Tunneling Current Research Articles

Determination of the Fermi velocity of graphene on MoS2 using dual mode scanning tunneling spectroscopy

We have measured the Fermi velocity of graphene on MoS2 using dual mode scanning tunneling spectroscopy. In our method, the tunnel current (I) and the derivate of the current to the gap width (dI/dz) are measured simultaneously as a function of sample bias (V) at a constant tip-substrate distance. Owing to the difference in lattice constants between graphene (2.461 Å) and MoS2 (3.161 Å), a moiré structure is found with a periodicity of 1.07 nm and a twist angle of 4°. Our experiments reveal that the electronic band structure of graphene remains intact in the bandgap region of MoS2. The Fermi velocity of graphene on MoS2 is determined to be 1.2±0.1×106 m/s.

Edge-Trimmed Nanogaps in 2D Materials for Robust, Scalable, and Tunable Lateral Tunnel Junctions

Lateral tunnel junctions are fundamental building blocks for molecular electronics and novel sensors, but current fabrication approaches achieve device yields below 10%, which limits their appeal for circuit integration and large-scale application. We here demonstrate a new approach to reliably form nanometer-sized gaps between electrodes with high precision and unprecedented control. This advance in nanogap production is enabled by the unique properties of 2D materials-based contacts. The large difference in reactivity of 2D materials’ edges compared to their basal plane results in a sequential removal of atoms from the contact perimeter. The resulting trimming of exposed graphene edges in a remote hydrogen plasma proceeds at speeds of less than 1 nm per minute, permitting accurate control of the nanogap dimension through the etching process. Carrier transport measurements reveal the high quality of the nanogap, thus-produced tunnel junctions with a 97% yield rate, which represents a tenfold increase in productivity compared to previous reports. Moreover, 70% of tunnel junctions fall within a nanogap range of only 0.5 nm, representing an unprecedented uniformity in dimension. The presented edge-trimming approach enables the conformal narrowing of gaps and produces novel one-dimensional nano-trench geometries that can sustain larger tunneling currents than conventional 0D nano-junctions. Finally, the potential of our approach for future electronics was demonstrated by the realization of an atom-based memory.

Bias Voltage Dependence of Sensing Characteristics in Tunneling Magnetoresistance Sensors.

One of the characteristic features of tunneling magnetoresistance (TMR) sensors is a strong influence of bias voltage on tunneling current. Since fundamental sensing characteristics of the sensors are primarily determined by the tunneling current, the bias voltage should impact these characteristics. Previous research has indeed showed the influence of the bias voltage on the magnetic field detection and sensitivity. However, the effect has not been investigated for nonlinearity and hysteresis and the influence of bias voltage polarity has not yet been addressed. Therefore, this paper systematically investigates the dependence of field sensitivity, nonlinearity, hysteresis and magnetic field detection of CoFeB/MgO/CoFeB-based magnetoresistance sensors on bias voltage magnitude and polarity. The sensitivity and field detection of all sensors improved significantly with the bias, whereas the nonlinearity and hysteresis deteriorated. The sensitivity increased considerably (up to 32 times) and linearly with bias up to 0.6 V. The field detection also decreased substantially (up 3.9 times) with bias and exhibited the minimum values for the same magnitude under both polarities. Significant and linear increases with bias were also observed for nonlinearity (up to 26 times) and hysteresis (up to 33 times). Moreover, not only the voltage magnitude but also the polarity had a significant effect on the sensing characteristics. This significant, linear and simultaneous effect of improvement and deterioration of the sensing characteristics with bias indicates that both bias voltage magnitude and polarity are key factors in the control and modification of these characteristics.

IMPROVEMENT OF SURFACE POTENTIAL ENERGY OF INDIUM TIN OXIDE THIN FILM MODIFIED WITH ORGANIC SEMICONDUCTOR MATERIAL BASED ON PHENYL GROUP

This study focuses on surface characterization of modified indium tin oxide (ITO) for potential applications as anode in organic electronics devices. [4-iodophenyl]-silanetriol molecules were coated by self-assembled monolayers method on ITO surface. Kelvin Probe Microscopy, Scanning Tunneling Microcopy and X-ray Photoelectron Spectroscopy techniques were used to analyze the modified ITO surface. The results show that organic semiconductor material based on Phenyl group enhance surface morphology and increase the surface potential energy of ITO around [Formula: see text][Formula: see text]meV and contribute the tunneling current that inject from Fermi energy level of the ITO. This study includes the influence of surface interactions on electrochemical and spectral features of compounds. The results are important in developing surface structures in amorphous layers, better understanding the mechanism of creation of such structures on ITO surface.

Surface structure of Gd20Co80 alloy

Gd 20 Co 80 alloy was studied applying experimental methods of spectral ellipsometry, atomic force and scanning tunneling microscopy. The experimental results exhibit the eutectic two-phase structure of this alloy. Clusters of a phase with a lower content had a lesser concentration of free carriers, which resulted in smaller tunnel currents during the measurements. To analyze the experimental data, a theoretical approach was developed, which was based both on the quantum-mechanical methods of configurational interaction and on three-diagonal Toeplitz matrices formalism. This approach allowed us to describe in detail the energy bands formation process in solid clusters with a relatively small quantity of atoms, which as a consequence enabled to describe the Shockley surface states as well as the existence of a surface layer with partially formed energy bands. Spectral-ellipsometric measurements of Gd 20 Co 80 alloy thin films confirmed a significant difference between the measured optical constants for 20-nm thick films and larger films. Quantum-mechanical molecular calculus allowed to obtain optical constants for several supercells of Gd-Co chemical compounds and confirmed the acquired experimental and theoretical results.

Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy.

Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics.

Broken-Gap PtS2/WSe2 van der Waals Heterojunction with Ultrahigh Reverse Rectification and Fast Photoresponse.

Broken-gap van der Waals (vdW) heterojunctions based on 2D materials are promising structures to fabricate high-speed switching and low-power multifunctional devices thanks to its charge transport versus quantum tunneling mechanism. However, the tunneling current is usually generated under both positive and negative bias voltage, resulting in small rectification and photocurrent on/off ratio. In this paper, we report a broken-gap vdW heterojunction PtS2/WSe2 with a bilateral accumulation region design and a big band offset by utilizing thick PtS2 as an effective carrier-selective contact, which exhibits an ultrahigh reverser rectification ratio approaching 108 and on/off ratio over 108 at room temperature. We also find excellent photodetection properties in such a heterodiode with a large photocurrent on/off ratio over 105 due to its ultralow forward current and a comparable photodetectivity of 3.8 × 1010 Jones. In addition, the response time of such a photodetector reaches 8 μs owing to the photoinduced tunneling mechanism and reduced interface trapping effect. The proposed heterojunction not only demonstrates the high-performance broken-gap heterodiode but also provides in-depth understanding of the tunneling mechanism in the development of future electronic and optoelectronic applications.

Effects of Process-Induced Defects on Polarization Switching in Ferroelectric Tunneling Junction Memory

The scaling effects of ferroelectric tunnel junction (FTJ) memory are investigated by evaluating remanent polarization ( ${P} _{\mathrm {r}}$ ), coercive field ( ${E} _{\mathrm {c}}$ ), and polarization switching speed. Through the analysis on dc tunneling currents and frequency responses of ${E} _{\mathrm {c}}$ and ${P} _{\mathrm {r}}$ for FTJs with various sizes, it is found that polarization switching mechanism (domain nucleation limit) is not changed by the width reduction. However, by the length scaling, domain wall spreading-limited switching becomes dominant by process-induced damaged edge regions, leading to the larger ${P}_{\mathrm {r}}$ and the faster switching speed.

Semiconductor-less vertical transistor with ION/IOFF of 106

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15–400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments.

Role of compensating current in the weak Josephson coupling regime: An extended study on excitonic Josephson junctions

Huang's experiment [Phys. Rev. Lett. $\bf 109$, 156802 (2012)] found, in the quantum Hall bilayer of the Corbino geometry, the interlayer tunneling currents at two edges are coupled to each other and one of two tunneling currents is referred to as the compensating current of the other. Our another work[arXiv:2006.15329] has explained this exotic coupling phenomenon as a result of excitonic Josephson effect induced by interlayer tunneling current. In this paper, we study the same setup -- excitonic Josephson junction -- but in the weak Josephson coupling regime, which occurs for large junction length. Interestingly, we find the compensating current drives the other edge to undergo a nonequilibrium phase transition from a superfluid to resistive state, which is signaled by an abrupt jump of the critical tunneling current. We also identify the critical exponent and furthermore offer more experimental prediction.

A MEMS Nanopositioner With Integrated Tip for Scanning Tunneling Microscopy

Slow Z-axis dynamics of Scanning Tunneling Microscope (STM) is a key contributing factor to the slow scan speed of this instrument. A great majority of STM systems use piezotube nanopositioners for scanning. The piezotube bulkiness along with the mass of STM tip assembly restrict the overall Z-axis bandwidth of the system to about 1 kHz. This limited bandwidth slows down the STM response to the sample topography changes. In this paper, we report a microfabrication process to build a Microelectromechanical-System (MEMS) nanopositioner for Z-axis positioning in STM with a tenfold bandwidth, and a similar range of motion. The MEMS device features an integrated nanometer-sharp in-plane Si tip, compatible with conventional batch fabrication processes. In addition, a novel electrical isolation technique is developed to electrically isolate the tip from the rest of this device. This enables us to provide a separate routing for tunneling current signal, enabling potential applications in parallelism. The fabricated MEMS device achieves 1.6 μm motion with its first in-plane resonance beyond 10 kHz. The capability of this MEMS nanopositioner to replace the Z-axis of STMs is demonstrated through establishing controlled and stable tunneling current on a graphite sample in ambient conditions. [2020-0345]

Multiplexed Silicon Nanowire Tunnel FET-Based Biosensors With Optimized Multi-Sensing Currents

In this study, silicon nanowire (SiNW) FET-based and SiNW tunnel FET (TFET)-based biosensors are co-integrated with CMOS circuits by using top-down approached and CMOS-compatible back-end process simultaneously. The possibility of multiplexed sensing is verified with the fabricated FET and TFET biosensors. For multiplexed-sensing, two separate sensing materials which react with two distinct bio-targets are formed by partially capping the gold on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film through a lift-off process. Then two bio-receptors which selectively combine to the gold and the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> are deposited. After the reaction of each biomolecule to each receptor, the changes of saturation and gate-induced-drain-leakage (GIDL) currents are monitored in the FET sensor. It is experimentally confirmed that two different biomolecules are independently detectable by the changes of the saturation and the GIDL currents in the FET sensor. To solve the dependence of the gold formation position on the sensitivity as well as the large current difference between the saturation and the GIDL currents, we demonstrated the TFET biosensor which uses the changes of tunneling and ambipolar currents generated in the source and the drain end. As a result, it is clearly revealed that two different biomolecules can be detected without interference, regardless of the position of the gold layer by the changes of the tunneling and the ambipolar currents with almost equivalent sensing current level.

Tunneling current-induced entanglement between electronic and vibrational modes in coupled molecules

The formation of entanglement between the electronic and vibrational subsystems of two interacting molecules localized between tunneling contact leads was theoretically analyzed using the Keldysh diagram technique. The time evolution of concurrence after ‘switching on’ the coupling between the molecules was investigated. It was revealed that non-zero concurrence can be present in the system in the resonant case, even if the molecules are connected by the leads. It was also shown that the stationary value of concurrence can be directly expressed by the stationary tunneling current. It reveals non-monotonic behavior with increasing coupling between the molecule’s electronic states. In the regime of small tunneling rates between the molecules and the leads, ‘switching on and off’ the coupling between molecules multiple times, while detecting one of the molecules’ charge states after each ‘on’ and ‘off’ cycle, results in the appearance of non-classical phonon statistics and opens the possibility of creating a vibrational mode in a Fock state.

Low-Power Vertical Tunnel Field-Effect Transistor Ternary Inverter

In this study, vertical tunnel FET-based ternary CMOS (T-CMOS) is introduced and its electrical characteristics are investigated using TCAD device and mixed-mode simulations with experimentally calibrated tunneling parameters. This new T-CMOS utilizes two different types of tunneling currents to form three different output voltage states: (1) source-to-drain tunneling current; and (2) conventional source-to-channel tunneling current. To form a half supply voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> ) output voltage during the inverter operation, the n-/p-type devices of the proposed T-CMOS are designed to have constant source-to-drain tunneling current regardless of gate voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) by using nitride spacer between gate and drain. Also, typical binary inverter operation is performed using the source-to-channel tunneling. In voltage transfer characteristics (VTC), it is confirmed that there is the clear half V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> state after matching the tunneling currents of the n-/p-type devices. It is revealed that the stable half V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DD</sub> state cannot be achievable if the currents are mismatched by gate workfunction, gate dielectric thickness, and interface trap variations, implying that the current matching between n-/p-type devices is crucial to obtain stable ternary operations.

Mechanism of dark current dependence on reverse voltage in mid-wavelength infrared HgCdTe mesa PIN avalanche diode

Mid-wavelength infrared HgCdTe avalanche photodiodes (APDs) are becoming increasingly significant in research and practical applications for the high linear gain, low excess noise and high quantum efficiency. However, the performance of HgCdTe APDs is largely limited by the voltage dependence of dark current. It has been well known that the tunneling current is the main component of the dark current of the p-i-n diode. However, the current contribution of different physical mechanism still needs to be indicated in different reverse voltages. In this paper, we mainly study the mechanism of the relationship between dark current and reverse voltage of mid-wavelength infrared $${\text{Hg}}_{1 - x} {\text{Cd}}_{x} {\text{Te}}$$ (x = 0.3) mesa p-i-n avalanche diode. Several physical models are used for the dark current numerical simulation. The results show that for detection of $${\text{Hg}}_{1 - x} {\text{Cd}}_{x} {\text{Te}}$$ mesa p-i-n detector under low temperature, dark current is mainly caused by tunneling current. With the reverse voltage increasing, the main component of dark current turns from trap-assisted tunneling current to direct band-to-band tunneling. The transition voltage is positively related to the energy level difference between trap energy level and conduction band. The results may provide a guideline to improve the performance of the mid-wavelength infrared APD detector by ranking the importance of different structural parameters.

Large In-Plane Piezoelectricity of 2D Janus Znbri: Multiscale&amp;nbsp;Prediction

The piezoelectric properties of two-dimensional (2D) Janus ZnBrI are predicted by the multiscale method. Based on the density functional theory, the electronic properties and piezoelectric coefficients of 2D Janus ZnBrI are calculated. It is found that the 2D Janus ZnBrI has a large in-plane piezoelectric coefficient d 11 , up to 145.85 pm/V, which is about 40 times that of 2D 2H-MoS 2 . Based on the finite element method and multi-physical field simulation, the large piezoelectric response of 2D Janus ZnBrI is shown intuitively. Based on the nonequilibrium Green's function, the device model of metal-semiconductor-metal is established, and the I-V response of the device model is calculated, which shows the modulation of piezoelectric effect on tunneling current. The simulation results show that 2D Janus ZnBrI is a 2D piezoelectric material with application potential. This multiscale prediction method can comprehensively predict other 2D piezoelectric materials.

The Nature of CVC Nonlinearity in Low-Voltage Scanning Tunneling Spectroscopy of Semiconductors

A new model of field emission in a scanning tunnelling microscope was developed. The model describes the tunnelling current from a surface of semiconductor (semimetal) and allows estimating the preexponential factor in the expression for the tunneling probability. It is shown that this factor is directly related to the degree of localization of the electron density and determines the shape of the local tunnel current-voltage characteristics (LTCVCs) at low voltages. The model allows separating the contributions of surface electronic states of different symmetry (dimension) of the tunnelling current. The practical application of the model is demonstrated by the example of mathematical processing of the LTCVCs of HOPG surface containing different structural defects.

A charge sensor integration to tunable double quantum dots on two neighboring InAs nanowires.

A single quantum dot serving as a charge sensor is integrated to scalable double quantum dots using local top finger-gate techniques on two neighboring pure-phase InAs nanowires. The single dot built on one nanowire capacitively couples one of the double dots constructed on another nanowire via a metal bridge gate. The charge occupation states of double quantum dots can be accurately monitored by the sensor even in a few-electron regime in which transport tunneling current through the double dots vanishes. In the tunneling spectroscopy of double dots, electron inter dot tunneling process is absent; however, it can be illustrated by the sensor in terms of a transconductance line between the two closest triple points. Thus, tunnel coupling strength between the double dots is quantitatively extracted from the detectable charge transition. The highly tunable multiple quantum dots with integrated charge sensors on InAs nanowires could be an essential building block for quantum information processing technology.

Precession coupled spin current in spin torque driven magnetic tunnel junctions

A spin-torque switchable magnetic tunnel junction contains two ferromagnetic electrodes across a barrier that supports spin-polarized tunnel current. The spin-torque induced magnetic switching of its more agile, or “free” layer provides the “write” mechanism. Often the dynamics of the non-switching “reference” layer is also important. Here, we illustrate such dynamics involving both the free and the reference layers by using an exchange-coupled two-macrospin-moment numerical model, described by a set of Landau–Lifshitz–Gilbert (LLG) equations, together with a stochastic Langevin-field for finite temperature. Damping-like spin-transfer torque is included for both moments. In steady-state, the coupled precession is shown to reduce effective spin-current delivered to the free layer due to a precessional resonant spin-current back flow. This back-flow of spin current preferentially affects the parallel state dynamics. It is not directly related to the reference layer’s thermal stability, nor its spin-torque switching threshold, as determined by the total anisotropy energy and magnetic volume. Rather, the spin-current reduction relates primarily to the matching of precession frequency between the free- and the reference-layer. Therefore, a desirable materials choice is to avoid anisotropy fields giving the free and the reference layer similar dynamic frequencies, so as to prevent such resonance-related spin-current loss.

Triggering Amino Acid Detection by Atomistic Resolved Tunneling Current Signals in Graphene Nanoribbon Devices for Peptide Sequencing

The transversal tunneling current flowing across a narrow nanogap is employed for amino acid recognition in polypeptides during their translocation across the nanogap. An ideal device, consisting of a nanogap in graphene nanoribbons, is considered for this purpose to exploit the ideal atomistic resolution of 2D electrodes. Using the nonequilibrium Green function scheme, based on the density functional theory, we have studied the trigger signal that can be collected from the backbone of some template peptides, showing that this signal is practically independent of the polarity of AAs considered. Both X (Asn, Ala, Asp, Ser) homopeptides and Gly–X heteropeptides have been considered, showing that the major role is played by the electrons injected through the CαH group and the partially resonant peptide bonds. Because of the smaller overall transversal size in relation to the graphene nanoribbon width, the X side chain is only partially involved in the electron injection for Gly–X heteropeptides, but no role is played by the polar ends. These results encourage the search of unique triggering signals related to the passage of each residue during translocation by atomic resolved tunneling currents.