Lateral Tunnel Junctions Research Articles

The spin transport properties of Ni(111)/SnSe/Ni(111) lateral tunnel junction

Spin-resolved transport properties of the armchair-edged SnSe lateral contact with two magnetic Ni(111) leads are investigated by density functional theory combined with nonequilibrium Green's function method. The spin-polarized currents, magnetoresistances and k-resolved transmission spectrums are simulated in spin parallel and antiparallel configurations. It is found that the heterostructure exhibits a stable spin injection efficiency which can reach up to 80% at low bias range in spin parallel configuration, suggesting the large spin-diffusion length of SnSe. The magnetoresistance can be maintained stably at about 40% at a wide bias range. These results make SnSe material possible to be used in spintronic devices.

The Spin Transport Properties of Ni(111)/Snse/Ni(111) Lateral Tunnel Junction

The Spin Transport Properties of Ni(111)/Snse/Ni(111) Lateral Tunnel Junction

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.

Tunnel field-effect transistors for sensitive terahertz detection

The rectification of electromagnetic waves to direct currents is a crucial process for energy harvesting, beyond-5G wireless communications, ultra-fast science, and observational astronomy. As the radiation frequency is raised to the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics becomes challenging and requires alternative rectification protocols. Here, we address this challenge by tunnel field-effect transistors made of bilayer graphene (BLG). Taking advantage of BLG’s electrically tunable band structure, we create a lateral tunnel junction and couple it to an antenna exposed to THz radiation. The incoming radiation is then down-converted by the tunnel junction nonlinearity, resulting in high responsivity (>4 kV/W) and low-noise (0.2 pW/sqrt{{rm{Hz}}}) detection. We demonstrate how switching from intraband Ohmic to interband tunneling regime can raise detectors’ responsivity by few orders of magnitude, in agreement with the developed theory. Our work demonstrates a potential application of tunnel transistors for THz detection and reveals BLG as a promising platform therefor.

Resistive switching in lateral junctions with nanometer separated electrodes

Resistive switching in lateral tunnel junctions is reported. Nanogap tunnel junctions made of Au/SiO2/Au and Au/TiO2/Au were patterned by electrical-beam-lithography (EBL) and a controlled electromigration process. Depending on the substrate material, different reproducible resistive switching characteristics were observed under vacuum conditions. While for TiO2 substrates bipolar resistive switching was observed, nanogap junctions on SiO2 substrates showed resistive switching characteristics with a negative differential resistance. The role of the substrate material with respect to the resistive switching behavior is discussed in the framework of the electrical breakdown. All experiments were performed under vacuum to suppress parasitic effects due to charged particles in ambient air. Nanogap resistive switching devices are promising candidates for densely integrated memresistive systems such as non-volatile resistive random memories (RRAMs), field programmable arrays (FPGAs), or artificial neural networks (ANNs).

Low energy dynamics of two-dimensional lateral tunnel junctions

We report on the low temperature tunneling characteristics of two-dimensional lateral tunnel junctions (2DLTJs) consisting of two coplanar two-dimensional electron systems separated by an in-plane tunnel barrier. The tunneling conductance of the 2DLTJ exhibits a characteristic dip at small voltages—consistent with the phenomenon of zero-bias anomaly in low-dimensional tunnel junctions—and a broad conductance peak at the Coulombic energy scale. The conductance peak remains robust under magnetic fields well into the quantum Hall regime. We identify the broad conductance maxima as the signature of the pseudogap in the tunneling density of states below the characteristic Coulomb interaction energy of the 2DLTJ.

Interaction Effects and Pseudogap in Two-Dimensional Lateral Tunnel Junctions

Tunneling characteristics of a two-dimensional lateral tunnel junction are reported. A pseudogap on the order of Coulomb energy is detected in the tunneling density of states (TDOS) when two identical two-dimensional electron systems are laterally separated by a thin energy barrier. The Coulombic pseudogap remains robust well into the quantum Hall regime until it is overshadowed by the cyclotron gap in the TDOS. The pseudogap is modified by the in-plane magnetic field, demonstrating a nontrivial effect of the in-plane magnetic field on the electron-electron interaction.

Ti/TiO x single-electron devices produced with a step-edge-cut-off (SECO) method

Single-electron current oscillations were for the first time observed in metal nanostructures produced by a step-edge-cut-off (SECO) method. The capacitance of an SECO-made Ti/TiOx tunnel junction was found to be as small as 10 aF at a linewidth of about 150 nm, which is nearly 50 times less than the capacitance obtained by shadow masking at the same linewidth. It is suggested that the produced lateral tunnel junctions are mesoscopic conductors with hopping conduction.

Mesoscopic electronic devices made by local oxidation of a titanium film covering gold islands

The local oxidation produced by the tip of an atomic force microscope scanning on a thin metallic film allows to define narrow oxide lines, thus providing a method to fabricate lateral tunnel junctions. In such devices, with rather thick tunnel junction barriers, the electrical transport is governed by thermally activated hopping rather than by direct electron tunneling. In this letter we show that tunneling barriers can also be produced with Ti films covering small gold islands. The gold islands significantly shorten the effective tunneling distance, allowing to observe temperature-independent electron tunneling across the lateral barriers. The mixed Ti/Au tunnel barriers reveal Coulomb blockade effects which may be used for single-electron devices consisting of a single oxide line.

Prospects for atomically ordered device structures based on STM lithography

A new approach to future nanodevice fabrication is proposed which could, if successful, realize practical 3-dimensional control over electron transport in silicon and silicon-based heterostructures down to the atomic scale. The process we envision consists of the following steps: (1) STM-induced desorption of hydrogen atoms to expose bare silicon dangling bonds on an H-terminated surface, (2) dosing with a few Langmuirs of PH 3, AsH 3, or B 2H 6 to deposit self-ordered arrays of dopant precursors onto the STM-exposed regions, (3) low-temperature silicon overgrowth using techniques designed to limit dopant redistribution to the atomic scale, (4) repetition to produce 3-dimensional electronic architectures. Reproducible electrical properties are made possible by the large Bohr diameter for bound state wavefunctions on the individual dopants, approximately 6.6 nm for electrons with in-plane mass m*=0.190 m 0, and preliminary calculations indicate that this same distance scale may apply to wavefunction coupling across lateral gaps between lithographically defined high-density donor/acceptor sheets. Potential advantages include: (1) elimination of disorder in random impurity doping, (2) greatly simplified all-UHV processing, (3) elimination of surface effects and offset charges through epitaxial encapsulation, (4) lateral tunnel junctions with reproducible impedance, and (5) structuring of electron transport in any desired 2D or 3D configuration.

Lateral tunnel junction produced by electron-beam-induced deposition

Electron-beam-induced deposition using a WF6 precursor molecule was applied to making metal/insulator/metal tunnel junctions for single-electron transport devices. Single wires 8 nm high and about 13 nm wide were produced on a SiO2 substrate with Au/Cr electrode pads and their conductance showed a rapid increase of about five orders of magnitude as electron-beam (EB) doses increased between 5 and 15 pC shot. To estimate the deposit thickness distribution, spatial thickness distribution in spot exposure was defined and obtained for specified EB doses. From this function, a single-wire resistivity at 230 and 300 K was determined to be 6×10−4 Ω cm at doses exceeding 15 pC/shot. Single-tunnel junctions, where space with a 2.5 nm increment was at the center of single wire, were produced. The electrical characteristics of these single junctions were fitted to a Fowler–Nordheim plot the absolute value of whose gradient gradually increased with increasing space width. The barrier height of this junction was estimated to be 0.17–0.2 eV, lower than that for SiO2/W junctions. This might be caused by the change from the metallic deposit to the insulator for the single wire as a function of the EB dose. This deposition technique enabled us to fabricate a transistor structure where dots were located in space and a side gate electrode was also deposited. The structure showed Coulomb oscillation even at 230 K and Monte Carlo simulation of this device showed reasonable agreement with the experiment, assuming appropriate circuit parameters of gate capacitance and tunnel resistance.

Metal point contacts and metal-oxide tunnel barriers fabricated with an AFM

We report the fabrication of atomic point contacts and lateral tunnel junctions by using anodic oxidation of thin metal films with an atomic force microscope. In situelectrical measurements were used as feedback to control the fabrication of metal nanowires that were subsequently anodized through their cross section to form point contacts and tunnel junctions. When the conductance of an Al device is reduced below ∼ 5×10 −4S it starts to decrease in discrete steps of ∼ 2 e 2/ h. In some devices we are able to stabilize the conductance at a value near 2 e 2/ hwhich corresponds to a single, atomic-sized conducting channel. Similar experiments on Ti devices result in a continuous decrease of the conductance and the formation of stable tunnel junctions. This continuous behavior is a result of the large series resistance and the small oxide barrier height of the Ti/TiO xsystem.

Fabrication of Lateral Tunnel Junctions and Measurement of Coulomb Blockade Effects

In order to operate single-electron devices at high temperatures, a fabrication process for laterally arranged double tunnel junctions is presented. The dimensions of the island can be reduced relatively easily down to the resolution limit of lithography, resulting in extremely low capacitances. The basic idea of the fabrication process is a very thin metallic nanowire which is made discontinuous by means of a sharp-edged groove in the substrate. Tunnel junctions are formed at either edge of the groove, and the tunnel barrier gap can be adjusted via the depth of the groove and the deposition parameters of the metal film. Coulomb blockade was observable, which is persistent up to temperatures above 77 K.