations at current densities similar to those measured in the InAs NWFET devices indicate that a significant temperature rise occurs in the channel due to enhanced phonon scattering that leads to the observed mobility degradation. Scanning transmission electron microscopy measurements on devices operated at high current densities reveal arsenic vaporization and crystal deformation in the subject nanowires.     

模板法合成分叉Ni纳米线阵列及其磁性能

首先采用三次阳极氧化法制备了具有Y形孔道的氧化铝(AAO)模板,然后采用直流电化学沉积法,在模板内成功合成了分叉Ni纳米线的有序阵列.通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)对所合成样品的晶体结构和形貌进行了表征测试.结果表明,制备的Ni纳米线分布均匀、排列有序,呈Y形分又结构,其...     

Position-controlled interconnected InAs nanowire networks

We demonstrate here a method for controlled production of complex self-assembled three-dimensional networks of InAs nanowires on a substrate, based on sequentially seeded epitaxial nanowire structures, or "nanotrees". A position-controlled array of trunk nanowires is first produced using lithographically defined Au particles as seeds. With these wires positioned along the proper crystallographic directions with respect to each other, nanotree branches grow toward neighboring trunks, connecting them together. Finally, we investigate the crystal structure of the interconnected nanotrees, demonstrating that branch growth after the contact with the second trunk has an epitaxial relationship to that trunk.     

High electron mobility InAs nanowire field-effect transistors

Single-crystal InAs nanowires (NWs) are synthesized using metal-organic chemical vapor deposition (MOCVD) and fabricated into NW field-effect transistors (NWFETs) on a SiO(2)/n(+)-Si substrate with a global n(+)-Si back-gate and sputtered SiO(x)/Au underlap top-gate. For top-gate NWFETs, we have developed a model that allows accurate estimation of characteristic NW parameters, including carrier field-effect mobility and carrier concentration by taking into account series and leakage resistances, interface state capacitance, and top-gate geometry. Both the back-gate and the top-gate NWFETs exhibit room-temperature field-effect mobility as high as 6580 cm(2) V(-1) s(-1), which is the lower-bound value without interface-capacitance correction, and is the highest mobility reported to date in any semiconductor NW.     

Shape-controlled Au particles for InAs nanowire growth

We present a study of InAs nanowire (NW) growth with shape-controlled Au seed particles. In comparison to more conventional spherical particles, the highly faceted, shaped Au particles are found to enhance the initial growth kinetics of InAs NWs at identical growth conditions. Analysis of the NWs after growth by transmission electron microscopy and energy-dispersive spectroscopy suggests that while In diffuses into the bulk of the shaped Au particles, in accordance with the vapor-liquid-solid (VLS) growth mechanism, the surface faceting is preserved. A key difference is that the shaped Au particles are characterized by a thicker In shell on their surfaces than the spherical Au particles, indicating that increased adsorption of In leads to the observed growth rate enhancement. On the basis of these results, we propose that our picture of VLS growth in regards to liquefaction and droplet formation is incomplete and that the initial particle morphology can be used to tailor NW growth.     

Stacked layers of InAs self-assembled quantum dots

Carrier injection and subsequent radiative recombination in two vertically stacked (but electronically only weakly coupled) layers of InAs/GaAs self-assembled quantum dots (SADs) embedded in the intrinsic region of a double hetero p-i-n structure was investigated by electroluminescence (EL) spectroscopy in the temperature range from 20 to 300 K. In such structures the filling of the SADs by charge carriers strongly depends not only on the applied voltage, but also on the relative position of the SAD layers within the i-region and on the temperature. The experimental data provide evidence of the dominant role of hole dynamics in the recombination processes in the stacks of SADs. The difference of the electronic structure of the SADs in the top and bottom layers is reflected by independent contributions of the two quantum dot layers to the electroluminescence from the SADs. The possibility to tune the emission spectra by varying the thickness of the GaAs layer between neighbouring SAD layers and by using the indium flush technique is demonstrated.     

Control of InAs nanowire growth directions on Si

We report on control of growth directions of InAs nanowires on Si substrate. We achieved to integrate vertical InAs nanowires on Si by modifying initial Si(111) surface in selective-area metal-organic vapor phase epitaxy with flow-rate modulation mode at low temperature. Cross-sectional transmission electron microscope and Raman scattering showed that misfit dislocation with local strains were accommodated in the interface.     

Self-aligned charge read-out for InAs nanowire quantum dots

A highly sensitive charge detector is realized for a quantum dot in an InAs nanowire. We have developed a self-aligned etching process to fabricate in a single step a quantum point contact in a two-dimensional electron gas and a quantum dot in an InAs nanowire. The quantum dot is strongly coupled to the underlying point contact that is used as a charge detector. The addition of one electron to the quantum dot leads to a change of the conductance of the charge detector by typically 20%. The charge sensitivity of the detector is used to measure Coulomb diamonds as well as charging events outside the dot. Charge stability diagrams measured by transport through the quantum dot and charge detection merge perfectly.     

The morphology of axial and branched nanowire heterostructures

We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.     

Optical properties of self-assembled InAs quantum dots grown on GaAs(211)A substrate

We have investigated the photoluminescence (PL) and growth properties of self-assembled InAs/GaAs quantum dots (QD) grown on (211)A-oriented GaAs substrate in a low coverage region. At the onset of the QD formation in the Stranski–Krastanov mode, structures of QD on (211)A substrate were quite different from those on (100) substrate. The uniformity of size distribution was better and the density was higher than that grown on (100) substrate. We found a PL peak at 1.32 eV when the InAs coverage was 1.57 ML. Another PL peak gradually appeared at 1.37–1.42 eV with increasing InAs coverage. The peaks at 1.32 and 1.37–1.42 eV were attributed to the emission from a defect-related QD and a typical QD, respectively. When the InAs coverage exceeded 1.89 ML, the QD density decreased with increasing InAs coverage, due to the coalescence of QD. The samples studied here showed PL spectra having a larger intensity and narrower full width at half-maximum compared with that grown on (100) substrate.     

Ge/Si nanowire mesoscopic Josephson junctions

The controlled growth of nanowires (NWs) with dimensions comparable to the Fermi wavelengths of the charge carriers allows fundamental investigations of quantum confinement phenomena. Here, we present studies of proximity-induced superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by superconducting leads. By using a top gate electrode to modulate the carrier density in the NW, the critical supercurrent can be tuned from zero to greater than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement in the radial direction, which results in stepwise increases in the critical current as a function of gate voltage. Transport measurements on these superconductor-NW-superconductor devices reveal high-order (n = 25) resonant multiple Andreev reflections, indicating that the NW channel is smooth and the charge transport is highly coherent. The ability to create and control coherent superconducting ordered states in semiconductor-superconductor hybrid nanostructures allows for new opportunities in the study of fundamental low-dimensional superconductivity.     

Breakdown enhancement in silicon nanowire p-n junctions

We demonstrate highly reproducible silicon nanowire diodes fabricated with a fully VLSI compatible etching technology, with diameters down to 30 nm. A contact technology based on recrystallized polysilicon enables specific contact resistances as low as rho approximately 10-7 Omega cm2. Our devices show a strongly diameter-dependent breakdown voltage at reverse bias, which we explain in terms of the influence of the surrounding dielectric. We suggest that this technology is suitable for incorporating nanowire-based functionalities into future integrated circuits.     

Optically-induced charge storage in self-assembled InAs quantum dots

We present results on spectrally resolved photo-resistance studies of optically-induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. The investigated structure designed for electron or hole storage in the QDs consists of a modulation doped two-dimensional channel which was grown on top of a layer of InAs QDs, separated by an asymmetric tunnel barrier. Our results show that optical QD charging with spectral resolution provides information on the charging dynamics and on the quantity and spectral dependence of stored charges in the QDs. This is a novel technique by which QD excitation spectra can be studied. Spectrally resolved storage effect measurements on electrons as well as on holes allowed to investigate thermal redistribution of carriers in the quantum dot layer. It was found that only at low temperatures carriers can be stored selectively over long time scales in the InAs QDs. The charge storage effect is observable for several hours at temperatures up to 170 K, for several seconds up to 250 K due to an increase in thermal emission of stored charges.     

Synthesis and structural characterization of single-crystalline branched nanowire heterostructures

We report the synthesis of three-dimensional single-crystalline branched nanowire heterostructures, where the backbones and branches are assembled with ZnS and CdS, respectively. Growth of branch and backbones with control over the compositions was enabled via sequential seeding of gold nanocluster catalysts. Elemental mapping data confirmed that branched nanowire heterostructures were synthesized with the intended chemical modulation, CdS branches on ZnS backbones. Transmission electron microscopy studies showed that the growth of heterostructure branches occurs epitaxially from the backbone while maintaining single-crystalline structure. This unique class of heterostructures holds great potential in assembling electronics and photonics in three dimensions.     

Direct electrical measurement of the self-assembled nickel silicide nanowire

We present results from the direct electrical measurement of an as-grown nanowire. The nickel silicide (NiSi) nanowire was spontaneously grown across a trench between two electrodes used for measurement. The NiSi nanowire, 58 nm in diameter and 2.9 microm in length, showed a low resistance characteristic of 147.9 Omega. This unique method is straightforward and does not require removal of a grown nanowire to be moved into a measurement environment.     

Optical properties of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices

Characterization of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices is presented. These structures are spontaneously generated during the epitaxial growth by metal–organic chemical vapor deposition and molecular beam epitaxy. Transmission electron microscopy reveals the structural details and electro-modulated reflectance is used to characterize the energy and anisotropy of the optical transitions in the lateral superlattices. We demonstrate several properties of these self-assembled structures: (a) the band gap energy can be changed by as much as 350 meV, (b) the polarization anisotropy of the lowest energy transition exceeds 90%, (c) the superlattice axis and the direction of the optical anisotropy can be oriented along two non-equivalent directions in the plane of the substrate, and (d) the valence band splitting between heavy- and light-hole transitions is significant. We discuss the difference between the samples from the two growth techniques. Finally, we theoretically model the electronic states in these lateral superlattices and we demonstrate that the difference in average InAs composition between the well and barrier can be as high as 35%.