Mitsuki Ito

Simultaneous fabrication of nanogap electrodes using field-emission-induced electromigration

We present a simple technique for simultaneous control of the electrical properties of multiple Ni nanogaps. This technique is based on electromigration induced by a field emission current and is called “activation.” Simultaneous tuning of the tunnel resistance of multiple nanogaps was achieved by passing a Fowler–Nordheim (F-N) field emission current through an initial group of three Ni nanogaps connected in series. The Ni nanogaps, which had asymmetrical shapes with initial gap separations in the 80–110-nm range, were fabricated by electron-beam lithography and a lift-off process. By performing the activation procedure, the current–voltage properties of the series-connected nanogaps were varied simultaneously from “insulating” to “metallic” via “tunneling” properties by increasing the preset current of the activation procedure. We can also simultaneously control the tunnel resistances of the series-connected nanogaps, which range from a resistance of the order of 100 TΩ–100 kΩ, by increasing the preset current from 1 nA to 30 μA. This tendency is quite similar to that of individually activated nanogaps, and the tunnel resistance values of the simultaneously activated nanogaps were almost the same at each preset current. These results clearly imply that the electrical properties of the series-connected nanogaps can be controlled simultaneously via the activation procedure.

High-throughput nanogap formation by field-emission-induced electromigration

High-throughput nanogap formation is reported for simultaneous fabrication of integrated nanogap arrays. Ten series-connected nanogaps with butterfly and bottle shapes were integrated by using electromigration induced by a field emission current (“activation”). Initially, ten series-connected butterfly-shaped nickel (Ni) nanogaps were fabricated with electron-beam lithography and lift-off processes. Activation with a preset current of 300 nA reduced the separation of the gaps to <10 nm. Similar results for bottle-shaped nanogaps indicated that integration of nanogaps using activation is not dependent on nanogap shape. The activation method was also used for the mass production of 30 identical nanogaps. Overall, the distance between the Ni nanogap electrodes was completely controlled by activation.

Structural tuning of nanogaps using electromigration induced by field emission current with bipolar biasing

We report a new method for fabrication of Ni nanogaps based on electromigration induced by a field emission current. This method is called “activation” and is demonstrated here using a current source with alternately reversing polarities. The activation procedure with alternating current bias, in which the current source polarity alternates between positive and negative bias conditions, is performed with planar Ni nanogaps defined on SiO2/Si substrates at room temperature. During negative biasing, a Fowler-Nordheim field emission current flows from the source (cathode) to the drain (anode) electrode. The Ni atoms at the tip of the drain electrode are thus activated and then migrate across the gap from the drain to the source electrode. In contrast, in the positive bias case, the field emission current moves the activated atoms from the source to the drain electrode. These two procedures are repeated until the tunnel resistance of the nanogaps is successively reduced from 100 TΩ to 48 kΩ. Scanning electron...

Resistive switching effects in electromigrated Ni nanogaps

Recently, resistive switches have been constructed from nanogap electrodes. We have already reported a simple method for the fabrication of nanogaps with well-controlled tunnel resistance, which is called “activation”. Since the activation is considered to be the method of transferring atoms across the nanogap, it is expected that a resistive switching effect is caused in the nanogaps controlled by the activation method. In this study, we explore a resistive switching effect of Ni nanogaps using the activation. First, by applying the activation, the tunnel resistance of Ni nanogaps was sufficiently decreased with increasing the field emission current passing through the nanogaps. Then, during the subsequent voltage sweep, the characteristic electrical properties exhibited current increase and decrease, showing the typical set (higher conduction) and reset (lower conduction) processes, respectively. Actually, set/reset state can be controlled with applying pulse voltages, and the conduction states were successfully read out. In this method, the endurance was more than 100 cycles and the resistive ratio was obtained to be about 103. The results suggest that resistive switching properties are successfully observed using nanogaps controlled with activation method.

Fabrication of single-electron transistors with electromigrated Ni nanogaps

We analyze single-electron transistors (SETs) fabricated with electromigrated Ni nanogaps using the Korotkov and Nazarov (KN) model. First, we investigate nanogap-based SETs consisting of multiple Ni islands placed between the source and drain electrodes by a field-emission-induced electromigration technique known as “activation.” After the activation procedure is performed using a preset current Is of 3 μA, the drain current-drain voltage characteristics of SETs with single-island structures are obtained and analyzed by using the KN model and considering the offset charges on the islands. We determine the fitting parameters obtained by the KN model from the electrical properties of the SETs. The parameters can be explained using the geometrical structures of the SETs that are observed in both scanning electron and atomic force microscopy images after the activation procedure. This approach allows the electrical and structural properties of the single-island structures of the SETs fabricated using the activation method to be determined.

Investigation of electromigration induced by field emission current flowing through Au nanogaps in ambient air

We developed a simple and controllable nanogap fabrication method called “activation.” In the activation technique, electromigration is induced by a field emission current passing through the nanogaps. Activation enables the electrical properties of Ni nanogaps in a vacuum to be controlled and is expected to be applicable to Au nanogaps even in ambient air. In this study, we investigated the activation properties of Au nanogaps in ambient air from a practical point of view. When activation was performed in ambient air, the tunnel resistance of the Au nanogaps decreased from over 100 TΩ to 3.7 MΩ as the preset current increased from 1 nA to 1.5 μA. Moreover, after activation in ambient air with a preset current of 500 nA, the barrier widths and heights of the Au nanogaps were estimated using the Simmons model to be approximately 0.5 nm and 3.3 eV, respectively. The extracted barrier height is smaller than that of 4.6 eV resulting from activation in a vacuum and much lower than the work function of bulk Au....

Controlling the tunnel resistance of suspended Ni nanogaps using field-emission-induced electromigration

The authors report on the ability to control the tunnel resistance of suspended Ni nanogaps by field-emission-induced electromigration. This method is called “activation.” Suspended Ni nanogaps are ideal for investigating activation because the leakage currents flowing through the substrates are suppressed in these structures. The tips of suspended Ni nanogap electrodes are isolated from the SiO2 substrates, so it is expected that the suspended Ni nanogaps act as isolated tunnel junctions during activation. After undergoing activation, the suspended Ni nanogaps clearly exhibited tunneling I–V properties. Furthermore, the authors were able to tune the tunnel resistance of the suspended Ni nanogaps using the activation method. When the applied voltage was swept, the device current switched between high- and low-resistance states. The results imply that activation is a viable method for modulating the electrical properties of suspended Ni nanogaps at the nanometer scale.

Simultaneous fabrication of nanogaps using field-emission-induced electromigration

We present a simple and easy technique for the simultaneous control of electrical properties of multiple Ni nanogaps. This technique is based on electromigration induced by a field emission current, which is so-called “activation”. The tuning of tunnel resistance of nanogaps was simultaneously achieved by passing a Fowler-Nordheim (F-N) field emission current through three initial Ni nanogaps connected in series. The Ni nanogaps having an asymmetrical shape with an initial gap separation of 80-110 nm were fabricated by electron-beam (EB) lithography and lift-off process. By performing the activation, current-voltage properties of series-connected nanogaps were simultaneously varied from “insulating” to “metallic” through “tunneling” properties with increasing the preset current of the activation. Furthermore, we can simultaneously control the tunnel resistance of the series-connected nanogaps ranging from the order of 100 TΩ to 100 kΩ with increasing the preset current from 1 nA to 30 μA. This tendency is quite similar to that of individually activated nanogaps, and it should be noted that tunnel resistance of simultaneously activated nanogaps was almost the same at each preset current. These results clearly imply that the electrical properties of series-connected nanogaps can be simultaneously controlled by the activation procedure.

Simultaneous arrayed formation of single-electron transistors using electromigration in series-connected nanogaps

A field-emission-induced electromigration method (activation) is reported for integrating single-electron transistors operating at T = 298 K. The field emission currents between the two opposite electrodes of each series-connected nanogap are tuned to accumulate Ni atoms within the gaps. For ten series-connected nanogaps, the resistance (VD/ID), obtained using the current-voltage (ID-VD) properties of these nanogaps during the activation procedure, is observed to decrease on activation. As a result, island structures are formed within the gaps, and the nanogap-based single-electron transistors can be integrated, when atom migration occurs at the tip of each nanogap electrode. After activating the ten series-connected nanogaps with a preset current, IS = 1 nA, current suppression (representative of coulomb blockade) is not observed in the fabricated devices. On the other hand, coulomb blockade, which depicts the charging and discharging of the nanoislands, can be observed at room temperature, after activation with a preset current, IS = 150 nA. Furthermore, the modulation properties of the coulomb blockade voltage by the gate voltage are also determined at room temperature. These results experimentally demonstrate the arrayed formation of ten single-electron transistors operating at room temperature, constituting a significant step toward the practical realization of single-electron-transistor-based systems.A field-emission-induced electromigration method (activation) is reported for integrating single-electron transistors operating at T = 298 K. The field emission currents between the two opposite electrodes of each series-connected nanogap are tuned to accumulate Ni atoms within the gaps. For ten series-connected nanogaps, the resistance (VD/ID), obtained using the current-voltage (ID-VD) properties of these nanogaps during the activation procedure, is observed to decrease on activation. As a result, island structures are formed within the gaps, and the nanogap-based single-electron transistors can be integrated, when atom migration occurs at the tip of each nanogap electrode. After activating the ten series-connected nanogaps with a preset current, IS = 1 nA, current suppression (representative of coulomb blockade) is not observed in the fabricated devices. On the other hand, coulomb blockade, which depicts the charging and discharging of the nanoislands, can be observed at room temperature, after activat...

A new computing architecture using Ising spin model implemented on FPGA for solving combinatorial optimization problems

Recently, the new computing architecture using Ising spin model has been attracting considerable attention. It is well known that the Ising spin model represents the physical properties of ferromagnetic materials in terms of statistical mechanics. In this model, the spin states are varied in order to minimize the system energy automatically, by the interaction between connected adjacent spins. The new computing scheme maps combinatorial optimization problems based on Ising model and solves these problems by using ground state search operations exploiting its convergence property. In this report, a new computing architecture using Ising spin model was implemented using field-programmable gate array (FPGA), and Ising computing using FPGA was investigated to solve combinatorial optimization problems.