Single-electron Charging Effects Research Articles

Tunneling and Optical Spectroscopy of InAs and InAs/ZnSe Core/Shell Nanocrystalline Quantum Dots

We combine scanning tunneling spectroscopy and photoluminescence excitation spectroscopy (PLE) to study the electronic level structure and single electron charging effects of InAs and novel InAs/ZnSe core/shell nanocrystal quantum dots. The two techniques provide complementary information on the electronic structure of these systems. In the tunneling spectra of core InAs nanocrystals grown by colloidal chemistry, 2–7 nm in diameter, we directly identify atomic-like electronic quantum dot states with s and p character as two- and sixfold charging multiplets. These measurements are correlated with the low temperature PLE data and, surprisingly, excellent agreement was observed between spacings of levels detected by the two techniques, yielding new information on the quantum dot level structure. The combined tunneling and optical spectroscopy approach was also applied to the study of InAs/ZnSe core/shell nanocrystals, which have a high fluorescence quantum yield in the near IR range. The tunneling spectroscopy shows changes in their electronic structure compared to the cores with the s–p gap closing in thicker shells. This observation is supported by PLE spectroscopy, establishing the effectiveness of the combined optical-tunneling spectroscopy approach in the study of quantum dots.

Electrical and structural properties of solid phase crystallized polycrystalline silicon and their correlation to single-electron effects

Single-electron transistors have been fabricated in solid phase crystallized polycrystalline silicon films deposited on SiO2 layers grown on silicon substrates. The single-electron transistors consist of lateral side-gated nanowires. A Coulomb staircase is observed at 4.2 K, which is fully modulated by the side-gate voltage. Two-period conductance oscillations are observed in nanowires fabricated on 10-nm-thick buried oxide layers, while single-period oscillations are observed in nanowires fabricated on 40-nm-thick buried oxide layers. The two-period oscillations are attributed to the formation of a charge layer in the silicon substrate. The single-electron effects are also studied as a function of the nanowire dimensions and annealing or oxidation treatments. The effects are correlated to the structure of the polysilicon film, characterized using transmission electron microscopy, Raman spectroscopy, and electron spin resonance analysis. These measurements demonstrate the significance of single-electron charging effects on electron transport in nanometer-scale complementary metal-oxide semiconductor systems.

Carrier Transport in Ultra-Thin Nano/Polycrystalline Silicon Films and Nanowires

ABSTRACTCarrier transport was investigated in two different types of ultra-thin silicon films, polycrystalline silicon (poly-Si) films with large grains > 20 nm in size and hydrogenated nanocrystalline silicon (nc-Si:H) films with grains 4 nm – 8 nm in size. It was found that there were local non-uniformities in grain boundary potential barriers in both types of films. Single-electron charging effects were observed in 30 nm × 30 nm nanowires fabricated in 30 nm-thick nc-Si:H films, where the electrons were confined in crystalline silicon grains encapsulated by amorphous silicon. In contrast, the poly-Si nanowires of similar dimensions showed thermionic emission over the grain boundary potential barriers formed by carrier trapping in grain boundary defects.

Defect-Tolerant Single-Electron Charging at Room Temperature in Metal Nanoparticle Decorated Biopolymers

Gold nanoparticles assembled on a biopolymer template (see Figure) between metal electrodes on an insulating substrate are shown to exhibit unambiguous single electron charging effects that are found to depend on the nanoparticle properties and the geometrical contraints imposed by the biopolymer. The results support the idea of using nanoparticles in conjuction with biomolecular organization to produce nanoscale systems with defect-tolerant current–voltage behavior.

Defect-Tolerant Single-Electron Charging at Room Temperature in Metal Nanoparticle Decorated Biopolymers

Gold nanoparticles assembled on a biopolymer template (see Figure) between metal electrodes on an insulating substrate are shown to exhibit unambiguous single electron charging effects that are found to depend on the nanoparticle properties and the geometrical contraints imposed by the biopolymer. The results support the idea of using nanoparticles in conjuction with biomolecular organization to produce nanoscale systems with defect-tolerant current–voltage behavior.

Single-electron charging effects in Si MOS devices

Single-electron charging of small quantum dot structures has been observed for many years. Only recently, however, have the effects been observed in Si device structures suitable for integration into other Si technologies. In this talk, the background of work in Si will be reviewed and its applications to unique device and circuit architectures will be discussed. Experimental results obtained using double-gated quantum dots embedded within a Si MOSFET will be discussed. While the present results are primarily at low temperature, they promise the application of single-electron technology to more integrated circuitry.

Single electron memory characteristic of siliconnanodotnanowire transistor

The authors have successfully fabricated an Si nanodot nanowire memory transistor using an inorganic SiO2 EB resist process for the formation of a 15 nm wide Si nanowire and an RTO process of an ultra-thin a-Si:H film for the ultra-small Si nanodot formation. In the fabricated Si nanodevice, a very large single electron charging effect, i.e. ΔVth of 0.72 V, is experimentally observed, and ΔVth of 2.2 V at 3 electrons is confirmed at room temperature. The developed technology may be useful for opening the way towards future Si nanodevices.

To Fill the Gap Between Si-ULSI and Nanodevices

A gap between silicon ULSI and nanodevices and a measure to fill the gap are described. It is shown that there are three phases in the development of silicon nanodevices. In the first phase where we are now, the quantum and single electron charging effects are just troublesome phenomena for ULSI. The most critical and important phase is the second phase, in which new physical phenomena are positively utilized in ULSI and nanodevices are merged into ULSI. The third phase will be the realization of silicon nanodevices that fully utilize the quantum and single electron charging effects. Some examples of favorable quantum and single electron effects that would break the miniaturization limitation of ULSI are reviewed.

TO FILL THE GAP BETWEEN SI-ULSI AND NANODEVICES

A gap between silicon ULSI and nanodevices and a measure to fill the gap are described. It is shown that there are three phases in the development of silicon nanodevices. In the first phase where we are now, the quantum and single electron charging effects are just troublesome phenomena for ULSI. The most critical and important phase is the second phase, in which new physical phenomena are positively utilized in ULSI and nanodevices are merged into ULSI. The third phase will be the realization of silicon nanodevices that fully utilize the quantum and single electron charging effects. Some examples of favorable quantum and single electron effects that would break the miniaturization limitation of ULSI are reviewed.

Magnetoresistance oscillations in double ferromagnetic tunnel junctions with layered ferromagnetic nanoparticles

Tunnel magnetoresistance in double tunnel junctions with layered ferromagnetic nanoparticles was investigated. The sample comprises two ferromagnetic electrodes (CoFe/Fe) separated by an Al/sub 2/O/sub 3/ insulating layer in which layered Co/sub 80/Pt/sub 20/ nanoparticles are embedded. The nanoparticles are ellipsoidal with an average diameter of 3.7 nm, and composes a well-defined layer. At 10 K, we observed magnetoresistance oscillation with respect to the bias voltage. The observed TMR oscillation, with a period of 1.6 mV, accompanied oscillations of the conductance. We consider that the conductance oscillation may originate from the single electron charging effect.

Single electron charging effect in individual Si nanocrystals.

AbstractWe present a detailed study of the electronics properties of individual silicon nano- crystals (nc-Si) elaborated by Low Pressure Chemical Vapor Deposition on 1.2 nm thick SiO2 grown on Si (100). The combination of ultra thin oxide layers and highly doped substrates allows imaging the hemispherical dots by Scanning Tunneling Microscopy. By analyzing the STM images, we deduce a size distribution, which ranges between 3 and 6 nm with a surface density around 1012 cm-2. Spectroscopic studies of single dots are made by recording the I(V) curves on the Si nanocrystal accurately selected with the metallic tip. These I(V) curves obtain on a single dot, exhibit Coulomb blockade and resonant tunneling effects. Coulomb pseudo gaps, Ec, between 0.15 and 0.2 V are measured for different dots. From the width and height of the staircases observed at bias greater than Ec, 60 meV and 40 pA respectively, capacitance of 0.5 to 1 aF and tunnel resistance of 3.5×108 and 5.7×109 Ohms are measured within the orthodox approximation for asymmetric junctions. We have determined, from experimental measurements, the energy of the first level confined in nc-Si.

Theory of electronic transport in semiconductor nanostructures.

AbstractWe review the orthodox theory of single charge tunneling in a semiconductor quantum dot and we extend it to treat both single electron and hole charging effects. We analyze recent tunneling spectroscopy experiments. We show that for sufficiently large bias voltages V, both electrons and holes can tunnel into the quantum dot, leading to specific features in the I(V) curve. We present detailed simulations of the I(V) curves based a tight binding method for the electronic structure. A very good agreement is obtained with available experiments on InAs nanocrystals, allowing a complete interpretation of the spectra. Finally, we make some predictions concerning Si nanocrystals.

強磁性ナノ粒子層を介した二重トンネル接合のＴＭＲ

Tunnel magnetoresistance (TMR) in double tunnel junctions with layered ferromagnetic nanoparticles was investigated. The sample comprised two ferromagnetic electrodes (CoFe/Fe) separated by an Al2O3 insulating layer in which a layer of Co80Pt20 nanoparticles was embedded. The nanoparticles were ellipsoidal with an average diameter of 3.7 nm, and composed a well-defined layer. The TMR and the tunnel resistance of the sample showed steep increases around 100 K, and weak temperature dependence below 50 K. The zero-bias TMR rose from 1 % at room temperature to 18 % at 20 K. Below 20 K, we observed magnetoresistance oscillation with respect to the bias voltage. The observed TMR oscillations, with periods of 1.6 and 15 mV, accompanied oscillations of the conductance. We consider that the conductance oscillations may originate from the coexistence of the discrete energy level of the nanoparticles and the discrete electrostatic potential due to the single-electron charging effect.

Fabrication and electron transport in multilayer silicon-insulator-silicon nanopillars

We report the fabrication of sub-0.1-μm-diam silicon nanopillars with a polycrystalline silicon and silicon nitride multilayer structure. The polycrystalline silicon layers, which are 20 nm thick and are separated by 2–3 nm-thick silicon nitride tunnel barriers, make potentially useful structures for the observation of single-electron charging effects. Measurements of electron transport at 4.2 K between contacts at the top and bottom of the pillars show a zero current region and current steps attributed to Coulomb blockade.

Single-electron effects in heavily doped polycrystalline silicon nanowires

We have observed single-electron charging effects in heavily doped polycrystalline silicon nanowires at 4.2 K. Wires of approximately 20 nm by 30 nm active cross section were defined by electron-beam lithography and thermal oxidation in standard polycrystalline silicon material. We have measured a Coulomb staircase and periodic current oscillations with gate bias, attributed to localized carrier confinement resulting from a statistical variation in the intergrain tunnel barriers. A sharp change in the current oscillation period is seen and we speculate that it is due to electrostatic screening of the gate bias by grain boundary defect states.

Single electron tunneling through Ge nanocrystal fabricated by cosputtering method

Electrical transport properties of thin SiO 2 films containing Ge nanocrystals have been studied. Steplike structures were observed on current–voltage characteristics. The structures changed depending on the size of nanocrystals. The observed step structures could be explained by a model considering the interplay between the resonant tunneling of electrons through the discrete electronic states of Ge nanocrystals and the single electron charging effect of Ge nanocrystals.

298 K operation of Nb/Nb oxide-based single-electron transistors with reduced size of tunnel junctions by thermal oxidation

We present the successful operation of Nb/Nb oxide-based single-electron transistors at room temperature. At first, devices were fabricated by scanning probe microscope based anodic oxidation technique. Then, the effective area of tunnel junctions was further shrunken by thermal oxidation. Ultrasmall tunnel junctions were easily obtained utilizing additional thermal oxidation process, and single-electron charging effects were observed by means of the modulation of Coulomb blockade voltages at room temperature.

Nonequilibrium Green's Function Approach to Resonant Magnetotunneling Through a Quantum Dot**The project supported by National Natural Science Foundation of China

Keldysh's nonequilibrium Green's function is applied to studying resonant magnetotunneling through a quantum dot. We propose a microscopic Anderson-impurity-like Hamiltonian in which two kinds of the on-site Coulomb interactions are introduced: the interaction of two electrons at the same Landau level with different spins, , and the interaction for the electrons between different Landau levels, . curves are obtained under different magnetic fields in asymmetrical structures which can display both the energy quantization and single-electron charging effect. The theoretical results are in good agreement with the experiments qualitatively. The linear-response conductance is also discussed.

Single-electron charging effects in Nb/Nb oxide-based single-electron transistors at room temperature

We have reported the single-electron charging effects in Nb/Nb oxide-based single-electron transistors (SETs) at room temperature (T=298 K). The SETs were first fabricated by a scanning probe microscope based anodic oxidation. Then, the miniaturization of tunnel junctions was performed by thermal oxidation. Ultra-low-capacitance tunnel junctions were easily obtained by utilizing both kinds of oxidation processes, which realizes room-temperature Nb-based SETs.

Observation of the single electron charging effect in nanocrystalline silicon at room temperature using atomic force microscopy

We have measured current–voltage (I–V) characteristics of individual surface oxidized nanocrystalline silicon (nc-Si) particles, which were grown in the gas phase of a plasma and which had well-defined grain sizes of less than 10 nm and a regular octahedron shape. The I–V characteristics were measured at room temperature using atomic force microscopy with conductive tips, which allows the grain size of nc-Si particles to also be measured directly. The measured I–V characteristics show staircaselike features. The period of the staircase increases with decreasing grain size, which is consistent with the single electron charging effect in nc-Si.