quasi-1D Channel Research Articles

Step-necking growth of silicon nanowire channels for high performance field effect transistors

Ultrathin silicon nanowires (diameter <30 nm) with strong electrostatic control are ideal quasi-1D channel materials for high-performance field effect transistors, while a short channel is desirable to enhance driving current. Typically, the patterning of such delicate channels relies on high-precision lithography, which is not applicable for large area electronics. In this work, we demonstrate that ultrathin and short silicon nanowires channels can be created through a local-curvature-modulated catalytic growth, where a planar silicon nanowires is directed to jump over a crossing step. During the jumping dynamic, the leading droplet undergoes significant stretching, producing a short necking segment of <100 nm in length, with a reduced diameter from approximately 45 nm to <25 nm. Compared to the FETs with uniform silicon nanowire channels, our step-necked silicon nanowire FETs exhibit substantially enhanced on/off current ratio Ion/off > 8 × 107 and a sharper subthreshold swing of 70 mV/dec, thanks to a stronger gating effect in the middle channel and markedly improved electric contacts at the thicker source/drain ends. These findings mark the pioneering experimental demonstration of catalytic growth acting as a deterministic fabrication method for precisely crafting engineered FET channels, ideally fitting the requirements of high-performance large-area displays and sensors.

Quasi-1D, rectangular-like and hexagonal moiré superlattices in exfoliated highly oriented pyrolytic graphite

Understanding the dynamics of structural collapse in hexagonal moiré superlattices is of importance towards the stabilization of exotic superconductive phenomena in quasi-1D superlattices. Here we investigate unusual exfoliation-induced structural transformations of hexagonal moiré superlattices, in a μm-thin lamella of highly oriented pyrolytic graphite (HOPG). By employing a combination of experimental techniques, namely transmission electron microscopy (TEM), high resolution TEM (HRTEM) and selective area electron diffraction (SAED) we systematically investigate the collapse of hexagonal moiré superlattices into rectangular-like and quasi-1D superlattices in an exfoliated lamella. Interestingly, the rectangular-like lattice reveals perpendicular super-periodicities D1 ~ 17 nm and D2 ~ 9 nm. The width of the quasi-1D channels was found to vary from ~5 nm to ~13 nm, with a super-periodicity D ~ 3 nm. Most importantly, systematic SAED acquisitions reveal a splitting of the electron diffraction signals into multiple components, consisting of doubled and tripled diffraction rings. We interpret these as a clear indicator of structural disorder which results from an interplay of exfoliation-induced structural transitions, namely interlayer-sliding, local layer-rotation (twisting) and significant wrinkling (strain) effects.

Performance improvement of planar silicon nanowire field effect transistors via catalyst atom doping control

Catalytic growth of silicon nanowires (SiNWs), mediated by metallic droplet, provide ideal quasi-1D channels to construct high performance field effect transistor (FET). However, the incorporation of catalytic metal atoms into SiNWs channels has significant impact on the FET characteristics, and thus needs to be better understood and controlled to fulfil its potential for high performance electronics. In this work, we focus on the effect of the incorporation of indium (In) catalytic atoms into planar SiNWs, grown via an in-plane solid-liquid-solid (IPSLS) mechanism, on the FET device performance. It is found that the initial high concentration of p-type In dopants in the as-grown SiNWs led to an equivalent boron (B) doping of 5×1018 cm−3. However, a simple step-wise annealing in oxygen at different temperatures, varied from 620 °C to 920 °C, can help to out-diffuse the dissolved In atoms into the surface SiO2 layer, and reduce the In concentrations down to <5×1017 cm−3, as verified by atom-probe tomography (APT) and four-point-probe measurements. This effective dopant control enables a remarkable device performance improvement of the Schottky barrier FETs, built upon an orderly array of parallel SiNWs of approximately 25 nm in diameter, where the current ratio (Ion/Ioff) has been substantially boosted from 105 to 108, while the subthreshold swing (SS) reduced from 400 mV/dec down to ∼100 mV/dec, with a hole carrier mobility increased from 10 to ∼75 cm2 V−1 s−1 as extracted according to a finite element analysis. These results pave the way for the employment of the catalytic IPSLS SiNWs to serve as high quality 1D channels for high-performance SiNWs-based electronics and sensors.

Strain-Induced Quasi-1D Channels in Twisted Moiré Lattices

Strain-Induced Quasi-1D Channels in Twisted Moiré Lattices

A robust ultra-microporous cationic aluminum-based metal-organic framework with a flexible tetra-carboxylate linker

Al-based cationic metal-organic frameworks (MOFs) are uncommon. Here, we report a cationic Al-MOF, MIP-213(Al) ([Al18(μ2-OH)24(OH2)12(mdip)6]6Cl·6H2O) constructed from flexible tetra-carboxylate ligand (5,5'-Methylenediisophthalic acid; H4mdip). Its crystal structure was determined by the combination of three-dimensional electron diffraction (3DED) and high-resolution powder X-ray diffraction. The structure is built from infinite corner-sharing chains of AlO4(OH)2 and AlO2(OH)3(H2O) octahedra forming an 18-membered rings honeycomb lattice, similar to that of MIL-96(Al), a scarce Al-polycarboxylate defective MOF. Despite sharing these structural similarities, MIP-213(Al), unlike MIL-96(Al), lacks the isolated μ3-oxo-bridged Al-clusters. This leads to an ordered defective cationic framework whose charge is balanced by Cl- sandwiched between two Al-trimers at the corner of the honeycomb, showing strong interaction with terminal H2O coordinated to the Al-trimers. The overall structure is endowed by a narrow quasi-1D channel of dimension ~4.7 Å. The Cl- in the framework restrains the accessibility of the channels, while the MOF selectively adsorbs CO2 over N2 and possesses high hydrolytic stability.

Monolayer-Assisted Surface-Initiated Schiff-Base-Mediated Aldol Polycondensation for the Synthesis of Crystalline sp2 Carbon-Conjugated Covalent Organic Framework Thin Films.

sp2 carbon-conjugated covalent organic frameworks (sp2c-COFs) with superb in-plane π-conjugations, high chemical stability, and robust framework structure are expected to be ideal films/membranes for a wide range of applications including energy-related devices and optoelectronics. However, so far, sp2c-COFs have been mainly limited to microcrystalline powders, and this consequently hampered their performances in devices. Herein, we report a simple and robust methodology to fabricate large-area, free-standing, and crystalline sp2c-COF films (TFPT-TMT and TB-TMT) on various solid substrates (e.g., fluorine-doped tin oxide, aluminum sheet, polyacrylonitrile membrane) by self-assembly monolayer-assisted surface-initiated Schiff-base-mediated aldol polycondensation (namely, SI-SBMAP). The resultant sp2c-COF films show lateral sizes up to 120 cm2 and tunable thickness from tens of nanometers to a few micrometers. Owing to the robust framework and highly ordered quasi-1D channels, the sp2c-COF membrane-based osmotic power generator presents an output power density of 14.1 W m-2 under harsh conditions, outperforming most reported COF membranes as well as commercialized benchmark devices (5 W m-2). This work demonstrates a simple and robust interfacial methodology for the fabrication of sp2c-COF films/membranes for green energy applications and potential optoelectronics.

Sub-angstrom noninvasive imaging of atomic arrangement in 2D hybrid perovskites.

Noninvasive imaging of the atomic arrangement in two-dimensional (2D) Ruddlesden-Popper hybrid perovskites (RPPs) is challenging because of the insulating nature and softness of the organic layers. Here, we demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable nonperturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. Moreover, such a joint technique also allows for the atomic-scale imaging of the electrostatic potential variation across the twin-domain walls, revealing alternating quasi-1D electron and hole channels at neighboring twin boundaries, which may influence in-plane exciton transport and dissociation.

Quasi-1D exciton channels in strain-engineered 2D materials.

Strain engineering is a powerful tool in designing artificial platforms for high-temperature excitonic quantum devices. Combining strong light-matter interaction with robust and mobile exciton quasiparticles, two-dimensional transition metal dichalcogenides (2D TMDCs) hold great promise in this endeavor. However, realizing complex excitonic architectures based on strain-induced electronic potentials alone has proven to be exceptionally difficult so far. Here, we demonstrate deterministic strain engineering of both single-particle electronic bandstructure and excitonic many-particle interactions. We create quasi-1D transport channels to confine excitons and simultaneously enhance their mobility through locally suppressed exciton-phonon scattering. Using ultrafast, all-optical injection and time-resolved readout, we realize highly directional exciton flow with up to 100% anisotropy both at cryogenic and room temperatures. The demonstrated fundamental modification of the exciton transport properties in a deterministically strained 2D material with effectively tunable dimensionality has broad implications for both basic solid-state science and emerging technologies.

Collective Dynamics in Quasi-One-Dimensional Hard Disk System

We present the results of molecular dynamic studies of collective dynamics in a system of hard disks confined to a narrow quasi-one-dimensional (quasi-1D) channel. The computer simulations have been performed for the specific channel width of 3/2 of disk diameter in which the disk arrangement at close packing resembles zigzag ordering characteristic of a vertically oriented two-dimensional (2D) triangular lattice. In such a quasi-1D system, which is intermediate between 1D and 2D arrays of hard disks, the transverse excitations obey very specific dispersion law typical of the usual optical transverse modes. This is in a sharp contrast both to the 1D case, where transverse excitations are not possible, and to the 2D case, where the regular shear waves with a propagation gap were observed. Other peculiarities of the dispersion of collective excitations as well as some results of disk structuring and thermodynamics of the quasi-1D hard disk system are presented and discussed for a range of hard disk densities typical for fluid and distorted crystal states.

Enhanced topological protection in planar quasi-one-dimensional channels with periodically modulated width

We study one dimensional (1D) and quasi-1D periodic structures as possible platforms for the emergence of Majorana bound states with enhanced robustness against disorder and system inhomogeneity. First, using a simple 1D model, we analytically derive the effective parameters characterizing the minibands generated by the periodic potential. We show that, for strong enough periodic potentials, the higher energy minibands hosting Majorana bound states have significant advantages compared to their counterparts in uniform systems, including increased topological gaps, enhanced robustness against disorder, and enlarged parameter space regions consistent with the presence of topological superconductivity. We identify the problem of engineering a strong enough periodic potential as a key roadblock to realizing efficient periodic 1D structures. To address this challenge, we propose an efficient implementation of the periodic potential based on quasi-1D channels realized in 2D semiconductor heterostructures proximity coupled to superconductor strips of periodically modulated width. Our numerical study of the modulated channel device shows excellent agreement with the simple 1D model, reveals a topological phase diagram that is quite insensitive to the details of the confining potential associated with screening by the superconductor, and demonstrates that engineering patterned 2D structures represents a powerful and versatile approach to realizing robust Majorana bound states.

Coulomb blockade in monolithic and monocrystalline Al-Ge-Al nanowire heterostructures

We report the realization of Ge single-hole transistors based on Al-Ge-Al nanowire (NW) heterostructures. The formation of these axial structures is enabled by a thermally induced exchange reaction at 350 °C between the initial Ge NW and Al contact pads, leading to a monolithic and monocrystalline Al-Ge-Al NW. The 25 nm-diameter Ge segment is a quasi-1D hole channel. Its length is defined by two abrupt Al-Ge Schottky tunnel barriers. At low temperatures, the device shows a single hole transistor signature with well pronounced Coulomb oscillations. The barrier strength between the Ge segment and the Al leads can be tuned as a function of the gate voltage VG. It leads to a zero conductance at VG= 0 V to a few quantum conductance at VG= –15 V. When the gate voltage increases from –5 V to –3 V, the charging energy is extracted and it varies from 0.39 meV to 2.42 meV.

Transport Studies of Epi-Al/InAs Two-Dimensional Electron Gas Systems for Required Building-Blocks in Topological Superconductor Networks.

One-dimensional (1D) electronic transport and induced superconductivity in semiconductor nanostructures are crucial ingredients to realize topological superconductivity. Our approach for topological superconductivity employs a two-dimensional electron gas (2DEG) formed by an InAs quantum well, cleanly interfaced with an epitaxial superconductor (epi-Al). This epi-Al/InAs quantum well heterostructure is advantageous for fabricating large-scale nanostructures consisting of multiple Majorana zero modes. Here, we demonstrate transport studies of building-blocks using a high-quality epi-Al/InAs 2DEG heterostructure, which could be put together to realize various proposed 1D nanowire-based nanostructures and 2DEG-based networks that could host multiple Majorana zero modes. The studies include (1) gate-defined quasi-1D channels in the InAs 2DEG and (2) quantum point contacts for tunneling spectroscopy, as well as induced superconductivity in (3) a ballistic Al-InAs 2DEG-Al Josephson junction. From 1D transport, systematic evolution of conductance plateaus in half-integer conductance quanta is observed with Landé g-factor of 17, indicating the strong spin-orbit coupling and high quality of the InAs 2DEG. The improved 2DEG quality leads to ballistic Josephson junctions with enhanced characteristic parameters such as Ic Rn and Iexc Rn, the product of superconducting critical current Ic (and excess current Iexc) and normal resistance Rn. Our results of electronic transport studies based on the 2D approach suggest that the epitaxial superconductor/2D semiconductor system with improved 2DEG quality is suitable for realizing large-scale nanostructures for quantum computing applications.

Extreme Reconfigurable Nanoelectronics at the CaZrO3 /SrTiO3 Interface.

Complex oxide heterostructures have fascinating emergent properties that originate from the properties of the bulk constituents as well as from dimensional confinement. The conductive behavior of the polar/nonpolar LaAlO3 /SrTiO3 interface can be reversibly switched using conductive atomic force microscopy (c-AFM) lithography, enabling a wide range of devices and physics to be explored. Here, extreme nanoscale control over the CaZrO3 /SrTiO3 (CZO/STO) interface, which is formed from two materials that are both nonpolar, is reported. Nanowires with measured widths as narrow as 1.2 nm are realized at the CZO/STO interface at room temperature by c-AFM lithography. These ultrathin nanostructures have spatial dimensions at room temperature that are comparable to single-walled carbon nanotubes, and hold great promise for alternative oxide-based nanoelectronics, as well as offer new opportunities to investigate the electronic structure of the complex oxide interfaces. The cryogenic properties of devices constructed from quasi-1D channels, tunnel barriers, and planar gates exhibit gate-tunable superconductivity, quantum oscillations, electron pairing outside of the superconducting regime, and quasi-ballistic transport. This newly demonstrated ability to control the metal-insulator transition at nonpolar oxide interface greatly expands the class of materials whose behavior can be patterned and reconfigured at extreme nanoscale dimensions.

Shubnikov-de Haas-like Quantum Oscillations in Artificial One-Dimensional LaAlO_{3}/SrTiO_{3} Electron Channels.

The widely reported magnetoresistance oscillations in LaAlO_{3}/SrTiO_{3} heterostructures have invariably been attributed to the Shubnikov-de Haas (SdH) effect, despite a pronounced inconsistency with low-field Hall resistance measurements. Here we report SdH-like resistance oscillations in quasi-1D electron waveguides created at the LaAlO_{3}/SrTiO_{3} interface by conductive atomic force microscopy lithography. These oscillations can be directly attributed to magnetic depopulation of magnetoelectric subbands. Our results suggest that the SdH oscillations in 2D SrTiO_{3}-based systems may originate from naturally forming quasi-1D channels.

Unique features of the generation-recombination noise in quasi-one-dimensional van der Waals nanoribbons.

We describe the low-frequency current fluctuations, i.e. electronic noise, in quasi-one-dimensional ZrTe3 van der Waals nanoribbons, which have recently attracted attention owing to their extraordinary high current carrying capacity. Whereas the low-frequency noise spectral density, SI/I2, reveals 1/f behavior near room temperature, it is dominated by the Lorentzian bulges of the generation-recombination noise at low temperatures (I is the current and f is the frequency). Unexpectedly, the corner frequency of the observed Lorentzian peaks shows strong sensitivity to the applied source-drain bias. This dependence on electric field can be explained by the Frenkel-Poole effect in the scenario where the voltage drop happens predominantly on the defects, which block the quasi-1D conduction channels. We also have found that the activation energy of the characteristic frequencies of the G-R noise in quasi-1D ZrTe3 is defined primarily by the temperature dependence of the capture cross-section of the defects rather than by their energy position. These results are important for the application of quasi-1D van der Waals materials in ultimately downscaled electronics.

Quasi-one-dimensional parahydrogen in nanopores

The low temperature physics of parahydrogen (ph2) confined in cylindrical channels of diameter of the order of 1 nm is studied theoretically by Quantum Monte Carlo simulations. On varying the attractive strength of the wall of the cylindrical pore, as well as its diameter, the equilibrium phase evolves from a single quasi-1D channel along the axis, to a concentric cylindrical shell. It is found that the quasi-1D system retains a strong propensity to crystallization, even though on weakly attractive substrates quantum fluctuations reduce somewhat such a tendency compared to the purely 1D system. No evidence of a topologically protected superfluid phase (in the Luttinger sense) is observed. Implications on the possible existence of a bulk superfluid phase of parahydrogen are discussed

Manipulating quantum channels in weak topological insulator nanoarchitectures

In $strong$ topological insulators protected surface states are always manifest, while in $weak$ topological insulators (WTI) the corresponding metallic surface states are either manifest or hidden, depending on the orientation of the surface. One can design a nanostep on the surface of WTI such that a protected helical channel appears along it. In a more generic WTI nanostructure, multiple sets of such quasi-1D channels emerge and are coupled to each other. We study the response of the electronic spectrum associated with such quasi-1D surface modes against a magnetic flux piercing the system in the presence of disorder, and find a non-trivial, connected spectral flow as a clear signature indicating the immunity of the surface modes to disorder. We propose that the WTI nanoarchitecture is a promising platform for realizing topologically protected nanocircuits immune to disorder.

Symmetries in the collective excitations of an electron gas in core-shell nanowires

We study the collective excitations and inelastic light scattering cross-section of an electron gas confined in a GaAs/AlGaAs coaxial quantum well. These system can be engineered in a core-multi-shell nanowire and inherit the hexagonal symmetry of the underlying nanowire substrate. As a result, the electron gas forms both quasi 1D channels and quasi 2D channels at the quantum well bents and facets, respectively. Calculations are performed within the RPA and TDDFT approaches. We derive symmetry arguments which allow to enumerate and classify charge and spin excitations and determine whether excitations may survive to Landau damping. We also derive inelastic light scattering selection rules for different scattering geometries. Computational issues stemming from the need to use a symmetry compliant grid are also investigated systematically.

Electrostatic effects in coupled quantum dot-point contact-single electron transistor devices

We study the operation of a system where quantum dot (QD) and point contact (PC) defined in a two-dimensional electron gas of a high-mobility GaAs/AlGaAs heterostructure are capacitively coupled to each other and to metallic single electron transistor (SET). The charge state of the quantum dot can be probed by the point contact or single electron transistor. These can be used for sensitive detection of terahertz radiation. In this work, we explore an electrostatic model of the system. From the model, we determine the sensitivity of the point contact and the single electron transistor to the charge excitation of the quantum dot. Nearly periodic oscillations of the point contact conductance are observed in the vicinity of pinch-off voltage. They can be attributed to Coulomb blockade effect in a quasi-1D channel because of unintentional formation of small quantum dot. The latter can be a result of fluctuations in GaAs quantum well thickness.

Semiclassical approach for spin dephasing in a quasi-one-dimensional channel

We derived the spin dephasing time taking Rashba and Dresselhaus spin-orbit interactions (SOI) into account in a quasi-one-dimensional (1D) system. Our analytical solution based on the semiclassical model describes temporal decay of the ensemble-averaged spin polarization, stability of the persistent spin helix state, and anisotropic spin dephasing in a quasi-1D channel, including the limit of a two-dimensional system. We herein proposed quantitative estimation of the cubic Dresselhaus SOI as well as other SOIs by optical measurement of the crystal orientation dependence of spin dephasing time in quasi-1D wires. The analytical approach shows good agreement with the Monte Carlo simulation.