Mamiko Yagi

Development of ballistic hot electron emitter and its applications to parallel processing: active-matrix massive direct-write lithography in vacuum and thin-film deposition in solutions

Abstract. Making the best use of the characteristic features in nanocrystalline Si (nc-Si) ballistic hot electron source, an alternative lithographic technology is presented based on two approaches: physical excitation in vacuum and chemical reduction in solutions. The nc-Si cold cathode is composed of a thin metal film, an nc-Si layer, an n+-Si substrate, and an ohmic back contact. Under a biased condition, energetic electrons are uniformly and directionally emitted through the thin surface electrodes. In vacuum, this emitter is available for active-matrix drive massive parallel lithography. Arrayed 100×100 emitters (each emitting area: 10×10  μm2) are fabricated on a silicon substrate by a conventional planar process, and then every emitter is bonded with the integrated driver using through-silicon-via interconnect technology. Another application is the use of this emitter as an active electrode supplying highly reducing electrons into solutions. A very small amount of metal-salt solutions is dripped onto the nc-Si emitter surface, and the emitter is driven without using any counter electrodes. After the emitter operation, thin metal and elemental semiconductors (Si and Ge) films are uniformly deposited on the emitting surface. Spectroscopic surface and compositional analyses indicate that there are no significant contaminations in deposited thin films.

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.

Deposition of thin Si and Ge films by ballistic hot electron reduction in a solution-dripping mode and its application to the growth of thin SiGe films

To enhance the usefulness of ballistic hot electron injection into solutions for depositing thin group-IV films, a dripping scheme is proposed. A very small amount of SiCl4 or GeCl4 solution was dripped onto the surface of a nanocrystalline Si (nc-Si) electron emitter, and then the emitter is driven without using any counter electrodes. It is shown that thin Si and Ge films are deposited onto the emitting surface. Spectroscopic surface and compositional analyses showed no extrinsic carbon contaminations in deposited thin films, in contrast to the results of a previous study using the dipping scheme. The availability of this technique for depositing thin SiGe films is also demonstrated using a mixture SiCl4+GeCl4 solution. Ballistic hot electrons injected into solutions with appropriate kinetic energies promote preferential reduction of target ions with no by-products leading to nuclei formation for the thin film growth. Specific advantageous features of this clean, room-temperature, and power-effective process is discussed in comparison with the conventional dry and wet processes.

In situ atomic force microscopy imaging of structural changes in metal nanowires during feedback-controlled electromigration

The authors present the real-time atomic force microscopy (AFM) imaging of structural changes in gold (Au) nanowires during the feedback-controlled electromigration (FCE) process. The resistance increases during the FCE process and is associated with drastic changes in the nanowire morphology, suggesting successful control of electromigration (EM) through the FCE scheme. Moreover, the AFM images taken after performing FCE indicate a redeposition of matter along the nanowire in the direction of the anode side. The grains show faceting structures at the anode side. Furthermore, to obtain quantitative information on the height of structures, cross-sections of the nanowire obtained from the AFM images during FCE were investigated. The height evolution of the narrowest part of the wire perpendicular to the electron flow was obtained, showing that void nucleation and void growth along the grain boundaries, which are located on the border of the nanowire, start in the vicinity of the nanowire constriction at the...

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...

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....

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...

Evolution of local temperature in Au nanowires during feedback-controlled electromigration observed by atomic force microscopy

Feedback-controlled electromigration (FCE) has been developed to enable more reproducible fabrication of nanoscale gaps between two metallic electrodes. However, there remains considerable uncertainty about some aspects of the FCE process. In this study, electromigration (EM)-induced mass transport in Au nanowires during the application of a voltage feedback technique was directly observed by in situ atomic force microscopy (AFM). The measured results unambiguously revealed a decrease in the cross-sectional area of the nanoconstriction early in the FCE process. In addition, the local temperature in the biased nanoconstriction was estimated using the diffusive heat transport relation. During FCE, the onset of EM occurred at local temperatures ranging from 420 K to 557 K in a room-temperature environment when the current density was held constant at 108 A/cm2. We found that the local temperature at the onset of EM increased in our results when the Joule heating power in the nanoconstriction was not constant...