Single Electron Tunneling Phenomena Research Articles

Electron transport in physically-defined double quantum dots on a highly doped silicon-on-insulator substrate

We study electron transport in physically-defined silicon quantum dots (QDs) on a highly doped silicon-on-insulator (SOI) substrate. We show that the QDs can be obtained as designed without unintentional localized states caused by fluctuating dopant potentials even when a highly doped SOI substrate is used. We observe the single electron tunneling phenomena both in the single QDs (SQDs) and in the double QDs (DQDs). The charging energy in the SQDs is ∼18 meV as estimated from the Coulomb diamond. This enables us to further estimate that the diameter of the SQDs is ∼35 nm, which is consistent with the designed fabrication specifications if the voltage condition is taken into account. A change of the charged state in the DQDs is detected using the SQD as a charge sensor. A periodic honeycomb-like charge stability diagram is obtained, which indicates that we achieved the fabrication of DQDs without unintentional localized states.

Physical Model for Reset State of Ta2O5/TiO2-Stacked Resistance Random Access Memory

We investigated the conduction mechanism of a Ta2O5/TiO2-stacked resistance random access memory (ReRAM) device and found that its highly resistive state can be attributed to tunnel barriers induced in the filament, since single-electron tunneling phenomena was observed in the current–voltage characteristics at low temperatures and the resistance depended only slightly on temperature. We also found that the largest tunnel barrier, whose resistance is more than 1000 times larger than the second largest one, is located at the interface between the Ta2O5 layer and TiO2 layer and that variation in the resistance was caused by variation in the tunnel barrier width.

Single-electron tunneling phenomena on preformed gold clusters deposited on dithiol self-assembled monolayers

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on preformed nanometer sized gold clusters. The clusters are deposited on Au(1 1 1) films that are coated with a self-assembled monolayer of 1,4-benzenedimethanethiol molecules, thereby creating a double barrier tunnel junction. In this configuration electrons can only move between the scanning tunneling microscope tip and the substrate through tunneling via the cluster. Due to the very small cluster size this results in pronounced single-electron tunneling effects. Anomalies in the I( V) curves measured above a single cluster can be consistently described in terms of charging of the double barrier junction for the majority of the particles. Both Coulomb blockade and Coulomb staircase with step widths in the 200–600 meV range were observed on various individual clusters. The results agree with the commonly used semiclassical theory of charging effects in a double barrier tunnel junction. Furthermore, for a limited number of clusters, additional conductance peaks are observed which are attributed to strong electron confinement inside the nanoscale clusters.

Single-electron transistors fabricated with sidewall spacer patterning

We have implemented a sidewall spacer patterning method for novel dual-gate single-electron transistor (DGSET) and metal–oxide–semiconductor-based SET (MOSET) based on the uniform SOI wire, using conventional lithography and processing technology. A 30 nm wide silicon quantum wire is defined by a sidewall spacer patterning method, and depletion gates for two tunnel junctions of the DGSET are formed by the doped polycrystalline silicon sidewall. The fabricated DGSET and MOSET show clear single-electron tunneling phenomena at liquid nitrogen temperature and insensitivity of the Coulomb oscillation period to gate bias conditions. On the basis of the phase control capability of the sidewall depletion gates, we have proposed a complementary self-biasing method, which enables the SET/CMOS hybrid multi-valued logic (MVL) to operate perfectly well at high temperature, where the peak-to-valley current ratio of Coulomb oscillation severely decreases. The suggested scheme is evaluated by SPICE simulation with an analytical DGSET model, and it is confirmed that even DGSETs with a large Si island can be utilized efficiently in the multi-valued logic.

Single-electron transistor based on a silicon-on-insulator quantum wire fabricated by a side-wall patterning method

We propose and implement a promising fabrication technology for geometrically well-defined single-electron transistors based on a silicon-on-insulator quantum wire and side-wall depletion gates. The 30-nm-wide silicon quantum wire is defined by a combination of conventional photolithography and process technology, called a side-wall patterning method, and depletion gates for two tunnel junctions are formed by the doped polycrystalline silicon sidewall. The good uniformity of the wire suppresses unexpected potential barriers. The fabricated device shows clear single-electron tunneling phenomena by an electrostatically defined single island at liquid nitrogen temperature and insensitivity of the Coulomb oscillation period to gate bias conditions.

Study on spin dependent tunneling and Coulomb blockade in granular systems with restricted tunneling paths

We describe the microfabrication process of a granular nanobridge structure consisting of electrodes separated by a nanometer-sized gap in which a thin insulating Co–Al–O granular film is filled, and discuss the observed Coulomb blockade with a clear threshold voltage ( V th) and the enhanced tunnel magnetoresistance of about 30% at a voltage slightly above V th. Recent results of our scanning tunneling microscopy studies to clarify the topographic features and single electron tunneling phenomena on the surface of Co–Al–O granular films are also presented.

Coulomb staircases by lateral tunneling between adjacent nanoclusters formed on Si surfaces

We have investigated the oscillatory tunneling current–voltage characteristics on metal nanoclusters formed on Sb-terminated Si(100) surfaces by using scanning tunneling microscopy/spectroscopy. Through the systematic investigation on a variety of cluster configuration environments, we suggest that the lateral tunneling between adjacent clusters dominantly contributes to the occurrence of the single electron tunneling phenomena. In the single clusters formed on Si surfaces, we detected only current oscillations, which must be distinguished from Coulomb staircases. Those results strongly suggest that Coulomb staircases should not originate from the direct conduction of electrons through Schottky junction between the single clusters and Si substrates in contrast to other previous reports.

Two-dimensional nanowire array formation on Si substrate using self-organized nanoholes of anodically oxidized aluminum

A highly ordered two-dimensional array of 48 nm Cu wires was successfully fabricated on Si substrate by the usage of anodic oxidation of aluminum (Al) for the first time. Anodic oxidation was carried out for Al sputtered film on Si substrate covered by a thin thermally oxidized SiO 2 film, which was very effective to protect Si substrate from anodic oxidation. A highly ordered array of nanoholes was formed by the two steps Al anodic oxidation, and finally Cu was deposited by electroless plating in nanoholes which aspect ratio was 2.5. The present method suggests possibility of a large area two-dimensional array of quantum dots or wires on semiconductor substrate, which are considered to be a key technology for future ULSIs operated by single electron tunneling phenomena.

Room temperature observation of single electron tunneling effect in self-assembled metal quantum dots on a semiconductor substrate

We report on the observation of room-temperature single electron tunneling phenomena in a metal-insulator-metal-semiconductor double-junction structure. The nanosized Ag dots were self-assembled on a Sb-terminated Si(100) surface, and the Coulomb gap and staircases were observed in the local current–voltage (I–V) measurements using scanning tunneling microscopy. The I–V characteristics exhibiting the single electron tunneling behavior vary significantly with the variation of the measurement position within the same Ag droplet. These phenomena are well described by the tip-dot(Ag)-Si double-junction picture.