Semiconductor quantum dots (QDs) are being regarded as the primary unit for a wide range of advanced and emerging technologies including electronics, optoelectronics, photovoltaics and biosensing applications as well as the domain of q-bits based quantum information processing. Such QDs are suitable for several novel device applications for their unique property of confining carriers 3-dimensionally creating discrete quantum states. However, the realization of such QDs in practice exhibits serious challenge regarding their fabrication in array with desired scalability and repeatability as well as control over the quantum states at room temperature. In this context, the current work reports the fabrication of an array of highly scaled Ge-nanowire (radius ~25 nm) based vertical metal-oxide-semiconductor devices that can operate as voltage tunable quantum dots at room temperature. The electrons in such nanowire experience a geometrical confinement in the radial direction, whereas, they can be confined axially by tuning the applied bias in order to manipulate the quantum states. Such quantum confinement of electrons has been confirmed from the step-like responses in the room temperature capacitance-voltage (C-V) characteristics at relatively low frequency (200 kHz). Each of such steps has observed to encompass convolution of the quantized states occupying ~6 electronic charges. The details of such carrier confinement are analyzed in the current work by theoretically modeling the device transport properties based on non-equilibrium Green's function (NEGF) formalism.
The graphene morphology regulated by nanowires patterned in parallel on a substrate surface is quantitatively determined using energy minimization. The regulated graphene morphology is shown to be governed by the nanowire diameter, the nanowire spacing and the interfacial bonding energies between the graphene and the underlying nanowires and substrate. We demonstrate two representative regulated graphene morphologies and determine critical values of the nanowire spacing, nanowire diameter and interfacial bonding energies at which graphene switches between the two representative morphologies. I…
Nominally identical permalloy nanowires, with widths down to 150 nm, were fabricated onto a single electron transparent Si$_{3}$N$_{4}$ membrane using electron beam lithography (EBL) and focused ion beam (FIB) milling. Transmission electron microscopy (TEM) experiments were performed to compare the nanostructures produced by these two techniques in what we believe is the first direct comparison of fabrication techniques for nominally identical nanowires. Both EBL and FIB methods produced high quality structures with edge roughness being of the order of the mean grain size 5 -10 nm observed in …
InSb nanowire arrays with different geometrical parameters, diameter and pitch, are fabricated by top-down etching process on Si(100) substrates. Field emission properties of InSb nanowires are investigated by using a nano-manipulated tungsten probe-tip as anode inside the vacuum chamber of a scanning electron microscope. Stable field emission current is reported, with a maximum intensity extracted from a single nanowire of about 1$μA$, corresponding to a current density as high as 10$^4$ A/cm$^2$. Stability and robustness of nanowire is probed by monitoring field emission current for about th…
GaAs was central to the development of quantum devices but is rarely used for nanowire-based quantum devices with InAs, InSb and SiGe instead taking the leading role. p-type GaAs nanowires offer a path to studying strongly-confined 0D and 1D hole systems with strong spin-orbit effects, motivating our development of nanowire transistors featuring Be-doped p-type GaAs nanowires, AuBe alloy contacts and patterned local gate electrodes towards making nanowire-based quantum hole devices. We report on nanowire transistors with traditional substrate back-gates and EBL-defined metal/oxide top-gates pr…
We have employed noise thermometry for investigations of thermal relaxation between the electrons and the substrate in nanowires patterned from 40-nm-thick titanium film on top of silicon wafers covered by a native oxide. By controlling the electronic temperature $T_e$ by Joule heating at the base temperature of a dilution refrigerator, we probe the electron-phonon coupling and the thermal boundary resistance at temperatures $T_e= 0.5 - 3$ Kelvin. Using a regular $T^5$-dependent electron-phonon coupling of clean metals and a $T^4$-dependent interfacial heat flow, we deduce a small contribution…
We report an electron-beam based method for the nanoscale patterning of the poly(ethylene oxide)/LiClO$_{4}$ polymer electrolyte. We use the patterned polymer electrolyte as a high capacitance gate dielectric in single nanowire transistors and obtain subthreshold swings comparable to conventional metal/oxide wrap-gated nanowire transistors. Patterning eliminates gate/contact overlap which reduces parasitic effects and enables multiple, independently controllable gates. The method's simplicity broadens the scope for using polymer electrolyte gating in studies of nanowires and other nanoscale de…
We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The hybrid devices can be utilized as tools to shed further light onto the origin of film dissipation and decoherence in phase-slip nanowire qubits, patterned entirely from disordered superconducting fi…
A good understanding of the electronic conduction processes through nanocontacts is a crucial step for the implementation of functional nanoelectronic devices. We have studied the current-voltage ($I$-$V$) characteristics of nanocontacts between single metallic RuO$_2$ nanowires (NWs) and contacting Au electrodes which were pre-patterned by simple photolithography. Both the temperature behavior of contact resistance in the low-bias voltage ohmic regime and the $I$-$V$ curves in the high-bias voltage non-ohmic regime have been investigated. We found that the electronic conduction processes in t…
Chip-scale integrated light sources are a crucial component in a broad range of photonics applications. III-V semiconductor nanowire emitters have gained attention as a fascinating approach due to their superior material properties, extremely compact size, and the capability to grow directly on lattice-mismatched silicon substrates. Although there have been remarkable advances in nanowire-based emitters, their practical applications are still in the early stages due to the difficulties in integrating nanowire emitters with photonic integrated circuits (PICs). Here, we demonstrate for the first…
In existing superconducting electronic systems, Josephson junctions play a central role in processing and transmitting small-amplitude electrical signals. However, Josephson-junction-based devices have a number of limitations including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability to drive large impedances, and (4) difficulty in controlling the junction critical current (which depends sensitively on sub-Angstrom-scale thickness variation of the tunneling barrier). Here we present a nanowire-based superconducting electronic device, which we call the nanocryotron (nTron), …
Rapid and reproducible assembly of aligned nanostructures on a wafer-scale is a crucial, yet one of the most challenging tasks in the incorporation of nanowires into integrated circuits. We present the synthesis of a periodic nanochannel template designed for electrochemical growth of perfectly aligned, rectangular nanowires over large areas. The nanowires can be electrically contacted and characterized in situ using a pre-patterned multi-point measurement platform. During the measurement the wires remain within a thick oxide matrix providing protection against breaking and oxidation. We use l…
We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation of pre-patterned GaN(0001) layers grown by hydride vapor phase epitaxy on Al$_{2}$O$_{3}$. Arrays with nanowire diameters and spacings ranging from 50 to 90 nm and 0.1 to 0.7 $μ$m, respectively, are simultaneously produced under identical conditions. The sublimation process, carried out under high vacuum conditions, is analyzed \emph{in situ} by reflection high-energy electron diffraction and line-of-sight quadrupole mass spectromety. During the sublimation process, the GaN(0001) surface va…
Identifying the two-dimensional (2D) topological insulating (TI) state in new materials and its control are crucial aspects towards the development of voltage-controlled spintronic devices with low power dissipation. Members of the 2D transition metal dichalcogenides (TMDCs) have been recently predicted and experimentally reported as a new class of 2D TI materials, but in most cases edge conduction seems fragile and limited to the monolayer phase fabricated on specified substrates. Here, we realize the controlled patterning of the 1T'-phase embedded into the 2H-phase of thin semiconducting mol…
The development of rotary nanomotors is crucial for further advancing NEMS technology. In this work, we report innovative design, assembly, and rotation of ordered arrays of nanomotors. The nanomotors were bottom-up assembled from nanoscale building blocks with nanowires as rotors, patterned nanomagnets as bearings, and quadrupole microelectrodes as stators. Arrays of nanomotors can be synchronously rotated with controlled angle, speed (to at least 18,000 rpm), and chirality by electric fields. Using analytical modeling, we revealed the fundamental nanoscale electrical, mechanical, and magneti…
The quantum-spin-Hall (QSH) phase of 2D topological insulators has attracted increased attention since the onset of 2D materials research. While large bulk gaps with vanishing edge gaps in atomically thin layers have been reported, verifications of the QSH phase by resistance measurements are comparatively few. This is partly due to the poor uniformity of the bulk gap induced by the substrate over a large sample area and/or defects induced by oxidation. Here, we report the observation of the QSH phase at room-temperature in the 1T'-phase of few-layer MoS2 patterned onto the 2H semiconducting p…
Epitaxial semiconductor-superconductor hybrid materials are an excellent basis for studying mesoscopic and topological superconductivity, as the semiconductor inherits a hard superconducting gap while retaining tunable carrier density. Here, we investigate double-quantum-dot devices made from InAs nanowires with a patterned epitaxial Al two-facet shell that proximitizes two gate-defined segments along the nanowire. We follow the evolution of mesoscopic superconductivity and charging energy in this system as a function of magnetic field and voltage-tuned barriers. Inter-dot coupling is varied f…
We evidence the influence of surface effects for InAs quantum dots embedded into GaAs photonic nanowires used as efficient single photon sources. We observe a continuous temporal drift of the emission energy that is an obstacle to resonant quantum optics experiments at the single photon level. We attribute the drift to the sticking of oxygen molecules onto the wire, which modifies the surface charge and hence the electric field seen by the quantum dot. The influence of temperature and excitation laser power on this phenomenon is studied. Most importantly, we demonstrate a proper treatment of t…
InGaAs gate-all-around nanowire MOSFETs with channel length down to 50nm have been experimentally demonstrated by top-down approach. The nanowire size-dependent transport properties have been systematically investigated. It is found that reducing nanowire dimension leads to higher on-current, transconductance and effective mobility due to stronger quantum confinement and the volume inversion effect. TCAD quantum mechanical simulation has been carried out to study the inversion charge distribution inside the nanowires. Volume inversion effect appears at a larger dimension for InGaAs nanowire MO…
High performance InGaAs gate-all-around (GAA) nanowire MOSFETs with channel length (Lch) down to 20nm have been fabricated by integrating a higher-k LaAlO3-based gate stack with an equivalent oxide thickness of 1.2nm. It is found that inserting an ultrathin (0.5nm) Al2O3 interfacial layer between higher-k and InGaAs can significantly improve the interface quality and reduce device variation. As a result, a record low subthreshold swing of 63mV/dec has been demonstrated at sub-80nm Lch for the first time, making InGaAs GAA nanowire devices a strong candidate for future low-power transistors.
Ambipolar currents in Germanium p-type nanowire Schottky barrier MOSFETs were calculated fully quantum-mechanically by using the multi-band k.p method and the non-equilibrium Green's function approach. We investigated the performance of devices with 100, 110, and 111 channel orientations, respectively, by varying the nanowire width, Schottky barrier height, and EOT. The 111 oriented devices showed the best performance. In comparison to Si as a channel material, Ge is more desirable because more current can be injected into the channel, resulting in steeper subthreshold slope and higher on-stat…
We review a series of works describing thermoelectric effects in gated disordered nanowires (field effect transistor device configuration). After considering the elastic coherent regime characterizing sub-Kelvin temperatures, we study the inelastic activated regime occurring at higher temperatures, where electronic transport is dominated by phonon-assisted hops between localised states (Mott variable range hopping). The thermoelectric effects are studied as a function of the location of the Fermi level inside the nanowire conduction band, notably around its edges where they become very large. …
Hole spins in silicon represent a promising yet barely explored direction for solid-state quantum computation, possibly combining long spin coherence, resulting from a reduced hyperfine interaction, and fast electrically driven qubit manipulation. Here we show that a silicon-nanowire field-effect transistor based on state-of-the-art silicon-on-insulator technology can be operated as a few-hole quantum dot. A detailed magnetotransport study of the first accessible hole reveals a g-factor with unexpectedly strong anisotropy and gate dependence. We infer that these two characteristics could enabl…
In this paper we review the theory of silicon nanowires. We focus on nanowires with diameters below 10 nm, where quantum effects become important and the properties diverge significantly from those of bulk silicon. These wires can be efficiently treated within electronic structure simulation methods and will be among the most important functional blocks of future nanoelectronic devices. Firstly, we review the structural properties of silicon nanowires, emphasizing the close connection between the growth orientation, the cross-section and the bounding facets. Secondly, we discuss the electronic…
Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite towards the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, we report on the growth of Ge wires on pre-patterned Si (001) substrates with controllable position, distance, length and structure. This is achieved by a novel growth process…
Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of the vapors. The non-stoichiometric supply of transition-metal precursors and chalcogen leads to poor control in products' location, morphology, crystallinity, uniformity and batch to batch reproducibility. While vapor-liquid-solid (VLS) growth involves molten precursors at the growth temperatures higher than their melting points. The liquid sodium molybdate can precipitate solid MoS2 monolayers when saturated wit…
We have investigated the stability and conductivity of unsupported, two dimensional infinite gold nanowires using ab-initio density functional theory (DFT). Two dimensional ribbon like nanowires, with 1-5 rows of gold atoms in the non-periodic direction and with different possible structures have been considered. The nanowires with > 2 rows of atoms exhibit dimerization, similar to finite wires, along the non-periodic direction. Our results show that in these zero thickness nanowires, the parallelogram motif is the most stable. A comparison between parallelogram and rectangular shaped nanowire…
Recent interest in topological quantum computing has driven research into topological nanowires, one-dimensional quantum wires that support topological modes including Majorana fermions. Most topological nanowire designs rely on materials with strong spin-orbit coupling, such as InAs or InSb, used in combination with superconductors. It would be advantageous to fabricate topological nanowires using Si owing to its mature technology. However, the intrinsic spin-orbit coupling in Si is weak. One approach that could circumvent this material deficiency is to rotate the electron spins using nanomag…
We report the controlled integration, via Dip Pen Nanolithography, of monolayer dots of ferritin-based CoO nanoparticles (12 Bohr magnetons) into the most sensitive areas of a microSQUID sensor. The nearly optimum flux coupling between these nanomagnets and the microSQUID improves the achievable sensitivity by a factor 100, enabling us to measure the linear susceptibility of the molecular array down to very low temperatures (13 mK). This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range o…
Doping is a common route to reducing nanowire transistor on-resistance but has limits. High doping level gives significant loss in gate performance and ultimately complete gate failure. We show that electrolyte gating remains effective even when the Be doping in our GaAs nanowires is so high that traditional metal-oxide gates fail. In this regime we obtain a combination of sub-threshold swing and contact resistance that surpasses the best existing p-type nanowire MOSFETs. Our sub-threshold swing of 75 mV/dec is within 25% of the room-temperature thermal limit and comparable with n-InP and n-Ga…
Majorana Zero Modes (MZMs) are prime candidates for robust topological quantum bits, holding a great promise for quantum computing. Semiconducting nanowires with strong spin orbit coupling offers a promising platform to harness one-dimensional electron transport for Majorana physics. Demonstrating the topological nature of MZMs relies on braiding, accomplished by moving MZMs around each other in a certain sequence. Most of the proposed Majorana braiding circuits require nanowire networks with minimal disorder. Here, the electronic transport across a junction between two merged InSb nanowires i…
We have analysed spin relaxation behaviour of various II-VI semiconductors for nanowire structure and 2-D channel by simulating spin polarized transport through a semi-classical approach. Monte Carlo simulation method has been applied to simulate our model. D'yakanov-Perel mechanism and Elliot-Yafet mechanism are dominant for spin relaxation in II-VI semiconductors. Variation in spin relaxation length with external field has been analysed and comparison is drawn between nanowire and 2-D channels. Spin relaxation lengths of various II-VI semiconductors are compared at an external field of 1kV/c…
One of the most promising platforms for one-dimensional topological superconductivity is based on semiconducting nanowires with strong spin-orbit coupling (SOC), where s-wave superconductivity is induced by proximity effect and an external Zeeman field drives the system into the topological superconducting phase with Majorana bound states (MBSs) at the end of the wire. During last years this idea has led to a great number of important experiments in hybrid superconductor-semiconductor systems, where the main signature is an emergent zero-bias peak (ZBP) in the differential conductance as the Z…
We report the capability to simulate in a quantum mechanical tight-binding (TB) atomistic fashion NW devices featuring several hundred to millions of atoms and diameter up to 18 nm. Such simulations go far beyond what is typically affordable with today's supercomputers using a traditional real space (RS) TB Hamiltonian technique. We have employed an innovative TB mode space (MS) technique instead and demonstrate large speedup (up to 10,000x) while keeping good accuracy (error smaller than 1 percent) compared to the RS NEGF method. Such technique and capability open new avenues to explore and u…
We present a systematic study of various ways (top gates, local doping, substrate bias) to fabricate and tune multi-dot structures in silicon nanowire multigate MOSFETs (metal-oxide-semiconductor field-effect transistors). The carrier concentration profile of the silicon nanowire is a key parameter to control the formation of tunnel barriers and single-electron islands. It is determined both by the doping profile of the nanowire and by the voltages applied to the top gates and to the substrate. Local doping is achieved with the realisation of up to two arsenic implantation steps in combination…
Helium-ion beams (HIB) focused to sub-nanometer scales have emerged as powerful tools for high-resolution imaging as well as nano-scale lithography, ion milling or deposition. Quantifying irradiation effects is essential for reliable device fabrication but most of the depth profiling information is provided by computer simulations rather than experiment. Here, we use atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB. By imaging the i…
The recent development of the superlattice nanowire pattern transfer (SNAP) technique allows for the fabrication of arrays of nanowires at a diameter, pitch, aspect ratio, and regularity beyond competing approaches. Here, we report the fabrication of conducting Si nanowire arrays with wire widths and pitches of 10-20 nm and 40-50 nm, respectively, and resistivity values comparable to the bulk through the selection of appropriate silicon-on-insulator substrates, careful reactive-ion etching, and spin-on glass doping. These results promise the realization of interesting nano-electronic circuits …
Thermally induced conductance jumps of metal nanowires are modeled using stochastic Ginzburg-Landau field theories. Changes in radius are predicted to occur via the nucleation of surface kinks at the wire ends, consistent with recent electron microscopy studies. The activation rate displays nontrivial dependence on nanowire length, and undergoes first- or second-order-like transitions as a function of length. The activation barriers of the most stable structures are predicted to be universal, i.e., independent of the radius of the wire, and proportional to the square root of the surface tensio…
We report on a theoretical study of quantum charge transport in atomistic models of silicon nanowires with surface roughness-based disorder. Depending on the nanowires features (length, roughness profile) various conduction regimes are explored numerically by using efficient real space order N computational approaches of both Kubo-Greenwood and Landauer-Buttiker transport frameworks. Quantitative estimations of the elastic mean free paths, charge mobilities and localization lengths are performed as a function of the correlation length of the surface roughness disorder. The obtained values for …
The singlet-triplet relaxation in nanowire-based quantum dots induced by confined phonons is investigated theoretically. Due to the quasi-one-dimensional nature of the confined phonons, the singlet-triplet relaxation rates exhibit multi-peaks as function of magnetic field and the relaxation rate between the singlet and the spin up triplet state is found to be enhanced at the vicinity of the singlet-triplet anti-crossing. We compare the effect of the deformation-potential coupling and the piezoelectric coupling and find that the deformation-potential coupling dominates the relaxation rates in m…
Crystallizing electrons with artificially patterned lattices
University of Arizona（亚利桑那大学）；University of Tennessee（田纳西大学）；Oak Ridge National Laboratory（橡树岭国家实验室）；National Institute for Materials Science（国立材料研究所）；University at Buffalo（布法罗大学）；Wyant College of Optical Sciences, The University of Arizona（亚利桑那大学怀恩特光学科学学院）
Confining two dimensional Dirac fermions on the surface of topological insulators has remained an outstanding conceptual challenge. Here we show that Dirac fermion confinement is achievable in topological crystalline insulators (TCI), which host multiple surface Dirac cones depending on the surface termination and the symmetries it preserves. This confinement is most dramatically reflected in the flux dependence of these Dirac states in the nanowire geometry, where different facets connect to form a closed surface. Using SnTe as a case study, we show how wires with all four facets of the <100>…
The ability to achieve near-unity light extraction efficiency is necessary for a truly deterministic single photon source. The most promising method to reach such high efficiencies is based on embedding single photon emitters in tapered photonic waveguides defined by top-down etching techniques. However, light extraction efficiencies in current top-down approaches are limited by fabrication imperfections and etching induced defects. The efficiency is further tempered by randomly positioned off-axis quantum emitters. Here, we present perfectly positioned single quantum dots on the axis of a tai…
We report comprehensive high-accuracy molecular dynamics simulations using the ReaxFF forcefield to explore the structural changes that occur as Au nanowires are elongated, establishing trends as a function of both temperature and nanowire diameter. Our simulations and subsequent quantitative structural analysis reveal that polytetrahedral structures (e.g., icosahedra) form within the "amorphous" neck regions, most prominently for systems with small diameter at high temperature. We demonstrate that the formation of polytetrahedra diminishes the conductance quantization as compared to systems w…
