Single-electron Charging Effects Research Articles

Periodic Coulomb blockade oscillations observed in single-layered Fe nanodot array

Clear Coulomb blockade characteristics of Fe nanodots in a multiple-nanodot array were achieved. The array was fabricated in a self-assembled manner by Fe deposition between the source and drain electrodes on thermally-grown SiO2/Si substrates. Current oscillations as a function of gate voltage were observed in almost all the devices having an average Fe film thickness (1.8–2.8 nm) less than the threshold thickness showing metallic conduction. The oscillations in major cases were complex owing to contributions from many dots; periodic current oscillations, apparently arising from a single dot, were seen with a certain probability (approximately 10%) at the Fe film thickness range from 2.4 to 2.8 nm which was close to the thickness showing metallic conduction. The periodic oscillations were analyzed by considering the Fermi–Dirac distribution function and this simple oscillation appeared from a certain single dot that existed among many other dots contributing to the background current. The phase of the oscillation period occasionally shifted. This phenomenon is explained by a single-electron charging effect from another dot far away from the single dot under consideration.

Thermodynamic and quantum bounds on nonlinear dc thermoelectric transport

I consider the non-equilibrium DC transport of electrons through a quantum system with a thermoelectric response. This system may be any nanostructure or molecule modeled by the nonlinear scattering theory which includes Hartree-like electrostatic interactions exactly, and certain dynamic interaction effects (decoherence and relaxation) phenomenologically. This theory is believed to be a reasonable model when single-electron charging effects are negligible. I derive three fundamental bounds for such quantum systems coupled to multiple macroscopic reservoirs, one of which may be superconducting. These bounds affect nonlinear heating (such as Joule heating), work and entropy production. Two bounds correspond to the first law and second law of thermodynamics in classical physics. The third bound is quantum (wavelength dependent), and is as important as the thermodynamic ones in limiting the capabilities of mesoscopic heat-engines and refrigerators. The quantum bound also leads to Nernst's unattainability principle that the quantum system cannot cool a reservoir to absolute zero in a finite time, although it can get exponentially close.

Electronic transport in silicon nanocrystals and nanochains

Si nanocrystals and nanochains, prepared by material synthesis, provide a means to define nanoscale devices using growth rather than lithographic techniques. Electronic transport in thin films of Si nanocrystals is influenced strongly by single-electron charging and quantum-confinement effects, and by the grain boundary regions between nanocrystals. This paper reviews electronic transport mechanisms in Si nanocrystal materials. These include thermionic emission of electrons across grain boundaries, space charge limited current, hopping transport, and single-electron charging effects. The fabrication of single-electron devices in Si nanocrystal thin films and nanochains is considered, particularly with regards to their operation at room temperature.

Escape dynamics of a few electrons in a single-electron ratchet using silicon nanowire metal-oxide-semiconductor field-effect transistor

Transport dynamics of a few electrons in a quantum dot are investigated in a single-electron ratchet using silicon nanowire metal-oxide-semiconductor field-effect transistors. Time-resolved measurements in a nanosecond regime are carried out to determine the escape times of the first, second, and third electrons from the quantum dot originally containing three electrons. The escape time strongly depends on the number of electrons due to the single-electron charging effect in the quantum dot, which makes it possible to achieve selective ejection of a desired number of electrons.

Resistance Fluctuations and Aharonov–Bohm-Type Oscillations in Antidot Arrays in the Quantum Hall Regime

Resistance fluctuation phenomenon in antidot lattices in the quantum Hall regime are studied. Magnetoresistance of finite antidot array systems in the quantum Hall plateau transition regime exhibits two types of oscillatory effect. One is the aperiodic resistance fluctuations (RFs) and the other is the Aharonov-Bohm (AB)-type oscillations. Their dependences on the magnetic field and the gate voltage are quite distinct. While the aperiodic RFs are attributed to the complex evolution of the conducting network of compressible channels, the AB-type oscillations are interpreted in terms of edge states formed around individual antidots. The self-consistent screening effect is important for the both phenomenon, whereas, the single electron charging effect plays a minor role in the present case.

Sequential tunneling and inelastic cotunneling in nanoparticle arrays

We investigate transport in weakly coupled metal nanoparticle arrays, focusing on the regime where tunneling is competing with strong single electron charging effects. This competition gives rise to an interplay between two types of charge transport. In sequential tunneling, transport is dominated by independent electron hops from a particle to its nearest neighbor along the current path. In inelastic cotunneling, transport is dominated by cooperative multielectron hops that each go to the nearest neighbor but are synchronized to move charge over distances of several particles. In order to test how the temperature-dependent cotunnel distance affects the current-voltage (I-V) characteristics, we perform a series of systematic experiments on highly ordered close-packed nanoparticle arrays. The arrays consist of ∼5.5 nm diameter gold nanocrystals with tight size dispersion, spaced ∼1.7 nm apart by interdigitating shells of dodecanethiol ligands. We present I-V data for monolayer, bilayer, trilayer, and tetralayer arrays. For stacks 2–4 layers thick we compare in-plane measurements with data for vertical transport perpendicular to the array plane. Our results support a picture whereby transport inside the Coulomb blockade regime occurs by inelastic cotunneling, while sequential tunneling takes over at large bias above the global Coulomb blockade threshold Vt(T) and at high temperatures. For the smallest measurable voltages, our data was fitted well by recent predictions for the temperature dependence zero-bias conductance due to multiple cotunneling. At finite but small bias, the cotunnel distance is predicted to set the curvature of the nonlinear I-V characteristics, in good agreement with our data. The absence of significant magnetic-field dependence up to 10 T in the measured I-V characteristics further supports the picture of inelastic cotunneling events where individual electrons hop no further than the nearest neighbor. At large bias, above the global Coulomb blockade threshold, the I-V characteristics follow power-law behavior with temperature-independent exponent close to two, predicted for sequential tunneling along branching paths that optimize the overall charging energy cost.

Electrochemical Gating in Scanning Electrochemical Microscopy

We demonstrate that scanning electrochemical microscopy (SECM) can be used to determine the conductivity of nanoparticle assemblies as a function of assembly potential. In contrast to conventional electron transport measurements, this method is unique in that electrical connection to the film is not required. The electrochemical potential of the assembly is set through the Nernst equation by the redox mediator present in solution in analogy to the gate electrode in traditional three-terminal transport experiments. In this proof-of-concept report, we show that the conductance of an assembly of small gold nanocrystals (NCs), so-called monolayer-protected clusters (MPCs), is strongly dependent on the electrochemical potential. This is due to the sub-attofarad capacitance of the MPC cores that gives rise to single-electron charging effects at room temperature.

Direct Measurement of Capacitance Parameters in Nanometer-Scale MOSFETs

A simple measurement method is proposed for extracting capacitances in nanometer-scale metal-oxide-semiconductor field-effect transistors (MOSFETs). The method utilizes two serially connected MOSFETs and an optional metal layer above the intermediate node between MOSFETs. Gate-drain overlap capacitance and capacitances around the intermediate node, including one related to the metal layer, can be obtained by measuring the transfer current when two MOSFETs are alternately turned on at high frequency. High sensitivity in the order of attofarad is demonstrated using silicon-on-insulator (SOI) MOSFETs with gate length of 140∼300 nm and channel width of 320 nm. The proposed method is useful not only in optimizing the high-frequency performance of the scale-down devices, but also in estimating the instability (i.e. kT/C noise) and single-electron charging effect in nanometer-scale circuits.

Theory of High Bias Coulomb Blockade in Ultrashort Molecules

We point out that single electron charging effects such as Coulomb Blockade (CB) and high-bias staircases play a crucial role in transport through single ultrashort molecules. A treatment of Coulomb Blockade through a prototypical molecule, benzene, is developed using a master-equation in its complete many-electron Fock space, evaluated through exact diagonalization or full Configuration Interaction (CI). This approach can explain a whole class of non-trivial experimental features including vanishing zero bias conductances, sharp current onsets followed by ohmic current rises, and gateable current levels and conductance structures, most of which cannot be captured even qualitatively within the traditional Self Consistent Field (SCF) approach coupled with perturbative transport theories. By comparing the two approaches, namely SCF and CB, in the limit of weak coupling to the electrode, we establish that the inclusion of strong-correlations within the molecule becomes critical in addressing the above experiments. Our approach includes on-bridge-correlations fully, and is therefore well-suited for describing transport through short molecules in the limit of weak coupling to electrodes.

A surface-gated InSb quantum well single electron transistor

Single electron charging effects in a surface-gated InSb/AlInSb QW structure are reported. This material, due to its large g-factor and light effective mass, offers considerable advantages over more commonly used materials, such as GaAs, for quantum information processing devices. However, differences in material and device technology result in significant processing challenges. Simple Coulomb blockade and quantized confinement models are considered to explain the observation of conductance oscillations in these structures. The charging energy (e2/C) is found to be comparable with the energy spectrum for single particle states (ΔE).

Heat Transistor: Demonstration of Gate-Controlled Electronic Refrigeration

We present experiments on a superconductor-normal-metal electron refrigerator in a regime where single-electron charging effects are significant. The system functions as a heat transistor; i.e., the heat flux out from the normal-metal island can be controlled with a gate voltage. A theoretical model developed within the framework of single-electron tunneling provides a full quantitative agreement with the experiment. This work serves as the first experimental observation of Coulombic control of heat transfer and, in particular, of refrigeration in a mesoscopic system.

Nanospintronics: when spintronics meets single electron physics

As spintronics goes nano, new phenomena are predicted resulting from the interplaybetween spin dependent transport and single electron physics. The long term goal ofmanipulating spins one by one would open a promising path to quantum computing.Towards this end, there is an ever-growing effort to connect spin tanks (i.e. ferromagneticleads) to smaller and smaller objects in order to study spintronics in reduced dimensions.As the dimensions are reduced, spin dependent transport is predicted to interplay withquantum and/or single electron charging effects. We review experiments andtheories on the interplay between Coulomb blockade and spin properties (namelymagneto-Coulomb effects) in structures where a single nano-object is connected toferromagnetic leads. We then discuss briefly future directions in the emerging field ofnanospintronics towards quantum dots, carbon nanotubes and single molecule magnets.

CARBON NANOTUBE BASED NONVOLATILE MEMORY DEVICES

The technology progress and increasing high density demand have driven the nonvolatile memory devices into nanometer scale region. There is an urgent need of new materials to address the high programming voltage and current leakage problems in the current flash memory devices. As one of the most important nanomaterials with excellent mechanical and electronic properties, carbon nanotube has been explored for various nonvolatile memory applications. While earlier proposals of "bucky shuttle" memories and nanoelectromechanical memories remain as concepts due to fabrication difficulty, recent studies have experimentally demonstrated various prototypes of nonvolatile memory cells based on nanotube field-effect-transistor and discrete charge storage bits, which include nano-floating gate memory cells using metal nanocrystals, oxide-nitride-oxide memory stack, and more simpler trap-in-oxide memory devices. Despite of the very limited research results, distinct advantages of high charging efficiency at low operation voltage has been demonstrated. Single-electron charging effect has been observed in the nanotube memory device with quantum dot floating gates. The good memory performance even with primitive memory cells is attributed to the excellent electrostatic coupling of the unique one-dimensional nanotube channel with the floating gate and the control gate, which gives extraordinary charge sensibility and high current injection efficiency. Further improvement is expected on the retention time at room temperature and programming speed if the most advanced fabrication technology were used to make the nanotube based memory cells.

Single electron charging and transport in silicon rich oxide

Single electron charging and single electron tunnelling effects were observed insilicon rich oxide (SRO). The devices used in this study have an Al/SRO/Simetal–oxide–semiconductor-like structure, where the SRO layer was deposited using lowpressure chemical vapour deposition. Two types of Si nanodots (NDs), interface NDs andbulk NDs, were identified by transmission electron microscopy measurements. Underelectric field, charges from the Si substrate are transferred into the interface NDs thatlocate at the interface, and each interface ND traps only one carrier. As the voltageincreases, conduction paths between the Al electrode and the silicon substrate are formed,and the conduction of electrons is via sequential tunnelling through the bulkNDs. Due to the Coulomb blockade effect, only one electron tunnels on eachnanodot at a specific electric field. The transport of the electrons through the Sinanodots is due to the Poole–Frenkel mechanism in the voltage regime studied.

Emerging nanoscale silicon devices taking advantage of nanostructure physics

This paper describes the present status of research on emerging nanoscale silicon devices that take full advantage of new physical phenomena which appear in silicon nanostructures. This new physics includes quantum effects that enhance the performance of MOS transistors and single-electron charging effects that add new function to conventional CMOS circuits. These physical phenomena may be used to extend the scaling and performance limits of conventional CMOS.

Preparation of arrays of metallic nanoparticles exhibiting coulomb blockade at room temperature: an approach based on the self‐organization of metal‐loaded diblock‐copolymers

AbstractThe self‐organization of HAuCl4 loaded poly (styrene)‐block‐poly (2‐vinylpyridine) (PS‐b‐P2VP) diblock‐copolymers resulting in hexagonally ordered arrays of Au nanoparticles is exploited to prepare a system exhibiting single electron charging effects at room temperature. For this purpose, the metallic nanoparticles are deposited on top of an Al2O3 tunnel barrier separating them from a Niobium thin film backelectrode grown on a c‐cut Sapphire substrate. The single electron system is completed by the metal tip of a scanning tunneling microscope (STM), which is positioned right above the nanoparticle and serves as the second electrode. The STM additionally allowed taking current‐voltage I (V) characteristics revealing a large gap around zero bias typical of a Coulomb blockade. This interpretation is supported by the good agreement between experimental I (V) curves and their theoretical description. Copyright © 2006 John Wiley & Sons, Ltd.

Controllable room-temperature metallic quantum dot

We demonstrate a technique for fabricating a controllable metallic dot coupled to metallic electrodes. This techniques enables one to explore the current-voltage characteristics while varying the distance between the dot and the leads. The fabrication technique combines atomic force microscope nanoscribing and nanomanipulation as well as electrochemical methods and enables very fine dot-lead coupling control. The measured current-voltage curves exhibit stable single-charge phenomena such as Coulomb blockade and Coulomb staircase which are clearly observed even at room temperature. This technique may provide an opportunity to study single electron charging effects in regimes that are currently unaccessible.

Quantum kinetic description of Coulomb effects in one-dimensional nanoscale transistors

In this paper, we combine the modified electrostatics of a one-dimensional transistor structure with a quantum kinetic formulation of Coulomb interaction and nonequilibrium transport. A multi-configurational self-consistent Green's function approach is presented, accounting for fluctuating electron numbers. On this basis we provide a theory for the simulation of electronic transport and quantum charging effects in nanotransistors, such as a gated carbon nanotube and whisker devices and one-dimensional CMOS transistors. Single-electron charging effects arise naturally as a consequence of the Coulomb repulsion within the channel.

Coulomb blockade oscillations in ultrathin gate oxide silicon single-electron transistors

Ultrathin oxide-gated (thickness ∼6nm) point-contact junctions have been fabricated to explore single-electron charging effects in strongly gate-dot-coupled polycrystallinesilicon transistors. Current–voltage (I–V) measurements show periodic current oscillations near room temperature. Analysis of the energy-level spacing relates the electron charging energy to a quantum dot of size ∼8nm, and also suggests electron tunneling is via the first excited state. These low-power ∼30pW and low-cost devices can be useful for the next generation nanoelectronics.

Coulomb oscillations based on band-to-band tunneling in a degenerately doped silicon metal-oxide-semiconductor field-effect transistor

We report Coulomb oscillations based on band-to-band tunneling through a valence band in silicon metal-oxide-semiconductor field-effect transistors. Degenerately p+-doped channel and n+-doped source/drain enables band-to-band tunneling, which can play a major role in the transport between the channel and source/drain. The formation of tunnel barriers and a quantum dot in a single-electron transistor structure originates from two p+–n+ tunnel junctions and a p+-doped channel with mesoscopic dimension, respectively. Coulomb-blockade oscillations with multiple peaks were clearly observed at liquid nitrogen temperature. Using the electrical and thermal characterization of the quantum dot, single-electron charging effect based on band-to-band tunneling is confirmed.